As I commented in my March 2025 web article, the 2024 growing season was incredibly wet, and as a consequence, I observed an uptick in foliar diseases such as apple scab and Septoria leaf spot of lilac.  But, it wasn’t only leaves that had problems due to the wet weather.  A lot of last year’s rain ended up in the soil, and the resulting wet soil conditions contributed to an increase in root rots and related diseases. 

Rot roots

Root rots are caused by a wide variety of organisms including (but not limited to) Rhizoctonia, Pythium, and Phytophthora,.  Rhizoctonia is a true fungus, while Pythium and Phytophthora fall into a group of fungi-like organisms called the water molds.   Water  mold root rot pathogens tend to be particularly aggressive and also tend to be long-lived.  They produce thick-walled resting spores (called oospores) that can survive for years, if not decades, in the soil.  Under hotter, drier conditions, these spores germinate to form hyphae (i.e., fungal threads) that can directly infect plant roots.  Under cooler, wetter conditions, oospores germinate in a different manner, eventually producing numerous smaller, swimming spores called zoospores.  These spores swim to plant roots where they infect.  Production of zoospores significantly increases a water mold’s ability to infect, leading to an increase in the number of infections and subsequent disease severity. 

Root rot pathogen infections directly lead to root decay and deterioration.  Roots are typically discolored (e.g., brown, black), as well as soft and mushy.  This decay interferes with a plant’s ability to take up water and nutrients.  Indirect effects due to root rots include leaf symptoms that might be misidentified as being due to poor fertility (e.g., yellowing or reddening), symptoms that might be mistaken for drought stress symptoms (e.g., marginal leaf browning), wilting and branch dieback, and, ultimately, plant death. 

Once symptoms of root rots are observed, often there is little one can do to correct the problem.  Root rots are best managed preemptively at the time that gardens and landscapes are established.  Modifying soils to improve drainage (e.g., by adding organic matter), and choosing plants that are adapted to the soil moisture conditions at a particular site are two ways to prevent root rot issues.  There are potentially fungicides that can be used for management of root rots.  However, for these treatments to be effective, early diagnosis of the problem (i.e., before there is extensive root damage) is critical.  Fungicides, for the most part, do not cure existing infections, but help protect new roots from being infected.  Before applying fungicides, proper identification of the organism(s) involved in a root rot problem is extremely important; the fungicides needed to manage fungal vs. water mold root rot pathogens are quite different.  Often, plants have problems with both fungal and water mold root rotters simultaneously, so use of multiple active ingredients is needed for control.  Note that root rot fungicides typically are not readily available to homeowners, so contracting with a professional pesticide applicator to make treatments  will likely be necessary.  As you can imagine, using fungicides for root rot control can get very expensive, very quickly. 

Related diseases

A variant of root rots that I encountered several times last summer and have already encountered in 2025, is bleeding cankers on trees (e.g., maple, beech, birch).  Typical symptoms of bleeding cankers are a series of wet, oozy spots on tree trunks.  Sometimes these areas are near the soil surface; other times, the oozing occurs farther up on the trunks.  The most typical cause of bleeding cankers on common landscape shade trees is Phytophthora (which I mentioned above in the context of root rots).  When people contact me about bleeding cankers, I request that they send me blobs of the ooze and/or small bits of the ooze-soaked bark (about the equivalent of a 2 inch by 2 inch bark piece).  I test this material with a dipstick serological test for Phytophthora.  This test uses similar technology to that used in home pregnancy tests and home COVID-19 tests. 

The real fun begins if I get a positive reaction from my initial testing (and many of the samples I have received have tested positive).  At that point, I have to forward the sample to the WI DATCP Plant Industry Lab for further testing.  One of the species of Phytophthora that can cause bleeding cankers is Phytophthora ramorum.  This organism causes sudden oak death/Ramorum blight and is highly regulated at both the state and federal level, because of how destructive it can be.  I need to make sure that samples that test positive for Phytophthora are not infected with Phytophthora ramorum.  Phytophthora ramorum has occasionally be detected by WI DATCP in nursery stock brought into Wisconsin from other states.  However, Phytophthora ramorum has never been detected in Wisconsin in established landscapes plants.  Luckily, none of the samples that I have submitted to WI DATCP has tested positive for Phytophthora ramorum.  Typically, the Phytophthora species present in my samples has been Phytophthora cactorum, a common Phytophthora species that can cause root rots on a wide range of plant species.

Need help?

Hopefully, you will never have problems with root rots or bleeding cankers in your gardens and landscapes.  If you do however, I encourage you to submit samples for a proper diagnosis.  If you have questions about any plant disease that you are seeing or questions about sample submission, please feel free to contact me at (608) 262-2863 or pddc@wisc.edu.  For additional information on plant diseases and their management, be sure to check out the UW-Madison PDDC website (https://pddc.wisc.edu/).  To keep up to date on clinic activities and resources, follow the PDDC on Facebook, Twitter (X), or Bluesky (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Keep on gardening, and have a great spring and summer!

The 2024 growing season was an amazing time for plant diseases.  Spring and early summer weather was particularly wet, which was favorable for development of an array of fungal plant diseases.  In addition, 2024 was a great year for insect activity, and certain insects that were prevalent last year (in particular aphids and leafhoppers) are notorious for moving pathogens from plant to plant.  Of the diseases that I saw at the UW Plant Disease Diagnostics Clinic last year, the following three are diseases that are likely to recur and have a significant impact in 2025. 

Apple scab

Last year was the best year for apple scab that I can think of in my 26+ years at the PDDC.  The wet weather that occurred as apple and crabapple trees began to leaf out in the spring was perfect for Venturia inaequalis (the apple scab fungus) to infect.  These early infections (characterized by dark, feathery-edged leaf spots) set trees up for an apple scab epidemic as wet weather continued later into the growing season.  Many trees defoliated due to apple scab by mid-summer.  The leaf debris from these trees is where the apple scab fungus has been hanging out for the winter.  As soon as rains start this spring and trees begin to leaf out, spores from this leaf litter will lead to new infections, and we’ll be off to the apple scab races once again. 

To limit apple scab development, remove and dispose of (by burning, burying, or hot composting) any leaf litter that is still hanging around your apple and crabapple trees.  If you have not recently thinned your apple or crabapple trees, consider doing this soon (as long as the buds on the trees have not yet begun to swell).  Thinning improves airflow that promotes more rapid drying of leaves when they get wet.  Drier leaves are a less favorable environment for the apple scab fungus to infect.  For details on pruning trees, check out “Pruning Deciduous Trees” available on the UW-Madison Division of Extension Horticulture website.  There are also fungicide treatments that can be used for apple scab control (see the UW Plant Disease Facts fact sheet on “Apple Scab” for details).  However, treating for apple scab requires multiple, carefully-timed fungicide applications, which can be difficult to achieve for many home gardeners.  If you are planning on planting a new apple or crabapple tree in 2024, make sure you select an scab-resistant variety to avoid apple scab problems in the future. 

Septoria leaf spot of lilac

After a hiatus in 2023 (most likely due to the hot, dry 2023 growing season), Septoira leaf spot of lilac was back with a vengeance in 2024.  Lilacs all over the state had leaves that first exhibited spotting, then turned partially or fully brown starting at the bottom of shrubs and working up the plant.  The culprit was a species of Septoria, a fungus related to (but distinct from) the organism that causes Septoria leaf spot of tomato.  The browning caused by Septoria on lilac tends to be quite dramatic, but luckily, the disease is not lethal. 

As with apple scab, good clean up of leaves that have fallen from affected lilac shrubs is the place to start in managing Septoria leaf spot.  Routine pruning/thinning (see “Pruning Deciduous Shrubs” for details) to open up lilac canopies and promote rapid drying of leaves will also help keep this disease at bay.  Use of fungicides (in particular chlorothalonil-containing products labeled for use on woody ornamentals) to manage the disease may also be useful.  However, early applications (as shrubs begin to leaf out) and additional regular follow-up applications (if there is wet weather) are critical for control of this disease.  Applying fungicides is a lot of work, and if you are unable to commit to the regular applications that are needed, then I recommend foregoing any applications at all. 

Aster yellows

Aster yellows is a phytoplasma disease that affects 300+ herbaceous plant species in 40+ plant families.  Phytoplasmas are bacteria-like organisms that certain insects (most commonly leafhoppers) move from plant to plant.  These insects feed in the phloem (i.e., the food-conducting tissue) of plants, and acquire phytoplasmas (and subsequently drop them off) as they feed.  In 2024, high numbers of aster yellows leafhoppers (the specific leafhopper that transmits the aster yellows phytoplasma) led to an increase in aster yellows.  Once infected, perennial plants remain infected, with the phytoplasma concentrating in plants’ crowns and roots in the fall, as plants go dormant for the year.  The phytoplasmas survive the winter in these tissues and recolonize new growth as it emerges in the spring.  Given the high incidence of aster yellows in 2024, I expect to see a lot of this disease in 2025.  Typical symptoms associated with aster yellows are plant stunting; yellow, orange, or purple leaves; distorted leaves; leafy, green flowers; and brooming (i.e., production for clusters of plant parts). 

Unfortunately, the only management for aster yellows is to remove infected plants.  You don’t have to be particularly careful about disposing of these plants.  Once plants have been dug up and have wilted, they are no longer attractive to leafhoppers (and other insects) that could potential move the pathogen to other plants. 

Need help?

Hopefully, my predictions for diseases that will be prevalent in 2024 will be wrong.  That said, you will likely encounter plant diseases of some kind this coming growing season.  If you need help in identifying whatever diseases you do encounter or need advice on plant disease control, please feel free to contact me at (608) 262-2863 or pddc@wisc.edu.  For more information on plant diseases in general and their management, be sure to check out the UW-Madison PDDC website (https://pddc.wisc.edu/).  To keep up to date on clinic activities and resources, follow the PDDC on Facebook, Twitter (X), or Bluesky (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Happy gardening, everyone!!

Another Garden Expo (this year officially “PBS Wisconsin’s Garden & Green Living Expo”) has come and passed.  The event was Friday, February 7 (noon until 7 pm), Saturday, February 8 (9 am until 6 pm) and Sunday, February 9 (10 am until 4 pm).  Yesterday, I finished my final tallies for the event and put my display materials to bed for another year.  It was a great show with total attendance just shy of 13,000. 

As in previous years, I sponsored the UW-Madison Plant Disease Diagnostics Clinic (PDDC) booth at Garden Expo.  Event organizers Amanda Balistreri and Lacey Richgels generously provided me with a double booth for my display at no charge.   I was located directly across from the large UW-Madison Division of Extension Horticulture booth, right near the entrance to the exhibit hall.  This year my booth had a LEGO theme to highlight the building block plant disease models that I formally launched at the end of 2024.  I displayed 10 of the models that I designed (apple scab, Armillaria root disease, bacterial soft rot, bird’s nest fungi, black knot, blossom end rot, cedar-apple rust, corn smut, powdery mildew, and silver leaf) and provided a flier on how to access online information on the models. 

In addition to my custom-designed models, I used official LEGO long-stem roses to decorate the booth and had a bouquet of LEGO sunflowers and daffodils as part of a memorial to Shelley Ryan.  Shelley is the former host of Wisconsin Public Television’s “The Wisconsin Gardener” and the reason Garden Expo exists.  I also used a LEGO poinsettia to discuss how these popular plants are actually diseased.  Poinsettias are infected with a bacterium-like organism called a phytoplasma, which gives the plants their compact, bushy look.  Non-infected poinsettias are lanky and tree-like. 

In my booth, I also provided free UW Plant Disease Facts fact sheets (118 titles) and used these to promote my collectable plant disease medallions (“Read a Fact Sheet!  Take a Quiz!  Earn a Medallion!  Collect Them All!”).  People could also check out my plant disease-themed limerick book (Limerickettsia) and pick up a flier on how to order the book to help support PDDC activities.  I also provided fliers on my department (the UW-Madison Department of Plant Pathology), my monthly “PDDC Plant Disease Talks”, and the “Ask Your Gardening Questions-Live” Q&A sessions that I participate in with colleagues from the UW-Madison Division of Extension Horticulture.  Finally, I provided brochures on how to submit samples to my clinic and brochures that summarize PDDC outreach efforts, as well as brochures from the UW Turf Diagnostic Lab and the UW Soil and Forage Lab. 

My Garden Expo odyssey this year began with set up on Thursday, February 6 (a 4-hour task).  I was back on Friday at 9 am to complete the final touches on my booth and to prep to answer questions (with Lisa Johnson of Extension Dane County) in a live broadcast of “Garden Talk with Larry Meiller” at 11 am.  This was the first time that Larry’s show had ever been broadcast from Garden Expo.  In addition to the radio show, I gave three talks:  “New and Emerging Plant Diseases”, “Growing Healthy Plants:  Basics in Plant Disease Management”, and “Top Ten Plant Diseases of 2024”.  Thanks to Lisa Johnson, as well as Laura Jull and Derrick Grunwald of the UW-Madison Department of Plant and Agroecosystem Sciences, and Jared Hanken, a recent graduate of the UW-Madison’s Department of Forest and Wildlife Ecology, who helped staff my booth when I was off giving presentations.  While I was hanging out at the PDDC booth, I had a constant stream of visitors and pretty much talked with and answered questions for visitors the entire time.  Adding to the festivities, I participated in Garden Expo’s Kid’s Passport program.  Kids could pick up a booklet in the exhibit hall and get their booklet stamped at select Garden Expo booths (including mine).  In addition to the stamp, I gave kids a specially designed, 3D-printed medallion (check out the icon at the beginning of this article to see what the medallion looked like).  A huge shout out to Ted Geibel for frantically printing 400 of these medallions in the two weeks prior to Garden Expo. 

By the end of the Garden Expo, I had distributed 2,829 fact sheets, 701 brochures/informational handouts of various kinds, 199 handouts for my talks, and 270 medallions.  All of the fact sheets, fliers, and brochures were not only educational in nature but were branded with the UW-Madison Division of Extension logo and/or the UW-Madison CALS logo, thus providing advertising for the UW-Madison. 

Overall, I had a wonderful weekend and feel like I provided a valuable service to the public.  I heard numerous positive comments about, and thanks for the services that I provide.  The building block plant diseases appeared to make a particular splash.  All of this positive feedback mentally refreshes me and is part of what keeps me motivated to do the work that I do.  PBS Wisconsin’s Garden & Green Living Expo continues to be, by far, the most important and enjoyable in-person outreach event that I do every year. 

If you’d like to learn more about the PDDC and all of the crazy projects I work on as I try to get people excited about plant diseases, feel free to check out the clinic website at https://pddc.wisc.edu.  Also, feel free to follow the clinic on Facebook, Twitter (X), or Bluesky (@UWPDDC), or subscribe to the clinic’s listserv (UWPDDCLearn) by emailing me at pddc@wisc.edu or (608) 262-2863. 

I’m smiling as I write this article and already planning ahead for Garden & Green Living Expo 2026!

Valentine’s Day is coming soon, and people are always looking for gifts to buy for their sweethearts.  Even if you don’t have a significant other, Valentine’s Day can be a time to buy something fun for yourself!  But, what to buy?  Tired of the typical and somewhat cliché chocolates and red roses?  If so, consider a novel plant disease-related gift that will hopefully put a smile on your or your loved one’s face. 

Floral bouquets

I just dissed rose bouquets, but there are options for making plant-disease inspired floral arrangements for Valentine’s Day. 

First, I suggest that you include fantail willow.  Every time I see this plant, I wonder if it is infected with a phytoplasma of some kind.  Phytoplasmas are bacteria-like organisms (often moved from plant to plant by leafhoppers) that colonize a plant’s phloem (i.e., food-conducting tissue).  Symptoms associated with phytoplasma infections can include flattening and curlicuing of branches, both of which are characteristics of fantail willow.  Someday, I’m going to buy some branches of this plant and test them for phytoplasmas in my clinic. 

Next, I would include tulips with striped, puckered, and/or fringed petals.  Nowadays, these floral characteristics are likely due to the genetics of specific tulip varieties.   However, these features pay homage to the Rembrandt and parrot tulips of the past that were infected with Tulip mosaic virus (an aphid-transmitted plant virus).  Virus-infected tulips contributed to “tulip mania” in The Netherlands in the 1600s.  People went gaga over tulip bulbs that produced these unusual flowers, paid way too much for single bulbs (e.g., the equivalent of the purchase price of a house or a seafaring vessel), and subsequently lost their shirts when the market for these bulbs crashed. 

Finally, I’d throw in a few snapdragons.  There’s nothing specifically plant pathological about snapdragons, but if you ever grow them in your garden and let them go to seed, you’ll notice that the mature seed pods look like tiny human skulls!  So, as Dr. Death, I find this plant very “on brand”. 

Nobel rot wines

Need a bottle of wine to go with your plant disease bouquet?  If so, consider noble rot wines.  These wines are produced from grapes that have been infected by Botrytis cinerea.  If growing conditions are wet for an extended period, then this fungus 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, then 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 adds 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).

Oud perfumes and colognes

Perfumes are a traditional Valentine’s Day gift, and oud perfumes are the plant disease versions that you can enjoy.  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 now 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.

Artisan wood bowls

If you know of someone who really loves cherry wood (a reddish wood appropriate for Valentine’s Day), 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 affectionately refer to this disease 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 that unfortunately, while beautiful, can be a bit pricey. 

Other types of bowls (or wooden art objects) that are made from diseased trees are those exhibiting spalting.  These wooden objects are produced from trees that are typically suffering from white rot, a kind of fungal wood rot.  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 into stunning artisan pieces with the decorative line patterns intact. 

Wood accessories and furniture

Do you or your significant other like to hike?  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 have a Valentine’s day vibe as they 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.  Fancy a diamond willow love seat anyone? 

Some good reading

Finally, I’ll finish my list by shamelessly promoting my recently published book, “Limerickettsia:  A Plant Pathologist’s Book of Verse”.  Poetry is the language of love, is it not?  So, why not gift yourself or your loved one a book of 52 disease-themed limericks (including a Valentine’s Day appropriate one about Tobacco rattle virus on bleeding heart), with additional prose information, photographs, and original artwork. 

Questions?

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

Happy Valentine’s Day, and good luck in love and life!

Note

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.

Earlier this year, one of my colleagues called me “giddy” as I was happily expounding about all of the diseases that I was seeing this year due to our wet weather.  The pinnacle of my euphoria came this month when I received a sample of one of my all-time favorite diseases, zonate leaf spot.  This disease is caused by several species of the fungus Cristulariella/Grovesinia, and occurs on a wide range of plants.

My first encounter with zonate leaf spot was back in 2018.  Interestingly, I received two samples of the disease on the same day that year, from two different Wisconsin counties (Marathon and Buffalo), and on different hosts (maple and grape).  I subsequently saw the disease in 2019 on industrial hemp from Langlade County, and the sample I received this year was another maple leaf sample originating in Dunn County. 

In my initial encounter with zonate leaf spot in 2018, a UW-Extension county educator sent me photos of maple leaves with symptoms (roughly round spots with a pattern of concentric rings) that were typical of what I had seen in online photos of the disease.  I REALLY wanted to see a physical sample (and offered to do the diagnosis for free), as zonate leaf spot had been on my plant disease “bucket list” ever since I first saw drawings of the reproductive structures of Cristulariella/Grovesnia (called conidiomata) in one of my reference books.  The Extension educator was quite accommodating. 

Once the physical sample arrived, I again noted typical zonate leaf spot lesions.  I scanned the lesions under my dissecting microscope and relatively quickly found conidiomata of the pathogen.  These structures are HUGE, are multicellular, and are tannish and pyramidal in shape.  Quite frankly, they look like tiny, tan Christmas trees popping up from the leaf surface.  If you saw these (and they are readily visible using a 20X hand lens), you might think they are insect droppings of some kind.  I ended up incubating the maple leaves in a moist chamber for several days, hoping to stimulate additional sporulation.  Interestingly, the fungus eventually formed black sclerotia (resting structures) that were readily visible to the naked eye.  In the recent maple sample that I received, I immediately found both conidiomata and sclerotia as soon as I pulled the sample from the bag. 

When I encountered zonate leaf spot in 2019 on hemp, I decided to go a step further in the identification of the fungus, as I was not able to find any reports of zonate leaf spot occurring on this host plant.  I plucked one of the conidiomata from the leaf surface and placed it on the center of a petri-plate filled with a potato-based growth medium.  The grew quite quickly on this medium.  Sue Lueloff, the molecular diagnostician in my clinic at the time, was able to able obtain a DNA sequence for the fungus, which matched sequences from known isolates of Grovesinia moricola. 

While I tend to get very excited when I see zonate leaf spot, as a home gardener, there’s not much to worry about.  This disease is similar to most other, more common leaf diseases in terms of management.  You begin by making sure to collect up and dispose of any leaves from infected trees after they drop in the fall.  The leaves can be burned (where allowed), buried, or hot composted.  Maintenance pruning of susceptible woody species can help with control of the disease as well.  This pruning opens up the plant canopy and improves airflow, which helps speed drying when leaves get wet.  This drier environment is less favorable for Cristulariella/Grovesnia to infect.  To be honest, zonate leaf spot does not appear to be a serious disease in most instances, so don’t panic if you think you see the disease.  Just let me know if you do, as you will make my day!  

If you have questions about zonate leaf spot (or any other plant disease) feel free to contact me at (608) 262-2863 or pddc@wisc.edu.  As always, for more information on plant diseases and their management, be sure to check out the UW-Madison PDDC website (https://pddc.wisc.edu/).  To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Go forth and look for supercool, bizarre diseases.  They are out there waiting for you to discover!

In last month’s web article, I talked about the myriad of fruit diseases that I’ve been seeing at the clinic.  This month, I thought I’d provide a summary of the vegetable diseases that I’ve been seeing.  Many of the vegetable samples that I’ve received have been from commercial growers, but diseases don’t discriminate and can affect both commercial- and home-grown vegetables alike. 

Root/Crown rots

Given our regular (and often plentiful) rains this year (it’s pouring as I write this article), it’s not surprising that vegetable root and crown rots have been an issue.  I have seen these diseases on peas, snap beans, and vine crops thus far this season.  Typical symptoms of root/crown rots can include stunted growth, leaf yellowing and browning (caused by inhibited water and nutrient uptake), and discolored and mushy roots.  The menagerie of organisms that I’ve recovered from root/crown rotted plants has been quite impressive.  I’ve seen pathogens such as Phytophthora, Aphanomyces, and Pythium (all fungi-like water molds), as well as Rhizoctonia and Fusarium (two true fungi).  Management of root rots in home gardens relies heavily on proper crop rotation, [i.e., making sure not to grow the same vegetable (or even vegetables in the same plant family) in the same area of your garden year after year after year].  This technique helps prevent the buildup of root/crown rot pathogens in the soil (where they tend to survive quite happily).  In particular, water molds produce thick-walled, long-lived resting spores (called oospores) that can survive for years in the soil.  For details on how to use crop rotation effectively in your home vegetable garden, check out the fact sheet here. 

Black rot

This disease of cruciferous crops (e.g., cabbage, broccoli, cauliflower, kale) seems to have exploded.  I have had four cabbage samples with this disease show up at the clinic in the past week.  Symptoms of black root often start at the edges of leaves with yellowing that progresses into wedge-shaped necrotic (i.e., dead) areas with yellow borders.  The points of the wedge-shaped diseased areas usually follow veins.  This disease can be very destructive and lead to substantial losses in commercial vegetable production.  The bacterium that causes this disease (Xanthomonas campestris pv. campestris) is typically introduced into a garden on contaminated seed or transplants.  Once the disease has occurred in a garden, the bacterium can survive in small bits of debris from infected plants that filter into the soil.  Crop rotation is a useful technique for managing this disease.  In addition, make sure you plant high-quality seeds that are less likely to harbor the bacterium.  If you have seed that you suspect may carry this pathogen, there are hot-water treatments that you can use to decontaminate the seed. 

Interestingly, in 2024, I have seen a large number of Xanthomonas diseases (aside from black rot) on a wide range of plants including lettuce, tomato, pepper, begonia, zinnia, hydrangea, amur honeysuckle, peach, and winter wheat.  This bacterial genus contains many common and destructive plant pathogens. 

Verticillium wilt

I have only seen a single vegetable example of this disease so far this season (on melon), but Verticillium wilt affects a wide range of vegetable crops including tomato, potato, pepper, eggplant, and vine crops such as cucumber, melon, squash, and pumpkin.  The fungus involved (typically Verticillium dahliae) survives readily in soil or associated with other plants (e.g., broad-leafed weeds) and infects susceptible vegetable plants through roots.  It then colonizes the xylem (i.e., the water-conducting tissue) in the plants and blocks it, preventing water flow from the roots to the aboveground plant parts.  This results in wilting and eventual plant death.  Use of nonsusceptible vegetables (e.g., corn, peas, snap beans) and use of Verticillium wilt-resistant varieties of susceptible vegetables (e.g., tomatoes) are common means of managing this disease. 

Veriticillium wilt is not only a vegetable disease, but it can also be a serious problem on woody and herbaceous ornamentals.  So far in 2024, I have diagnosed this disease on Japanese maple (a very common host), catalpa (another common host), gas plant, and buttonbush.  My buttonbush detection this year is only the second time in my 26+ year career that I have seen Verticillium wilt on this host.  And, I have never seen the disease on gas plant before. 

Powdery Mildews

As with Verticillium wilt, I have only seen a single case of vegetable powdery mildew this year, and that was on cucumber.  I have also seen limited powdery mildew on other non-vegetable plants this year, with detections only on pear, serviceberry, and delphinium.  Part of the reason I’ve likely not seen a lot of powdery mildew samples in the clinic is that many people are familiar with these diseases.  They readily recognize the powdery white growth associated with powdery mildews and don’t need my help with an ID.  In addition however, we’ve had a very wet growing season in 2024.  Powdery mildew fungi don’t perform well when there is a lot of rain and leaves are wet.  These fungi prefer dry leaves and high humidity.  Limited rain and muggy conditions (which favor powdery mildews) are typical in Wisconsin in July and August, but not so much in 2024.  We’ve had regular rains this year, so powdery mildew infections have been suppressed. 

Interestingly, in one of the powdery mildew samples that I received, I found Ampelomyces, a fungus that parasitizes powdery mildew fungi.  Ampelomyces provides natural biological control of these diseases.  Interestingly, this fungus has been commercialized and is the active ingredient in certain fungicides that are used for powdery mildew control. 

What vegetable diseases are you seeing in your garden?  

If you find something interesting and need help in identifying the problem, please contact me at (608) 262-2863 or pddc@wisc.edu for advice.  As always, for more information on plant diseases and their management, be sure to check out the UW-Madison PDDC website.  To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

The 2024 field season is in full swing, and samples have been coming into the clinic at a breakneck pace.  As I think about the samples that have been submitted thus far, I’m struck by how may fruit samples I’ve looked at.  Here’s a summary of the fruit diseases I’ve been seeing. 

Apple scab

Wow!  In my (almost) 26 years in the clinic, I’ve never seen a year as good for apple scab as 2024.  We’ve had perfect weather for an apple scab epidemic.  We had substantial rains around the time that trees were first leafing out, and this is the optimal time for initial infections to occur via spores released from apple and crabapple leaf litter.  These early rains also prevented people from being able to make protective fungicide applications to their apples and crabapples (if they were so inclined).  We continued to have rains throughout the spring, and that provided the leaf wetness needed to allow additional infections to occur via spores that were produced from the initial infections.  I have received so many calls and received so many samples from clients complaining that their apple and crabapple leaves are turning blotchy and brown, and then falling from trees.  This is virtually all due to apple scab.  Expect to see more leaf loss as we head into the summer. 

Cedar-apple rust

On apple and crabapple trees that haven’t defoliated due to apple scab (i.e., that are resistant to that disease), I have been seeing a lot of yellow/orange spotting.  This includes one of the crabapples in my front yard.  This spotting is caused by one of the Gymnosporangium rusts, most likely cedar-apple rust.  Cedar-apple rust is a fungal disease, where the pathogen spends part of its life on junipers (where it produces a brown, brain-like galls that sprout bright orange, gelatinous growths) and part of its life cycle on apples and crabapples where it leads to yellow/orange leaf spots.  I received several photos of the gelatinous juniper phase of this disease in mid-spring.  We had a lot of rain around that time, so I’m not surprised to now see a lot of leaf spotting.  Watch your apples and crabapples for this disease, and if you happen to see a variant where the spotting is red/fuchsia rather than yellow/orange, give me a shout and send me some photos.  You could be dealing with a new Gymnosporangium rust (red star rust) that recently has been found in Wisconsin.  I’m attempting to track this disease, so if you’re in a county where red star rust has not been formally confirmed, I’ll ask you to send in a leaf sample for testing. 

Fire blight

I talked about this disease in my May web article (Wisconsin’s Wild Weather Woes), so check out that article for details.  As I predicted, I’ve been seeing an uptick in the number of cases of fire blight in 2024.  My most interesting fire blight cases have been on quince, a fruit crop that I rarely see. 

Taphrina diseases

I have seen three of these diseases so far this season, caused by three different species of the fungus Taphrina.  Two of the diseases have been on stone fruits.  Peach leaf curl (caused by Taphrina deformans) leads to distorted (sort of bubbly-looking) and typically colorful (creamy white, light green, or pink) areas on peach and nectarine leaves.  Plum pockets (caused by Taphrina pruni) affects plum fruits causing them to become enlarged and hollow.  Early in the development of this disease, the fruits are spongy, but they eventually dry and become brittle.  Finally, I’ve seen oak leaf blister (caused by Taphrina caerulescens), where the fungus leads to irregular, puckered areas on leaves.  Management of severe cases of peach leaf curl and plum pockets (you can just ignore oak leaf blister) often involves making an application of a copper-containing fungicide to branches (where the fungus overwinters) after leaf drop in the fall or before leaf emergence in the spring. 

Bacterial canker

You can think of this disease as being the “fire blight” of stone fruits (i.e., cherry, plum, peach, apricot).  The bacteria involved in this disease can infect through flowers causing them to wither and die.  This phase of the disease is often referred to as blossom blast.  On branches and trunks, the pathogens often gain entry via wounds (e.g., cold injured tissues, pruning wounds), and you will typically see blobs of sap oozing from the infected areas.  These blobs can range from large and obvious to small, subtle, and nondescript.  Aggressive and timely pruning of infected branches is critical for management of this disease.  Pruning will prevent movement of bacteria into the main trunk, which will ultimately lead to tree death. 

Root/crown rots

I can see these diseases on virtually any fruit crop, but my most recent case was on raspberries.  The client complained that his raspberries were stunted and had purplish foliage, two symptoms that I often associate with root/crown rots on raspberry.  I tested root/crown tissue from his plants for Phytophthora and sure enough, the plant tested positive.  Phytophthora (a type of fungus-like water mold) is the most common organism that I see causing root/crown rot issues on raspberries.  It produces thick-walled resting spores that can survive for years in the soil and becomes particularly active and problematic when soils are wet.  Unfortunately, there are few management options for home raspberry growers because of how aggressive this pathogen is and how long it can survive in a garden.  My typical recommendation is to establish a new raspberry patch in a new location with new plants. 

Hopefully, your fruit crops are healthy and growing well, but if not please contact me at (608) 262-2863 or pddc@wisc.edu for advice.  For more information on plant diseases in general and their management, be sure to check out the UW-Madison PDDC website (https://pddc.wisc.edu/).  To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

While 2023 was excessively hot and dry, Wisconsin’s spring 2024 weather has been particularly wet.  That wet weather has come in the form of regular rounds of severe weather.  The third week in May saw a particularly violent set of storms move through the state with heavy rains, high winds (including tornadoes), and hail.  With these severe storms, there came extensive physical damage to many trees across the state.  Some trees lost large branches, and others completely succumbed to the winds and toppling over.  These were the immediate effects of the storms, but other delayed effects are yet to come.  Here are three diseases that I suspect we will see an increase in over the summer in the aftermath of May’s severe weather.

Oak wilt

Oak wilt is a lethal fungal disease that affects oaks of all kinds.  The disease most rapidly develops in oaks in the black oak group (those oaks with pointed-lobed leaves).  White oak group oaks (those with rounded-lobed leaves) are also killed by the disease, although typically more slowly than red oaks.  The fungus that causes oak wilt (Bretziella fagacearum) colonizes the xylem (i.e., the watering-conducting tissue) of infected trees, leading to blockages that prevent movement of water to branches.  Branches wilt, and trees eventually die from the disease.  Introduction of the oak wilt fungus often first occurs via sap beetles, which are attracted to wounded oak trees.  Wind damage to oaks during our recent storms, high sap beetle activity, and the prevalence of the oak wilt fungus in oak trees across the state will likely lead to spread of the fungus and an increase of oak wilt this year.  Watch for flagging (i.e., dying) branches on oaks, and if you see this symptom, consider submitting a sample to the PDDC for oak wilt testing. 

Diplodia shoot blight and canker

Back in 2017, an EF3 tornado cut an 83-mile path through four northern Wisconsin counties (Polk, Barron, Rusk, and Price).  The following year, I started receiving red pine samples from areas near where the tornado had traveled.  The trees, which had survived the storm, started to exhibit extreme needle browning and eventual tree death.  When I examined needles and branches from these trees, I noted extensive numbers of fruiting bodies (i.e., reproductive structures) of Diplodia, the fungus that causes Diplodia shoot blight and canker.  Although these trees had survived the 2017 storms, they apparently were damaged by the high winds, and these wounds provided entry points for Diplodia.  The fungus subsequently colonized and killed the trees.  Given wind damage from our recent storms, I am concerned that we will see a repeat of this scenario in 2024.  So, watch your red, Austrian, mugo and jack pines for needle browning and branch dieback.  If you see these symptoms, Diplodia may be the cause. 

Fire blight

The bacterium (Erwinia amylovora) that causes this disease is most commonly introduced into susceptible trees (e.g., apples, crabapples and pears) via honeybees.  These insects pick up the bacterium when they feed on a combination of sap and bacterial cells that oozes from infected tree branches.  Honeybees drop off the bacterium in the flowers of healthy trees as they pollinate.  Alternatively, Erwinia amylovora can gain entry into trees via wounds created during stormy weather.  In particular, I have seen an increase in fire blight after hail.  During our recent stormy weather, certain areas of the state experienced quite large hail that did significant damage.  These wounds could provide easy access for the fire blight bacterium.  If you see rapid dieback of apple, crabapple, or pear branches over the next few weeks, be sure to contact me about testing for the disease. Early detection, followed by pruning to remove infected branches is critical to get this disease under control.  

Need Help?

Hopefully, you will not see any of the diseases that I have described above.  However, if you see evidence of these or any other plant disease problems, feel free to contact me at (608) 262-2863 or pddc@wisc.edu for advice.  For more information on plant diseases and their management, be sure to check out the UW-Madison PDDC website (https://pddc.wisc.edu/).  To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Our early warm weather has me chomping at the bit to visit local greenhouses to spend money on my favorite annuals and perennials.  Greenhouse growers do their best to produce plants that are healthy and vigorous, but even with the best of care, plants can sometimes end up with unintended disease issues.  As consumers, we need to be diligent and watch for potential issues to make sure we don’t bring diseases home to our ornamental and vegetable gardens.  Here are a few tips on what to look for when buying plants to avoid problems. 

Select plants that are vigorously growing, but aren’t overly leggy. 

Stunting in plants can be caused by several types of diseases including root rots and a variety of plant viruses (more on virus diseases in a bit).  To get a better sense if plants have a root rot problem, carefully pop plants out of pots and look for root discoloration.  Healthy roots should be white, but if you see roots that are brown or black, there’s a good possibility that root rot pathogens are present.  That said, root discoloration can also be caused certain non-disease issues such as heat or salt injury, and plants with these sorts of injuries should be avoided as well.

Leggy plants may be healthy, but often this leggy growth is wimpy and easily injured, particularly as plants are moved and transplanted.  Whenever you injure plants, the wounds you create can provide entry points for a variety of plant pathogens, particularly bacterial pathogens. 

Avoid plants with leaf browning. 

Browning around the edges of leaves typically indicates that plants have been stressed for water.  This marginal browning could mean that the plants weren’t watered enough during production but can also be another symptom of root rots. 

Brown leaf spots can arise due to a variety of disease-causing organisms.  If leaf spots are roughly circular, then the most likely cause is some sort of fungal pathogen (and there are a myriad of fungi that can cause leaf spots and blights).  If you see spots with concentric rings (i.e., looking like a target), then you could be dealing with certain viral pathogens like Impatiens necrotic spot virus (INSV) or Tomato spotted wilt virus (TSWV).  These viruses are common in greenhouse settings and are easily moved around by thrips (a type of insect). 

If you notice angular leaf spots (i.e., spots bordered by veins, giving the spot a straight-edged look), check to see if there is a yellow halo bordering the dead tissue.  If so, you’re likely dealing with a bacterial disease of some kind.  If the angular spots don’t have a halo, then you could be seeing symptoms cause by foliar nematodes. 

Whether spotting is caused by a fungus, a bacterium, a virus, or a nematode, don’t bring these symptomatic plants home.  All of these pathogens can potential spread to other plants in your garden. 

Avoid plants with odd leaf coloring.

Some plants naturally have variegated foliage and this is a highly prized horticultural feature.  However, if you notice plants with unexpected blotchy light and dark-colored leaves, yellow lines or yellow ring patterns, avoid these.  These sorts of symptoms are often caused by plant viruses such as Tobacco mosaic virus (TMV), Hosta virus X (HVX), Cucumber mosaic virus (CMV), and Tobacco rattle virus (TRV).  Once introduced into a garden, plant viruses can easily be spread to other plants by touch (TMV), gardening tools (virtually any plant virus), insects such as aphids (CMV), and naturally occurring soil-borne nematodes (TRV). 

Avoid plants with fuzzy growth on the leaves.

Fuzzy growth typically indicates a fungal or water mold infection of some kind. 

White fuzzy growth on upper and lower leaf surfaces can indicate a powdery mildew problem.  Powdery mildews are ubiquitous and readily develop in humid conditions that are common in greenhouse settings.  These diseases are often relatively cosmetic, but on certain plants (e.g., cucumbers, squash, pumpkin, bee balms, phlox), powdery mildews can cause substantial leaf browning and loss. 

White, gray, or purplish fuzzy growth on the undersides (and only the undersides) of leaves is characteristic of downy mildew diseases.  I routinely find downy mildew on basil wherever this plant is sold.  The first symptom of the disease that I notice is typically leaf yellowing.  When I see this symptom on basil, I then flip the leaves over to look for typical fuzzy gray/purple growth on the undersides.  Downy mildews tend to be more destructive than powdery mildews, so it’s best not to bring these diseases home on your newly purchased plants. 

Don’t panic. 

Most plants available at your local greenhouse and nursery will be healthy and disease free.  With just a little care, you can avoid those rare diseased plants that may pop up, and end up buying healthy, pathogen-free plants that will provide months, if not years, of gardening enjoyment. 

Need help?

For more information on plant diseases and their management, check out the UW-Madison PDDC website (https://pddc.wisc.edu/).  Alternatively, if you have disease questions, feel free to call (608) 262-2863 or email pddc@wisc.edu for advice and guidance.  To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe). 

Happy shopping everyone!

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

Southern blight  

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

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

Oak wilt

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

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

Thousand cankers disease

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

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

Happy spring and happy disease hunting, everyone!