Category Archives: Monthly Column

December: Ho! Ho! Ho! A Plant Health Horror for the Holidays!

As the December holiday season nears, one of the traditions for many folks is to decorate their homes with festive greens.  When selecting wreaths and garlands to using in decorating, keep in mind that you may not only be bringing pine boughs and holly into your home, but also plant pathogens.

Of particular concern has been the boxwood blight fungus (Calonectria pseudonaviculata).  Boxwood blight is a devastating, lethal disease of boxwood, one of our most popular landscape shrubs.  Pachysandra (a common groundcover) is also susceptible to the disease.  Boxwood blight was first identified in the eastern US in 2011 and this past summer was reported for the first time in Wisconsin in a nursery in Kenosha County.  The Wisconsin Department of Agriculture, Trade and Consumer Protection is currently trying to contain and (hopefully) eradicate the infestation.

While the greatest potential for introducing the boxwood blight pathogen into an area is through movement of infected landscape shrubs, there have also been reports in other states (e.g., Indiana) of the pathogen having been found in holiday wreaths festooned with sprigs from infected boxwood shrubs.  These wreaths were produced in North Carolina and then shipped to other states for sale.  The manufacturers unwittingly shipped the boxwood blight pathogen with their wreaths.  A similar scenario could potentially occur in Wisconsin, and if contaminated wreaths are not handled properly, extensive spread of the boxwood blight pathogen could result.

So, what can you do?  First, don’t be afraid to enjoy a holiday wreath; just consider buying one that does not contain boxwood sprigs.  If you are unsure whether a wreath you have already purchased contains boxwood, assume that it does (just to safe) and dispose of it appropriately.  One option is to burn the wreath, if your municipality allows this.  Otherwise, double bag your wreaths in garbage bags, seal them up and place them in your municipal garbage to be landfilled.  Be careful to watch for any leaves or branches that may have fallen from the wreaths and collect these up and dispose of them as well.  In particular, you want to make sure that no potentially contaminated material ends up near boxwood shrubs that you have in your yard.  Under NO circumstances should you attempt to compost any suspect boxwood materials.  Again, if you are uncertain if you are decorating with boxwood, assume that you are and act accordingly.  This is a situation where you can have a huge impact in helping prevent the spread of an economically important disease-causing organism.

If you have questions about boxwood blight (or other plant diseases), feel free to contact the PDDC at (608) 262-2863 or  Also watch for a new University of Wisconsin Garden Facts fact sheet on boxwood blight that should be available on the PDDC website ( early in 2019.  Also feel free to follow the PDDC on Facebook and Twitter @UWPDDC to receive updates on emerging diseases (such as boxwood blight) and their management.

November: Thank Heaven for New Plant Diseases

Turkey on PlatterOne of the traditions of the Thanksgiving season, is to contemplate the past year and express thanks for positive aspects of our lives.  Thinking of this concept in the context of plant diseases, I thought that in this month’s web article, I would discuss new diseases that I saw in the clinic in 2018 that make me thankful for having a job that is always stimulating and never dull.

Cranberry false blossom.  The most seasonally appropriate of the new diseases that I encountered this year was cranberry false blossom.  This is not a new disease of cranberry and was originally first described in the 1920’s in cranberries in Wisconsin.  However, until this year, the disease has not been observed for decades in the state.  Cranberry false blossom is caused by a phytoplasma (i.e., a bacterium-like organism) that is transmitted by the blunt-nosed leafhopper.  Typical symptoms include odd-shaped, discolored and sterile flowers, excessive branching of vines (called brooming) and early fall reddening of foliage.  Sue Lueloff [the Plant Disease Diagnostics Clinic (PDDC) Assistant Diagnostician], working with Patricia McManus (the UW-Madison/Extension fruit pathologist) and Lindsay Wells-Hansen of Ocean Spray, is working to get a better sense of how widespread a problem this disease may be in commercial cranberry bogs in the state.

Bacterial streak of corn

Bacterial streak of corn.  This disease, caused by the bacterium Xanthomonas vasicola pv. vasculorum, was first reported in the US in Nebraska in 2016 and was found in Wisconsin this past summer.  Sue Lueloff was again instrumental in confirming this disease, working with Damon Smith (the UW-Madison/Extension field crop pathologist), as well as with scientists at USDA APHIS.  Typical symptoms of bacterial streak include linear, necrotic (i.e., dead) stripes (typically with a bit of a yellow halo) on affected corn leaves.  The long-term impact of bacterial streak on corn production in Wisconsin is not clear.  Damon and other corn researchers will be monitoring and assessing the disease in the coming years.  Currently USDA APHIS scientists are attempting to complete Koch’s postulates with an isolate of the bacterium that they recovered from Wisconsin corn samples.  If this is successful, publication of a first report of the disease for Wisconsin will follow in a scientific journal (something that always looks good on resume).

Verticillium wilt of VerbenaI am always watching for Verticillium wilt on new host plants, but this find caught me a bit off guard.  A local greenhouse submitted Verbena leaves to my clinic in mid-summer, complaining that branches on affected plants had died back.  As I microscopically examined the leaves, I noted fungal spores and conidiophores (i.e., specialized spore producing fungal threads) that were consistent with those of Verticillium (the fungus that causes Verticillium wilt).  But the conidiophores were odd, looking beefier than those of Verticillium that I had seen in the past.  I was able to grow a pure culture of the organism from the leaves and turned this over to Sue who once again did her molecular diagnostic magic.  She identified the Verticillium as Verticillium nonalfalfae, a species I had never before encountered.  I have plans to try to complete Koch’s postulates with this organism, and if successful, I will be able to publish a first report ever of Verticillium wilt on Verbena. 

Zonate leaf spot of maple

Zonate leaf spot.  This disease, caused by the fungus Grovesinia moricola (formerly Cristulariella moricola), has been on my plant disease bucket list for years, ever since I saw drawings of the microscopic, tree-like reproductive structures of the fungus in one of my plant disease references.  This summer, Brianna Wright, the Marathon County UW-Extension horticulture educator, emailed me photos of maple leaves with circular necrotic spots with concentric rings.  My fungey senses (the plant pathology equivalent of Spiderman’s spidey senses) immediately went off, and I begged Brianna to send me a sample of leaves.  She graciously did, and sure enough, there were the itsy-bitsy tree-like structures characteristic of Grovesinia.  Interestingly, the exact same day Brianna’s maple leaves arrived, I also received a grape leaf sample for another part of Wisconsin with exactly the same pathogen and disease.  Oh, the irony.  It took me 20 years to see this disease for the first time and then I received two samples on the same day.

So there you have it, a sprinkling of the diseases that make me thankful to be a plant disease diagnostician.  That said, as I reread this article, it dawns on me that what I am even more thankful for is having Sue Lueloff as a colleague in my clinic.  Her molecular diagnostic skills have greatly enhanced the services that the Plant Disease Diagnostics Clinic (PDDC) has been able to provide over the past year.  Happy Thanksgiving to her and to all of you!

To learn more about common diseases and disease management, explore the PDDC website ( and in particular, check out the fact sheet section of the website.  Also follow the PDDC on Facebook and Twitter @UWPDDC to receive updates on emerging diseases and their management.

October: Dr. Death’s Halloween

Jack O Lanterns

October hosts my favorite holiday of the year, Halloween.  Call me sentimental, but how can you go wrong (having a moniker like “Dr. Death”) with celebrating a holiday that caters to things that creep and crawl in the night.  Most people think what I enjoy and do professionally is pretty weird and I have to say there’s a part of me that revels in sharing with the public the bizarre (well, I’d say “cool”) things that I get to see every day.  Halloween fits in perfectly with what I readily admit is my somewhat warped and twisted world viewpoint.  What can I say?  I love my job.  And luckily, my clients have embraced (or at least tolerate) my rather eccentric world viewpoint.

In the spirit of the Halloween season, here are my votes for the top three most disgusting and horrifying diseases.

Bacterial Soft RotIf you like slimy, oozy, stinky plant diseases, this is the one for you.  Bacterial soft rot is caused by several bacteria in the genera Pectobacterium, Dickeya, and Clostridium (amongst others).  The characteristic of these diseases is the collapse and liquefication of plant parts.  I most commonly see this disease (but not exclusively) in vegetables like potatoes, carrots, cucurbits (e.g., pumpkins and squash) and cole crops (e.g., broccoli and cabbage).  If you’ve ever had potatoes liquefy in your refrigerator (or worse yet, your cupboard), you’ve experienced the joys of bacterial soft rot.  Soft rot bacteria liquefy plants parts by producing enzymes called pectinases.  These enzyme digest plant pectin, the “glue” that holds plant cells in place and helps give plants their shape.  Once the pectin is gone, the plant structure collapses into a soupy mess.  EWWWWW!!!  Oftentimes this slimified plant tissue has a rather fetid stench.  In particular in my mind, soft-rotted broccoli has the ultimate, vomit-inducing bouquet.  The worst part about working with bacterial soft-rotted materials is that after a while you get used to the smell.  But it seeps into your clothing and when you get on the bus to go home, people move away from you because they think you have a personal hygiene problem.

Armillaria Root DiseaseI most commonly see this disease associated with woody trees and shrubs.  The fungi involved (several species in the genus Armillaria) typically infect through roots (often wounded roots) and colonize up into the trunks under the bark where they form a thin, creamy white fungal masses called a mycelial fans.  Eventually Armillaria produces mushrooms (called honey mushrooms) which help with its reproduction.  The reason I find this disease very creepy is that it produces root-like (or shoestring-like) structures called rhizomorphs.  These grow outward from an infected plant, “looking for” other plants to infect.  I’m anthropomorphizing a bit here, but the fact that Armillaria can grow from tree to tree, is quite disturbing to me.  And even if the trees are removed, the fungus is still in the soil.  Amanda Gevens, the UW-Madison/Extension vegetable pathologist, recently told of her experience with Armillaria when she grew potatoes recently cleared forest land.  As she harvested her potato tuber, she found that they were covered with rhizomorphs.  CREEPY!!!  Armillaria can infect and kill trees (and other plants) over large areas.  There’s actually a super-colony of Armillaria in the Upper Peninsula of Michigan covering 37 acres.  The nearby town of Crystal Falls celebrates “Humongous Fungus Fest” every August in honor of this organism.

Common Corn SmutAs I’ve said before, I LOVE plant diseases, but I have to say this fungal disease of corn really grosses me out.  I think this is because I encountered this disease as a kid when I helped my maternal grandparents on their farm every summer in central Illinois.  I would often run across corn ears with the swollen, tumorous masses (galls) characteristic of common smut.  I don’t know which stage was worse, the early stage where the galls were pasty, zombie gray, or the later stage where the masses converted to powdery mass of spores that crumbled and blew away.  In my adult years, I have come to have a greater appreciation of this disease as the early, fleshy, zombie phase is sold in high end restaurants as a culinary delicacy under its Native American name “huitlacoche”.  But even so, the horrible childhood memories of this disease linger.  SHIVER!!!  And another cautionary tale in the context of smut. . .Be sure when doing internet searches on smut diseases to include the pathogen name (in the case of common corn smut, Ustilago maydis) in your search.  If you search on just “smut”, you will end up at a number of very, um, interesting websites that have NOTHING to do with plant diseases.

So there is my Halloween plant disease hall of fame.  Enjoy.  And once you think you’re grossed out by these beauties, try doing an internet search on “cordyceps, insects and photos”.  You won’t be able to sleep for days.  But that’s an article for PJ Liesch, my colleague in the UW-Madison Insect Diagnostic Lab.  MWAH-HAH-HAH!!!!!

To learn more about common diseases and disease management, explore the Plant Disease Diagnostics Clinic (PDDC) website ( and in particular, check out the fact sheet section of the website.  Also follow the PDDC on Facebook and Twitter @UWPDDC to receive updates on emerging diseases and their management.

September: Water, Water, Neverwhere

WaterThere is a certain irony of me writing an article about watering plants after Madison, WI (where I live) recently received record amounts of rainfall and, as I write this article, is experiencing flooding in many areas of the city.  However, as September arrives, the psychology of many gardeners seems to be that as we enter the fall, plants really don’t need a lot of water.  Nothing could be farther from the truth.  Fall is a critical period for watering, particularly for evergreens, which require sufficient internally stored water to make it through harsh winter conditions without dehydrating and suffering from winter burn.

Wet LeavesEstablished trees, shrubs and herbaceous ornamentals (i.e., plants that have been in the ground three years or more) typically require roughly one inch of inch per week.  New transplants (i.e., plants that have been in the ground less than three years) require slightly more water (on the order of one and a half to two inches per week).  If Mother Nature does not cooperate with natural rain, then gardeners should apply supplemental water with a soaker or drip hose in as much of the root zone of plants as possible.  Often, gardeners have limited ability to water this large of an area (note that the roots of a tree can extend on the order of three to five times the height of the tree from the trunk).  In such situations, I recommend concentrating watering at the drip lines of trees and shrubs (i.e., the edges of where branches extend).  Deciduous trees and shrubs (i.e., those that drop their leaves or needles) should be watered up until they begin to turn their normal fall color.  Evergreens, on the other hand, should be watered up until the ground freezes or there is a significant snowfall.  To get a rough estimate of how much water is being applied, sink empty tuna cans in the soil so the lips of the cans are level with the soil surface.  Place a section of hose over the can and see how much water collects.  When there is an inch of water in the can, then roughly one inch of water has been applied in that area.

MulchAlso consider mulching properly to help keep plants properly hydrated.  Turf is a great competitor with other plants for water, so I typically recommend removing turf from around other plants (particularly trees and shrubs) out to at least their drip lines.  These areas should be mulched with a high quality mulch (my favorites are shredded oak bark mulch and red cedar mulch).  I typically recommend roughly one to two inches of mulch on a heavier, clay soils and a bit more (say, roughly three inches) on a lighter, sandy soil.

Watering CanWith proper watering and mulch, you can help maximize the likelihood that your plants will survive harsh winter conditions and come through healthy, happy and ready to vigorously grow the following growing season.  As always, if you have questions about this topic or about plant diseases and their management in general, feel free to contact me at (608) 262-2863 or

P.S.:  I can’t use the reference “Neverwhere” in the title of my article without plugging the book from which this word originates.  “Neverwhere” is one of my favorite fantasy books, penned by the amazing Neil Gaiman.  Check it out!

August: Free Plant Disease Testing Through the PDDC

While the Plant Disease Diagnostic Clinic (PDDC) typically charges a small fee for processing plant disease samples, there are certain diseases where diagnoses are performed for free.  These diseases typically fall into four categories:

  1. Diseases that have not been documented in Wisconsin, but should they be introduced could cause serious damage, and thus early detection is critical for proper management.  The disease that currently falls into this category is thousand cankers disease of walnut.  This disease has been, for all practical purposes, lethal on black walnut wherever it has been found.  The pathogen involved is a fungus (Geosmithia morbida) that is transported by and introduced into walnut trees by the walnut twig beetle.  Neither the fungus nor the insect has been found to date in Wisconsin, but I am watching carefully for both.  If you see declining walnut trees with yellow leaves and a thin canopy, particularly with small, pin-sized holes on larger diameter branches, get a sample to the PDDC.  Invoke the words “thousand cankers disease” and the diagnosis is free.
  2. Newly introduced diseases of regulatory importance where documenting how widespread these diseases are can be important for eradication and limit of spread.  Boxwood blight falls into this category.  This disease is incredibly destructive to boxwoods (lethal in many cases) and was first documented in southeast Wisconsin in late July of this year.  Typical symptoms include black spots on leaves and stems, progressing to defoliation, dieback and oftentimes shrub death.  Unfortunately, boxwoods tend to be prone to a variety of dieback issues (winter burn being the most common), so I have been telling everyone for several years, “If your boxwood has branch dieback, send a sample to the PDDC, invoke the words ‘boxwood blight’ and get your free diagnosis.”  Submissions of these samples are even more critical now that the disease is in the state to figure out how widely distributed this disease is.
  3. New diseases to that state that aren’t necessarily of serious concern (at least at this point), but should be documented to keep track of their distribution and/or provide samples for researchers.  Corn tar spot falls into this category.  Many of you are likely familiar with tar spot of maple.  This tree disease (caused by the fungi Rhytisma americanum and Rhytisma acerinum) has been common in Wisconsin for years.  However, in 2016, a visually similar disease (caused by the fungus Phyllachora maydis) was found on corn.  At this point, corn tar spot has not been a particularly serious disease, but Diane Plewa (my diagnostic counterpart at the University of Illinois) is studying this disease as part of her PhD research and is interested in obtaining samples of the disease from as many locations as possible.  So, if you see corn tar spot in Wisconsin, fill out a corn tar spot survey form, send the form and your sample to the PDDC for a free initial ID (including your complete mailing address so I can get a report to you) and I will forward the sample to Diane.
  4. Diseases of extreme economic importance that recur every year.  Late blight of tomato and potato falls into this category.  Wisconsin is the third largest grower of potatoes in the US and also has a thriving fresh market tomato industry.  Late blight can be devastating to both (it did cause the Irish potato famine of the 1840’s and 1850’s).  Knowing when the pathogen (Phytophthora infestans) arrives and perhaps even more critically which type(s)/variant(s) of the pathogen is(are) in the state (there are many) are important for choosing appropriate fungicides for control.  Every year I offer free late blight testing, and I forward positive samples to Amanda Gevens, the UW-Madison/Extension vegetable pathologist for typing.  Most of the time late blight is not the problem (typically Septoria leaf spot or early blight), but better to be safe than sorry.  Get those tomato and potato samples in for your free diagnosis.  The first report of late blight for 2018 just came through this week.

Be sure to take advantage of the PDDC’s free services while they last, but also remember that the PDDC needs paying samples as well to help fund the clinic’s operation.  So submit early and often!  As always, if you have questions about plant diseases and their management, or PDDC activities and services, feel free to contact me at (608) 262-2863 or

July: 20 Years in the Life of a Plant Disease Diagnostician

July 1, 2018 marks my 20th anniversary as director of the UW-Madison/Extension Plant Disease Diagnostics Clinic.  It really seems just like yesterday that I started at the clinic.  I remember being so excited about being asked to interview for the position, but terrified that I wouldn’t be hired because my diagnostic background was very limited.  I felt better after I gave my interview seminar and Tom German (a virologist in my department) commented how he didn’t see how I could have given a better talk.  Craig Grau (the department field and forage drop Extension specialist and my boss at the time) was also incredibly supportive, and Jennifer Parke (my previous boss in the department) wrote me (from what I was told) a “perfect” letter of recommendation.

Eventually the stars aligned, I was hired, and I was off to the plant disease diagnostic races on July 1, 1998.  I had only two weeks of overlap with Sr. Mary Francis Heimann (my predecessor in the clinic) and I tried to sop up as much of her knowledge as I could in that short period of time.  After that, it was sink or swim.  In particular, I was forced to learn a lot about ornamental diseases (the bulk of my samples even to this day) very quickly.  Everyone in my department, and also Phil Pellitteri (the UW-Madison/Extension insect diagnostician in the UW-Madison Department of Entomology), was very supportive as I consulted with folks about plants, diseases and insect pests that I was unfamiliar with.  I have learned a lot with everyone I have interacted with over the past 20 years and continue to do so even now.

The PDDC’s physical facilities have evolved over the years as well.  My original clinic space was a small lab and office on the second floor in Russell Labs.  I remember one day when so many samples arrived that I had to leave them in a pile on the floor because there wasn’t enough counter space to organize them.  And then there was the 8 ft. Douglas fir that I had to drag into the hall for several days so I could work in the lab and then haul it back into the lab each night so the custodial staff wouldn’t haul it to the dumpster.  Eventually, I moved to the clinic’s current location in Rm. 183 Russell Labs, about three times the size of my original space.  With the arrival of soybean rust into the US in 2004, I added clean space in the basement of Russell Labs so I could pursue the molecular diagnostics needed to detect the pathogen.  I added a new office just a few years ago and now have about four times my original space.  With renovations, I and my staff have created an efficient and productive work space.

And speaking of staff, I have had the best over the years.  The first addition to the clinic was Lynn Williamson, a returning adult undergraduate, who worked for several years as a student hourly in the PDDC.  As my funding became more stable (with increased clinic revenues and federal funding through the National Plant Diagnostic Network), Ann Joy (who I had worked with previously in the department) joined the clinic as the Assistant Diagnostician, providing general support and initiating our foray into molecular diagnostics.  With Ann’s retirement, Sean Toporek joined the clinic as her successor and expanded the PDDC’s molecular diagnostics over his roughly two year tenure.  With Sean’s decision to pursue graduate school (his MS at the time and now his PhD), Sue Lueloff joined the clinic and our molecular diagnostic program has exploded.  Over the years, an army of dedicated undergraduates have worked (and kept me young) in the lab culminating this year with John Lake (my student hourly) and Stephanie Salgado (a Memorial High School intern hired through the TOPS/AVID program).  Ann Joy continues her presence in the clinic doing data entry and Dixie Lang recently joined the group to provide her magical IT expertise and clinic website ( and social media (@UWPDDC on Facebook and Twitter) support.  Everyone I have worked with over the years including new Plant Pathology faculty and PJ Liesch (Phil Pellitteri’s successor in the Insect Diagnostic Lab), continue to help me learn and do my job.

Clinic activities have expanded over the years.  In addition to diagnosing plant diseases (on average about 1500 samples per year), the clinic provides outreach on plant diseases throughout the state and also nationwide.  I routinely give disease talks (a record 104 in 2017) particularly providing support for home gardeners and professionals in the horticulture arena.  I am particularly grateful to county UW-Extension agents/educators (like Lisa Johnson of Dane County UW-Extension and Diana Alfuth of Pierce County UW-Extension) who have been willing to collaborate with me to provide programming.  I’ve had the pleasure of doing television (on the late Shelley Ryan’s “Wisconsin Gardener”) and radio (on Larry Meiller’s “Garden Talk”) under the moniker “Dr. Death” (a nickname that I acquired at Garden Expo years ago and that makes me smile every time I hear it).  I am also pleased to have been involved in the development of the “University of Wisconsin Garden Facts/Farm Facts/Pest Alert” fact sheet series (  I continue to enjoy instructional activities at the UW-Madison including helping Bryan Jensen with his “IPM Scout School” course and conducting my summer “Plant Disease Diagnostics Practicum” course.

It’s been a great 20 years at the PDDC and I don’t see myself retiring anytime soon.  I still have too much I would like to do.  Diagnostics, my outreach activities and my fact sheet work still call.  In addition, I am currently working on plans for an outdoor plant disease laboratory (in collaboration with James Steiner of the UW-Madison Department of Planning and Landscape Architecture) that I would like to see to completion.  And it has always been my goal, time permitting, to have a more active research program in my department documenting new pathogens (new hosts for Verticillium anyone?).  I am excited as I enter my third decade in the PDDC and look forward to whatever challenges come my way.

As always, if you have questions about plant diseases and their management, or PDDC activities and services, feel free to contact me at (608) 262-2863 or

June: Stormy Weather Ahead – Pathogens on the Wind

Trees in WindJune 1 marks the beginning of Hurricane season in the Atlantic and while full-blown hurricanes do not reach Wisconsin, their effects (and those of other seasonal winds) can have an influence on plant diseases.

Soybean rustA somewhat recent example of an apparent direct effect of a hurricane was the introduction of the Asian soybean rust fungus (Phakopsora pachyrhizi) into the United States in 2004.  Prior to 2004, this fungus had been well-established in South America (after an initial introduction in 2001) and caused substantial losses in soybean production in Brazil.  US soybean producers had been watching this disease closely and were concerned that the pathogen would hopscotch from island to island through the Caribbean and eventually make its way to the US.  The introduction of soybean rust however occurred quite abruptly in 2004.  The speculated method of introduction was by Hurricane Ivan which skirted the coast South American in September and then made its initial landfall in the US in Alabama (as a hurricane) and then made a second, later landfall in Louisiana (as a tropical depression) after reforming following a looped track through Maryland, and then eventually the Florida peninsula.  Soybean rust was first confirmed in Louisiana in November of 2004, roughly two months after Hurricane Ivan.  Losses due to soybean rust in the US have never approached those seen in Brazil, but the disease continues its presence in the southern US to this day.  While soybean rust has never been reported in Wisconsin, spores of the pathogen have been documented in Wisconsin, apparently having been blown into the state by southerly winds.  Fortunately these spores have never led to a soybean rust outbreak.

Black stem rust of wheat:  If you find the idea that spores of the soybean rust fungus can make it all the way from the southern US to Wisconsin amazing, I present for your consideration another amazing example of long distance movement of a pathogen via wind:  black stem rust of wheat.  The fungus that causes this disease (Puccinia graminis) is an alternating rust that requires two very radically different plants, wheat (a grass) and barberry (a broad-leafed shrub), to complete its life cycle (including sexual reproduction).  During this life cycle, spores produces on wheat infect barberry and spores produced on barberry infect wheat.  Attempts (and very successful ones) were made to eradicate barberry from wheat-producing regions of the US starting in 1918.  The thought behind barberry eradication was that eliminating this plant would prevent the black stem rust from completing its life cycle, and thus eliminate the disease.  What folks didn’t count on was a third type of spore that the fungus produces, one that is produced on wheat and reinfects wheat.  This spore type (called a urediniospore) is produced year around in the southern US on wheat, and urediniospores can blow from the south into more northern wheat-producing areas (including Wisconsin) every growing season.  This movement is so well documented that it’s been dubbed the Puccinia Pathway.  Although eliminating barberry did not totally eliminate black stem rust, it did severely limit sexual reproduction of Puccinia graminis.  This is important, because it’s during sexual reproduction that recombination of fungal genes occurs that can lead to new variants of the pathogen that can overcome resistance genes in commercially-grown varieties of wheat.  Genetic resistance is a major means of controlling black stem rust.  Less pathogen sexual reproduction means that resistant wheat varieties tend to be effective longer.

Coneflowers with aster yellows (right) often have deformed, discolored flowers.

Aster yellowsMy final example of a windborne pathogen is an indirect one.  Aster yellows is a disease caused by a bacterium-like organism called a phytoplasma (specifically the aster yellows phytoplasma).  This organism does not survive on its own in the environment, but will survive inside infected living plants.  The host range of the aster yellow phytoplasma is very broad including over 300 plants in roughly 40 plant families.  In addition to residing in infected plants, the aster yellows phytoplasma also can survive in association with certain insects, particularly the aster leafhopper.  This insect does not survive Wisconsin winters, but does overwinter in the southern US.  During the growing season, aster leafhoppers can fly (and/or be blown) to Wisconsin, and some carry with them the phytoplasma.  As the leafhoppers feed in a plant’s phloem (it’s food-conducting tissue), they drop off the phytoplasma, and once in the plant, the phytoplasma induces a wide range of very bizarre symptoms.  These include, but are not limited to, yellow leaves, curled and cupped leaves, leafy-green flowers, tufts of white hairy roots (particularly on carrots), mini-tubers on the branches of infected potato plants and an off flavor to certain edible plants (like carrot).  The year 2012 was a particularly good year for aster yellows.  The season got started early (March), aster leafhoppers arrived early and in larger numbers than usual, and a higher percentage of the leafhoppers carried phytoplasmas than normal.  There were symptoms of aster yellows EVERYWHERE.  I was in plant pathologist’s heaven.  I also distinctly remember talking about this disease in a presentation in Iron County, WI.  The county Extension educator who was hosting me came up after my talk and told me how her little boy, who had always liked carrots, refused to eat them that year because they “tasted funny”.  She said after seeing the photos in my talk, she realized all of her carrots had aster yellows.  Her son had been able to tell.  Amazing!

So, as you enjoy the breezes of late spring and summer, remember that on those breezes are the seeds (or should I say spores) of plant diseases and destruction, some coming from nearby, some coming from afar.  As always, if you have questions about plant diseases and their management, feel free to contact me at (608) 262-2863 or

May: Rattling the Cage for Tobacco Rattle Virus

It’s been a long winter and now temperatures have warmed to the point that spring emphemerals in my backyard are beginning to emerge and bloom.  As their leaves begin to appear, I am on the lookout for symptoms of tobacco rattle caused by Tobacco rattle virus (TRV).

Tobacco rattle virus-infected plants often have leaves with yellow line patterns.
Tobacco rattle virus-infected plants often have leaves with yellow line patterns.

I know I have TRV in my garden and I am reasonably certain that I introduced the virus via a bluebell that a fried gave me many years ago.  The plant showed interesting line patterns on the leaves and a bit of leaf distortion.  If I had been a “good” gardener, I would have thrown the plant away as it had obvious symptoms of a viral disease.  Instead, I was a “good” plant pathologist and plopped the plant into one of my beds and let it do its thing.  Over the years I have seen symptoms of TRV in numerous plants in my backyard flowerbeds.  I have volunteer Canada goldenrod plants that have lightning bolt (think Harry Potter’s forehead scar), yellow line patterns on their leaves every year.  About 10 years ago, I noticed a similar line pattern on leaves on my bleeding heart, odd blotchy color and crinkly of leaves on my bloodroot, and dimpling and distorted leaves on my twinleaf.  I had my visual diagnosis of TRV confirmed by diagnosticians at the Wisconsin Department of Agriculture, Trade and Consumer Protection, who had just started testing for the virus in nursery/greenhouse stock.  My bleeding heart in particular was quite positive for the virus.

The diagnosis and symptoms of TRV fascinated me as a plant pathologist.  However, they horrified me as a gardener because virus-infected plants often decline over time and typically stop blooming as the virus redirects plant energy and nutrients from producing more plant tissue and setting new flower buds to producing more viral particles.  Interestingly though, after that one year of dramatic symptoms 10 years ago, my plants (other than the volunteer goldenrod) have been conspicuously lacking in any symptoms of TRV.  Several years ago, I began testing for TRV in my own clinic and last year I noticed that the stock of positive control material for my test was getting low and I needed TRV-infected tissue to generate more.  My bleeding heart was huge, lush, blooming profusely and totally asymptomatic, but I thought, “What the heck,” let’s test the plant again for TRV.  Lo and behold the test lit up like the proverbial Christmas tree as positive for TRV.  So did the symptomatic goldenrod from my yard.  I had my positive control material.

All of the blathering above about TRV is well and good, but what are the take home messages?

  • Beware of plants showing viral symptoms.  No matter what the virus, these plants can be bad news because they can serve as a source of a virus that eventually end up in other plants.  Interestingly, TRV is transmitted by stubby root nematodes, microscopic worm-like organisms that feed on roots of infected plants, pick up the virus, and then transmit the virus once they feed on the roots of healthy plants.  TRV can also be transmitted mechanically via contaminated tools (e.g., shovels, knives, etc.) used to divide plants.  Nematode-transmitted viruses are somewhat unusual, but mechanically-transmitted viruses are very common.  Another common way that certain viruses (but not TRV) can be moved about is by insects (aphids and thrips are notorious movers of plant viruses).  Some viruses can even be transmitted by touch!
  • Even healthy-looking plants can be infected with TRV.  As my bleeding heart demonstrates, plants that look healthy and bloom profusely can be have a viral problem.  TRV has been a real issue in the perennial plant industry as the virus has a wide host range (including but not limited to the plants I have already mentioned as well as peony, astilbe, coral bells and relatives, and columbine) and often the plants show no symptoms.  The onus is on plant propagators to supply healthy virus-free plants, but often they do not.  So consumers buy TRV-infected, asymptomatic perennials, and happily plant them in their gardens only to have the virus rear its ugly head in other plants as it spreads.  Asymptomatic plants can particularly be a problem if you plant them near a commercial potato field.  Potato is a host for TRV.  The virus does not cause foliar symptoms, but leads to necrotic (i.e., dead) flecks and arcs in potato tubers.  If these tubers are sliced and fried, you end up with potato chips with black spots.  Thus commercial potato producers (FYI, Wisconsin is the third largest potato producer in the US) are very worried about this virus.
  • Proper sanitation is critical for managing this (and other viruses).  Watch for any symptomatic plants and immediately remove and destroy them (by burning, burying or hot composting).  Unfortunately, you may still have asymptomatic plants and the only way to check them for TRV is to have them professionally tested.  This is not an inexpensive test (my clinic currently charges $35 for TRV-testing).  Also, be careful to decontaminate anything (e.g., tools, working surfaces) that may have come in contact with infected plants.  Soapy solutions work best.  I typically recommend a solution that is 10% shampoo (make sure the label says the shampoo contains sodium lauryl sulfate) and 1% Alconox® (a laboratory detergent) in water.

All of this said, you may decide you think TRV-infected plants look cool (I do!) want to leave them in place.  TRV-infected plants are actually quite beautiful.  But be aware that I DO NOT recommend this if you live near a commercial potato field.  And even in urban areas, your neighbors may not be happy with you if the virus spreads to their plants.  Luckily I have neighbors who are tolerant of my plant pathological eccentricities.  You may not be so lucky!  As always, if you have questions about plant diseases and their management, feel free to contact me at (608) 262-2863 or

P.S.:  Happy Belated Robigalia (April 25), the Roman festival of the god of rust!

April: April Showers Bring…Plant Diseases (Yay!)

It appears that spring is slowly arriving, and with the spring typically comes regular, often frequent rain showers.  The upside to this moisture is that it helps thaw the ground and stimulate plants to grow.  The downside however can be that this moisture provides a favorable environment for plant diseases to develop.

Lower stem collapse of Zinnia seedlings due to damping-off.
Lower stem collapse of Zinnia seedlings due to damping-off.

Damping-Off:  If you like to plant early when soils are colder and moisture is high, you may run into problems with damping-off.  Damping-off pathogens (e.g., Pythium, Rhizoctonia, Fusarium) are found, at least at some level, in many soils and when combined with wet conditions and young, tender seedlings, death and destruction can be the result.  Watch for plants that never emerge (the seed rot or pre-emergence phases of damping-off) or those that do and then fall over onto the soil surface with collapsed lower stems (the post-emergence phase of the disease).  You can often avoid damping-off by planting later when soils are warmer and there is slightly drier weather.  The warmer soil stimulates plants to grow rapidly out of early stages of growth when they are most susceptible to infection.  Using a good seed fungicide treatment (often commercial seeds are pretreated prior to packaging) can also help prevent the disease.  Just be sure to handle any fungicide-treated seed according to the directions on the package to minimize any direct exposure to the fungicide.

Brown discoloration of roots typical of root rots.
Brown discoloration of roots typical of root rots.

Root Rots:  Root rots are caused by many of the same organisms that cause damping-off including Phytophthora, Pythium, Rhizoctonia and Fusarium.  Root rots differ from damping-off however in that affect older herbaceous and woody plants.  The pathogens destroy root tissue, thus reducing water uptake, and that can eventually translate into dieback, general plant decline and, in extreme cases, death.  Root rot organisms tend to perform better in wet soils, and Pythium and Phytophthora actually reproduce more efficiently under cooler, wetter conditions.  So, making sure soils are well drained can be critical for root rot prevention.  Adding organic matter to heavier, clay soils to improve drainage prior to establishing a landscape can have a big impact long term on reducing root rot problems.  Also, making sure to mulch properly can help moderate soil moisture to a level that makes root rots less likely.  I typically recommend using approximately one to two inches of a high quality mulch (e.g., shredded oak bark mulch or red cedar mulch) on heavier, clay soils and roughly three inches of mulch (perhaps up to four inches) on lighter, sandier soils.  The mulch should be applied out to at least the dripline of trees and shrubs (i.e., the edge of where the branches extend) and kept away (approximately four to six inches) from tree trunks and crowns of shrubs.

Symptoms of tar spot on silver maple leaves.
Symptoms of tar spot on silver maple leaves.

Leaf Spots and Blights:  Spring rains can also have a huge effect on the severity of many types of leaf spots and blights like anthracnose, tar spot and apple scab.  If extended rainy periods arrive when leaves are first emerging, then numerous infections can occur early and that can translate into severe disease and possibly even defoliation later in the summer.  Luckily trees seem to tolerate at least some defoliation, and long term effects due to leaf diseases are often minimal.  However, defoliation year after year can stress plants to the point where they become susceptible to more serious diseases (e.g., Armillaria root disease) and insect pests (e.g., two-lined chestnut borer).  While we have no control over Mother Nature and the rain she brings in the spring, using other disease management strategies can help lessen the effects of wet spring weather.  Careful cleanup and destruction (by burning, burning or hot composting) of plant debris in the fall can significantly reduce leaf pathogen carryover.  Proper pruning of trees to promote better air penetration, allowing for more rapid drying of foliage can also help reduce problems with leaf diseases.  For certain diseases like apple scab, growing a resistant apple or crabapple variety may be your best option.  And finally, in certain situations, use of preventative fungicide treatments may be warranted to keep leaf diseases in check.

So, as you dream of those warm spring rain, dream of them in moderation.  As with most things in life, balance is the key.  Hope for enough rain to get your plants to grow, but not enough to lead to disease problems.  As always, if you have questions about plant diseases and their management, feel free to contact me at (608) 262-2863 or


March: The Irish – Good Luck in Life, Bad Luck in Plants

As March arrives, being in part Irish by ancestry, my thoughts tend towards St. Patrick’s Day and as a plant pathologist, I imagine what havoc plant disease might cause for the holiday.


ShamrocksA major symbol of St. Patrick’s Day is the shamrock.  While several plants can be called shamrocks, the most common plant to be so-named is white clover (Tifolium repens).  This plant was once a common component of lawns (in combination with grasses such as Kentucky bluegrass) and served the important function of enriching soil with nitrogen.  Interestingly, it’s not the clover plant itself that is instrumental in this nitrogen enrichment process.  Actually, the credit goes to the bacterium Rhizobium which colonizes the roots of clover (along with the roots of other plants in the pea family) and causes formation of nodules (swellings) on the roots.  Inside the pinkish, elongate nodules, Rhizobium takes nitrogen gas (which is very common in the air) and converts it to a form of nitrogen that is more easily used not only by the bacterium, but by the clover plant it colonizes.  In exchange for this ready supply of nitrogen, the clover plants provide Rhizobium with sugars (produced through photosynthesis) that it needs to grow and reproduce.

Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.
Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.

This interesting symbiosis between clover and Rhizobium, can be disrupted by the plant pathogenic nematode Meloidogyne, more commonly known as the root-knot nematode.  Nematodes are small (typically microscopic) worm-like organisms.  Many nematodes are beneficial, but root-knot nematode infects the roots of a variety of plants (including clover) causing damage.  Root-knot nematode females tunnel into roots and set up feeding sites.  In the process of feeding, they secrete saliva that stimulates root cells to grow larger than normal, grow faster than normal, and divide like crazy.  This uncontrolled growth leads to a tumor-like swelling on the infected root (called a gall or knot).  Formation of the galls can interfere with root function (i.e., movement of water and nutrients to leaves and stems above ground) and can also interfere with proper nodulation by Rhizobium.  Thus plants with root-knot nematode often look stunted and discolored due to nutrient deficiencies caused by the presence of the pathogen.  You’re not going to find a lot of four-leafed clover leaves on plants with root-knot.



The food that comes to my mind as a symbol of St. Patrick’s Day is corned beef and cabbage.  While I can’t say too much plant pathological about beef, cabbage is another matter.  The primary disease that I can think of that would prevent you from enjoying your cooked cabbage is black rot.  I have seen an amazing increase in the incidence of this bacterial disease over the past five years or so.  The disease not only affects cabbage, but virtually all types of brassicas, the group of plants that includes cabbage, broccoli, cauliflower, kale, rutabaga and turnip, as well as weed plants such as shepherd’s purse and wild mustard.  Often the causal bacterium (Xanthomonas campestris pv. campestris) comes into a garden on contaminated (but asymptomatic) seed or transplants.

Black rot causes V-shaped yellow and brown/ dead areas in affected leaves. (Photo courtesy of Amanda Gevens)
Black rot causes V-shaped yellow and brown/ dead areas in affected leaves. (Photo courtesy of Amanda Gevens)

Eventually wedge-shaped yellow, then dead areas develop on leaves or other plant parts leading to deterioration of the plant.  Black rot can be followed by soft rot (another bacterial disease), leading to even more extensive damage.  It’s not a happy day when cabbage with black rot and soft rot arrives in my clinic.  The stench is overpowering!  Good debris clean up, decontamination of gardening tools, proper weed control, proper vegetable rotation, and hot-water seed treatments can all help in managing this disease.


And no discussion of the Irish would be complete without a mention of late blight, the cause of the Irish potato famine.  This devastating disease wiped out the Irish potato crop for several years in the 1840’s and 1850’s.  For a variety political and social reasons, potato was the primary food of the Irish during this period.  Loss of the crop due to late blight led to the starvation of over 1 million Irish and the emigration of over 1 million more Irish, many of them to the US.  I am sitting, writing this article in Madison, WI due to this disease.

Late Blight on Potato Tubers, photo courtesy of Prof. Amanda Gevens
Late Blight on Potato Tubers. (Photo courtesy of Amanda Gevens)

Even today, late blight can have a huge negative impact on both commercial and home garden potato (and tomato) production.  Without proper treatment the disease can wipe out entire potato and tomato patches/fields in a matter of a few days.  It is critical therefore to identify any occurrences of the disease in Wisconsin as early in the growing season as possible and also identify which variant(s) (and there are many) of the pathogen is(are) causing problems.  For that reason, my clinic provides free diagnoses for late blight for anyone growing potatoes and tomatoes in Wisconsin.  All you need to do to get the free diagnosis is send in a potato or tomato sample and invoke the words “late blight” and the diagnosis is free.  Even if you don’t think your potato or tomato problem is late blight, send in a sample, mention “late blight” and I’ll provide a diagnosis and management recommendations for free.  You can send samples to:

Plant Disease Diagnostics Clinic
Department of Plant Pathology
University of Wisconsin-Madison
1630 Linden Drive
Madison, WI  53706-1598

As always, if you have questions, feel free to contact me at (608) 262-2863 or

With that, go forth, wear green, drink green beer, think about the contributions that the Irish have made to US culture and of course, don’t forget about the all-important plant diseases.  Happy St. Patrick’s Day!!

Shamrock and Claddagh