All posts by ddlang

March 2021: Dampening Damping-Off: Tips on Seed Starting to Avoid Disease

Germinating Seed IconAfter a long, cold winter, it’s time to start growing plants from seeds for the upcoming growing season.  Damping-off is a common disease that can prevent seed-starting success.  Here are tips that can help prevent damping-off from being a problem.

Damping Off
Damping Off

Buy high quality seed from a reputable source

High quality seeds are less likely to carry damping-off organisms.

Use pasteurized soil

Pasteurized soil has been steam treated to kill pathogens.

Use clean pots/containers

Store new pots or flats in sealed plastic bags to prevent possible pathogen contamination prior to use.  When reusing pots, soak them in 10% bleach (1 part of a disinfecting bleach, 9 parts water) for 30 minutes, then rinse well to remove bleach residues.

Plant seeds at the proper depth

This will promote quick germination and rapid growth of seedlings out of the early stages of growth when they are most susceptible to damping-off.

Start seeds at higher temperatures

This will again help plants grow out of their susceptible phase quickly.  Consider using a seed-starting heat pad (available at your local garden center), particularly if you start seeds in a colder part of your home (e.g., a basement).

Don’t overwater

Damping-off organisms are more active in wet soils.  Water enough to keep seedlings alive, but keep plants a bit on the dry side to slow development of damping-off pathogens.

Seed starting can be a fun way to start the gardening season.  With just a little extra effort, you can prevent damping-off from dampening your gardening efforts.

Need more information?

For more information on damping-off and its management, check out University of Wisconsin Garden Facts XHT1124 (Damping-Off), available at or

Tobacco Mosaic

Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.
Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.

What is tobacco mosaic?  Tobacco mosaic is a common viral disease of worldwide distribution that affects over 200 species of herbaceous and, to a lesser extent, woody plants.  Common hosts include tobacco, solanaceous vegetables (e.g., pepper, tomato) and vining vegetables (e.g., cucumber, melon, squash), as well as a wide range of ornamentals (e.g., begonia, coleus, geranium, impatiens, million bells, petunia).  The disease has its biggest impact on vegetables, where it can reduce yield and affect quality to the point that commercial crops cannot be marketed.

What does tobacco mosaic look like?  Symptoms of tobacco mosaic vary in type and severity depending on the plant infected, plant age, the variant of the virus involved, and environmental conditions.  On leaves, typical symptoms include blotchy light and dark areas (called mosaic); cupping, curling, elongation (strapping), roughening, wrinkling and other growth distortions; and smaller than normal size.  Fruits may have a blotchy color, ripen unevenly, be malformed or have an off flavor.  Entire infected plants are often stunted.  Other viral diseases like cucumber mosaic (see UW Plant Disease Facts D0036, Cucumber Mosaic) can cause symptoms similar to tobacco mosaic.  Often, multiple viral diseases can simultaneously affect a single plant.  Certain herbicide exposures (see UW Plant Disease Facts D0060, Herbicide Damage), nutrient deficiencies or toxicities, high temperature and even insect feeding can also cause similar symptoms.  Proper diagnosis of tobacco mosaic requires lab testing.

Where does tobacco mosaic come from?  Tobacco mosaic is caused by Tobacco mosaic virus (TMV), the first virus ever identified.  Numerous variants (strains) of the virus have subsequently been described.  TMV survives in infected plants (including viable seeds), as well as in debris from these plants.  Plant-based products (most notoriously tobacco products) can harbor the virus.  TMV is very stable and can survive for long periods of time; there are reports of TMV surviving and remaining infectious after 50 years in storage at 40°F.  Because of its stability, TMV can survive on and be picked up from hands, clothing, gardening tools, work surfaces and any other object (e.g., door knobs) that gardeners may handle.

TMV is highly transmissible and is commonly spread by handling infected plants, then healthy plants.  Spread via gardening tools is also very common.  No specific insects spread TMV (the way that aphids spread Cucumber mosaic virus).  However, bees and chewing insects (e.g., grasshoppers) can transmit TMV through casual contact or their feeding as they move from plant to plant.

Leaf growth distortions caused by tobacco mosaic.
Leaf growth distortions caused by tobacco mosaic.

How do I save a plant with tobacco mosaic?  There is no cure for tobacco mosaic.  Once infected, plants remain infected for life, and typically the virus spreads throughout the plant from the point of infection.  Infected plants and any associated debris should be burned (where allowed by local ordinance) or double-bagged and disposed of in a landfill.  DO NOT compost plants with this disease.  Thoroughly decontaminate any items that have come into contact with infected plants or their debris by treating them for a minimum of one minute with:

  • 2.75 tablespoons Alconox® (a lab detergent) plus 2.5 tablespoons sodium lauryl sulfate (SLS), also known as sodium dodecyl sulfate (SDS), in one gallon of water, or
  • 14 dry ounces of trisodium phosphate in one gallon of water.

These ingredients can be ordered on the internet.  If you decide to use SLS (SDS), be sure to wear gloves, safety goggles and a dust mask, and mix the solution in a well-ventilated area as SLS (SDS) is a known skin and eye irritant.  Once treated, rinse items with sufficient water to remove any residues.  Also, thoroughly wash your hands with soap and water, and launder any clothing that you wore while disposing of infected plants and debris.

How do I avoid problems with tobacco mosaic virus in the future?  Inspect plants prior to purchase for any symptoms of tobacco mosaic, and DO NOT buy symptomatic plants.  Purchase seed from a reputable supplier that routinely inspects their seed-producing plants for symptoms of viral (and other) diseases.  If you use tobacco products, DO NOT use them around plants.  Also, wash your hands thoroughly with soap and water prior to handling plants, and consider wearing freshly laundered clothing when gardening.  Finally, decontaminate (as described above) any items that might harbor TMV to help prevent spread.  Even if you do not use tobacco products, routine handwashing and decontamination of gardening tools and other items can help prevent tobacco mosaic from being a problem.

For more information on tobacco mosaic:  Contact your county Extension agent.

February 2021: Deep Freeze Search and Destroy

Magnifying Glass and Hammer IconIn 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.

Pruning tips

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!!

Need help?

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: Bypassing Plant Pathogens: Promoting Tree and Shrub Health Through Proper Pruning

Pruning in the winter can reduce the risk of disease-causing organisms infecting trees and shrubs through pruning cuts.
Pruning in the winter can reduce the risk of disease-causing organisms infecting trees and shrubs through pruning cuts.

Although it doesn’t seem like the optimal time to be gardening, February is actually a great time to be out pruning your trees and shrubs to make them more structurally sound and aesthetically pleasing.

Why prune now? 

Whenever you prune, you create wounds that potentially can serve as entry points for disease-causing fungi and bacteria.  If you prune in the spring and summer (when it’s warmer and often wetter), these organisms are very active and more likely land on fresh pruning cuts and infect.  When the weather is colder and drier (as it tends to be in February in Wisconsin), disease-causing organisms are much less active and the chances of them infecting though pruning cuts is much reduced.

How do I go about pruning? 

Check out University of Wisconsin Garden Facts XHT1013 (Pruning Evergreens), XHT1014 (Pruning Deciduous Trees) and XHT1015 (Pruning Deciduous Shrubs) for pointers on how to prune.  Prune only when it’s dry, and decontaminate pruning tools between cuts (or at a minimum between each tree or shrub) by treating them with 70% alcohol (e.g., rubbing alcohol right out of the bottle, spray disinfectants containing ~70% alcohol) or (in a pinch) 10% bleach.  Decontaminating tools kills off disease-causing organisms that you might pick up as you prune.  Once done pruning, if you’ve used bleach, be sure to thoroughly rinse your tools, and oil them to prevent them from rusting.

By pruning regularly and taking a few simple precautions as you do, you will end up with trees that are beautiful, structurally sound and healthy.

January 2021: Taking a Close Look at 2020

Magnifying Glass Icon2021 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

Tomato Spotted Wilt of Potato

What is tomato spotted wilt?  Tomato spotted wilt is a common viral disease of worldwide distribution that can affect over 1000 plant species.  Economically important hosts include a wide range of vegetables, fruits, field crops, and ornamentals.  Many weeds are also potential hosts.  Tomato spotted wilt is especially important in greenhouse production where it can cause significant economic losses in horticultural and floral crops.  In potato production, tomato spotted wilt is uncommon but of enough concern that routine monitoring for the disease in greenhouse and field settings is warranted.

Leaf spots with concentric rings (left) and stem streaking (right) typical of potato plants due to Tomato spotted wilt. Photos courtesy of G. J. Holmes (left) and Joshua Kunzman (right).
Leaf spots with concentric rings (left) and stem streaking (right) typical of potato plants due to Tomato spotted wilt. Photos courtesy of G. J. Holmes (left) and Joshua Kunzman (right).

What does tomato spotted wilt look like?  Symptoms of tomato spotted wilt vary widely depending on host species, host variety and when during development a plant is infected.  Typical symptoms on leaves include yellow, brown or black ringspots (i.e., ring-like areas of discolored tissue).  On some hosts, the ringspots can form a target-like pattern.  Distorted leaf growth can also be a typical symptom.

On potato leaves, tomato spotted wilt can lead to necrotic (i.e., dead) areas with or without yellow haloes.  These symptoms can resemble symptoms of early blight (see UW Plant Disease Facts D0046, Early Blight).  Black streaks on petioles or stems and branch tip dieback are also common symptoms on potato (as well as other hosts).  Potato tuber symptoms include surface rings or dark patches, and internal discolored patches, rings, spots or flecks.

Where does tomato spotted wilt come from?  Tomato spotted wilt is caused by Tomato spotted wilt virus (TSWV), which is closely related to Impatiens necrotic spot virus (INSV), the cause of impatiens necrotic spot (see UW Plant Disease Facts D0067, Impatiens Necrotic Spot).  TSWV is found in the sap of infected plants and is most commonly spread by thrips (see University of Wisconsin Garden Facts XHT1022, Managing Thrips in Greenhouses).  At least eight thrips species can be involved in TSWV transmission.  Thrips larvae acquire the virus as they feed on infected plants, then transmit the virus as they move to and feed on healthy plants.  Once thrips acquire the virus, they can transmit it for their entire lifespans.  The severity of tomato spotted wilt depends on the size and activity of thrips populations at a location, as well as on the number of infected plants (often weeds, but potentially infected potatoes as well) serving as reservoirs for the virus.  TSWV can also be transmitted mechanically (e.g., on tools used to trim branches or cut potato tubers), but this method of transmission is much less common than transmission by thrips.

How can I save plants with tomato spotted wilt?  There is no known cure for tomato spotted wilt.  Infected plants should be removed and destroyed to eliminate a reservoir for the virus that can subsequently contribute to spread to other plants.  Infected plants can be burned (where allowed by local ordinance), deep buried or hot composted.

Surface and internal tuber symptoms due to Tomato spotted wilt . Photos courtesy of Joshua Kunzman.
Surface and internal tuber symptoms due to Tomato spotted wilt . Photos courtesy of Joshua Kunzman.

How can I avoid problems with tomato spotted wilt in the future?  Prevent introducing TSWV and thrips into your greenhouse by carefully inspecting any new plants for potential problems.  Test suspect plants for TSWV using dipstick tests [available from Agdia, Inc. (] or by submitting a sample to the UW-Madison Plant Disease Diagnostics Clinic (  Remove and dispose of any infected plants.

To prevent new infections via thrips, remove weeds in and outside of greenhouses to eliminate TSWV and thrips reservoirs.  Place 400-mesh screens on vents to limit thrips movement.  Also limit thrips movement on clothing by avoiding colors (e.g., pink, blue, yellow, white or green) that can attract thrips.  Monitor for thrips using blue and yellow sticky cards placed above plants throughout the greenhouse and near doors and vents.  Use chemical and/or biological control products to control thrips.  See University of Wisconsin Garden Facts XHT1022, Managing Thrips in Greenhouses, for details on what products to use and when to use them.  Multiple applications will likely be necessary because thrips eggs are not killed by insecticides.  If you find plants that you suspect are infected with TSWV, remove and dispose of these plants immediately.

To prevent mechanical transmission of TSWV, be sure to routinely decontaminate any items (e.g., pruners, knives, pots, work surfaces) that come into contact with plants by treating them for a minimum of one minute with a solution of one of the following:

  • 2.75 tablespoons Alconox® (a type of lab detergent) plus 2.5 tablespoons sodium dodecyl sulfate (SDS) [also known as sodium lauryl sulfate (SLS)] in one gallon of water, or
  • 14 dry ounces of trisodium phosphate in one gallon of water.

In field settings, tomato spotted wilt does not appear to be a significant problem on potato (although it can be a significant problem on other crops).  Use of resistant or tolerant potato varieties may reduce yield loses, but unfortunately there is little information on which potato varieties are resistant/tolerant to the disease.  Removing symptomatic plants can reduce potential reservoirs of TSWV, but may not be feasible.  More important in field settings may be to keep weeds under control.  Current research suggests that TSWV does not persist long-term in the field unless there are perennial weeds to serve as TSWV reservoirs.

For more information on tomato spotted wilt:  Contact your county Extension agent.

December 2020: Launching into 2021

Rocketship IconI think it’s safe to say that 2020 was a surreal year for everyone, including those of us here at the Plant Disease Diagnostics Clinic.  COVID-19 fundamentally changed how the PDDC conducted business (e.g., check out my March 2020 Monthly Column for details on how the clinic handled sample submissions in 2020).  As I look ahead to 2021 (where COVID-19 is still likely to loom large), I am trying to adapt how the PDDC functions so that I can continue to provide timely and informative services to my clients around the state.  Below are some thoughts on how the PDDC will function in 2021.

Sample submissions

Submitting samples to the PDDC in 2021 will be similar to 2020.  Due to COVID-19 safety concerns, having student hourly help in the clinic likely won’t be possible, and that will limit the number of physical samples that clinic staff (myself and Sue Lueloff, the PDDC molecular diagnostician) will be able to handle.  To manage sample volume, I will be requesting that clients initially submit photos of their plants using the PDDC online submission form.  Whenever possible, I will provide a disease diagnosis based on these photos.  If an accurate diagnosis is not possible, I will then request a follow-up physical sample.  As in 2020, in-person submissons will not be allowed, but you will be able to mail samples to the clinic.  As always, a completed  PDDC sample submission form should accompany each sample.

New in 2021 will be a $20 fee for all digital submissions.  If a follow-up physical sample is needed, this digital fee will be credited to any fees charged for processing the physical sample.  For a complete listing of PDDC fees, check out the PDDC Clinic Fee Schedule.  Note that as a public service, the PDDC will continue to offer free testing for plant diseases of regulatory importance (e.g., Ralstonia wilt, sudden oak death, boxwood blight) and for plant diseases that pose significant risks to Wisconsin’s agricultural economy (e.g., late blight).

Educational Outreach

COVID-19 curtailed PDDC in-person educational outreach in 2020.  In 2021, I will be taking greater advantage of distance education platforms like Zoom to provide education.  I am planning to partner with county Extension educators across Wisconsin to offer monthly plant disease presentations.  The goal is to to give these presentations a local flavor by offering the first opportunity to participate to residents of the host and surrounding counties.  Eventually however, participation will be open to residents statewide.  On January 13, 2021, I will be partnering with Darrin Kimbler of Extension Iron County for the first of these talks.  I’ll be presenting on Early Season Landscape Diseases.  Stay tuned for additional details on this presentation and others in the series.

In addition to these monthly presentations, I am willing to entertain requests to provide other talks throughout the state.  I really LOVE talking to the public about plant diseases, and I have been beefing up my repertoire of talks in anticipation of getting requests.  Check out the Talks for the General Public section of the PDDC website for details on some of my favorite talks.

Also, if there is interest, I am considering sponsoring statewide online plant disease Q&A sessions once a month from May through September.  These sessions will be informal discussions of whatever plant disease-related topics are of interest to whomever chooses to participate.  I’ll be providing additional details on these sessions as we get closer to the 2021 growing season.

Online content

As always, watch for new and revised University of Wisconsin Garden Facts/Farm Fact/Pest Alerts on the PDDC website.  Winter/early spring is definitely a prime time for me to put on my editor’s hat and crank out fact sheets.  I currently have fact sheets on container gardening and pruning tomatoes waiting in the wings.  I will also continue to post web columns (like this one) each month.  In addition, I will be adding a new online feature called Plant Pathology Pointers, which will provide short, timely advise on plant diseases and their management.  Watch for these to start early in January 2021.

Have any suggestions?

Finally, I am always looking for new ideas on how to better serve my clients around the state.  If you have thoughts that you’d like to share, please contact me at  To keep up-to-date on PDDC services and educational resources, feel free to follow the clinic on Twitter or Facebook (@UWPDDC).  Or alternately, put in a request (by emailing to subscribe to the clinic’s listserv, UWPDDCLearn.

Happy Holidays, everyone!  Please be safe, and stay healthy!

January 2021: Saying Bye-Bye to Boxwood Blight: Proper Disposable of Holiday Wreaths

Holiday wreaths containing boxwood sprigs can be a source of the fungus that causes boxwood blight. (Photo courtesy of Steven Shimek, Minnesota Department of Agriculture)
Holiday wreaths containing boxwood sprigs can be a source of the fungus that causes boxwood blight. (Photo courtesy of Steven Shimek, Minnesota Department of Agriculture)

As the holiday season wraps up and you begin to put away your decorations for the year, be aware that certain perishable holiday items need to be disposed of properly.  In particular, be careful of how you dispose of holiday wreaths and other garlands that contain boxwood sprigs.

Why the concern? 

Boxwood-containing holiday decorations have been found to be a potential source for the fungus that causes boxwood blight.  This disease has been devastating boxwood plantings in the East for nearly a decade and was recently (starting in 2018) found in Wisconsin in both nursery stock and in home landscapes plantings.  While documented cases of boxwood blight in Wisconsin have been relatively few, it’s important to do everything possible to reduce spread of the boxwood blight fungus.  Boxwood is THE most valuable woody ornamental produced and used in landscapes in the US.

What can I do? 

When disposing of your holiday wreaths and garlands, check to see if they have boxwood sprigs.  If you are unsure, assume that they do.  Dispose of any suspect materials by burning them if that is allowed in your local municipality.  If burning is not possible, bury the materials at least two feet underground (most likely impractical in Wisconsin in January) or double bag the materials in sturdy garbage bags and have them landfilled.

Have questions?

For more information on boxwood blight and its management, check out University of Wisconsin Garden Facts XHT1265 (Boxwood Blight), available at or

November 2020: Houseplant Horrors

Houseplant IconAs cold temperatures arrive in Wisconsin, many diehard plant enthusiasts are now concentrating on inside gardening activities involving their favorite houseplants.  In some cases, people have been growing their houseplants outdoors for the summer and recently (or perhaps not so recently) brought them back indoors to avoid plummeting fall temperatures.  In other cases, gardeners have visited their local greenhouses to pick out brand new plants to add to their indoor collections.  In either case, disease-causing organisms may be coming along for the ride.  Here are some of the common diseases that you may encounter as you garden indoors this winter.

Powdery mildews

I have talked about powdery mildews before on numerous occasions, but typically in the context of plants growing in outdoor settings.  One of the plants where I see powdery mildews indoors is rosemary.  Rosemary plants are often grown outdoors for the summer where they become infected and then begin to show severe symptoms once they are brought into the kitchen for the winter.  Higher humidity from cooking and dishwashing most likely contributes to development of the disease.  Relatively uniform, white, powdery growth on leaves is pretty typical for the disease on this host.  Management often involves cutting the plant back almost to the ground and allowing regrowth, in addition to placing the plant in a new location with lower humidity.  I also occasionally see powdery mildew on jade plants where typical symptoms are dark, sunken spots on leaves.  It’s only when you look carefully that you see a subtle network of white hyphae (i.e., fungal threads) of a powdery mildew fungus in the center of these dark areas.  Plucking off the infected leaves (typically there aren’t a lot) often takes care of the problem.

Root rots

Virtually any plant grown indoors may end up with a root rot problem.  Plants taken outside for the summer can become infected if soil from an outdoor garden accidentally gets splashed (e.g., via a hard rain) into a pot.  Garden soils typically contain at least some level of root rot pathogens.  Greenhouse-grown plants can harbor root rot pathogens as well, as these organims can survive in production facilities and retail greenhouses for years.  Plants may not show symptoms initially, but once brought into a home setting where owners tend to overwater, root rot pathogens can become very active.  They will cause root decay below ground and wilting above ground.  Root rots can eventually kill plants.  Often by the time indoor gardeners notice root rot symptoms, damage is so severe that discarding plants is the best option.  Plastic pots should be thrown away with the plants  Clay or ceramic pots are salvageable if you wash the pots well (to remove any remaining soil), then soak them for roughly 30 minutes in 10% bleach to kill off root rot pathogens.  Whenever you decontaminate pots, be sure to rinse thoroughly after treatment to remove bleach residues.  To minimze future root rot issues, cut back on watering, providing enough water to keep plants happy and growing vigorously, but not so much that root rot pathogens become active.

Bacterial leaf spots and blights

I see bacterial leaf spots and blights on a wide range of plants grown indoors including geraniums (brought in from outdoors to overwinter), poinsettias (purchased for the holiday season), and more traditional houseplants such as dieffenbachia, Chinese evergreen, elephant ear and philodendron (all in the Arum family).  These diseases are typically caused by bacteria in the genus Xanthomonas.  Plants initially may harbor sub-symptomatic levels of these bacteria, but eventually high enough populations develop to cause disease.  Typical symptoms can include angular, necrotic leaf spots (i.e., dead areas where veins border the dead tissue leading to very straight edges) or dead areas along leaf margins.  Dead tissue is typically surrounded by a distinct yellow halo.  Bacterial diseases are notoriously hard to manage.  You can remove symptomatic leaves, but healthy leaves typically still harbor the pathogen.  These healthy leaves often eventually development symptoms.  When bacterial diseases are a problem, you are typically left with the choice of living with the disease or throwing the plant out.  As I outlined above in my discussion of root rots, I recommend throwing out plastic pots and decontaminating clay and ceramic pots by bleaching them.

Need help or more information?

Whether I’m working with outdoor or indoor plants, there’s never a dull moment at the PDDC.  If you are having plant disease problems of any kind and need help diagnosing these problems, feel free to contact me.  For the PDDC’s current policy on sample submission, including submission of digital photos, check out the following link.  As always, be sure to check out the PDDC website for timely information on plant diseases.  Also, feel free to follow the clinic on Twitter or Facebook (@UWPDDC) to receive timely PDDC updates.  Or alternately, put in a request to subscribe to the clinic’s new listserv (UWPDDCLearn) by emailing

As always, hang in there, be safe, and stay healthy!