All posts by hudelson

July 2023: 25 Years. . . 25 Cool Diseases

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


Leaf/Fruit Diseases

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

Powdery Mildews

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

Tar Spot

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

Taphrina Diseases

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

Gall Diseases

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

 Black Knot

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

Common Corn Smut

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

Crown Gall

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

Leafy Gall

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

Rust Diseases

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

Gymnosporangium Rusts

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

Fir Broom Rust

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

Elderberry Rust

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

White Pine Blister Rust

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

 Canker Diseases

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

 Golden Canker

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

Eastern Filbert Blight

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

Blister Canker (Nailhead Canker)

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

Miscellaneous Diseases

 Not all diseases are easily categorizable!


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

Dead Man’s Fingers

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

White Mold

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

Fertility Issues

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


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

Blossom End Rot

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


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


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

Fairy Rings

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

Slime Molds

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


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

Bird’s Nest Fungi

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


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

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

June 2023: Weeding is Fundamental!

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

Weeds compete for nutrients and water

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

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

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

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

Weeds can serve as a reservoir for disease-causing organisms

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


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


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

May 2023: Jewels in the Crown

Crown IconThere was a fair amount of pomp and circumstance earlier this month surrounding the coronation of King Charles III of England. As I was reading the news articles, I began to imagine how plant diseases might fit into a coronation ceremony. I doubt that British royals would find any of the diseases that I deal with particularly appealing. However, in typcal Dr. Death fashion, I found three diseases that, in my eccentric world view, could fit into a ceremony to crown a plant pathologist king or queen. I hope you enjoy my selections.

Crown Rots

The most destructive of the “crown” diseases are crown rots. The fungi and fungi-like water molds involved in these diseases often infect though a plant’s roots and eventually destroy a plant’s crown (i.e., the part of the plant where the roots and above-ground plant parts converge). Plant death is a common result. Crown rot pathogens prefer wet conditions; thus, crown rots tend to be more prevalent in heavier (e.g., clay) soils, in low areas, and in wet growing seasons. Management of crown rots involves improving soil drainage by adding organic matter (e.g., compost, leaf litter) to heavier soils and/or creating raised beds. Proper mulching (not more than two inches on heavier soils) can help prevent these diseases as well. Finally, for plants of high economic or sentimental value, fungicide treatments are a possibility. For treatments to be effective however, proper identification of crown rot pathogen(s) is critical, as some crown rot fungicides target fungi, others water molds.

Crown Gall

I got excited recently when I was removing leaf litter from my parents’ wintercreeper and caught sight of greenish-white, tumor-like blobs (galls) on the main trunk of the shrub. These blobs are typical of crown gall, a disease caused by the soil-borne bacterium Agrobacterium tumefaciens. This bacterium has a very wide host range; I commonly see crown gall on wintercreeper, rose shrubs, and apple trees. Often tumors form in the crowns of affected plants, but root galls are also common. Management of crown gall involves removing and destroying infected plants and avoiding planting susceptible plant species in areas where the disease has been observed.

Interestingly, when Agrobacterium tumefaciens infects a plant, it injects a small piece of its DNA (i.e., genetic material) into plant cells, where this DNA inserts into plant chromosomes. This bacterial DNA codes for enzymes that produce a variety of interesting chemical compounds. These compounds hijack plant cell growth and cause cells to grow fast and divide like crazy, leading to characteristic crown gall tumors. Other enzymes make opines, a class of chemicals that are a favorite food of the bacterium. Scientists eventually discovered that they could manipulate Agrobacterium tumefaciens DNA and incorporate plant genes into it. With these plant genes in place, the bacterium then could be used to infect a different type of plant, thus moving plant genes from one plant to another. Via this process, genetically engineered/modified plants were first produced.

Crown Rust

This is perhaps the most fitting plant disease for this month’s article, as the name of the causal fungus is Puccinia coronata. The species designation (coronata) refers to projections on the fungus’ club-shaped reproductive structures that give the clubs a crown-like appearance. Home gardeners most commonly see crown rust on turf; if you have ever walked through your lawn and ended up with orange shoes, you’ve encountered this disease. The orange powder is sporulation that allows the fungus to reinfect turfgrass. Interestingly, Puccinia coronata needs two host plants to complete its life cycle. The second host plant for Puccinia coronata is buckthorn, the invasive tree that takes over understories of wooded areas. Puccinia coronata produces a second type of spore in the spring that drifts from turfgrass to buckthorn, infecting leaves and green branch tissue of the tree. These infections lead to yellow leaf spots and yellow, distorted branch growth. These yellow areas produce yet another spore that drifts back to turfgrass, completing the fungus’ life cycle. Management of crown rust involves eradicating buckthorn, as well as regular mowing and optimal nitrogen fertilization of lawns to help remove the fungus and help turfgrass outgrow the disease.


Can you think of any other plant diseases that are fit for a plant disease king or queen? If so, let me know. Also, if you have questions about the diseases discussed above and/or how to submit samples to the clinic, feel free to give me a shout. As always, you can reach me at or (608) 262-2863. Long live plant diseases!!

April 2023: The Best Things in Life Are Free!

Free IconThe Plant Disease Diagnostics Clinic (PDDC) typically charges a small fee for providing diagnoses of plant diseases and disorders.  However, there are certain instances where the PDDC waives fees.  In particular, if there are plant diseases that are new to the state and/or potentially have severe negative economic impacts to Wisconsin agriculture, the PDDC does not charge to provide a diagnosis.  Early detection of new diseases can help in limiting and slowing spread, and may even provide opportunities to eradicate a disease.  Early detection of established, but economically important, diseases can help provide growers critical information for disease management.

The following are diseases that the PDDC will be providing free diagnoses for in 2023.

Late Blight

This disease can have a huge impact on the commercial potato industry in Wisconsin (the third largest potato-producing state in the US).  Identifying this disease as early as possible and determining which variant(s) of the pathogen (and there are many) has(have) arrived in the state are critical for providing timely and appropriate control recommendations to commercial potato producers.  Because the disease can affect tomatoes as well as potatoes, I encourage home gardeners to watch for this disease in their vegetable gardens.  If you see any suspicious leaf spotting on tomato or potato leaves or on tomato fruits, submit a sample for free testing.  Just indicate when you submit that you are concerned about late blight.

Boxwood Blight

This disease was first detected in Wisconsin in 2018 but has been devastating boxwood plantings in the eastern US since 2011.  The PDDC is continuing to map the movement of boxwood blight in the state and added Door County to the official boxwood blight county map in 2022.  If you see dark leaf spots, followed by leaf collapse and branch dieback on boxwood shrubs, get a sample to the clinic for a free diagnosis.  The problem may just be winter burn, but if it is boxwood blight, I’d like to know.

Japanese Apple Rust

I more commonly call this disease lipstick rust, and there is a move afoot to change the official name to red star rust (a literal translation of the original Japanese name of the disease).  This disease is a new Gymnosporangium rust for Wisconsin, having first been reported in the state by WI DATCP in 2021.  To date, lipstick rust has been confirmed in Dane, Kenosha, Milwaukee, Outagamie, Ozaukee, Portage, Racine, Sheboygan, and Waukesha Counties.  Watch for the red or fuchsia-colored spots on apple and crabapple leaves characteristic of lipstick rust.  If you see spots of this type, I suggest first submitting digital photos to the PDDC.  If the symptoms look consistent with lipstick rust, and there has been an official confirmation of the disease in your county, I’ll make a diagnosis from the photos.  If the symptoms look consistent, and there has not been an official confirmation of lipstick rust in your county, I’ll request that you submit a physical sample for DNA sequencing (the method of choice for confirming new cases of the disease).  Either way, the diagnosis is free.


If you have questions about these diseases and/or how to submit your samples, feel free to give me a shout.  You can reach me at or (608) 262-2863.

March 2023: Reading is Fun(gi)damental

Book IconI have spent a substantial portion of my career writing about plant diseases. When I started at the UW-Plant Disease Diagnostics Clinic in 1998, one of the first things articulated by county Extension educators was their need for short, concise, and to-the-point fact sheets on a variety of horticulture topics. This need led to the development of what are now known as the UW Plant Disease Facts, a series of one page fact sheets (targeted toward an adult, home-gardener audience) that cover a range of plant disease topics. I serve as the editor of this series and have authored or co-authored roughly two-thirds of the 130 titles.

When the COVID-19 pandemic hit in 2020, and I was stuck at home, writing was an increasingly important activity that I could use to fill my time.  I began to think about how I might engage a younger audience and get grade, middle and high school students interested in plant diseases. My thoughts drifted to a limerick that my coworkers (Ann Joy and Nancy Kurtzweil) and I had written back in my days with the UW Ginseng Research Program (waaaaaaaaaaaaaaay back in the Jurassic Period). The limerick:

There once was a farmer Ontarian,
The bane of his life Alternarian.
Its cure was a smash,
And brought lots cash,
And made him a wealthy agrarian.

had been taped to a refrigerator in my clinic for decades. That bit of verse made me wonder if limericks, with all of their fun and silliness, might be a way to capture the attention of a younger audience and get them immersed in the wondrous world of plant diseases.

Limerickettsia CoverThus was born Limerickettsia: A Plant Pathologist’s Book of Verse. This book, just published this year, contains 52 plant disease-themed limericks with accompanying prose descriptions, and full-color photographs and original artwork. The book includes limericks about diseases caused by all of the major types of pathogens (i.e., fungi, bacteria, viruses, nematodes, phytoplasmas and even parasitic plants), as well as limericks about things that look like plant diseases (e.g., bird’s nest fungi, slime molds, stink horns) but are not. With each limerick, you get to learn about what the plant disease looks like, how the organism that causes the disease does its thing, and tips on plant disease control. For some of the diseases (e.g., grain rusts, late blight of potato, Dutch elm disease), there are also tidbits about how the diseases have impacted human history. If you are intrigued, check out the Limerickettsia page on the UW-PDDC website.

If you have any questions about Limerickettsia: A Plant Pathologist’s Book of Verse or any of the other educational resources available through the UW Plant Disease Diagnostics Clinic, don’t hesitate to contact me. You can reach me at or (608) 262-2863.

Happy reading!!

February 2023: Recovering from Garden Expo 2023

PDDC Booth IconI have just finished catching up from the 2023 PBS Garden and Landscape Expo (Friday, February 10 through Sunday, February 12). My booth displays are back in storage, and I’ve documented all of the various education materials that I provided for visitors. This was my first in-person Garden Expo post-COVID. I have to say the weather for the actual event was perfect (sunny and warm), after a challenging set-up day of snowy weather and hazardous driving. Attendance at this year’s Garden Expo was 13,584, with visitors not only from Wisconsin, but from all around the Midwest.

This year, I coordinated and staffed the UW-Madison Plant Disease Diagnostics Clinic (PDDC) booth. Garden Expo organizers Amanda Balistreri and Heather Robbins were very generous in allowing me to have a double booth for my display at no charge. This spacious area provides lots of room for Garden Expo visitors to move about, look at disease specimens and photos, browse free fact sheets (116 titles this year), talk to booth denizens (myself included), and watch segments that I did over the years with Shelley Ryan on WPT’s Wisconsin Gardener. I really like visitors to know that Garden Expo exists because of Shelley and her show.

I was at Garden Expo all three days (noon until 7 pm on Friday, 9 am until 6 pm on Saturday and 10 am until 4 pm on Sunday). Many thanks go to Lisa Johnson of Extension Dane County, Diana Alfuth of Extension Pierce and St. Croix Counties, and longtime horticulture friend, Lis Friemoth (aka The Garden Hoe) for helping staff the booth. A special shout out goes to Tim Lorenz of the UW Russell Labs Hub for helping transport and unload my booth materials on Thursday and Lis Friemoth for helping pack up and load my van at the end of the show on Sunday.

During the three days, I gave three talks (New and Emerging Plant Diseases, Vegetable Diseases and Growing Healthy Plants: Basics in Plant Disease Management) and helped answer questions with Lisa Johnson Larry Meiller’s in-person Garden Talk session on Saturday morning. I also had a steady stream of visitors to the PDDC booth all three days and pretty much talked with and answered questions for folks the entire time. In fact, this is the busiest I have been at my booth for many, many years. I distributed 4,319 fact sheets, 720 brochures/informational handouts of various kinds and 248 handouts for my talks. The top ten fact sheets selected by booth visitors were (in alphabetical order):

Blossom End Rot
Home Vegetable Garden Fungicides
Lawn Disease Quick Reference
Plant Problems to Watch for in 2023
Powdery Mildew (Deciduous Woody Ornamentals)
Powdery Mildew (Herbaceous Ornamentals)
Powdery Mildew (Vegetables)
Septoria Leaf Spot
Ten Common Plant Diseases/Disorders You Can Identify by Eye
Vegetable Disease Quick Reference

All of these materials were not only educational in nature but were branded with the UW-Madison Division of Extension logo and/or the UW-Madison CALS logo and thus provided advertising for UW-Madison.

All-in-all, I had a great, productive weekend, and I think provided a valuable service to the general public. By the time the weekend was over, I physically felt like I had been hit by a truck, but mentally and emotionally, I felt rejuvenated due to all of the positive feedback that I received. I heard numerous positive comments about, and thanks for the work that I do (and more generally about the outreach work that UW as an organization does). PBS Garden & Landscape Expo is by far the most important in-person event that I do every year and, quite frankly, my absolute favorite work activity. I encourage any of you who have not attended Garden Expo to consider doing so in 2024. If you do attend, be sure to stop by the PDDC booth and say hello.  I will be there!


If you have questions about PDDC outreach activities and diagnostic services, feel free to contact the PDDC by email at or by phone at (608) 262-2863.  Also, feel free to check out the PDDC website ( for additional details on plant diseases and their management, as well sample submission.  You can follow the PDDC on Twitter and Facebook (@UWPDDC) or email to subscribe to the PDDC listserv, UWPDDCLearn, to receive updates on clinic services and educational materials.

January 2023: Plant Disease Diagnostics Clinic Services for 2023

Turquoise Microscope Icon2023 has arrived, and I’m already thinking ahead to the deluge of samples that are likely to arrive at the UW Plant Disease Diagnostics Clinic (PDDC) this summer.  PDDC personnel continue their efforts to balance providing high quality services to the public with keeping clinic staff and clients healthy and safe.  The following are current guidelines for submitting samples and tapping into the PDDC’s outreach programs and resources.  Our world is ever evolving, and there will likely be changes in how the PDDC functions as 2023 unfolds.  Be sure to check this document routinely for updates.

Plant Disease Diagnostics

The PDDC continues to offer digital diagnoses, and I really encourage you to submit digital photos of your plant disease problem before submitting a physical sample.  Use the PDDC online form for photo submissions.  In some situations, I will be able to provide a diagnosis and management recommendations based on your photos.  There is a $20 fee for this service.  If I cannot make a diagnosis from your photos, I will not charge the fee, but I will use the photos to provide you suggestions on what sort of physical sample to submit as a follow-up.  Note that if I charge a digital fee and you end up submitting a follow-up physical sample, the digital fee will be credited towards any lab fees for the physical sample.

You can submit physical diagnostic samples either by mail or in person.  If you opt to drop off a sample in person, PDDC staff will not likely be available for in-person consultations.  While I am committed to providing personalized service to my clients whenever possible, the PDDC is short-staffed at this time, and PDDC staff members need to concentrate their efforts on sample processing in order to keep up with incoming samples and provide timely diagnoses.

The PDDC provides an area for physical sample drop off in the hall outside the clinic.  Check for the large, black, wood shelving unit in the alcove just down the hall from the clinic door.  You can drop your sample(s) there.  Be sure to fill out a submission form to include with each sample.  You can find forms in the wall pocket to the left of the clinic door.  The form is also available online if you would like to print and fill it out ahead of time to speed up the submission process.  Please print neatly on the form and make sure to include your complete mailing address (with zip code), your phone number and your email address.  If you are a commercial client, please include the name of the business that you work for.  At this time, I am emailing virtually all reports and invoices.  If you do not have email however, just let me know, and I definitely can and will send out a hard copy of your report and invoice.


I will be providing more in-person plant disease presentations in 2023.  However, due to the cost of traveling (both in terms of time and monetary expense), I will continue to encourage clients to consider virtual presentations (e.g., via Zoom) whenever possible and practical.  If you are interested in having me provide a presentation for a group that you are affiliated with, click here to check out a list of topics that I routinely speak on.  Email me at or call me at (608) 262-2863 to discuss what talk might be best suited for your audience and to schedule a date and time.

In addition in 2023, I will be sponsoring monthly Zoom presentations on a range of plant disease topics.  Click here to check out these presentations and sign up if you are interested.

Online Resources

Via the PDDC website, I will continue to provide weekly updates of my PDDC diagnoses (the Wisconsin Disease Almanac), as well as web articles (hopefully monthly, but frequency will depend on clinic sample volume).  Also check out the PDDC website for the UW Plant Disease Facts (formerly the plant disease-related titles of the University of Wisconsin Garden Facts).

I will be recording the monthly PDDC plant disease talks (mentioned above) and will eventually be working these into online playlists (with full transcripts to fulfill UW-Madison accessibility requirements) that I will make available on my nascent YouTube channel (@UWPDDC).  My Fundamentals of Plant Diseases presentation is currently available as a playlist on my YouTube channel.

Outreach teasers

I am currently working on several new outreach projects.

Limerick Book

First is my book, Limerickettsia:  A Plant Pathologist’s Book of Verse.  This is a book of 52 plant disease-related limericks with supplemental prose discussions of the diseases and original artwork.  I am in the final throes of completing this and getting it printed.  April 3, 2023 update:  Now complete!!  Click here for details.

Plant Disease Medallions

I am also working on the UW Plant Disease Facts Medallions project.  For each of the 130 fact sheets in the UW Plant Disease Facts series, I have created electronic medallions and quizzes.  Eventually, you will be able to read one of the fact sheets and take the corresponding quiz.  If you get a score of 100%, you will be emailed the corresponding medallion.  Collect them all!  Hopefully, this project will be launched to the public this winter or early spring.  April 3, 2023 update:  Now complete!!  Click here for details.

Building Block Plant Diseases

Finally, I am developing building block kits (akin to LEGO kits) of common plant diseases (with supplemental educational materials).  I have designed models for blossom end rot, powdery mildew, downy mildew of grape, apple scab, silver leaf, Armillaria root disease, and (most recently) common smut of corn.  I am now working on getting the blocks for the kits 3-D printed so that I will have prototypes available for view by this spring.  October 27, 2023 update:  The blossom end rot building block kit is set to debut in early December at the Monona Public Library courtesy of the UW-Madison Department of Plant Pathology’s graduate student outreach group, What’s Eating My Plants (WEMP).

Need information on the PDDC? 

As new PDDC resources become available, I will announce their availability via Twitter (@UWPDDC) and Facebook (@UWPDDC), or via my clinic listserv, UWPDDCLearn (email me to subscribe to this).  In addition, you can always contact me by phone at (608) 262-2863 or email at

Have a great 2023 everyone!

December 2022: A Plant Pathologist’s Holiday Shopping List

Present Icon‘Tis the Season to buy holiday gifts.  I often have a difficult time coming up with ideas for what to buy for friends and family who seem to have everything.  Similarly, I’m a frustrating person to buy for; I just have too much stuff.  If you’re trying to come up with that perfect gift, and socks, a scarf or a necktie won’t do, consider a nifty plant disease-related gift that may put a smile on a loved one’s face.  I’m not talking about a greeting card or t-shirt with a plant disease design or pithy plant disease quote but actual diseased plant materials that you can gift to a friend.

Artisan wooden bowls

There are a couple of variations on this gift idea.  If you know of someone who really loves cherry wood (me, me!!), then consider finding a local woodworker who makes bowls from burls that form on the trunks of cherry trees.  These masses of wood (galls) are often the result of a fungal infection by Apiosporina morbosa, the black knot pathogen.  When infections occur on smaller branches, I refer to this disease affectionately as poop-on-a-stick, and the galls are not of much use.  However, larger black knot trunk galls are highly prized for their amazing grain patterns and can be used to make distinctive one-of-a-kind bowls.

The other types of bowls (or other wooden art objects) that are produced from diseased trees are those exhibiting spalting.  These wooden objects are produced from trees that are suffering from wood rot (typically white rot).  These rots are fungal in nature.  As the fungus colonizes the wood and begins to degrade it, dark lines can form at the interface between healthy and diseased wood.  If caught at the right time (before substantial degradation has occurred), the diseased wood can be shaped in to stunning artisan pieces with the decorative line patterns intact.

Walking sticks

Do you have a hiker friend who needs a gift?  If so, then consider a diamond willow walking stick.  Diamond willow is not a type of willow tree, but the result of certain species of willows being infected by canker-causing fungi (in particular the fungus Valsa).  Infection leads the formation of diamond-shaped sunken areas (cankers) on affected branches.  These sunken areas exhibit a distinctive light and dark (often reddish) contrast between healthy and diseased tissue.  The diamond shapes and color combination make the infected branches highly prized for making not only walking sticks but also furniture.


For a less expensive disease-related gift (the wood gifts described above can be quite pricey), consider giving a friend or loved one a poinsettia.  “What’s diseased about a poinsettia?” you may ask.  Well, the poinsettias that you buy at your local greenhouse or floral shop are infected with phytoplasmas.  Phytoplasmas are bacteria-like organisms that colonizes a plant’s food-conducting tissue.  Poinsettias, in their native habitat, are tall (we’re talking 10 ft. or more) and lanky.  The presence of phytoplasmas stunts infected poinsettia plants and makes these plants produce extra branches, thus yielding the beautiful, compact, bushy poinsettias that we have come to love.


For the gourmand on your shopping list, a can of huitlacoche (alternatively called cuitlacoche) might be the perfect stocking stuffer.  Huitlacoche is the polite name for common smut of corn.  This is a fungal disease where infection occurs through corn silks as the plants flower.  The fungus causes corn kernels to enlarge into huge, pasty gray masses (galls again).  Eventually, the interior gall tissue converts into powdery spores.  But, if you catch the galls when they are still solid and fleshy, you can use them for cooking.  Huitlacoche is a common ingredient in traditional Mexican cuisine, and many high-end restaurants now offer it on their menus.  While fresh huitlacoche is always a better choice, canned huitlacoche is readily available in specialty food stores, as well as online.


If you have questions about any of the disease gifts discussed above, or need additional ideas for plant pathology related gardening gifts, feel free to contact the PDDC by email at or by phone at (608) 262-2863.  Also, feel free to check out the PDDC website ( for additional details on plant diseases and their management, as well sample submission.  You can follow the PDDC on Twitter and Facebook (@UWPDDC) or email me to subscribe to the PDDC listserv, UWPDDCLearn, to receive updates on clinic services and educational materials.

Happy Holidays and good luck with your shopping!


Links in this article lead to sites that are for illustrative purposes only and are not an endorsement of any particular vendor or item for sale.

Update 12/9/22 

Here’s another plant disease gift idea I stumbled across (although there are some caveats with this one). . .

Oud perfumes and colognes 

Oud (also spelled oudh) refers to a dark, fragrant resin that is produced in the heartwood of agarwood trees (Aquilaria spp.) in response to infection by the fungus Phialophora parasitica.  Only a small percentage of agarwood trees (maybe 2%) become infected.  The combination of sap and fungus has a scent that evokes leather, saffron and smoke.  Unfortunately, oud has historically has been so valued (for a variety of purposes in addition to perfume production) that agarwood trees have been overharvested to the point where many species are endangered.  There is a move afoot to more sustainably produce oud by cultivating agarwood trees and artificially inoculating them.  So, if you decide that an oud perfume is your gift of choice, I suggest making sure the oud therein is from a sustainable source.

Update 12/14/22:  

And Santa’s plant pathology gift list grows longer (courtesy of Patty McManus, the former UW-Madison fruit pathologist) . . .

Noble rot wines 

Have a wine connoisseur who needs a holiday gift?  Then, consider noble rot wines.  These wines are produced from grapes that have been infected by Botrytis cinerea.  This fungus, if growing conditions are wet for extended periods of time, can devastate a grape crop, rotting the fruits on the vine.  However, if there is only a short wet period (which promotes infection), followed by drier conditions, the fungus causes a dehydration of the grapes without destructive decay.  This raisining of the grapes causes a concentration of sugars, and wines produced from noble rot grapes tend to be sweeter wines.  The presence of Botrytis cinerea also appears to add to the flavor profile of the wine, giving noble rot wines subtle hints of honey, beeswax and/or ginger.  Common noble rot wines include (but are not limited to) Tokaji (from Hungary/Slovania), Sauternes (from France) and Beerenauslese (from Germany/Austria).


March 2022: Checking Out the New Gym (or Flexing Your Muscles on a New Wisconsin Disease)

Barbell IconI’ve been around long enough at the PDDC (25 years next summer) that most of the diseases that I see at the clinic are old friends.  Rhizosphaera needle cast, oak wilt, Verticillium wilt and many others are part of the cast of plant disease characters that I see in Wisconsin every year.  It’s relatively rare that I see brand new diseases.  In recent years, boxwood blight, Neopestalotiopsis leaf spot and fruit rot of strawberry and zonate leaf spot (a disease that had been on my bucket list for years) are new diseases that have crossed my doorstep.  In 2022, I’m watching for yet another new disease:  Japanese apple rust.

Japanese apple rust is a type of Gymnosporangium rust caused by the fungus Gymnosporangium yamadae.  This fungus is native to Asia (specifically China, Korea and Japan) and was first detected in the United States in Delaware and Pennsylvania in 2008 (with a first formal published report in 2009).  In 2021, Sam Fieweger of the Wisconsin Department of Agriculture, Trade and Consumer Protection (WI DATCP) Plant Industry Bureau Lab confirmed the first detection of the disease in Wisconsin.  The disease was identified on a crabapple tree in a nursery in southeast Wisconsin.

Like other Gymnosporangium rusts, Japanese apple rust is an altnernating rust.  The causal fungus requires two different host plants to complete its life cycle.  It spends half of its life on a wide range of apples/crabapples (Malus spp.) and the other half on Chinese juniper (Juniperus chinensis, Japanese garden juniper (Juniperus chinensis var. procumbens) and Sargent juniper (Juniperus chinensis var. sargentii).

Japanese apple rust on juniper.  Photo courtesy of Nancy Gregory, University of Delaware.
Japanese apple rust on juniper.  Photo courtesy of Nancy Gregory, University of Delaware.

Typical symptoms of Japanese apple rust are not radically different than those of other Gymnosporangium rusts.  On juniper hosts, the disease leads to the formation of relatively small branch galls that ooze orange gelatinous arms/masses in the spring.  These galls/masses look (at least to the casual observer) not that different from those produced by cedar-apple rust (CAR) and (particularly) cedar-hawthorn rust (CHR), two common Wisconsin Gymnosporangium rusts.  On apple/crabapple hosts, symptoms of Japanese apple rust are again similar to those of CAR and CHR:  large, brightly-colored leaf spots.  In the cases of CAR and CHR, the spots are yellow to orange.  In the case of Japanese apple rust, the spots are fuchsia to red.  On the undersurface of the leaves where the spots are located, short spiny structures eventually form.  These are reproductive structures that produce spores that reinfect the juniper hosts.

Japanese apple rust on crabapple.  Photo courtesy of Nancy Gregory, University of Delaware.
Japanese apple rust on crabapple.  Photo courtesy of Nancy Gregory, University of Delaware.

The presence of Japanese apple rust in Wisconsin puts a wrench in the recommendations that I typically make for managing Gymnosporangium rusts.  Chinese junipers are not great hosts for CAR, CHR and cedar-quince rust (the other common Gymnosporangium rust in Wisconsin), so I have historically recommended these junipers for clients who are desperate to grow junipers and apples, crabapples and hawthorns near one another.  If (more likely when) Japanese apple rust becomes more prevalent, Chinese junipers will no longer be a good choice to grow alongside apples and crabapples.  In the future, management of Gymnosporangium rusts will likely have to rely on not growing junipers and apples, crabapples and hawthorns anywhere near each other.

Because Gymnosporangium yamadae is not native to the US, Japanese apple rust is of regulatory importance in Wisconsin (and also elsewhere in the US).  There is interest in determining how widespread the disease may be in the state.  For that reason, I will be providing free diagnostics (as long as I am financially able – the testing is a bit pricey) for suspect Japanese apple rust samples.  The easiest hosts to verify the disease on will likely be apples or crabapples.  So, please watch for those fuchsia to red spots.  If you see anything suspicious, please submit digital photos via my online digital diagnostics form.  Let me know in the “Message” field that you suspect Japanese apple rust, and there will be no fee.  If your photos look suspicious, I’ll request a follow-up physical sample for additional lab testing.

Have questions?

If you have questions about Japanese apple rust or submitting a Japanese rust sample (or need advice about plant diseases in general), feel free to contact the PDDC by phone at (608) 262-2863 or email at  Also check out the PDDC website ( for additional details on plant diseases and sample submission.  Feel free to follow the PDDC on Twitter and Facebook (@UWPDDC) or email me to subscribe to the PDDC listserv, UWPDDCLearn, to receive updates on clinic services and educational materials.

Good luck with your new workout!


March 2022: To Reuse Pots or Not Reuse Pots – That is the Question

Clay Pot IconI have recently seen comments and discussions on social media about reusing pots from greenhouse or nursery-purchased plants.  In some instances, there has been discussion of returning pots to nurseries and greenhouses for businesses to reuse.

I’m definitely all for reusing pots whenever possible.  It’s an environmentally sound thing to do.  However, keep in mind that you need to be cautious when reusing pots as they potentially can be sources of disease-causing organisms.  These pathogens may be carryovers from plants that you originally purchased in the pots, or they may be pathogens that the pots picked up later as the pots have be stored in your garden or elsewhere on your property.  In particular, damping-off and root rot pathogens can be found in many garden soils, so anytime pots come in contact with garden soil (and it doesn’t have to be a lot), they can potentially become contaminated.  And, it’s not just pathogens that can be an issue.  Insect pests and (gulp!) jumping worms could also potentially be carried via contaminated soil on recycled pots.

So, if you have a collection of old pots and want to reuse them (or even more importantly want to give them to your local nursery or greenhouse), take some precautions prior to reuse or donation.

Clean your pots thoroughly with soapy water

to remove any bits of soil clinging to the surfaces of the pots that might be harboring pathogens. An added bonus of using an initial cleaning with soapy water is that this should help deactivate plant viruses that might be hanging out on your pots.

Soak your pots in a 1.0-1.5% sodium hypochlorite  solution for 20-30 minutes

Sodium hypochlorite is the active ingredient in disinfecting bleach.  This treatment will help kill pathogens that remain on the surfaces of the pots.  Be cautious when using bleach to avoid contact with skin, eyes and clothing.

Thoroughly rinse the pots to remove bleach residues

that can be toxic to the new plants that you are trying to grow in the pots.

Note that the treatment outlined above may not be successful in all situations.  Bleach treatments tend to work better on clay or ceramic pots but tend to be less successful for plastic pots.  Be that as it may, seriously consider decontaminating you pots prior to reuse or donation to local businesses.  This will help limit, as much as possible, plant pathogen carryover and spread.

Need more information?

For more information on plant diseases and their management, check out the UW Plant Disease Diagnostics Clinic website ( or contact PDDC staff at or (608) 262-2863.