Category Archives: Monthly Column

March 2024: Warming Wisconsin Weather

Rising Temperature IconI’ve been pretty amazed by the recent temperatures in Wisconsin.  Here in Madison, we hit 70°F in late February.  There have been some colder days this winter, but in general temperatures have been warmer than I remember, particularly compared to what I recall from when I moved to Madison in the mid-1970s.  The warm winter of 2023-2024 followed an extraordinarily warm and dry summer in 2023.  In the context our warmer weather trends, I’ve been trying to think of how plant diseases might be impacted.  Three diseases come to mind that likely could be more problematic in the coming years if warmer conditions continue. 

Southern blight  

This disease is by far the most destructive fungal disease that I see in home landscapes.  The fungus that causes Southern blight (Athelia rolfsii) has a wide host range, and virtually any herbaceous plant in the path of this fungus is likely to be killed.  The fungus can also kill small trees under the right conditions.  The first time I encountered Southern blight was about 20 years ago when it popped up in a flowerbed at the Allen Centennial Garden.  By the time I visited the garden to make a diagnosis, the fungus had killed off every plant in about a 40 square foot area.  As I examined the dead plants, I noted characteristic sclerotia (i.e., resting structures) of the fungus (they look like Osmocote® pellets) all over the bases of the dead plants and in a layer across the mulch in the bed. 

Early in my diagnostic career (i.e., the late 1990s), I occasionally saw Southern blight in Wisconsin.  However, because the pathogen is adapted to tropical and sub-tropical climates, it did not survive the harsh Wisconsin winters at that time.  Extended periods where high temperatures were in the negative digits were common, and these temperatures killed the fungus.  Nowadays, with our relatively mild winters, A. rolfsii seems to be better able to overwinter in the state.  That’s a huge problem because the fungus is easily moved around (via those pesky sclerotia), and if it is not killed by cold weather, then there is increased risk of spread and subsequent wholesale plant destruction. 

Oak wilt

Oak wilt has been present in Wisconsin for the entirety of my diagnostic career, and I have watched the Wisconsin Department of Natural Resources map its spread in the state over the years.  Historically in Wisconsin, sap beetles have moved the oak wilt fungus (Bretziella fagacearum) from location to location.  These insects are attracted to wounded trees and drop off B. fagacearum as they feed on sap in the wounds.  The fungus infects, leading to blockages of a tree’s water-conducting tissue, subsequent branch dieback, and eventual tree death.  Once an oak tree is infected, B. fagacearum can move to nearby oak trees through root grafts. 

I am concerned that our warming weather pattern could contribute to increased risk for oak wilt in a couple of ways.  Warmer summer temperatures are often associated with an increase the frequency and severity of summer storms.  More (and more violent) storms increase the risk of damaged oak trees, which increases the risk of sap beetles visiting these trees and dropping off the oak wilt fungus.  In addition, sap beetles are not the only insect that can move the oak wilt fungus around.  Certain oak bark beetles have been documented to move the fungus as well.  These bark beetles have traditionally had a more southern native range and have not been found extensively in Wisconsin.  However, PJ Liesch, the UW-Madison insect diagnostician, recently commented to me that Pseudopityophthorus minutissimus (one of the oak bark beetles involved in transmission of the oak wilt fungus) has been very abundant in Wisconsin over the last few years.  He’s seen lots of this insect in oak samples that have come into his lab.  How important oak bark beetles are in transmitting Bretziella fagacearum in Wisconsin at this time is not known.  However, they will likely become increasingly important as they become more established and abundant in the state. 

Thousand cankers disease

This fungal disease of black walnut trees has not be reported in Wisconsin to date; the closest state with confirmed reports is Indiana.  Efforts to prevent the introduction of the pathogen (Geosmithia morbida) into Wisconsin currently involve limiting importation of raw walnut wood into the state.  The hope is to prevent introduction of G. morbida, as well as the walnut twig beetle (Pityophthorus juglandis).  This insect moves the thousand cankers disease fungus from tree to tree.  P. juglandis is native to Arizona, California, and New Mexico, but in the last decade or so has been introduced into states east of the Mississippi River (e.g., Indiana, Ohio, Tennessee).  While movement of walnut wood currently poses the greatest threat for introduction of G. morbida and P. juglandis into Wisconsin, milder long-term weather patterns open up the possibility of eventual natural spread of P. juglandis (carrying G. morbida) into the state.  I think it’s only a matter of time before thousand cankers disease pops up in Wisconsin. 

As our long-term weather pattern changes, I’ll be on the lookout for new diseases and changes in established diseases in Wisconsin.  I suggest that you watch your own gardens and landscapes for new diseases that you have not encountered in the past.  You’ll likely see changes.  If you’d like help in identifying your new finds, feel free to contact the PDDC for help.  Check out the clinic website at for details on sample submission, or call (608) 262-2863 or email for advice and guidance.  To keep up to date on clinic activities and resouces, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Happy spring and happy disease hunting, everyone!

February 2024: Garden Expo Afterglow

Afterglow IconThe 2024 PBS Wisconsin Garden and Landscape Expo (February 9 – 11) has come and gone.  While I spent last week physically recuperating from Garden Expo (standing on a concrete floor gets rougher each year), my mind and spirit came away from the event rejuvenated and invigorated.  The weather was perfect with four warm, snow-free days (including set up day) that attracted almost 15,000 people to the event. 

As I do each year, I coordinated and staffed the UW-Madison Plant Disease Diagnostics Clinic (PDDC) booth at Garden Expo.  Organizers Amanda Balistreri and Heather Robbins again generously provided me with a double booth for my display at no charge.   This year I had an endcap booth in a prime location near the entrance to the exhibit hall.  The configuration of the booth (i.e., with visibility from three sides) provided me even more space than normal to display and highlight the full range of services and educational materials that I provide.   The booth allowed visitors the opportunity to get up close and personal with plant disease specimens and photos; to peruse and walk away with free UW Plant Disease Facts fact sheets (116 titles again this year); and to check out my plant disease-themed limerick book, building block plant diseases, and UW Plant Disease Facts medallions.  I also provided fliers on the monthly PDDC Plant Disease Talks that I organize, as well as the Ask-the-Experts Q&A sessions I participate in with colleagues from the UW-Madison Division of Extension Horticulture Program.  Finally, I played a looped video of segments that I filmed over the years with Shelley Ryan on Wisconsin Public Television’s The Wisconsin Gardener.  It’s been 10 years since Shelley’s passing, and it’s comforting to know that the outreach event that she and her show inspired is still going strong. 

As always, I was at Garden Expo all three days (noon until 7 pm on Friday, 9 am until 6 pm on Saturday, and 10 am until 4 pm on Sunday).  A special shout out goes to Chad Teubert of the UW Russell Labs Hub for helping transport and unload my booth materials on Thursday at the Alliant Energy Center, and for helping unload things back at Russell Labs the Monday after Expo.  Also, thanks go to Lisa Johnson of Extension Dane County who helped staff my booth when I was off giving talks. 

During the course of Garden Expo, I gave three talks:  Vegetable Diseases, Growing Healthy Plants:  Basics in Plant Disease Management, and Ten Diseases of Native Plants (and Non-Natives, Too).  I also helped answer questions with Lisa Johnson at Larry Meiller’s in-person Garden Talk session on Saturday morning.  I had a constant stream of visitors through the PDDC booth all three days (Friday was unusually busy this year) and pretty much talked with and answered questions for folks the entire time.  I distributed 4,805 fact sheets, 924 brochures/informational handouts of various kinds, and 228 handouts for my talks.  For the first time, I had to reprint some fact sheets early Sunday morning, because I had run out of certain titles (e.g., Blossom End Rot) by the end of the day on Saturday.  All of these materials were not only educational in nature but were branded with the UW-Madison Division of Extension logo and/or the UW-Madison CALS logo, thus providing advertising for the UW-Madison. 

I had a exhilarating, productive weekend and provided a valuable service to the public.  I heard numerous positive comments about, and thanks for, the services that I provide.  This positive feedback is what keeps me motivated to do the work that I do.  The 2024 PBS Wisconsin Garden and Landscape Expo continues to be, by far, the most important in-person outreach event that I do every year and, quite frankly, my absolute favorite work activity. 

If you’d like to learn more about the PDDC and the services it provides, feel free to check out the clinic website at  Also, feel free to follow the clinic on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv (UWPDDCLearn) by emailing me at or (608) 262-2863. 

Here’s to looking forward to Garden Expo 2025!

January 2024: New Kits on the Block

Building Block IconMark your calendar!  January 28, 2024 is LEGO* Day.  I have been a big fan of LEGO in my personal life for a long time, and I was particularly excited when, several years ago, LEGO came out with their “Botanical Collection”.  I must admit that I went a bit crazy and overindulged in purchasing (and building) every plant-themed kit that became available. 

One day, as I putting together one of the LEGO botanical kits, I thought, “If you can have kits for building plants, why couldn’t you have kits for building diseased plants?”  Thus was born what I now refer to as “Building Block Plant Diseases”. 

The first step in creating the plant disease kits that I envisioned was to design models.  Luckily, an IT friend directed me to a computer program that allowed me to 3-D model plant diseases in building block form.  Using this program, I designed building block versions of several plant diseases including apple scab, Armillaria root disease, blossom end rot, cedar-apple rust, common smut of corn, grape downy mildew, powdery mildew, and silver leaf

After all of this computer-modeling, I next wanted to build a physical model of at least one of the diseases.  I focused on blossom end rot of tomato, as I was able to find and purchase pre-fabricated blocks that would allow me to build this model.  I also ended up designing and building a healthy tomato fruit to provide a comparison with the diseased fruit .  I have to say, it was pretty exciting to see one my “theoretical” building block plant diseases become a physical reality. 

My ultimate goal was to use my kits as educational tools, so I next worked on developing materials to supplement and enhance the building block models.  For my blossom end rot kit, I designed instruction manuals for both the diseased and healthy tomato fruits, as well as adult– and kid-friendly fact sheets to teach users about the disease/disorder.  To add educational value, I developed a blossom end rot word search game to include in the kit. 

Given that my building block kits are targeted toward a youth audience (not a demographic that I normally work with), I then needed to find partners who could get the my kits into the hands of kids.  I was fortunate in that two groups showed interest.  First was the “What’s Eating My Plants” or WEMP graduate student group in my department.  This group does extensive outreach to schools and other kid-friendly venues in the Madison area.  WEMP debuted my blossom end rot kits at a program the Monona Public Library in December 2023.  WEMP was also instrumental in translating all of my written materials, so that a Spanish version of the kit would be available.  The other folks that I have been partnering with extensively are UW-Madison Division of Extension 4H educators.  These educators will be incorporating the blossom end rot kit into a curriculum that they are developing to teach kids about the history and uses of tomatoes, as well as about tomato-related careers.  Eventually, blossom end rot building block kits will be availble in every county in Wisconsin. 

If the idea of “Building Block Plant Diseases” piques your interest, and you are interested in seeing my blossom end rot building block models, I will have the models on display at my booth at the PBS Wisconsin Garden and Landscaping Expo at the Alliant Energy Center in Madison, February 9-11, 2024.  Feel free to stop by booth #533/#604 to take a look. 

So, what’s next?  Well, the big challenge for bringing other plant disease building block kits to fruition is that prefabricated blocks are not available for the other models that I have designed.  A possible work around for this problem would be to 3-D print the blocks that I need.  I currently have a 3-D printer in my basement, and with help from the IT friend that I previously mentioned, I hope to engage in a frenzy of block printing activity this winter and have physical versions of at least some of the other plant diseases that I have modeled by the end of the year. 

“Building Block Plant Diseases” has been a really fun and exciting project to work on.  If you would like to keep up to date on developments on this project, be sure to follow the PDDC on Twitter (aka X)  or Facebook (both @UWPDDC), or subscribe to the PDDC listserv, UWPDDCLearn (just email me to join).  As always, you can also contact me by phone at (608) 262-2863 or email at

With that, go forth and build!!


*DISCLAIMER:  References to LEGO products in this article is not an endorsement of these products by the University of Wisconsin or a criticism of other similar products.  Mention of LEGO is solely for the purpose of illustrating the process that led to the development of “Building Block Plant Diseases”. 


December 2023: 2023 in Review

Review IconIt’s the last day of 2023 and time to look back and see what went on during the year at the Plant Disease Diagnostics Clinic (PDDC).  In my mind, I see 2023 as probably the most creative year of my entire career.  While I continued to perform my routine diagnostics tasks, I also initiated several new outreach projects that were (at least to me) incredibly exciting.  Here’s a rundown of the highs and lows in the clinic for the year. 

Plant disease diagnostics

Diagnosing a Plant Disease at the Microscope
Diagnosing a Plant Disease at the Microscope

2023 was, quite frankly, a horrible year in terms of sample submissions.  I completed 1005 samples (a combination of physical and digital samples) during 2023, the lowest number ever during my tenure as director of the PDDC.  I received samples from 60 of Wisconsin’s 72 counties, as well as from six additional states in the US.  Most of the samples were ornamentals (i.e., trees, shrubs, herbaceous plants), but there were a fair number of vegetable, fruit, and field and forage crop samples as well.  I’d like to give a big shout out to Dante Tauscheck, the new molecular diagnostician at the PDDC, who helped with the processing of many of these samples. 

In large part, the low sample number in 2023 was due to our extremely dry weather during the growing season.  Plant disease-causing organisms (particularly fungi) are more active in wetter weather, and we just didn’t have much rain throughout Wisconsin in 2023.  Diseases/disorders that did make a big splash in 2023 included fire blight, powdery mildew, and chlorosis.  The uptick of all of these was likely influenced by our dry weather.  Bees move the fire blight bacterium from tree to tree, and these insects tend to be more active in dry weather.  Powdery mildew fungi are known to prefer dry (i.e., no rain) but humid conditions, and that’s exactly the sort of weather we had this past summer.  And finally, lack of soil moisture likely inhibited uptake of iron and manganese by plants, thus exacerbating the chlorosis problems that we see every year throughout much of the state due to high soil pH.    


PDDC Booth at Garden Expo 2023
PDDC Booth at Garden Expo 2023

I notched 64 outreach presentations/talks during the year.  Three of those talks occurred at WPT’s Garden and Landscape Expo.  2023 marked my return to that event for the first time since the COVID pandemic started in 2020.  At Garden Expo, in addition to providing talks, I sponsored the PDDC booth, where I answered questions pretty much non-stop and distributed over 4000 UW Plant Disease Facts.  I also teamed up with Larry Meiller of WPR’s Garden Talk and Lisa Johnson of Extension Dane County to do a Q & A session.  It was a great three days.  For additional details on my experiences at the 2023 Garden Expo, check out my February web article.  

In 2023, I also debuted my PDDC Plant Disease Talks series.  I provided talks once a month on a wide range of plant disease topics and had 1465 participants over the course of the year.  While targeted toward WI residents, I ended up having participants from across the US (New York to Oregon, Minnesota to Arizona).  The talks provided general education for the public, as well as continuing education units (CEUs) to help professional arborists and Master Gardeners maintain their certifications.  I have to give a shout out to Ted Geibel and Lisa Johnson for helping behind the (Zoom) scenes to help make these talks a success.  The talks were popular enough that I will be continuing the series in 2024

Across all 64 of my presentations, including three visits to Garden Talk with Larry Meiller/The Larry Meiller Show, I helped over 210,000 people learn about plant diseases over the course of 2023. 

Online written resources

Sample Plant Disease Medallions
Sample Plant Disease Medallions

Via the PDDC website, I provided weekly updates of my clinic diagnoses (the Wisconsin Disease Almanac).  In addition, I wrote 13 web articles during the course of the year.  My favorite was probably 25 Years. . . 25 Cool Diseases, commemorating my 25th anniversary (July 1) as director of the PDDC.  Also housed on the PDDC website were the 130 titles in the UW Plant Disease Facts series.  A new twist for the fact sheets in 2023 was the debut of the Fact Sheet Medallions project.  For each fact sheet, I wrote a quiz and created a decorative electronic medallion.  You can now read a fact sheet, take the corresponding quiz, and once you get 100% on the quiz, you earn the corresponding medallion (automatically delivered to you via email).  So far, participants have taken over 1700 quizzes and earned over 450 medallions.  I know of one person who has collected all 130.  Earning medallions is not only fun, but it also qualifies as CEUs for Master Gardeners.  I have to give a special thank you to Dixie Lang, IT wizard extraordinaire, who helps maintain my clinic website and also figured out how to create the online quizzes and automation of medallion delivery that made the medallion project possible. 

The book! 

Limerickettsia: A Plant Pathologist's Book of Verse
Limerickettsia: A Plant Pathologist’s Book of Verse

I was so excited to publish my first book in 2023.  Titled Limerickettsia:  A Plant Pathologist’s Book of Verse, it contains 52 plant disease-related limericks with supplemental prose discussions of the diseases, photos, and original artwork.  I debuted the book at Garden Expo and have been selling the book as a fundraiser for the clinic since then.  Larry Meiller was kind enough to feature my book on his radio show (thanks Jill Nadeau for doing the interview!), and that’s helped with sales.  The book will never be a New York Times Bestseller (I’ve sold just over 160 copies thus far), but that was never the point.  My goal was to create a fun, alternate way for people (particularly a younger demographic) to learn about plant diseases.  I think I’ve accomplished that goal. 

Other crazy outreach

Building Block Blossom End Rot
Building Block Blossom End Rot

Another of my projects that came to partial fruition in 2023 was the plant disease building block kits that I started working on during the COVID pandemic.  I completed the blossom end rot kit this year, which includes LEGO pieces to construct diseased and healthy tomato fruits, instructions manuals, adult- and kid-oriented fact sheets, and a word search game.  The graduate student outreach group (What’s Eating My Plants or WEMP) in my department at the UW-Madison debuted the kits at the Monona Public Library in December.  I am also working with Extension 4H educators to provide kits to each county in Wisconsin in 2024.  I’m working on other kits as well (e.g., powdery mildew, downy mildew of grape, apple scab, silver leaf, Armillaria root disease, and common smut of corn) and that involves using 3-D printing to generate the parts that I need.  Hopefully, physical mock ups of these kits will be available sometime in 2024

Want to know more about the PDDC? 

All in all, 2023 was a great year at the PDDC.  As we get into 2024, and I develop new PDDC resources, I will announce their availability via Twitter (@UWPDDC) and Facebook (@UWPDDC), or via my clinic listserv, UWPDDCLearn (email me to subscribe to this).  In addition, you can always contact me by phone at (608) 262-2863 or email at

Happy New Year!

November 2023: Plant Pathology Playlist

Music IconYears ago, I participated (with three other Extension colleagues) in a weekly Saturday morning horticulture Q&A radio show in Milwaukee, WI.  One of the fun aspects of the show (other than being able to wax poetic about plant diseases on the air) was that I was allowed to select songs, snippets of which would be played as the show went in and came out of commercials.  Each month, I’d pick a theme (“celestial bodies” anyone?) and select seven or eight songs that both fit the theme and my musical tastes.  For this month’s web article, I continue this tradition by sharing some of my favorite plant-titled songs (with links to recordings) and providing commentary on diseases that might affect the referenced plants.

A Morning with the Roses

It’s hard to avoid “rose” songs in music.  They seem to be everywhere just like roses shrubs are everywhere in garden settings.  This “rose” song by Richard Dworsky is my favorite.  It’s an instrumental piece and one of the first New Age songs I ever encountered.

The most common disease of roses is black spot.  This disease affects rose leaves and canes, with classic symptoms being feathery-edged black leaf spots.  On susceptible rose varieties, the disease can be so severe that shrubs will defoliate.  For the causal rose grower, I suggest dealing with the disease by simply only growing black spot-resistant varieties.  Routine thinning of shrubs to promote better airflow and create a drier environment is another useful management strategy.  For hardcore rose growers, use of preventative fungicide treatments on particularly susceptible varieties may be needed to keep the disease under control.

Honeysuckle Rose

This Fats Waller/Andy Razaf song is part of the Great American Songbook, a compilation of “. . . the most important and influential popular songs and jazz standards from the early 20th Century. . .” (per the Great American Songbook Foundation).  The definitive version of this song for me (probably because it’s the first version I heard as a kid) is by Lena Horne (I could listen to her sing the telephone book).  I also really love versions by contemporary jazz singer Jane Monheit and the amazing and tragic Eva Cassidy (I suggest listening to her cover of another plant-themed song, Sting’s “Fields of Gold”, as well).

Although this is another “rose” song, we’re going to concentrate, disease-wise, on the honeysuckle part of the title.  Every year, I enjoy watching my parents’ vining honeysuckle develop powdery mildew, the most common disease that I see on this plant.  By the end of the summer, the plant’s leaves are powdery and white, but as with most plants, the disease is primarily a cosmetic issue and causes little actual damage.  My parents’ honeysuckle blooms profusely and attracts hummingbirds (which my parents can watch for hours on end).  The twisted vines also provide shelter for English sparrows (much to my parents’ chagrin).  Management of powdery mildews on most plants, in my mind, involves developing the ability to ignore the diseases, given their cosmetic nature.  On more susceptible plant species (e.g., phlox and beebalms), growing resistant varieties and thinning plants to increase airflow and reduce humidity (the driving environmental factor for powdery mildew development) can help manage these diseases.

Willow Song

I’m not a huge opera fan, but this aria from “The Ballad of Baby Doe” by Douglas Moore and sung by the marvelous Beverly Sills really mesmerized me when I stumbled across it years ago.  Baby Doe was one of Sills’ signature roles (although I will always remember her best for her guest appearance on “The Muppet Show”).  If opera isn’t your thing, then consider as an alternate willow song, Gordon Lightfoot’s “Pussywillows, Cat-Tails”.

Probably the most common diseases that I see on willows (usually weeping willows) are canker diseases.  There are a wide range of canker fungi that can infect and girdle willow branches, leading to branch dieback.  Often weeping willows grow rapidly and outgrow significant damage from canker diseases.  However, if management is needed, I suggest pruning four to six inches below obviously dead areas on branches.  Always be sure to prune only when it is dry, and be sure to disinfest pruning tools between cuts by dipping them in bleach diluted to a final concentration of 0.5% sodium hypochlorite (the active ingredient) or (even better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants can be used as a source of alcohol as well.  Just be sure to check the ingredient list of the disinfectant that you select to make sure it contains roughly 70% alcohol.  If you decide to use bleach, be sure to thoroughly rinse your tools after you are done pruning and oil them to prevent rusting that can be caused by bleach use.  You can dispose of branches by burning (where allowed) or burying them.


This song was written by Pat Alger, Ralph Murphy, and Ralph Murthy, and I found it on country singer Kathy Mattea’s “Lonesome Standard Time” album.  Mattea is one of my favorite country artists from the 1980s and 1990s.  I challenge you to listen to her recording of “Where Have You Been?” and not a shed a tear.

If you talk about seeds, then from a disease standpoint, you need to discuss damping-off.  This disease has both a seed decay phase (where seeds rot before germinating) and a seedling phase (where seedlings collapse and die just after emergence).  Damping-off can be caused by several fungi and water molds, with the water mold Pythium probably the most common organism involved.  To prevent damping-off, use pasteurized potting mixes/soil, decontaminate pots, germinate seeds at warm temperatures, and keep soil moisture on the dry side as seeds germinate and seedlings emerge.

Dusty Miller

I doubt that this traditional bluegrass song is about dusty miller plants, but I had to include the song in my list because one version of it was recorded by Alison Krauss.  “Dusty Miller” showcases Krauss’ prowess with a fiddle; she’s world-class.  I encourage you to explore her other music starting with her big hit (with her band Union Station), a cover of the Paul Overstreet/Don Schlitz song “When You Say Nothing at All”.  Her voice is ethereal and angelic.  There’s a reason she’s won 26 Grammy Awards (including two album of the year awards).

I rarely see dusty miller in the clinic, but when I do, the problem is typically a root rot of some kind.  Root rots tend to be caused by the same organisms that cause damping-off (discussed above).  At least some level of root rot pathogens can be found in most garden soils, so management of these diseases tends to involve moderating soil moisture.  Root rot organisms tend to be more active in wet soils, so making sure not to over-water and over-mulch can help prevent root rots from being an issue.  Most established plants require roughly one inch of water per week during the growing season.  Mulch usage varies depending on soil type.  For heavy soils (e.g., clay), use one to two inches of a high quality mulch (I like shredded oak bark mulch and red cedar mulch).  On light soils (e.g., sand), use three to four inches of mulch.

The Wind that Shakes the Barley

This adaptation (by Robert Dwyer Joyce) of a traditional Celtic song is on the album of the same name by Canadian national treasure Loreena McKennitt.  Possessed of a haunting soprano voice and known for her harp accompaniments, I have enjoyed McKennitt’s work ever since hearing her perform “Penelope’s Song” on NPR in 2007.

Barley is not a plant that home gardeners typically grow, but as part of my diagnostic responsibilities, I often receive agricultural crop samples such as barley and wheat.  A common disease of these grain crops is barley yellow dwarf, a viral disease caused by Barley yellow dwarf virus (BYDV)This virus is aphid transmitted.  The virus causes yellowing of barley and wheat leaves, particularly the flag leaf (the leaf just below the developing grain head).  This leaf is incredibly important for producing the nutrients needed for proper filling of grain heads.  Thus, the disruption and yellowing caused by BYDV can lead to substantial yield losses.  Management of the disease often entails modifying planting times.  For fall-sown wheat varieties, late planting after aphid populations have declined for the growing season is recommended to limit infections.  For spring sown wheat varieties, early planting is recommended.  This allows substantial time for plants to grow before aphids arrive and infections can occur.  Late infections have a lesser impact on yield.

Moments in the Woods

This song is from “Into the Woods”, perhaps my favorite Stephen Sondheim musical (I’m a huge Sondheim fan in general).  The added bonus of the version of the song linked above is that it’s sung by one of my favorite contemporary singers, Sara Bareilles, who starred in the 2022 Broadway revival of the show.  If you like Bareilles’ voice, I suggest checking out “Gravity” (and other songs) from her “Little Voice” album.

Thinking of plant diseases that I might encounter during a “moment in the woods”, black knot immediately comes to mind.  This is the disease that I affectionately call poop-on-a-stick, because the causal fungus induces formation of feces-like galls on branches of cherry and plum trees.  At this time of the year, even with an absence of leaves, I can ID these trees in wooded settings just based on black knot symptoms.  Pruning out infected branches on trees in landscape settings can help manage the disease.  However, certain individual cherry or plum trees can be so susceptible to black knot (and have so may galls), that I recommend “basal pruning” (i.e., removal) and replacement.

Prairie Trains

The final song on my playlist is a shoutout to my friend and colleague, Extension Dane County’s Lisa Johnson.  I’ve know Lisa since high school and watched her develop as a music composer and performer over the years.  This song, from her “The Season” album, is an ode to the many trips that she and I made in our undergraduate days to a prairie remnant (now long gone) sandwiched between the stretch of Hwy. 26 and the adjacent railroad track that ran between Fort Atkinson and Jefferson, WI.  I have many fond memories of those expeditions and the prairie plants that we stumbled upon.

One of the prairie plants that Lisa mentions in her song is big blue stem, and back in 2022 I received photos of this grass suffering from culm smut.  This is a fungal disease where spores of the causal fungus infect the plant’s flowers causing the formation of a fleshy gall.  This gall eventually degrades into a powdery mass of blackish fungal spores that are blown to other big blue stem plants where they initiate additional infections.  The pathogen not only infects the seed heads, but it eventually systemically colonizes the rest of the plant.  Infected plants become stunted, and can continue to produce flower galls and fungal sporulation for a time.  Eventually the plants decline to the point where they no longer bloom and eventually die out.  Interestingly, there is speculation that this disease plays a role in the normal process of plant succession in prairies.  That said, if you love your big blue stem, removing infected plants as soon as you see them is important to prevent spread of the pathogen to other big blue stem in your planting.

I hope you’ve enjoyed this month’s musical plant disease adventure.  If you have your own favorite plant-titled or plant-themed songs, I’d love to hear about them.  Feel free to contact me at or (608) 262-2863 with your suggestions.  Happy Thanksgiving everyone!

October 2023: Poison Apples for the Halloween Season

Poison Apple IconOctober is National Apple Month and also host to my favorite holiday of the year, Halloween.  (Does the latter really surprise you given my my moniker, Dr. Death?)  So, in honor of both events, I decided that for this month’s web article, I’d talk a bit about diseases that can adversely affect apple fruits and make them less than optimal for use in making pies and seasonal treats like caramel apples.  The pathogens involved in these diseases do not produce compounds toxic to humans, so technically the infected apples are not poison per se.  However, the damage these pathogens cause often makes apple fruits unusable.

Apple Scab

Apple Scab
Apple scab. (Photo courtesy of UW-Madison PDDC)

This disease is the most common that I see causing issues on apple fruits.  The causal fungus (Venturia inaequalis) commonly infects apples leaves, often leading to defoliation in susceptible apple varieties.  Once leaf infections occur, the fungus can eventually infect fruits leading to symptoms that range from blackish surface blemishes to fruit growth distortions (if infections occur early in fruit development).  Of the apple fruit diseases, apple scab is the most cosmetic in my mind.  Fruits are often usable if peeled.  The downside to the disease is that consumers expect perfect, perfect fruit and are not accepting of fruits with apple scab lesions.  Also, scab-blemished fruits typically do not store as well as unblemished fruits.  For this reason, commercial apple growers spend a lot of time, effort, and money spraying apple trees with fungicides to prevent this disease.

Gymnosporangium Rusts

Gymnosporangium Rust
Gymnosporangium rust. (Photo courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University,

The fungi involved in these diseases (which include cedar-apple rust, cedar-hawthorn rust, and cedar-quince rust) must infect both woody rosaceous plants (most commonly, apples and hawthorns), and junipers to complete their life cycles.  Red cedar (Juniperus virginiana) is a particularly susceptible juniper host.  I most commonly see Gymnosporangium rust symptoms on leaves, where bright yellow to orange spots (about the size of a dime or nickel) form.  But, fruit infections on hawthorn (where you will see spiny, salmon-colored fruit) are also very common.  I have seen fewer apple fruit infections, but they do occur, and the lesions tend to have a spiny appearance.  Infected fruits are often frightening and fascinating at the same time.

Fire Blight

Fire Blight
Fire blight. (Photo courtesy of University of Georgia Plant Pathology, University of Georgia,

2023 was a banner year for fire blight in my estimation.  I diagnosed more fire blight this year than any other year in my 25 years at the PDDC.  The bacterium that causes the disease (Erwinia amylovora) commonly gains entry into trees through flowers.  Honeybees pick up the bacterium as they feed on oozing sap from infected branches.  The bees then drop the pathogen off as they pollinate.  Erwinia amylovora can rapidly colonize branches leading to significant dieback, and even tree death.  Interestingly this year, I received samples of apple fruits (not full developed, but well on their way) with blotchy, rotted areas that were quite sticky.  These areas tested positive for Erwinia amylovora.  This was a first for me.  I had never seen fireblight on apple fruits before.

Black Rot

Black Rot
Black rot. (Photo courtesy of University of Georgia Plant Pathology, University of Georgia,

I don’t see this fungal fruit disease often, but I commonly see the pathogen involved (a Sphaeropsis species) cause cankers and dieback on apple branches.  Often, when clients think they have a fire blight problem, the real problem is the canker phase of black rot.  The black rot pathogen also causes frogeye leaf spot on apple leaves.  Interestingly, the pathogen does not sporulate well on leaves, but it sporulates profusely on branches and fruits.  Fruit symptoms are often large, roughly circular, dark areas of softened tissue.  If you look closely within the decayed tissue, you may notice a series of concentric rings showing how the fungus has colonized the fruit.  You may also notice tiny, pimple-like structures embedded in the rotten tissue.  These are the fruiting bodies (i.e., reproductive structures) of the pathogen, filled with relatively large, oblong, brown spores.

Bitter Rot

Bitter Pit
Bitter pit. (Photo courtesy of University of Georgia Plant Pathology, University of Georgia,

Bitter rot is caused by fungi in the genus Colletotrichum (typically Colletotrichum gloeosporioides or Colletotrichum acutatum).  This genus contains many fungi that cause anthracnose leaf diseases.  On apple, the bitter rot pathogens are primarily fruit pathogens (leaf symptoms are only rarely observed), leading to symptoms that look somewhat similar to those observed with black rot.  Bitter rot lesions are often large, roughly circular, and somewhat tannish to brown in color.  Concentric rings are typically quite obvious with the diseased tissue, and pimple-like fruiting bodies abound.  Individual spores of the bitter rot pathogens are smaller than those of the black rot pathogen, oblong and colorless.  En masse on the surfaces of fruits however, these spores can have a pink, salmon, or orange color.

Management of apple fruit diseases varies from disease to disease, but in general involves:

Cleaning up leaf debris and rotted fruits

Removal of these materials eliminates overwintering sites for many of the disease-causng organisms discussed above.  Leaf debris removal is particularly important for management of apple scab, but not particularly important for the Gymnosporangium rusts (because the spores that infect apple trees come from junipers).  Dispose of leaf and fruit debris by burning (where allowed), burying, or hot composting.

Pruning routinely

Pruning removes infected branches that can serve as a source of pathogens (e.g, those that cause fire blight, black rot, and bitter rot).  In addition, routine pruning of a healthy tree opens the canopy and promotes drier conditions that are less favorable for infections to occur.  For branches with probable fungal infections, pruning approximately four to six below obviously dead tissue will likely be adequate to remove pathogens.  If fire blight is of concern however, more aggressive pruning, 12 inches below obviously diseased branch tissue, is needed.  Be sure to prune only when it is dry, and be sure to disinfest your pruning tools between cuts by dipping them in bleach diluted to a final concentation of 0.5% sodium hypochlorite (the active ingredient) or (even better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants can be used as a source of alcohol as well.  Just be sure to check the ingredient list of the disinfectant that you select to make sure it contains roughly 70% alcohol.  If you use bleach, be sure to thoroughly rinse and oil tools after you are done pruning to prevent rusting that can be caused by bleach use.  Dispose of branches by burning (where allowed) or burying them.

Eliminating alternate hosts

This technique is most important for managing Gymnospoangium rusts.  Removing junipers near apple trees will help break the life cycle of Gymnosporangium fungi and substantially reduce or even elminate these rust diseases.  Unfortunately removing junipers may not be practical.  I have seen some recommendations for commercial apple producers that advise removing junipers within a two mile radius of apple producing areas!  This would not be possible in a home gardening setting.  For apple scab, removing susceptible crabapple trees may be helpful in controling scab on fruit bearing trees by eliminating a potential source of fungal spores.

Planting resistant varieties

When buying new apple trees, I always recommend that home fruit growers select varieties that have resistance to both apple scab and fire blight.  These two diseases are typically the most problematic for home gardeners.

Using fungicide treatments

I recommend this option only as a last resort and only if you have had your apple disease problems formally diagnosed.  By knowing which apple diseases you commonly encounter, you can develop a fungicide treatment regime tailored to your specific situation.

Now that I’ve spent the afternoon writing about apple fruit diseases, my tummy is rumbling.  It’s time for me to track down a caramel apple.  As always, if you have questions about plant diseases, don’t hesitate to contact me at or (608) 262-2863.  Enjoy the last colorful days of autumn, everyone!

September 2023: Plant Disease Management and Organic Gardening

Organic Gardening Icon - Green Bag with Gardening Implements and SeedsSeptember is National Organic Month, so I thought I’d use this month’s PDDC web article to explore techniques for plant disease management that fall within the philosophy of organic gardening.  This philosophy tends to steer gardeners away from the use of chemicals for plant disease control.  There are many disease management techniques that are appropriate for organic gardening, and quite frankly, when I am making control recommendations for virtually any client, the primary techniques that I recommend are organic in spirit.  These include:

Practicing good garden clean up

Disease-causing organisms often survive in the remains of infected plants.  Removing and properly disposing of this debris can help reduce pathogen populations in a garden or landscape.  Whether you do this clean up in the fall after plants have gone dormant for the year or in the spring before plants begin to emerge for the new growing season is up to you.  There are pros and cons to either choice.  Disposal methods for this debris include burning (not the most environmentally friendly option), burying (a somewhat laborious task), or hot composting (probably the best technique if done properly).

Using resistant plant varieties

Individuals of a particular plant species can be highly variable in terms how they react to disease-causing organisms.  Plant enthusiasts have exploited this variability by watching for plants that develop less severe symptoms and interbreeding these healthier individuals to develop disease resistant plant varieties.  I often recommend that home gardeners plant apple trees that have been bred for resistance to apple scab and fire blight, or rose shrubs that have been bred for resistance to black spot.  Monarda, phlox, and cucumber varieties that are resistant to powdery mildew can be useful in home gardens as well.  One downside to using resistant varieties, is that these plants may not have other horticultural characteristics (e.g., flower color, size, scent, flavor) that gardeners are looking for.

Buying healthy plants and seeds

You can easily introduce disease-causing organisms into a garden via infected (or otherwise contaminated) plant materials.  Carefully inspect new plants for evidence of disease issues (e.g., leaf spotting, sunken areas on branches or trunks, fuzzy growth on upper or lower leaf surfaces, etc.), and avoid buying problematic plants.  Seeds can also be a source of disease-causing organisms but can be much more difficult to assess, as they may not exhibit obvious symptoms.  Try to purchase seeds from a reputable grower/company (although even the best of growers can occasionally have disease issues).  If you have a suspect batch of seed (particularly vegetable seed), consider using hot water treatments to help eliminate disease-causing organisms.  These treatments can be particularly effective for controlling seed-borne bacteria.

Planting trees and shrubs (and plants in general) in the right environment

Plants that are under environmental stresses tend to be more prone to disease issues.  Before purchasing plants, make sure they are well adapted to the light, moisture, and fertility conditions at your location.  Put the right plant in the right place.  Pagoda dogwood is a tree that is often sited poorly in urban landscapes.  I see these trees in the middle of an open yard with grass growing up to the trunk where there is a lot of light, excessive heat, and limited water. Pagoda dogwood is an understory tree that prefers shady, cool, moist conditions.  When planted in a hot, dry, sunny environment, this tree tends to be more prone to developing golden canker, a serious and often lethal fungal disease.  Also, be cautious about planting pin oaks or red maples.  In much of Wisconsin, these trees are prone to chlorosis, because soil pH is so high that the trees have trouble taking up adequate iron (pin oak) or manganese (red maple).

Using proper plant spacing and thinning of trees and shrubs

Plant diseases tend to be more of an issue when plants are crowded.  Crowded plants trap humid air, which slows leaf drying.  Wetter leaves favor infection by disease-causing fungi and bacteria.  Planting herbaceous plants farther apart, routinely dividing large clump plants (e.g., peonies), and regularly pruning trees and thinning shrubs can help create a drier environment that is less favorable for disease development.  Regular pruning also removes diseased branches, thus eliminating a source of pathogens and reducing pathogen spread.

Watering properly

Avoid using sprinklers for watering your garden, as this method wets leaves and creates a favorable environment for fungal or bacterial infection.  Use of overhead watering can eliminate any benefits you might gain by properly spacing and pruning/thinning plants (as described above).  Instead of a sprinkler, water with a drip or soaker hose that applies water directly to the soil and keeps moisture off of leaves.

Keeping weeds under control

Weeds compete with garden and landscape plants for nutrients, leading to stress that can predispose plants to infection.  Weeds also crowd other plants, trapping moisture and creating an environment conducive to infection and disease development.  Finally, weeds can serve as reservoirs for disease-causing organisms that can eventually move from the weeds to your favorite ornamentals and vegetables.  So, weed, weed, weed.  Fewer weeds translates into a healthier garden.

The points that I’ve outlined above are just a few of the many techniques that you can employ to achieve a healthier and more aesthetically appealing garden or landscape.  If you have questions about these or any other plant-disease-management techniques, feel free to contact me at or (608) 262-2863.

Now, go forth and garden!!

August 2023: Bugging Out Over Plant Diseases

Insect IconThis growing season has largely been a bust when it comes to plant diseases.  Our dry weather has not been favorable for diseases (particularly leaf diseases) to development.  On the other hand, 2023 has been a banner year for insects, and PJ Liesch, the UW insect diagnostician, has been very busy identifying insect problems.  As it turns out, plant diseases and insects are not totally unrelated.  There are a fair number of disease-causing organisms that can be moved from plant to plant by insects, and it’s these insect-vectored pathogens, and the diseases that they cause, that I have been seeing a lot of this year.

Fire blight

In my June 2021 web article (Fired Up About Fire Blight), I bemoaned the fact that often when people see dying branches on apple, crabapple, and pear trees, they assume the dieback is due to fire blight.  Often it is not.  But, this year I have seen an uptick of fire blight cases.  I suspect that, in part, this is due to increased transmission of the fire blight bacterium (Erwinia amylovora) by honeybees.  In the absence of rains during flowering, which tends to discourage their activity, honeybees are out pollinating a wide range of plants, including apples, crabapples, and pears.  Honeybees also visit branches where they feed on oozing sap.  On trees suffering from fire blight, this sap often contains the fire blight bacterium.  Honeybees become contaminated with the pathogen and carry it to apple, crabapple, and pear flowers, where it can infect.  As I attempt to diagnose fire blight, I often check dying apple, crabapple, and pear branches to see if infections appear to have started near flower clusters.  If so, my fire blight radar goes off.

Among the numerous fire blight cases this year, I had two interesting/sad ones.  The first case (more in the interesting category) was fire blight on mountain-ash.  Although called an “ash”, this tree (the one with the clusters of bright orange fruits) is in the same plant family as apples, crabapples, and pears, and it is a known host for fire blight.  The sample that was submitted to my clinic was from a tree that had numerous dying branches, and it appeared that infections had occurred through flowers.  The second case (definitely in the sad category) involved an apple grower who had numerous dwarf trees that he had espaliered.  Many of them exhibited dieback, and all of the samples that he submitted tested positive for fire blight.  Unfortunately, my recommended pruning for fire blight (i.e., cut 12 inches below where there are obvious symptoms/dieback) functionally meant that the grower had to remove and destroy many of his trees.

Bacterial wilt of cucurbits

I have had a number of vine crop (e.g., cucumber, melon) samples arrive at the clinic with wilting symptoms.  Fungal diseases like Verticillium wilt and Fusarium wilt, as well as water mold diseases like Phytophthora root and crown rot, can cause wilting symptoms.  However, for many of the cucurbit samples in 2023, not only was there wilting, but there was also significant scarring on the vines from cucumber beetle feeding.  These insects can carry Erwinia tracheiphila, the bacterial wilt pathogen, in their intestinal tracts.  As they feed, cucumber beetles defecate in their feeding wounds, inoculating plants.  The bacterium colonizes the water-conducting tissue of the plants and blocks it off, leading to reduced water movement from the roots to the vines, and thus wilting results.

Diagnosis of bacterial wilt can be relatively easy on certain types of vine crops (particularly cucumber).  You cut a wilted vine off near the base of a plant, then cut a roughly six to eight inch vine segment from that severed vine, giving you two vine sections.  You then hold the two vine segments apart for about 10 seconds (allowing liquid to ooze from the severed water-conducting tissue), then rub the ends of the cut vine segments together gently for another 10 seconds, then very slowly pull the two vine sections apart.  If you see thin, spider web-like strands pull between the two vine pieces, you’re dealing with bacterial wilt.  Management of this disease primarily depends on keeping cucumber beetles under control.

Insect-transmitted viruses

There are many viruses that can adversely affect plant growth and productivity.  A fair number of these viruses are insect transmitted.  Early in the year, when many of my samples come from commercial greenhouses, I tend to see thrips-transmitted viruses.  These include Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV)Interestingly, this year I detected TSWV in commercially grown potato plants, where the virus produced necrotic (i.e., dead) spots that were reminiscent of early blight symptoms.

Aphids also commonly transmit plant viruses.  In herbaceous ornamentals and vegetables, Cucumber mosaic virus (CMV) is the aphid-transmitted virus that I most commonly see.  This virus causes blotchy color (i.e., mosaic) or line patterns on leaves, as well as growth distortions.  In recent correspondence with Damon Smith, the UW-Madison field and forage crop pathologist, I learned that he has been seeing another aphid-transmitted virus, Barley yellow dwarf virus (BYDV), in many wheat fields this year.  In wheat, BYDV causes yellowing of the upper leaves of the plant, particularly the flag leaf (i.e, the leaf just under the developing grain head).  Infection by the virus can lead to significantly reduced yields.  BYDV can also infect other “grass” crops including barley, oats, and corn.  I had a suspicious corn sample arrive at the clinic in late July, and I suggested the client submit a sample to another lab for BYDV testing (I do not test for this virus at the PDDC).

The diseases described above just scratch the surface of plant diseases where there is a potential insect connection.  If you would like read more, check out the UW Plant Disease Facts on ash yellows, aster yellows, Dutch elm disease, ergot, oak wilt, plum pox, soybean vein necrosis disease and thousand cankers disease.  After reading, if you if you have questions about these or any other plant diseases, feel free to contact me at or (608) 262-2863.  I am always here to help.

July 2023: 25 Years. . . 25 Cool Diseases

Twenty-fifth Anniversary IconJuly 1 marked my 25th anniversary as director of the University of Wisconsin Plant Disease Diagnostics Clinic.  In celebration of that milestone, this month’s web article highlights 25 diseases, disorders, and oddities that look like diseases, but aren’t.  All of these tickle my fancy as a geeky plant pathologist.  I am emphasizing diseases/disorders/oddities that can be identified by eye.  Where I have a UW Plant Disease Facts fact sheet on the disease/disorder/oddity, I provide a link.  Where I don’t, I provide more extensive comments and a link to a photo.  For several of the entries, I wax nostalgic about unusual encounters I’ve had with the diseases/disorders/oddities over the years.


Leaf/Fruit Diseases

The diseases in this section are all fungal and very common.  I’ve made a fair amount of money diagnosing these diseases in my 25 years in the clinic.

Powdery Mildews

Powdery mildews are one of my favorite types of fungal diseases due to the very ornate microscopic structures that they produce.  Powdery mildews are mostly cosmetic diseases but can be the bane of cucurbit, phlox, beebalm, and ‘Diablo’ ninebark growers.

Tar Spot

These incredibly visual diseases often pop up on maples, most commonly silver and Norway maple.  When I first started in the PDDC, I tended to see these diseases primarily in counties that bordered Lake Michigan.  Nowadays, I can find them anywhere in the state.

Taphrina Diseases

The most common of these diseases is peach leaf curl, which gives peach leaves a colorful, seersucker sort of look.  The more bizarre versions of these diseases cause fruits to swell and become spongy.  Plum pockets and what I have dubbed chokecherry pockets are examples.  I encountered this latter disease for the first time in 2023.  I love seeing new diseases!!

Gall Diseases

Gall diseases lead to the overgrowth of affected plant parts.  Some are fungal, some are bacterial.  All are quite cool!

 Black Knot

This is the fungal disease that I affectionately call “poop-on-a-stick”.  I can’t tell you how many of my former students have told me that they remember this disease because of my use of this rather risqué name.

Common Corn Smut

Is this disease bad or good?  It all depends on your point of view.  If you’re a sweet corn grower, you really don’t want to see this disease at all.  If spores of the fungus get into canned corn, they give the corn an off flavor.  If you run a restaurant however, you may be quite pleased to find this fungus on your sweet corn, so that you can harvest it and include it on your menu as huitlacoche.

Crown Gall

This is the classic bacterial disease that kick-started today’s industry of genetically modifying/engineering plants.  This industry was first gaining traction back when I was in graduate school (i.e., the Jurassic Period).

Leafy Gall

Think of this disease as crown gall on steroids.  You get the tumorous growth of crown gall but with the added bonus of tons of tiny leaves and shoots.  After reading about this disease for years, I finally received a sample for the first time in 2022.

Rust Diseases

Rust diseases are a large of group of diseases caused by closely related fungi that are very host specific.  Some rust fungi complete their life cycles on a single type of plant.  Others require two, often very different, plants to complete their life cycles.  I think rusts are very interesting and tend to get a smile on my face when these diseases arrive at the clinic.

Gymnosporangium Rusts

I find this group of rusts, which includes cedar-apple rust, cedar-hawthorn rust, and cedar-quince rust, very attractive, particularly given that many of the diseases that I see routinely are rotting, slimy, and rather (ahem) odiferous.  Gymnosporangium rusts are always suitable for inclusion in a plant disease bouquet to be used at a summer field day.  Other diseases. . . not so much.

Fir Broom Rust

This is an alternating rust that infects both fir trees and chickweed.  I have never seen the disease on chickweed, but the symptoms and signs on fir are very distinctive.  Infection leads to production of a massive cluster of branches (i.e., a broom) with pale, wimpy needles that produce masses of yellow, powdery spores.  Optimally, you would try to eradicate any chickweed near your fir trees to control this disease.  Unfortunately, this weed is so common and inconspicuous that eradication is difficult.  Management of fir broom rust more typically involves pruning out the branch masses in fir trees as they form.  The first time I encountered this disease was when a client sent me photos of his fir trees.  The photos were of such high resolution that I could zoom in and see sporulation on the needles!  My client could tell I was very excited about seeing this disease for the first time, and, kind soul that he was, he dropped off the sample on one of his trips through Madison from the Eau Claire area.

Elderberry Rust

The symptoms of this disease remind me of a huge, yellow banana slug hugging an elderberry branch for dear life.  This disease is definitely in the running for my #1, favorite plant disease.

White Pine Blister Rust

This disease is potentially lethal in white pine as the causal fungus can girdle the main trunks of white pine trees.  In the spring, the fungus produces the blister-like reproductive structures on white pine that gives the disease its name.  Many years ago, I had a VERY long phone conversation with a client who wanted to start a currant farm in northern Wisconsin in the middle of white pine forest.  That’s a really bad idea given that currants (along with gooseberries) are the alternate host for the white pine blister rust fungus.  I don’t think I convinced my client to abandon this idea, and I’ve often wondered over the years if she ever started her currant farm and if so, what the consequences were.

 Canker Diseases

 These diseases girdle branches and cause branch dieback.  I have seen a lot of these diseases over the years.

 Golden Canker

This disease is specific to pagoda dogwood, particularly those trees improperly sited in hot, dry, full-sun locations.  This is another disease that is “attractive” in my rather warped worldview.  Like with Gymnosporangium rusts, I often include this disease in festive disease bouquets.

Eastern Filbert Blight

This disease is the bane of European hazelnut, and I often see it on Harry Lauder’s walking stick.  I really like how the fruiting bodies (i.e., reproductive structures) of the causal fungus pop out of the dead branches.  It looks like an insect marched along the length of the branch leaving tiny footprints in its wake.

Blister Canker (Nailhead Canker)

This disease tends to be an issue on stressed apples and crabapples (although I have also seen it on serviceberry).  The causal fungus infects larger branches and tree trunks, and eventually forms clusters of large, round, black pads (technically called stromata) in amongst the bark.  Embedded in the stromata are fungal reproductive structures that contain elongate sacks with eight dark-colored spores.  This disease is awesome as a teaching tool because if preserves nicely at room temperature with little fuss.

Miscellaneous Diseases

 Not all diseases are easily categorizable!


This disease has had a significant impact on human history, due to the toxic compounds that the causal fungus produces.  Check out “St. Anthony’s fire” or “ergotism” online if you’d like to read more on this.  In a very personally satisfying PDDC moment, I diagnosed this disease and helped a horse owner save her horses.  The horses were in declining health because they had been grazing in their paddock on grasses that had this disease.  The fix was any easy one.  The client just had to mow the grass to remove the infected seed heads and then had to be sure to mow again anytime the grass began to flower.

Dead Man’s Fingers

This is a root rot pathogen that I most commonly see on stressed trees and shrubs.  The most hysterical photo of dead man’s fingers that I’ve seen looked like someone’s toes were sticking out from under a log.

White Mold

This is one of the more destructive fungal diseases that I see, causing problems across a wide range of herbaceous plants.  Look for the mouse-dropping-like resting structures of the causal fungus on (and inside) infected plant tissue.  If you see these, they are a dead giveaway that you’re dealing with white mold.

Fertility Issues

 Not all plant health problems are true diseases where a pathogen (i.e., a disease-causing organism) is involved.


This is a classic disease of pin oak and birch where the trees are suffering from an iron deficiency.  In red maple, a manganese deficiency is typically the problem.  Lack of these nutrients in the soil is often not the issue, but overly high soil pH (which makes these nutrients less available for plant uptake) is.

Blossom End Rot

People tend to tend to think of tomatoes when they think of blossom end rot, but any vegetables that’s botanically a fruit (e.g., peppers, eggplant, cucumbers, squash) can suffer from this disorder.  A calcium deficiency in the fruits is the problem.  A lack of calcium in soils is typically not the issue but a lack of water (e.g., from drought or uneven watering) more commonly is.


 Sometimes what I see are not diseases, but some other sort of amazing oddity. . .


This is a parasitic plant that grows prolifically, covering parasitized plants in what looks like a mass of yellow or orange spaghetti.  The most awesome example of dodder I’ve ever seen was back in my undergraduate days when I found an entire lot next to the Fort Atkinson, WI sewage treatment plant covered with the plant.  You could make out shapes of shrubs under the growth.  Mind boggling!

Fairy Rings

Fairy rings can cause aesthetic issues on golf courses and in home lawns but are relatively cosmetic issues.  Their most classic form is a ring of mushrooms growing in a grassy area.  If you believe in ancient lore, you may want to stake out these rings when you find them to watch for dancing fairies on a moonlit night.

Slime Molds

Slime molds come in many shapes and forms.  The most common slime mold gardeners encounter looks like a neighborhood dog upchucked on newly spread mulch.  My personal favorites are a) one that looks like a cluster of tiny cattails and b) one that looks like a spherical birdcage under the microscope.  I have to give a shout out to Marilyn Hanson, my high school biology teacher, who introduced me to slime molds (and fungi as well) and was instrumental in setting me on the path that led to me becoming a plant pathologist.


I have had a number of somewhat awkward conversations with clients about these common garden fungi, some of which look like a certain part of the male anatomy (thus the awkwardness).  In addition to their odd form, stinkhorns (as the name implies) have a rather unpleasant odor.

Bird’s Nest Fungi

“Super cute” are the best words to describe these fungi.  Their reproductive structures look like tiny bird’s nests complete with eggs!  Watch for these in clusters in mulched flowerbeds.


These fantastic organisms are a symbiotic combination of a filamentous fungus, an alga (often a blue-green alga, more accurately referred to as a cyanobacterium), and in certain instances a yeast (a non-filamentous type of fungus).  Please don’t bemoan seeing lichens growing on the trunks of your trees (or anywhere else).  If you see lots of different types of lichens in your area, that’s an indication of good air quality.

I hope you’ve enjoyed this list and my trip down memory lane.  If you have questions, feel free to contact me at or (608) 262-2863.  Enjoy the rest of your summer!!

June 2023: Weeding is Fundamental!

Weeding Tools IconMark your calendar. June 13th is National Weed Your Garden Day. It’s time to get out into your garden to remove those unwanted plants growing in amongst your prized herbaceous ornamentals and tasty vegetables. Not only will weeding make your garden look more tidy and beautiful, it will also help make your favorite garden plants healthier. How do weeds negatively impact your garden plants’ health? Let’s look at some ways.

Weeds compete for nutrients and water

All plants in a garden use and compete for available soil nutrients and water to grow, flower, and set fruit and seed. Nutrients and water used by weed plants are not available to be used by ornamentals and vegetables that we are attempting to grow. This leads to smaller plants, fewer flowers, and reduced vegetable yields. In addition, plants stressed for nutrients and water because of competition from weeds are less likely to be able to produce compounds that they can use to fend off infections by the myriad of plant pathogens in the environment. This can contribute to an increase in infections and disease, leading to a further reduction in plant quality and yield.

Weeds create an environment that is more favorable for infections to occur

Weeds, like all garden plants, produce foliage. Thus, high weed pressure in a garden will lead to a denser plant canopy that will reduce airflow. When leaves get wet (e.g., when it rains), this lack of air movement will increase the length of time that it will take for the leaves to dry. Longer periods of leaf wetness will provide more time for spores of fungal plant pathogens to germinate and infect, thus increasing the likelihood that many types of leaf diseases will develop.

In addition, plants (including weeds) transpire. Transpiration is a natural loss of water from leaves as they take in carbon dioxide and release oxygen during photosynthesis. Dense plant canopies trap this released moisture, creating humid conditions around leaves. High humidity tends to promote sporulation of many disease-causing fungi, which can lead to an increase in additional infections. For some disease-causing fungi (e.g., powdery mildew fungi), high humidity (rather than leaf wetness) is the environmental factor that promotes spore germination and infection.

Weeds can serve as a reservoir for disease-causing organisms

Many disease-causing organisms are very host specific, in that they can infect only a single type of plant or only a small group of very closely related plants. Other pathogens have broad host ranges and can infect many types of plants. Weeds (particularly perennial weeds) can serve as reservoirs where these broad-host-range pathogens can overwinter and subsequently spread to garden plants. In particular, I worry about weeds harboring viral pathogens such as Tobacco mosaic virus (TMV), Cucumber mosaic virus (CMV), Impatiens necrotic spot virus (INSV), and Tomato spotted wilt virus (TSWV). These viruses can survive in weed hosts and can subsequently be moved be moved from plant to plant by insects, on gardening tools, or even by just handling plants. Weed species can also serve are reservoirs for the phytoplasma that causes aster yellows, and they can help keep fungal pathogens such as Verticillium (the cause of Verticillium wilt) at elevated levels in garden soils. By weeding, you can eliminate sources many disease-causing organisms and help prevent pathogen survival and spread.


So, as you get some spare time in the coming weeks, be sure to spend a few minutes in your garden removing those pesky, unwanted plant species. You will not only end up with a more beautiful garden, but ultimately a healthier garden as well.


If you have questions about weeds and how they can impact diseases in your yard, feel free to give me a shout. As always, you can reach me at or (608) 262-2863. Go forth and weed!!