All posts by ddlang

February: Wacky Wisconsin Winter Weather

After last week’s sobering, subzero weather, I began to think about all of the ways winter in Wisconsin is designed to make the life of plants, particularly woody ornamentals, difficult.


Snow IconSnow can be a mixed blessing.  I like to see a few inches of snow on the ground, because snow actually has an insulating effect.  Without a snow layer, soil temperatures can become quite cold far down into the soil profile and this can lead to cold injury to plant roots.  Such injury can outright kill plants, or alternatively lead to a slow, painful death where plants leaf out, but leaves rapidly dry up and die because there is a lack of a functional root system to take up water to supply to the emerging leaves.

Excessive snow can also be problematic.  In particular, I see situations where white cedars (arborvitaes) become so laden with snow that a variety of problems can arise.  In extreme instances, the weight of the snow may be so heavy that branches will snap.  In other instances, the snow simply causes the branches to bend downward.  This may seem innocuous, but if the snow doesn’t melt away and the plants are weighted for an extended period, branches may not spring back into their normal position and the shrubs end up deformed.

Cold Temperatures

Cold IconLate January’s deep freeze here in Wisconsin likely led to significant plant damage.  Plants have particular ranges of temperatures that they can tolerate (usually described in terms of their USDA Hardiness Zone).  If temperatures drop outside of this optimal range, physical injury to branches and trunks, and even plant death can result.  As I mentioned above, lack of snow cover can make cold injury worse by allowing for additional cold injury to roots.  Another contributing factor can be that many gardeners want to “push the envelope” and grow plants (often exceptionally beautiful trees and shrubs) that are not rated for their hardiness zones, but for warmer environments.  Often these marginally hardy plants will do well for many years, until they experience an extreme winter.  We’ll have to wait to find out how severe this winter’s damage has been until spring arrives.  At that point, we’ll be able to see how many trees and shrubs don’t leaf out or are stone dead.  I’m expecting plants like Japanese maple, magnolia, redbud and many types of fruit trees (particularly peach and apricot) to be hardest hit.


Ice IconNow that temperatures have warmed a bit, I’m seeing some areas of Wisconsin experiencing “wintry mixes” of precipitation.  Often this means freezing rain, which can coat branches and, depending on the duration of the rain and the specific temperature, lead to thick layers of ice that can be so heavy that they cause branches to break.  Some gardeners, in an effort to prevent this breakage, attempt to knock ice from branches, but this technique runs the risk of damaging overwintering buds.  I have fond memories (read EXTREME sarcasm here) of an ice storm in the mid-1970’s that caused substantial tree damage, paralyzed Madison and left my family without power for five days.  We spent a lot at the mall.


Wind IconHigh winds can cause extensive plant dehydration.  I most commonly see this as a problem on evergreen trees and shrubs.  I think of windy conditions, alone or in concert with cold temperatures, as being particularly damaging on Alberta spruce, boxwood and white cedar (arborvitae).  Watch for brown or bleached needles and branch dieback on these plants, particularly as they come out of dormancy in the spring.  This damage in often referred to as winter burn.  In extreme situations, high winds can physically damage and break off branches.

Now that I’ve totally depressed you by considering all of the possible adverse effects of winter weather, let’s try to put a sunny spin on things and think of the upside:  All of the winter death and destruction provides ample opportunities to plant new and exciting trees and shrubs (and even herbaceous plants), and watch these new plants grow and mature.  🙂

For addition information on the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at


Gymnosporangium Rusts

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UW Plant Disease Facts


Authors:   Brian Hudelson, UW-Madison Plant Pathology
Last Revised:   03/01/2024
D-number:   D0058

What are Gymnosporangium rusts? 

Gymnosporangium rusts are a group of closely related diseases caused by fungi that infect both junipers (in particular red cedar) and woody plants in the rose family such as, but not limited to, apple, crabapple, hawthorn and quince.  These fungi must infect both types of plants to complete their life cycles.  The most common Gymnosporangium rusts found in Wisconsin are cedar-apple rust, cedar-hawthorn rust and cedar-quince rust.  The names of these diseases are somewhat misleading, given that all three diseases can affect multiple rosaceous hosts in addition to those referenced in their names.

Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).
Yellow cedar-apple rust spots on an apple leaf (left) and slimy, orange, gelatinous cedar-apple rust galls on a juniper branch (right).

What do Gymnosporangium rusts look like? 

On junipers, symptoms of Gymnosporagium rusts vary.  Cedar-hawthorn and cedar-apple rust fungi induce formation of irregularly-shaped brown galls, with cedar-hawthorn rust galls tending to be smaller in size (approximately 1/8 to 9/16 inch in diameter) than cedar-apple rust galls (approximately 1/4 to 2 inches in diameter).  Both types of galls produce distinctive slimy, orange, gelatinous appendages in the spring.  In contrast, the cedar-quince rust fungus causes juniper branch swellings.  Orange spores ooze from these swollen areas in the spring.

On rosaceous hosts, Gymnosporangium rust symptoms also vary.  Symptoms of cedar-hawthorn rust and cedar-apple rust appear in mid to late May, typically as circular, yellow-orange areas on leaves.  Eventually, tube-like structures (that have a fringe-like appearance) form on the undersides of leaves beneath the yellow spots.  Symptoms of cedar-quince rust typically become obvious later in the summer (most commonly on hawthorns) as swollen, spiny branches and/or fruits.

Where do Gymnosporangium rusts come from? 

Several fungi in the genus Gymnosporangium cause Gymnosporangium rusts. These include Gymnosporangium juniperi-virginianae (cedar-apple rust), Gymnosporangium globosum (cedar-hawthorn rust), and Gymnosporangium clavipes (cedar-quince rust).  These fungi overwinter in infected branches and galls on junipers.  Spores oozed from the infected branches or produced in the gelatinous gall appendages drift to rosaceous hosts leading to leaf and fruit infections.  Similarly, spores produced in the tube-like structures/spines on rosaceous leaves and fruits drift to junipers leading to new branch infections and additional gall formation.

How do I save a tree or shrub with Gymnosporangium rust? 

Gymnosporangium rusts are primarily cosmetic diseases that make susceptible plants unattractive, but rarely have long-term detrimental effects.  Gymnosporangium rusts on leaves can, for all practical purposes, be ignored.  Gymnosporangium rusts on juniper branches can be easily managed by pruning approximately four to six inches below swollen areas or galls.  Rosaceous hosts with infected branches can be pruned similarly.  Be sure to decontaminate pruning tools between cuts by treating them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants) or 10% bleach.  Decontaminating tools will prevent movement of rust fungi from branch to branch or from plant to plant during pruning.  If you use bleach, be sure to thoroughly rinse and oil your tools after pruning to prevent rusting.

Cedar-quince rust on hawthorn fruit.
Cedar-quince rust on hawthorn fruit.

How do I avoid problems with Gymnosporangium rusts in the future? 

The best way to avoid Gymnosporangium rusts is to not grow junipers (particularly red cedar) and susceptible rosaceous hosts close to one another.  In urban settings where yards are small however, keeping both hosts adequately separated may be impossible.  Where Gymnosporangium rusts have consistently been a problem, consider using evergreens (e.g., pine, fir, spruce) and flowering trees and shrubs (e.g., cherry, plum, lilac) that are immune to these diseases.  If you decide that you want to mix junipers with apple, crabapple, hawthorn, and quince on your property, check at your local nursery for resistant varieties that will satisfy your landscaping needs.  In general, Chinese junipers (Juniperus chinensis) tend to be relatively resistant to Gymnosporangium rusts.

Fungicides treatments are also available to control Gymnosporangium rusts, although such treatments should be considered only as a last resort.  Among fungicides marketed for use by home gardeners, those containing chlorothalonil, copper, mancozeb, myclobutanil, propiconazole, and sulfur are labeled for use for Gymnosporangium rust control.  These products may be useful for controlling Gymnosporangium rusts on rosaceous hosts, but will likely not be effective if used on junipers.  For optimal control on rosaceous hosts, apply treatments when flower buds first show color, when half of the flowers are open, at petal-fall, seven to 10 days after petal fall and finally 10 to 14 days later.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the products(s) in the safest and most effective manner possible.  In particular, be sure that you select appropriate products when treating trees and shrubs with edible fruit.  If you decide to use propiconazole or myclobutanil, alternate use of these active ingredients with use of at least one of the other active ingredients listed above (but DO NOT alternate propiconazole with myclobutanil) to help minimize potential problems with fungicide-resistant strains of Gymnosporangium rust fungi.

For more information on Gymnosporangium rusts: 

Contact the University of Wisconsin Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 or

This Fact Sheet is also available in PDF format:

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An EEO/Affirmative Action employer, University of Wisconsin-Madison Division of Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. This document can be provided in an alternative format by calling Brian Hudelson at (608) 262-2863 (711 for Wisconsin Relay).

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Thanks to Laura Jull, Darrin Kimbler, Sharon Morrisey, Charlene Schmidt and Janet Van Zoeren for reviewing this document.

A complete inventory of UW Plant Disease Facts is available at the University of Wisconsin-Madison Plant Disease Diagnostics Clinic website:

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January: 2018 in Review

Calendar at AngleAs the New Year rolls in, it’s time to reflect on the past year and all that happened at the PDDC.

Clinic staff processed 1282 samples, down roughly 11% from 2017.  These samples came from 62 of Wisconsin’s 72 counties, as well as FL, IA, ID, IL, ME, MI, MN, MO, NM and TX.

While samples numbers were down, the complexity of the samples seemed to be on the rise.  In particular, requests for molecular diagnostics increased this year and I have to give kudos to Sue Lueloff, the PDDC’s Assistant Diagnostician, for stepping up and handling all of these samples.  Sue tested numerous samples for phytoplasmas, an incredibly complex process involving identifying DNA sequences of these bacteria-like organisms.  She confirmed the presence of the cranberry false blossom phytoplasma, a pathogen that hasn’t been documented in WI for many decades.  Sue was also involved in the first detection in Wisconsin of Xanthomonas vasicola pv. vasculorum, the corn bacterial streak pathogen.  Sue also confirmed the presence of Verticillium nonalfalfae, a rather odd species of Verticillium (at least in my experience), in Verbena.  PDDC staff still need to complete Koch’s postulates with this fungus, but if successful, this will be the first report of Verticillium wilt of Verbena.

While Sue handled the molecular side of things at the PDDC, I concentrated on handling the more classical side of the diagnostic process that involved culturing (i.e., growing pathogens from plant tissue) and microscopy.  I spent a lot of time identifying Phyllochora maydis, the corn tar spot fungus (a recent addition to the pantheon of fungal plant pathogens in the state).  I also got to see a new fungus that has been on my personal bucket list for years:  Cristullariella (cause of zonate leaf spot).  This is a rather uncommon disease that I saw in 2018 on both maple and grape.  I also continued to provide digital disease diagnostics via email, through the UW-Extension PlantDOC online diagnostic website, and through the Association of Specialty Cut Flower Growers Facebook page.  I logged roughly 1800 exchanges in the process of handling online plant disease inquiries.

2018 PDDC Educational StatisticsPDDC outreach activities hit an all-time high in 2018.  I did 107 talks/presentations/workshops in 31 Wisconsin counties (virtually all of these in-person visits).  My biggest outreach event was again Wisconsin Public Television’s Garden Expo.  I spent three days at the event, gave two talks on diseases of vegetables and helped answer questions with Lisa Johnson of Dane County UW-Extension at Larry Meiller’s Garden Talk session (always a blast!).  As always, I had a steady stream of visitors to the PDDC booth, talking with and answering questions for folks the entire time.  I distributed 10,300 University of Wisconsin Garden Facts fact sheets (a record), 663 brochures/informational handouts of various kinds and 154 handouts for my talks.  Across all programs in 2018, I interacted with over 230,000 people.  Again, a big thanks goes out to Larry Meiller for having me on his radio show with its awesome listenership.

2018 marked my 20th anniversary at the PDDC.  I really couldn’t have accomplished what I have over those 20 years (and what I continue to accomplish) without the support of a number of people.  I already mentioned Sue Lueloff (molecular diagnostician extraordinaire) above.  Also part of my team are Ann Joy (who does data entry that is instrumental in keeping federal funds through the National Plant Diagnostics Network flowing into the clinic), Dixie Lang (who makes the PDDC website look beautiful and work smoothly), Laurie Ballentine of the Russell Labs support staff (who prints handouts and prints and folds clinic brochures), and finally John Lake (just graduated from the UW-Madison with a degree in Plant Pathology) and Stephanie Salgado (now a senior at James Madison Memorial High School, my alma mater), my superhero student hourlies who processed the bulk of PDDC samples this year and kept me from going insane.  A special congratulations to John and his wife Michelle on a new addition to their family (Daphne Day) who arrived just before Christmas!

I’m not sure what 2019 will bring, but let’s see what happens!  Bring it on!

For addition information on the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at

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
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 Verbena

I 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

Zonate leaf spot of maple
Zonate leaf spot of maple

 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 Banner

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 Rot

Bacterial Soft Rot of Potato
Bacterial Soft Rot of Potato

If 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 Disease

Armillaria Rhizomorphs on a Potato Tuber
Armillaria Rhizomorphs on a Potato Tuber

I 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 Smut

Common Smut of Corn
Common Smut of Corn

As 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

Splashing 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 Plants with a 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:

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.

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.

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.

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 Blowing in the 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 rust

A 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.

Aster yellows

Coneflowers with aster yellows often have deformed, discolored flowers.
Coneflowers with aster yellows often have deformed, discolored flowers.

My 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