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 pddc@wisc.edu or (608) 262-2863.  Enjoy the rest of your summer!!