Category Archives: Monthly Column

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

I’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.

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.

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.

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!


February 2022: Optimizing Your Diagnosis at the PDDC

Here at the Plant Disease Diagnostics Clinic (PDDC), we strive to provide the best possible diagnoses of plant disease problems.  PDDC staff use a range of techniques to do this including visual and microscopic examination of plant tissue, incubation of tissue in moist chambers to coax fungal pathogens to sporulate, culturing techniques to grow pathogens from tissue, serological tests to detect proteins specific to certain pathogens, and DNA/RNA (i.e., genetic material) detection techniques.  The PDDC’s ability to provide a high quality diagnoses using these techniques however, is dependent on receiving a high quality sample to work with.

Here are some pointers on how to provide an optimal sample so that PDDC staff can provide an accurate, timely diagosis.

Submit a sample when you first see a problem.  Diagnosing plant diseases as early as possible allows for greater flexibility in management of diseases once they are identified.  So, contact PDDC staff as soon as you see a problem.  That said, disease-causing organisms may not produce structures that we need to see to make a diagnosis (e.g., fruiting bodies, spores) early in disease development.  We may need to work with you and have you repeatedly sample and submit materials over time to accurately diagnosis your plant disease problem.

 Consider submitting photos prior to a physical sample.  Sometimes, we can provide a reasonably accurate diagnosis based on photos (tar spot anyone?).  If not, photos can provide useful information on the sort of physical sample that you can send to the PDDC for a more detailed analysis.  You can use the online form at to submit photos.  If for some reason that form doesn’t work for you, feel free to send photos to the clinic email address (  If we are able to provide a diagnosis from your photos, there will be a $20 digital diagnosis fee.  If we make a preliminary diagnosis from the photos but suggest a follow-up physical sample submission for additional testing, the $20 digital fee will be credited towards any lab fees for your follow-up sample.

When submitting a physical sample, make sure you provide the appropriate plant part.  If you have any questions about what to submit, contact the PDDC for advice.  Submitting the wrong plant part can lead to an inaccurate or delayed diagnosis.

For herbaceous plants (e.g., herbaceous ornamentals, vegetables), sending in an entire plant may be the best course of action.  In some situations, symptoms that you see on the leaves of plants are an indication of a problem in the root system.  With trees and shrubs, submitting an entire plant is not practical.  For these plants, submitting subsets of leaves/needles, branches and/or roots will likely be more appropriate.

Whatever you end up sending, send A LOT.  Often, we need to perform multiple tests to diagnose a problem.  We don’t want to run short on tissue.  For leaves, send half a dozen to a dozen (or more) showing a range of symptoms.  For branches, send three or more symptomatic branches (with attached leaves or needles where appropriate).  For roots, send a large handful of the small, fibrous roots.

In particular, appropriate branch selection can be critical for accurately diagnosing vascular wilt diseases such as Dutch elm disease, oak wilt and Verticillium wilt.  For these diseases, select branches that have recently wilted/died back.  DO NOT submit branches that easily snap off.  These branches have been dead too long and cannot be accurately tested for vascular wilt pathogens.  If you suspect Verticillium wilt, select symptomatic branches from as low on the tree as possible as the pathogen that causes this disease infects through the roots.  Choose branches that are roughly one inch in diameter.  Larger diameter branches (particularly from oak trees) tend to have thick bark that is difficult to remove without contaminating underlying tissue where vascular wilt pathogens reside.  This contamination will interfere with growing pathogens from branch tissue and can also interfere with DNA-based tests for pathogen detection.  Sometimes, clients like to send in trunk sections for testing for vascular wilts.  These sorts of samples can work, but only if the trunk slices are no more that about one inch thick.  We need to be able to easily pop off the bark from these slices with minimal contamination of the wood underneath.

Submit your sample as quickly as possible to the PDDC.  If possible, collect samples just before you mail them or drop them off in person.  If there is going to be a delay in submission, keep samples as cool as possible.  High temperatures can kill certain pathogens and can degrade herbaceous plant tissues, leading to what we not-so-affectionately refer to as “slime in a bag”.  Degraded samples make diagnostic testing more difficult, if not impossible.  Mail samples by overnight mail when possible.  If mailing via regular mail, please mail early in the week (Monday or Tuesday) so that samples do not sit around in a mail facility over a weekend.  Click here for details on how to package samples to make sure they arrive at the PDDC in good shape.

Help us, help you.  Providing us with a high quality sample can go a long way in us providing you with a high quality diagnosis.  Let’s work together to make this happen.  If you have questions about submitting a sample (or 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 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 and happy plant disease sleuthing!

January 2022: Plant Disease Diagnostics Clinic Services for 2022

As we start 2022, COVID-19 continues to inform how the UW Plant Disease Diagnostics Clinic (PDDC) provides services to the public.  Clinic personnel continue to try 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.  Given that the COVID-19 situation is ever evolving, there will likely be changes in these guidelines as the year progresses.  Be sure to check this document routinely for updates.

PLANT DISEASE DIAGNOSTICS:  In 2020, I started offering formal digital diagnoses, and that service will continue in 2022.  I really encourage you to submit digital photos of your plant disease problems before submitting physical samples.  Use the online form at  If I can see enough of what’s going on in your photos, I will provide you a diagnosis with management recommendations and charge the $20 digital diagnosis fee.  If I cannot make a definitive diagnosis, I will not charge the fee, but I will use the photos to help me provide you with suggestions on what sort of physical sample to submit as a follow-up.  Note that if I charge the digital fee and you end up submitting a follow-up physical sample, the digital fee will be credited towards any lab fees for your physical sample.

You can submit physical samples for diagnosis either by mail or in person.  For increased safety, I suggest submitting by mail.  If you opt to drop off a sample in person, PDDC staff will not be available for an in-person consultation.  While I am committed to providing personalized service to my clients, continuing issues with COVID-19 suggest that limiting person-to-person contact is prudent.  Also note that wearing a mask is required in all UW-Madison buildings at this time.

The PDDC provides a sample drop off location 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 your 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 fill it out ahead of time to speed up the submission process.  Please print neatly and make sure to include your complete mailing address, your phone number and your email address.  At this time, I am emailing virtually all reports and invoices.  If you do not have email however, I definitely can send out a hard copy of your report and invoice.

PRESENTATIONS:  I was hoping to start providing in-person PDDC presentations in 2022, but with COVID-19 cases increasing at this time, I have cancelled the few in-person talks that I already had scheduled in early 2022 and will be limiting presentations to virtual presentations (e.g., via Zoom) until further notice.  If you are interested in having me provide a presentation, 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.

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 revised and rebranded UW Plant Disease Facts (formerly the plant disease-related titles of the University of Wisconsin Garden Facts).  I am also working on converting some of my in-person talks [Fundamentals of Plant Diseases and The Science (and Art) of Plant Disease Diagnosis] into online training modules with voiceovers.  As these new resources become available, I will announce their availability via Twitter, Facebook (@UWPDDC), or via my clinic listserv, UWPDDCLearn (email me to subscribe to this).

CONSULTATIONS:  As always, if you have questions about plant diseases, feel free to contact me by phone at (608) 262-2863 or email at

Stay safe everyone and all the best for 2022!

July 2021: Summer Doldrums – Wilted Tomatoes in the Garden

Tomato IconI have recently received a slew of questions about wilted tomatoes in home gardens.  Here are the top five reasons that tomatoes can wilt based on samples that I have received in my clinic over the years.

Walnut toxicityOne of the lessons that I have learned after doing plant disease diagnostics for over 20 years is that when a home gardener consults me about wilting tomatoes, the first question I should ask is, “Do you have a walnut tree near your vegetable garden?”  More times than not, the answer is “Yes” and the walnut tree is the cause of the problem.  Black walnuts produce toxins (exuded by roots and produced in leaves and fruits) that adversally affect a wide range of plants,  Tomatoes are particularly sensitive and are often die from the exposure.  Anytime that tomatoes are grown in the root zone of a walnut tree (which extends three to five times the height of the tree from the trunk), problems can arise.  Cutting down walnut trees will not solve the problem in the short term, because roots from the cut tree can continue to exude toxins for 15 to 20 years.  Often the best recourse when walnut trees are present in a landscape is to grow tomatoes in raised beds or in pots to keep tomato roots as far above walnut roots as possible.

Drought stress:  In 2021, lack of rain has been a potential cause for wilting in tomatoes and virtually every other plant.  Most established plants require about one inch of water per week.  When rain is insufficient (as it has been in much of Wisconsin this year), it’s important to apply supplemental water to plants with a soaker or drip hose.  Proper watering can not only prevent wilting in tomatoes, but it can also help improve calcium uptake and reduce problems with blossom end rot.  Using an inch or two of a high quality mulch (my favorites are shredded oak bark mulch and red cedar mulch) around plants can help retain moisture and lessen wilting issues.  Mulching around tomatoes also helps reduce movement of spores (produced in bits of old tomato debris in the soil) of the fungi that cause Septoria leaf spot and early blight.

Bacterial canker:  The bacterium that causes this disease (Clavibacter michiganensis subsp. michiganensis – THERE’S a mouthful) is seedborne, so gardeners typically introduce this pathogen into their gardens on contaminated tomato seeds or transplants.  Plants initially look healthy, but the bacterium eventualy colonizes, discolors and disrupts the water-conducting (vascular) tissue inside the plant, leading to wilting.  Infections can lead to long, somewhat subtle cracks in stems and ultimately less subtle open wounds (i.e., cankers) in stems near the soil line.  Another telltale symptom of the disease can be ghostly-white spots with a darker center (called bird’s-eye spots) on tomato fruits.  Removal and destruction of infected plants, and rotation away from susceptible vegetables (e.g., tomatoes and peppers) for several years in the affected area of a garden are typical management strategies.

Verticillium wiltMany gardeners are familiar with this disease in the context of the death and destruction it brings to woody trees and shrubs.  However, Verticillium, the cause of Verticillium wilt, is an equal opportunity destroyer and can kill a wide range of herbaceous plants as well, including popular vegetables such as solanacoues crops (e.g., tomato, potato, eggplant, pepper) and vine crops (e.g., cucumber, squash, pumpkin).  This fungus is routinely found in the soil and can build up over time if susceptible vegetable crops are grown over and over again in an area where the fungus is located.  Verticillium infects through the roots and colonizes and plugs a tomato’s (or other plant’s) water-conducting tissue, leading to wilting.  Discoloration of a tomato plant’s vascular tissue is a typical symptom of this disease, but stem cracks and cankers are notRotation can be useful as a control strategy for Verticillium wilt, although it is less effective than for bacterial canker because of the wider host range for Verticillium (including many weeds).  For tomatoes, use of resistant varieties can also be useful.  To identify resistant varieties, look for a “V” after the variety name on a tomato seed packet or in the variety description in your favorite seed catalog.

Fusarium wilt.  This disease is very similar to Verticillium wilt except for the fungus involved.  For Fusarium wilt, Fusarium oxysporum f. sp. lycopersici is the culprit.  Fusarium oxysporum is a large fungal species with many special forms (that’s what “f. sp.” stands for), each one adapted to infect a specific host plant or a very small range of host plants (e.g., vine crops).  Fusarium oxysporum f. sp. lycopersici is specific to tomatoes and will not infect other vegetable crops.  If you ever encounter this disease, rotation away from tomatoes in the affected area for several years should work well as a management strategy.  In addition, you can use resistant tomato varieties.  Look for one or more ‘F’s” after the variety name.

As you can imagine, figuring out the exact reason your tomatoes are wilting can be challenging, particularly if there is disease involved.  For help with proper diagnosis of tomato wilts (and other plant problems in general), contact the PDDC at or (608) 262-2863.

To find out more about the clinic and its activities, check out the PDDC website.  To keep up-to-date about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

June 2021: Fired Up About Fire Blight

FireIconforJune 2021 Monthly ColumnIt‘s the time of year where I am once again getting questions about apple and crabapple trees with dead branches.  Often, in these situations, clients assume that their trees are suffering from fire blight.  This bacterial disease has received a lot of press over the years and can be a serious problem.  However, fire blight is definitely not the only reason that branches on apples and crabapples die.

There are myriad of fungal diseases that can lead to branch dieback.  In particular, fungal canker diseases can be an issue.  I often find fungi like Cytospora, Phomopsis and Sphaeropsis in dead apple or crabapple branches.  These fungi, like the fire blight bacterium, locally infect and girdle branches (thus leading to branch death), often in a somewhat random pattern in a tree.  Sphaeropsis is particularly common.  This fungus not only infects branches but can also infect fruits (causing black rot) and leaves (causing frogeye leaf spot).  More systemic fungal diseases like root and crown rot, Armillaria root disease and black root rot (dead man’s fingers) can also lead to branch dieback.  The pathogens involved in these diseases infect and disrupt root and trunk function, preventing proper water movement from roots to branches.  This lack of water leads to branch death, often over a fairly substantial portion of the tree canopy.

Environmental stresses can also lead to branch death in apples and crabapples.  Drought can lead to branch dieback symptoms similar to those caused by the systemic diseases I described above.  Cold injury can also be a contributing factor.  Growing a non-hardy apple or crabapple variety often leads to dieback issues.  Even on hardy varieties, branch dieback can occur if cold snaps occur in the spring right as or just after trees leaf out.  Lack of snow cover (which insulates soil) coupled with extremely cold winter temperatures can lead to physical injury to roots, which in turn limits water uptake, leading to branch dieback.

So, with all of these potential causes of branch dieback on apples and crabapples, how can you tell if your tree is suffering from fire blight?  The answer is, “It’s not easy!”  People often claim that fire blight leads to a branch with a shepherd’s crook (a downward bend at the branch tip).  However, after years of seeing dead and dying apple and crabapple branches, I just don’t consider this a reliable symptom for diagnosing fire blight.  To me, a shepherd’s crook just indicates that the branch didn’t get enough water and wilted.  That could be due to any of the causes I outlined above.  And conversely, I have seen cases of fire blight where branches don’t have a shepherd’s crook.  What I tend to look for as I’m attempting to diagnose fire blight is oozy material (a combination of sap and bacterial cells) that seeps from affected branch.  I also look for some indication that the infection may have started where flowers were attached.  I look for this latter indicator because trees are often inoculated with the fire blight bacterium by bees that carry the bacterium and drop it off in the flowers as they pollinate.  Even when I see these symptoms, I will only diagnose fire blight if I have evidence that the fire blight bacterium is present.  There are dipstick serological tests (these use the same technology as home pregnancy kits) that I use to confirm the presence of the fire blight bacterium.  If I don’t find evidence of the bacterium, I look for other possible causes of the branch dieback.

So, why do I really need to know if branch dieback is really due to fire blight?  It all comes down to management.  If fire blight is the cause, I recommend very aggressive pruning (roughly 12 inches below where there are obvious symptoms).  The fire blight bacterium can move rapidly down a branch under the bark, so you want to make sure to prune down far enough to remove all of the bacterium.  Fungal pathogens tend to move less rapidly, so you can get by with pruning roughly six inches below where there are obvious symptoms.  If the problem is a root disease of some kind, pruning will not resolve the problem.  Fungicide treatments to the roots may be needed in some instances, or there may be ways of reducing tree stress that slow down the progression of these types of diseases.

It all comes down to the fact that if you don’t know what the underlying problem is with your tree, it is unlikely that you will be able to fix the problem.  So, get a proper diagnosis and then tailor your management strategy to the specific problem(s) you are facing.  Without a proper diagnosis, you can spend a lot of time, effort and money, and not improve the health of your trees one bit.

For help with proper diagnosis of plant problems, contact the PDDC at or (608) 262-2863.  To find out more about the clinic and its activities, check out the PDDC website.  To keep up-to-date about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

May 2021: Rampant, Ravaging Rusts

May 2021 Column IconI really love this time of the year as plants emerge once again after their long winter naps.  I also love the fact that I now get to start looking once again for some of my favorite plant diseases.  At the top of the list at this time of the year are the rust diseases.  Rusts encompass a large group of fungal diseases, where the fungi produce brightly-colored (yellow to orange to bown) spores.  Each rust fungus has a very specific host range.  The following are a few of the rusts (one on a broad-leafed tree, one on a conifer and one on an herbaceous plant) that I have recently seen either through the clinic or in my own yard.

Crown Rust Buckthorn
Crown Rust on Buckthorn

Crown rust:  Crown rust is classic rust of grass species such as turfgrass and oats.  If you’ve ever walked through your lawn and gotten orange shoes, you’ve encountered this disease.  The fungus that causes crown rust is Puccinia coronata, which has several variants adapted to infect specific grass hostsThe disease and pathogen names come from the look of the resting spores of the fungus.  These spores have spikes that give them the appearance of a crown.  At this time of the year, I don’t see crown rust on grass or oats (that comes later in the growing season), but I see it on a second host (called the alternate host) of the fungus, buckthorn.  Buckthorn is actually required by the crown rust fungus to complete its life cycle.  Puccinia coronata causes yellow-orange, powdery patches on the buckthorn leaves and green stems, and I actually use crown rust as an ID feature for buckthorn.  If I see seedlings that I think are buckthorn, but I’m not quite sure, I look for the characteristic orange patches of crown rust to confirm.  And if you need another reason get rid of buckthorn, in addition to this plant being incredibly invasive, here it is.  If you remove buckthorn, you will prevent the crown rust fungus from completing its life cycle and reduce the severity of the disease on turf and oats.

Weirs Cushion Rust
Weirs Cushion Rust on Spruce

Weir’s cushion rustThis is rust disease of spruce that I see infrequently, but I just received a sample of it this past week in the clinic.  I was over the moon!  (Yes, I know I’m weird and lead a very sheltered life.)  The fungus that causes this disease is Ceropsora weirii (formerly Chrysomyxa weirii), a single-host rust fungus that only requires spruce to complete its life cycle.  Infection leads to yellow banding on one-year-old needles.  Within these bands in the spring (typically April or May), fruiting bodies (i.e., reproductive structures) of Ceropsora weirii form yielding orangish spores that blow to newly emerging needles where the fungus infects.  The fruiting bodies are easily visible with a hand lens or even with the naked eye.  Eventually the infected3 needles brown and drop off the tree.  Management of Weir’s cushion rust can be a challenge and typically involves use of fungicides to protect newly emerging needles.

Mayapple Rust

Mayapple rust:  I’d like to give a shout-out to Brenda Dahlfors, Master Gardener Program Coordinator with University of Illinois Extension for sending me photos of this cool rust.  The fungus involved here is another species of Puccinia, Puccinia podophylli.  This is another single-host rust, where the fungus that only infects mayapples.  The disease is most visible in the spring when bright orange, powdery patches develop on the undersides of leaves.  On the upper leaf surfaces above these patches, you will see yellow spots/discolored areas.  The orange spores produced by the pathogen reinfect mayapple plants, causing additional disease.  The bright orange patches tend to fade to a duller brown as they age and convert to producing brown overwintering spores.  These overwintering spores germinate in the spring to produce yet another type of spore that causes the initial infections in the spring.  Careful removal of infected leaves and plant debris (burn, bury or hot compost this material) combined with fungicide sprays where appropriate is the typical management strategy for this disease.

These are just a few of the cool rust diseases that you may encounter as you are out and about.  Watch for these and other rusts, and enjoy them when you find them.  There are the most visually colorful and attractive diseases that I see.  For additional information on the PDDC and its activities, check out the PDDC website.  To learn about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

April 2021: Tomato in the Basement, Canary in the Coalmine

Canary IconI have the coolest job on the planet.  Everyday, I get to help people with their plant disease problems.  This may entail helping someone learn how to grow healthy, nutritious vegetables to feed their family or assisting a grieving family select the just the right tree to plant to serve as a lasting memorial for a recently lost loved one.  On occasion, I get to help Wisconsin farmers avoid severe economic losses due to plant diseases or help prevent the introduction of federally regulated plant pathogens that potentially can have negative impacts nationwide  This month, I’d like to share with you a diagnostic case where a proper diagnosis had the potential to save someone’s life.

I recently received photos from a home gardener who was growing tomato transplants indoors.  She was concerned that her plants were not doing well and losing leaves.  Her photos showed plants with leaves that were cupping downward and showed twisted petioles and other growth distortions.  I was immediately suspicious that the plants had been exposed to ethylene.  Ethylene is a gas that is a plant growth hormone that can be very beneficial for proper plant development; in particular, ethylene is important in fruit ripening.  But in other situations, when plants are exposed at the wrong time or at too high of a concentration, ethylene can have negative effects, in fact exactly the sort of symptoms I was seeing in my client’s photos:  distorted plant growth and premature leaf loss.

Tomatoes Damaged by Ethylene
Tomatoes suffering from ethylene exposure. Photo courtesy of Kristine Meixensperger.

After an exchange of several emails, the puzzle pieces started to fall into place.  My client had been growing her tomatoes in the basement (not uncommon for many gardeners) next to the boiler that provided heat for her home.  As the weather warmed up, she moved the plants to her garage where she parks her car and where she has a full kitchen.  She had been cooking in this kitchen recently to provide a bit of additional warmth for her plants.  Both her boiler and stove burn propane.

At this point, alarm bells were going off.  If propane burners malfunction and don’t burn propane completely, one of the breakdown products of this incomplete combustion is ethylene.  You can also find ethylene in exhaust fumes from motor vehicles, in the smoke produced by wood-burning stoves and as a contaminant in natural gas.  I suggested to my client that she should have her boiler and stove checked immediately for problems.  One or both of these (and possibly also fumes from her car) were likely the source of ethylene that was causing problems for her tomatoes.  She emailed back to tell me that what I had told her made perfect sense as her tomatoes nearer the boiler had more severe symptoms than those farther away.  Another sentence from this email became the inspiration for the title of this article:  “So the tomatoes plants in the basement acted like a canary in a coalmine.”

I told her that she was spot on with her analogy, and at that point, I gave potentially even more serious news.  In addition to producing ethylene, malfunctioning propane burners (and other types of heating systems) also can produce carbon monoxide, a potentially deadly gas.  According to the CDC, approximately 50,000 people visit hospitals with carbon monoxide poisoning each year and at least 430 of these people die from this poisoning.  Luckily, my client had a carbon monoxide detector near the boiler and it hadn’t gone off.  But, the unit was old, and my client indicated that our conversation had made her realize that she needed to replace that unit.

What if she hadn’t had a carbon monoxide detector?  Then, those distorted tomatoes would have been her first hint that a potentially deadly carbon monoxide situation was developing.  Similarly, if she had had distorted tomatoes growing near a natural gas-fueled furnace, that could have indicated a natural gas leak, another potentially lethal situation.

Ah, the power of a lowly vegetable and a bit of knowledge about how they grow!

For additional information on the PDDC and its activities, check out the PDDC website.  To learn about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

March 2021: Gardening Ideas for the 2021 Growing Season

Plant Light BAs the temperatures begin to warm and the snow melts, gardeners are chomping at the bit to be outdoors working away.  Thoughts tend towards “What should I plant this year?” and “When should I plant and seed?”  Those are great questions, but there are other aspects of gardening that I think are important to consider.  This month, I’d like to share what I think about at this time of year as I try to prepare for a successful growing season.

Garden clean-up.  I often talk about the importance of fall clean-up for plant disease management.  Plant pathogens often overwinter in debris from infected plants left over from the previous growing season.  Removing this material from a garden helps eliminate a source of pathogens that can reinfect plants during the current growing season.  While fall is a great time to do this removal, there are a variety of reasons why gardeners might choose not to do clean-up in the fall.  Some people just don’t have the time.  Some like to use leaf litter to insulate flower beds.  Others like to maintain plants that have died back for winter visual interest in their gardens or as overwintering sites for beneficial insects.  If you’re one of the people who likes keep plant debris around for the winter, spring is the time to take care of this material.  Try to remove the debris before plants begin to produce new growth, and burn (where allowed), bury or hot compost it.

Watering concerns.  This can be a tough time of year for many evergreens (particularly yews and boxwoods), as they tend to be prone to winter burn.  Sometimes winter burn develops during the winter months, but spring is a prime time for symptoms to develop as plants start to photosynthesize and thus lose more water through stomates on needles and leaves.  So, as the ground begins to thaw and new needles and leaves begin to emerge, make sure evergreens are receiving sufficient water.  Established plants (those planted three years or more) require approximately one inch of water per week from rain or from supplemental watering with a drip or soaker hose placed at their driplines (i.e., the edges of where the branches extend).  Newly transplanted plants (those planted within the past three years) require roughly two inches of water per week.

Decontaminating pots.  If you grow plants in pots or other containers, and particularly if you have issues with root rots or other diseases caused by soil-borne pathogens, you should seriously consider decontaminating your containers before reusing them this year.  Empty any soil from the containers and discard the soil (especially if the plants previously grown in the containers have had disease issues), wash the containers thoroughly to remove any remaining soil, then soak the containers for 30 minutes in a 10% bleach solution (one part of a disinfecting bleach and nine parts water).  Rinse the containers thoroughly to remove any bleach residues and you’re ready to plant.  Note that this techniques works best for clay or ceramic pots, but may not be reliable for plastic pots.  Sometimes the best way to decontaminate plastic pots is to throw them away.

Mapping vegetable gardens.  For long-term success with vegetable gardening, crop rotation is a must.  Growing the same (or related) vegetables in the same spot in your vegetable garden year after year is a great way to build up disease-causing organisms in the soil.  These pathogens can cause problems for years, if not decades, to come.  So, if you haven’t been mapping out where you plant your vegetables each year, make this the year when you start doing that.  Buy some graph paper, draw your vegetable garden space to scale and preplan where you will plant your various vegetables this year.  Keep this plan as a reference so that next year when you create your 2022 map, you can properly rotate your vegetables to new areas in your garden.  If possible, don’t grow the same (or related) vegetables in the same area for a period of at least three to four years.

Keeping a garden journal.  Also, make this the year that you start documenting what goes on in your garden.  Record information on when plants emerge or begin to leaf out, and when they flower.  Keep track of the weather including temperatures, rain (and snow), significant storm events (hail, high winds, driving rains) and note when you see particular insects and diseases in your garden.  This sort of information can be very useful (particularly after you have several years’ data) in predicting insect pest and disease activity and thinking ahead about how to fend off these sorts of problems.

I hope these pointers help you have a successful 2021 gardening season.  For additional information on the PDDC and its activities, check out the PDDC website.  To learn about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

February 2021: Deep Freeze Search and Destroy

Magnifying glass and hatchetIn this month’s Plant Disease Pointers, I discussed the advantages of pruning trees and shrubs in the winter to increase structural soundness and overall aesthetics.  Winter is also a great time to inspect trees and shrubs for certain diseases and, where needed, prune out these problems.  Diseases that can be corrected, at least in part, by winter pruning include canker and gall diseases.

Canker diseases:  There are a wide range of fungal and bacterial pathogens that infect branches and eventually lead to branch dieback.  With some of these diseases (e.g., Diplodia shoot blight and canker, white pine blister rust), the pathogens initially infect through needles.  With others (e.g., fire blight), the pathogens enter through flowers.  Once in the plant, these pathogens work their way relatively rapidly down branches and can cause significant damage.  Catching these diseases early and pruning out affected branches can reduce the overall damage that they cause.  If not managed properly and removed, these pathogens can eventually reach and girdle the main trunk, thus killing the tree.

Other canker diseases tend to be more localized in their effect.  For some, like Nectria canker, the pathogens often enter through wounds (e.g., pruning wounds, wounds from storm damage).  For others, like Thyronectria canker of honeylocust or Cytospora canker of spruce, direct infection of branches appears to be the norm.  With these diseases, the pathogens progress somewhat slowly, causing localized sunken areas (a “classic” canker symptom) around the point of infection.  Eventually these diseases will progress to the point where the entire circumference of the branch is affected, which leads to branch death.  Movement of these pathogens into the main trunk tends to be a slower process, although if left unchecked, these organisms can eventually cause significant damage as well.

Gall diseases:  The classic diseases in this category include black knot of Prunus species (particularly plum and cherry) and the Gymnosporium rusts like cedar-apple rust and cedar-hawthorn rust.  These diseases typically do not cause branch dieback but can reduce the aesthetic appeal of infected trees and shrubs.  In the case of black knot, you will see fairly large black masses (what I call “poop-on-a-stick”) on infected branches.  These are particularly visible in the winter when there is no foliage to hide them.

Galls associated with Gymnosporium rusts are much smaller and more subtle.  They look like tiny brown brains that form on the branches of junipers, particularly Eastern red cedar.  In the winter, if you don’t look carefully, you might miss these.  In the spring however, the galls reach the pinnacle of their visual glory when they sprout gelatinous, orange arms/masses that produce spores.  These spores infect certain trees and shrubs in the rose family (e.g., apple, crabapple, hawthorn, quince, pear and serviceberry) leading to brightly-colored leaf spots (in the case of cedar-apple rust and cedar-hawthorn rust) or spiny fruits and branch galls (in the case of cedar-quince rust).  Spores produced in these diseased leaves, fruits and branches eventually infect junipers completing the life cycle of the pathogen.

Once either canker or gall diseases become established, pruning is the method of choice for management, and wintertime is a great time to do this pruning.  Symptoms are often more visible during the winter months, and pruning in colder, drier winter weather tends to reduce the risk of infections through pruning cuts.  For diseases caused by fungal pathogens, I suggest pruning four to six inches below where you see obvious symptoms.  For diseases caused by bacteria, I suggest pruning more aggressively, roughly 12 inches below where symptoms are visible.  When pruning in the winter, it may seem that decontaminating tools is not necessarily.  However, I recommend decontamination no matter when you prune.  Treat pruning tools between cuts for at least 30 seconds with 70% alcohol (e.g., rubbing alcohol straight out of the bottle), a commercial disinfectant that contains roughly 70% active ingredient or 10% bleach.  If you decide to use bleach, be sure to thoroughly rinse your tools after you are done pruning and oil them to prevent rusting that can be caused by bleach use.  You can dispose of branches by burning (where allowed) or burying them.

So, as temperatures start to hover in the upper 20s or lower 30s this winter, think about pruning your trees.  Remove diseased branches, and at the same time, prune out healthy branches to improve the structural integrity and aesthetic appeal of your trees.  All of this said, please DO NOT prune when it is excessively cold:

  • Pruning diseased branches in winter:  GOOD!
  • Frost bite and freezing to death:  BAD!!

For additional information on the PDDC and its activities, check out the PDDC website.  To learn about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

January 2021: Taking a Close Look at 2020

Magnifying Glass2021 has arrived and I can’t say that I’m sad to see 2020 gone.  Last year was incredibly challenging for everyone due to COVID-19.  I am very grateful to still have job and to be able to do the work that I love.  Here’s how things shook out in 2020 at the PDDC.

Clinic staff processed a whopping 2381 samples, up 58% from 2019 and an all-time record for my tenure at the PDDC.  Samples came from 69 of Wisconsin’s 72 counties, as well as 21 additional states (AZ, CA, CO, FL, GA, ID, IL, IA, ME, MD, MI, MN, MO, NV, NM, NY, ND, OH, SD, TN and WA) and three foreign countries (Canada, New Zealand and the United Kingdom).  Much of the increase in sample numbers resulted from the clinic formalizing and substantially promoting digital diagnostics for the first time.  This was necessitated by COVID-19, which limited clinic staffing (specifically student hourly help) and curtailed the clinic’s capacity to process physical samples.  In addition, having several wet seasons in a row prior to 2020, as well as a wet early 2020 season, helped promote a wide range of plant diseases.  People, sequestered at home for much of the year, seemed to take notice of and were curious about the diseases in their gardens and landscapes and as a consequence asked for more help with identifying the problems they observed.  No matter what the cause of the increase in sample numbers, I was certainly kept busy (and out of mischief) for the year.

In 2020, as in previous years, the PDDC expanded its molecular (i.e., DNA-based) diagnostic offerings.  One disease of note that was detected this year using molecular diagnostics was Potato mop top virus (a first report for Wisconsin)This virus is transmitted by the organism (a type of slime mold) that causes powdery scab.  As always, whenever I discuss the PDDC’s molecular efforts, I have to give a shout out to Sue Lueloff, the PDDC’s Assistant Diagnostician.  Without Sue, molecular diagnostics at the PDDC would not exist.  As in 2019, Sue not only tested routine clinic samples but also worked with the Wisconsin Department of Natural Resources (WI DNR) to test tree samples from around the state for phytoplasmas.

In other 2020 diagnostic highlights, Ralstonia wilt reared its ugly head once again in the US with an initial detection in Michigan.  The last occurrence of this disease in the US was in 2004.  The pathogen that causes Ralstonia wilt (Ralstonia solanacearum race 3, biovar 2) was introduced on contaminated geranium cuttings brought into the US from Guatemala and is of concern because of its potential to spread and cause severe disease on potatoes.  In fact, this bacterium is so destructive that the US government has listed it as a select agent, with potential to be weaponized by terrorists and used against US agriculture.  In 2020, potentially contaminated geranium cuttings were shipped to 650+ greenhouses in 44 states, with 19 greenhouses in Wisconsin involved.  Luckily there were no positive detections the disease in Wisconsin greenhouses.  My involvement with testing for Ralstonia wilt came in the latter half of 2020 through collaborative work with Dr. Caitilyn Allen, the UW-Madison’s world expert on Ralstonia wilt.  She was contacted by the geranium producer in Guatemala (through USDA APHIS) who was involved in the 2020 outbreak, to test current stock (for 2021 geranium production) for Ralstonia solanacearum race 3, biovar 2 contamination.  Dr. Allen’s group had insufficient staffing/resources to process the 1500 samples requested and ended up partnering with the PDDC to use the clinic’s Maxwell automated nucleic acid extraction system.  Using this equipment allowed Dr. Allen’s group to quickly obtain DNA samples from the plants that were subsequently tested for the bacterium.  Fortunately, all of the materials tested negative.

Other PDDC outreach activities were somewhat curtailed due to COVID-19.  I did end up giving 70 talks/presentations/workshops in at least 16 Wisconsin counties.  Many of these presentations were provided via Zoom with participants coming from multiple counties and sometimes the entire state.  My biggest outreach event in 2020 was Wisconsin Public Television’s Garden and Landscape Expo.  I spent three days at the event, gave three talks and helped answer questions with Lisa Johnson at two Q&A sessions (one hosted by WPR’s Larry Meiller).  I had a steady stream of visitors to the PDDC booth (newly redesigned and rebranded given Extension’s merger with the UW-Madison) and talked with and answered questions for visitors the entire time.  I distributed 4,023 University of Wisconsin Garden Facts fact sheets, brochures and other informational materials at the event.  Across all outreach programs in 2019, I interacted with almost 223,737 people (interestingly just a slight decrease from 2019).  As always, a big thanks goes out to Larry Meiller for having me on his radio show with its awesome listenership.

And finally, I can’t emphasize enough that the accomplishments of the PDDC are not a solo effort.  I have amazing help, including Sue Lueloff (molecular diagnostician extraordinaire mentioned above) Ann Joy (data entry expert who keeps federal funds flowing from the National Plant Diagnostics Network), Dixie Lang (IT support wizard who makes the PDDC website look beautiful and keeps the PDDC database running and up to snuff), Laurie Ballentine of the Russell Labs Hub staff (who never says no and happily prints, folds and otherwise produces all of the written handouts I use for my outreach efforts), and Alex Mikus (an undergraduate here at the UW-Madison who was able to help process samples in the clinic prior to the onset of COVID-19).

2020 is over – Phew!  Let’s see what 2021 has in store!

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