Category Archives: Monthly Column

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

December 2020: Launching into 2021

RocketshipI 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.

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!

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 mildewsI 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 rotsVirtually 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.

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!

October 2020: Autumn Angst

October AngstAutumn has arrived in Wisconsin, with days shortening and temperatures cooling.  As gardeners begin putting their gardens to bed for the winter, they have been coming across what they consider alarming developments on their herbaceous ornamentals, deciduous trees and evergreens.  Luckily, most of what they have been seeing is innocuous and non-life-threatening.  Here is a rundown of some of the issues that I’ve been hearing about.

Herbaceous ornamentals.  Many clients have commented on an uptick is what appear to be foliar diseases on a wide range of herbaceous perennials.  While I have certainly seen a number of fungal and bacterial diseases on herbaceous plants over the course of the summer, much of the dieback I have been seeing in September and October has been natural dieback as plant start to go dormant for the year.  As the days shorten, perennial plants start moving nutrients from leaves and into crowns and roots where these nutrients can be stored for the winter.  Leaves yellow and brown as a consequence of this nutrient movement.  These changes can occur quickly and look very dramatic and disease-like, but this is normal for this time of the year.

From a disease standpoint, I like to point out that as plants go into “winter mode” at this time of the year, pathogens do as well.  For plants with phytoplasma diseases like aster yellows, as these plants transport nutrients into their roots and crowns, they also concentrate phytoplasmas in these tissues, where the organisms overwinter.  Powdery mildew fungi often overwinter as hyphae (i.e., fungal threads) in the overwintering buds of perennial plant hosts.  Above-ground plant debris is another place where a variety of plant pathogens can survive the winter.  For that reason, I routinely emphasize the importance of garden cleanup to remove these materials and eliminate a source of fungal spores that can infect next year’s plants.  Most gardeners traditionally do cleanup in the fall, but there can be reasons (e.g., improving winter appeal of a garden, leaving overwintering sites for important plant pollinators) for doing this cleanup in the spring.  Cleaning up before new leaves emerge in the spring is critical however, for good good disease control.  Burning (where allowed), burying or hot composting are typical ways of disposing of old plant debris.

Deciduous trees and shrubs.  Leaf diseases on trees and shrubs have been quite prevalent this past summer, but as gardeners have begun to rake leaves, one particular disease, tar spot, has been generating a number of questions for the PDDC.  Tar spot is a fungal disease, characterized by formation of black, tarry spots on leaves of maples.  These spots appear to be more visible (and thus disconcerting to gardeners) in the fall, most likely because the spots are more easily visible against leaves that have turned bright fall red or yellow, compared to the dark green of leaves in the summer.  There are two variations of tar spot that occur in Wisconsin.  On native silver and red maples, the tar spot fungus is Rhytisma americanum, which causes large, solid black, raised areas that look as though someone left a thumbprint in the middle of the tarry spot.  On Norway maple (a European maple species), the tar spot fungus is Rhytisma acerinum, a non-native fungus that causes large, diffuse (spotted-looking), flat, black areas.  Both types of tar spot are cosmetic.  Good cleanup of the infected leaves (as described above) should provide adequate control of the disease.

Evergreens.  I have recently been getting numerous questions about yellow or orange/brown needles on white pines and arborvitaes (as well as occasionally on other evergreens).  The discolored needles are typically very vibrant in color and are, for the most part, interior, older needles.  The timing of the color change (September into October), as well as the intense color and location of the affected needles, points to this being something called seasonal needle drop.  Seasonal needle drop is a natural needle color change and loss that evergreens can go through in the fall.  It is equivalent to the color change that we are used to seeing on broad-leaved trees (like oaks and maples) every autumn.  While sometimes dramatic, seasonal needle drop is normal and not detrimental to trees and shrubs.

Are you seeing what you consider alarming developments in your garden or landscape?  Hopefully not, but if you are, and need help diagnosing these problems, feel free to contact the PDDC.  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

Hang in there, be safe, and stay healthy everyone!

September 2020: Perennial Planting Problems

Crossed ShovelsBecause of the COVID-19 pandemic, I have been doing a large number of digital diangoses this summer.  While I have always felt that working with physical samples is the best way to diagnose disease issues, there has been one area where having access to photos has been of great benefit:  diagnosing nondisease issues causing general decline and dieback of woody ornamentals, particularly deciduous trees.  Seeing so many photos has really educated me in just how many tree issues have nothing to do with diseases, but everything to do with improper plant selection and planting.  This month, I would like to share some of what I have learned after seeing this plethora of photos.

Plant the right tree in the right location.  Many tree problems that I have diagnosed this summer have to do with use of trees that are not well-adapted to the sites where they are planted.  For trees to be successful, I can’t emphasize enough how critical it is that the site conditions (e.g., soil pH, light, temperature, moisture) match with the conditions preferred by the particular tree that is to be grown at the site.  I constantly see trees such as pin oaks and red maples planted in locations where the soil pH is too high, leading to problems with chlorosis.  Similarly, I see trees like pagoda dogwood (an understory tree that prefers shady, cool, moist condtions) planted in the middle of yards in full sun, with grass growing up to the trunk.  The stress from excessive sun and heat, as well as water stress from competition with turf, makes pagoda dogwood prone to golden canker, which can eventually kill the tree.

Start small.  People seem to want an instant “finished” landscape filled with mature, full-sized trees.  While planting large trees is easy to do (or at least easy to have done professionally), keeping these trees alive after planting is another issue.  I can’t tell you how many times I have chatted with folks who have planted large trees, only to have them die.  They then replace these trees with other full-sized trees, only to have these replacements die as well.  And on and on and on.  What people don’t realize is that when a tree is dug at a nursery, a large percentage of its root system (up to 60%) is left behind.  This root loss puts a tree under incredible stress.  The bigger the tree is, the biggerer the stress and the lower the probability that the tree will survive transplanting.  Personally, I don’t like transplanting trees much over four feet tall.  I have found that smaller trees survive better.  Often by starting small, you can end up with a well-established, large tree in the same time period as transplanting and replacing multiple, full-sized trees.

Prepare transplants properly.  Many people end up buying balled and burlaped trees, and a big mistake they make is to not remove the burlap, underlying wire basket and wires/cords/strings on these plants.  Burlap and wire baskets do not break down rapidly (as is often the claim) and can interfere with proper root growth.  Burlap exposed above ground can wick water away from trees, leading to water stress.  Wires, cords and strings can girdle trunks, eventually killing trees.

Plant at the correct depth.  I have seen numerous photos of trees that have been planted too deeply.  The trunks of these trees look like telephone poles as they enter the ground.  Ideally, the root flare (i.e., the part of the trunk that widens to form the roots) should be visible just above the soil line.  With many balled and burlaped trees, removing soil from the top of the root ball will be necessary to expose the root flare.

Overly deep planting increases the likelihood of girdling roots.  These are roots that instead of growing outward from the trunk, grow around the trunk.  If girdling roots form and are left in place, the trunk will eventually come into contact with these roots, and the roots will compress the water-conducting tissue under the trunk’s bark.  This will inhibit water movement from the roots into the branches, leading to canopy thinning, branch dieback and tree decline.  Stress from girdling roots can also make trees (particularly maples) more prone to frost cracks, the vertical cracks that are often found on the southeast sides of tree trunks.  Frost cracks can provide entry points for wood rot fungi that do additional damage and structurally weaken trees, making them more prone to snapping off or blowing over in high winds.

Personally, I like to plant bare-root trees, because I think they are easier to plant properly.  I can easily see the root flare (and get it positioned properly), and I can orient roots at planting to prevent formation of girdling roots.

Mulch properly.  I often see trees with grass growing right up to the trunk.  Grass is very efficient at taking up water and preventing it from getting to trees.  I suggest removing turf out to the drip line of a tree (i.e., the edge of where the branches extend) and mulching this area with a high quality mulch (e.g., shredded oak bark mulch or red cedar mulch).  Use one to two inches of mulch if you have a heavier (e.g., clay) soil, and three to four inches if you have a lighter (e.g., sandy) soil.  Keep the mulch about four inches away from the trunk.

Water, water, water.  Homeowners often water new transplants for a few weeks, but then believe the trees are well-established enough that they no longer need to water.  In reality, new transplants need LOTS of water for a LONG time.  I typically recommend that new transplants (anything planted within roughly the past three years, maybe even long for larger transplants) receive about two inches of water per week from the time they bud out in the spring, through the summer and into the fall up until they start to turn their normal fall color (for deciduous trees) or until the ground freezes or there is a significant snowfall (for evergreens).  If Mother Nature doesn’t cooperate, I suggest watering at the drip lines of trees using a drip or soaker hose.

Ask for help.  Hopefully, the pointers above will help you successfully transplant trees and keep them healthy and vigorous.  If you run into disease problems or other issues as you grow your trees, and need help diagnosing these problems (or problems of any other kind of plant for that matter), feel free to contact the PDDC.  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


Hang in there, be safe, and stay healthy everyone!

August 2020: Green Grow the Lilacs – NOT!

LilacLilacs are one of my favorite spring-flowering shrubs.  When sited properly (in full sun) and pruned/thinned regularly, they reliably produce a bounty of beautiful blooms each spring in a myriad of colors.  And oh that scent!  Is there really anything that smells better than the scent of lilacs in full bloom?

In most years, lilacs tend to be relatively disease free.  However, this year, I have seen a fair number of disease issues on lilacs that have ranged from cosmetic to lethal.  Here’s a round up of common lilac diseases to watch for.

Powdery MildewLilacs are the poster child (in my mind) for this disease.  They routinely exhibit the white, powdery growth on their leaves that is characteristic of powdery mildew, but tend to show no adverse effects from the disease.  The best management strategy for powdery mildew on lilac is to develop a healthy ability to ignore the disease during the growing season.  Then in the fall, be sure to collect up the leaf debris from your lilacs, and burn (where allowed), bury or compost the leaf debris.  Even with diligent leaf clean up, don’t expect a powdery mildew-free lilac next year.  The pathogen has a sneaky way of surviving in the overwintering buds of the plant.

Bacterial BlightThis disease tends to be more of an issue early in the growing season, often just as lilacs are beginning to leaf out.  The bacterium that causes bacterial blight (Pseudomonas syringae pv. syringae) often is part of the normal microflora that lives on lilac leaves, occurring at a low enough level not to cause any disease issues.  If the population increases however (often in response to wetter weather), the bacterium can lead to leaf tissue necrosis (i.e., death).  The dead areas (sometimes in discrete spots, other times in larger blotches) typically have yellow halos.  To complicate matters, Pseudomonas syringae pv. syringae has a protein in one of its membranes that mimics an ice crystal.  So, if the bacterium is present on lilacs in high numbers and a cold snap occurs, plants tend to be more prone to cold injury, particularly branch tip dieback.  Management of bacterial blight includes good fall clean up (as described above) and pruning in the case of branch dieback.  Prune four to six inches below obviously dead areas on branches.  Be sure to prune only when it is dry, and be sure to disinfest your pruning tools between cuts by dipping them in 10% bleach or (even better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants can be used as a source of alcohol as well.  Just be sure to check the ingredient list of the disinfectant that you select to make sure it contains roughly 70% alcohol.  If you 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 the use of bleach.  You can dispose of branches by burning (where allowed) or burying them.

Septoria Leaf Spot:  This has been the number one disease this season on lilac (and is towards the top of my disease list for all plants).  I have received numerous inquiries about lilacs with leaves that have partially or fully browned starting at the bottom of the shrub and working up the plant.  The culprit in this browning appears to be a species of Septoria, a fungus related to (but distinct from) the organism that causes Septoria leaf spot of tomato.  While the browning caused by this disease is quite dramatic, the disease is not lethal.  If you look carefully at the branches with symptomatic leaves, you should be able to find healthy leaf buds at the base of the petioles of this year’s leaves.  These buds are ready to sprout next spring to produce a new crop of leaves.  Good fall clean up (see above) is the place to start in managing Septoria leaf spot.  Routine pruning/thinning to open the canopy and promote rapid drying of leaves when they get wet will also help keep this disease at bay.  If we continue to see wetter summers, this disease may become a chronic, severe problem, and use of fungicides to manage the disease may become necessary.  However, I suspect we will cycle into a series of drier summers over the next couple of years, and if we do, I would expect the serverity of this disease to decrease, and no fungicide treatments will be needed.

Lilac Witches’ Broom:  This disease is caused by the same bacterium-like organism (called a phytoplasma) that causes ash yellows.  The phytoplasma is introduced into a lilac’s food-conducting tissue (i.e., phloem) by leafhoppers.  Once in the plant, the phytoplasma leads to a yellowing of foliage, stunting of the entire plant, and over-production of lateral branches (i.e., brooming).  Infected plants typically decline and die over a period of several years,.  There is no way to get rid of phytoplasmas, so removal and replacement of infected shrubs is the management strategy of choice.

Verticillium WiltVerticillium wilt is the most rapidly lethal of the lilac diseases, and the disease that I most commonly see on Japanese tree lilac.  The fungus that causes this disease (tyically Verticillium dahliae for lilac) is found in the soil and infects plants through the roots.  It colonizes the water-conducting tissue (i.e., xylem) inside the plant and blocks it off.  This prevents water movement from the roots to the branches, leading to wilting.  Typically the wilting starts in a single branches.  As the disease progresses, a localized cluster of branches wilts, and eventually, the entire tree/shrub dies.  Verticillium dahliae has a wide host range, so if you have a lilac with this disease, you need to be very careful about what you choose as a replacement.  Conifers are your best best, although a limited selection of deciduous trees and shrubs (tricolor beech anyone?) ban be used.

NEED HELP?  If you need help diagnosing any of the lilac diseases described above (or diseases of any other kind of plant for that matter), feel free to contact the PDDC.  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

Hang in there, be safe, and stay healthy everyone!

P.S.:  Bonus points if you recognize where this month’s article title comes from.

July 2020: Elderberry Whine – Rust, Rust Everywhere!

Rust Fungi2020 has been a banner year for rust diseases.  I can’t remember a year where I have seen so many rusts on so many different plants:  apple, ash, buckhorn, crabapple, elderberry, hawthorn, Jack-in-the-pulpit, juniper (including red cedar), mayapple, raspberry, spinach, spruce, violet and white pine.  Rust diseases are caused by a closely related group of fungi, with each fungus in the group highly adapted to causing disease on specific plants.  These fungi are incredibly complex.  They can produce from two to five different types of spores depending on the particular rust fungus, and their life cycles fall into two major categories:  autoecious and heteroecious.

Autoecious rust fungi are able to complete their entire life cycles on a single plant host.  A classic rust disease that falls into this category is rose rust.  This rust produces one of my favorite spores, a teliospore (i.e., a resting spore) that looks like a corndog.  While I have yet to see rose rust in 2020, I have seen several other autoecious rusts including mayapple rust, Jack-in-the-pulpit rust and orange rust of black raspberry.  Orange rust is particularly destructive.  Infected plants become stunted and spindly, and they do not produce flowers or fruit.  Removal and destruction of infected plants is the only effective management strategy for this disease.

Heteroecious rust fungi require two different plants in order to complete their life cycles.  The most common heteroecious rusts that I see every year are the Gymnosporangium rusts.  These rusts spend part of their life cycles on junipers (particularly red cedar) and the remainders of their life cycles on certain woody rosaceous plants such as apple, crabapple and hawthorn.  I actually think the symptoms that these diseases cause (i.e., bright yellow leaf spots on rosaceous hosts) are quite pretty, giving the trees some additional aesthetic appeal after their flowers have faded and fallen for the year.  Other heteroecious rusts that I have seen this year include white pine blister rust (hosts:  white pine and gooseberries/currants) and crown rust (hosts:  buckthorn and turfgrass).

I have to give a special shout out to elderberry rust, a heteroecious rust that alternates between elderberry and sedges.  Symptoms on elderberry are some of the most awesome that I get to see of any disease.  Infection by the rust fungus causes formation of a swollen areas (called galls), as well as growth distortions.  The galls eventually produce a bright yellow spores (these are the spores that infect the alternate sedge host), making the galls look like banana slugs.  TOTALLY COOL!!

I really can’t wait to see how many more rusts I can check off my plant disease bucket list this summer.  If you see any rust diseases (or any cool diseases in general), feel free to email me photos.

NEED HELP?  If you need help diagnosing plant diseases, feel free to contact the PDDC.  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

Hang in there, be safe, and stay healthy everyone!