Category Archives: Monthly Column

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!

June 2020: Cherries and Peaches and Plums, Oh My!

FruitThese days, digital photos of diseased plants are arriving fast and furious in the Plant Disease Diagnostics Clinic (PDDC) email inbox.  While clients have been having problems with many different types of plants, I have been receiving a large number of photos of stone fruits.  Ornamental and fruit-bearing varieties of cherries, plums and peaches seem to be having a rather tough year this year.  Several of the diseases that adversely affect stone fruits are fairly straightforward to diagnose by photo.  Below are the common diseases of cherries, plums, and peaches that I have been seeing thus far this season.

Peach Leaf curlPEACH LEAF CURLThe name of this fungal disease is quite descriptive.  Infected peach leaves become curled and puckered, and often have a combination of a green, creamy-white and fuchsia color.  Peach leaf curl seems relatively cosmetic, but repeated occurrences of the disease over time can reduce the quantity and quality of fruit.  Typically with leaf diseases, I recommend good fall clean up of leaves for management.  Unfortunately, this strategy does not work for pearch leaf curl, as the pathogen (Taphrina deformans) overwinters on peach branches.  Management of the disease relies of use of fungicides (e.g., copper-containing products) applied either after leaf drop in the fall or prior to bud swell in the spring.

Peach Leaf curlBLACK KNOTThis disease, which is specific to plants in the genus Prunus (e.g., cherries and plums), is what I affectionately refer to as “poop-on-a-stick”.  It really does look as though some pesky animal has defecated on the branches of affected trees and shrubs.  The fungal pathogen involved (Apiosporina morbosa) induces formation of black, gnarly swollen areas (called galls or knots) on infected branches.

Unfortunately, once the knots form, the only method of management is to remove the growths by pruning.  For fungal diseases, I typically recommend pruning roughly four to six inches below the diseased area.  When pruning, be sure to decontaminate tools between cuts by treating them for at least 30 seconds with 70% alcohol (e.g., straight rubbing alcohol), a commercial disinfectant or 10% bleach.  Spray disinfectants work as well (as long as they contain roughly 70% active ingredient).  Just spray tools until they drip and then allow them to air dry.  When using bleach, be sure to rinse tools completely after pruning and then oil them to prevent rusting.  Dispose of black knot galls by burning (where allowed) or burying them.  In some situations, there will be so many galls in a tree that my recommendation is what I call “basal pruning” or “a single pruning cut at the ground level”.  You remove the affected trees and replace it with non-susceptible plants.

Bacterial CankerBACTERIAL CANKERProbably the most serious of the diseases that I have seen on stone fruits this year is this one.  The pathogens involved (two variants, called pathovars, of the bacterium Pseudomonas syrinage) infect branches causing branch dieback.  From infected areas, sap emerges and gelatinizes on branch surfaces.  For bacterial canker, timely pruning of diseased branches is critical for management, as the pathogens can rapidly colonize infected branches and move into the main trunks of trees where they can girdle the trunks, killing the trees.

Prune at least 12 inches below visible dieback on affected branches and again dispose of branches by burning (where allowed) or burying them.  Decontaminate tools as described above for black knot.  When bacterial canker occurs in main trunks, tree removal and replacement is the only real option.


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!

May 2020: Toxic Plant Disease Olympics

Toxic Plant OlympicsMost days, I really love my job.  I am well-known for my love of plant disesaes and I tend to get giddy when plant samples arrive at the PDDC.  There is always the possibility with each new package that I will become reacquainted with an old disease friend (e.g., cedar-apple rust) or that I will be introduced to new disease friend that I’ve been wanting to meet for years (e.g., zonate leaf spot).

Others days, I open a package and my shoulders sag, and I let out sigh.  This most often occurs when the sample potentially has a disease/pathogen that is regulated by either the state or federal government.  These diseases are often fascinating in and of themselves, but the paperwork involved with their diagnosis can be soul crushing.  Right now in Wisconsin, there are three diseases on my radar that fall into this dreaded category.  This month’s web article is devoted to these medal-winning diseases that keep me up at night.

Leaf spots typical of boxwood blight on boxwood sprigs in a holiday wreath. (Photo courtesy Purdue PPDL)
Leaf spots typical of boxwood blight on boxwood sprigs in a holiday wreath. (Photo courtesy Purdue PPDL)

In the scheme of things, boxwood blight is not bad as regulated diseases go.  Boxwood blight was introduced into Wisconsin in 2018 through contaminated nursery stock and is regulated at the state level.  The Wisconsin Department of Agriculture, Trade and Consumer Protection (WI DATCP) monitors boxwood blight’s spread and is currently attempting to eradicate the disease as it rears its ugly head, particularly in nurseries.  I first encountered boxwood blight last summer when a landscape maintenance professional submitted a sample from a boxwood shrub planted at a Madison area residence.  Once I made my diagnosis, I immediately contacted WI DATCP so that they could follow up with the homeowner regarding containment and eradication.

Boxwood blight typically first shows up as distinct spots appearing on leaves in the lower canopy of boxwood shrubs.  Most boxwood varieties are very susceptible to the disease and rapidly defoliate and die.  Pachysandra, a common ground cover, is also susceptible.  If you want to see how devastating this disease can be, do an internet search on “boxwood blight” and your favorite state along the eastern seaboard (e.g., North Carolina, Virginia, Maryland).  You will find photos of landscapes where every boxwood has been wiped out.  For additional details on this disease, check out our boxwood blight pest alert.

Rapid wilting and die back of branch tips can be a symptom of ramorum dieback.
Rapid wilting and die back of branch tips can be a symptom of ramorum dieback.

Sudden oak death (I prefer the name Ramorum blight) was first described in California in the 1990’s and has killed millions of oaks in that state.  Because of its destructive potential, the disease/pathogen is regulated at the federal level by the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS).  There has been movement of the disease/pathogen over the years into other states (again through movement of nursery materials), and in 2019, WI DATCP inspectors found the disease on an azalea in a nursery in Wausau.  Unfortunately, azaleas from the same supplier were distributed to nurseries around Wisconsin, and many were sold to homeowners before WI DATCP became aware of potential problems.  Another possible introduction of the disease/pathogen on red ‘Double Knockout’ roses also occurred in 2019.  I have not yet had this disease arrive in my lab, but I did prescreen several samples for the disease last summer.  Based on my preliminary testing, I forwarded two suspicious samples to a second lab for another round of testing.  If those samples had tested positive at this second lab (luckily they didn’t), they would have been sent to yet another lab (a USDA APHIS facility) for a final round of testing.

Unfortunately the symptoms of sudden oak death/Ramorum blight are not readily distinguishable from other diseases.  Branch dieback, nondescript leaf browning and eventually plant death can be typical symptoms.  See our sudden oak death pest alert for additional details on this diease.

Yellowing and wilting characteristic of Ralstonia wilt. Photo courtesy of WI DATCP
Yellowing and wilting characteristic of Ralstonia wilt. Photo courtesy of WI DATCP

This is the granddaddy of regulated diseases that I have encountered over the years.  One variant of the bacterium that causes this disease (Ralstonia solanacearum race 3, biovar 2) causes a devastating disease of potatoes (called brown rot) and was classified in the early 2000’s as a select agent by the federal government.  This means that the pathogen is recognized as having the potential to be weaponized and used in bioterrorism attacks against US agriculture.  Ralstonia wilt was first detected on geraniums in Wisconsin (on a plant submitted to the PDDC) in 1999 with additional introductions on this crop through 2004.  In March of 2020, the disease/pathogen was detected after a 16 year absence, this time on Fantasia® ‘Pink Flare’ geraniums in Michigan.  This variety of geranium was also distributed to greenhouses in 38 other states including Wisconsin.  USDA APHIS is currently leading efforts to eradicate potentially contaminated plants and to decontaminate affected greenhouses.  The PDDC has the capacity to detect the bacterial species involved in the disease (but not the specific race and biovar) using the plant disease equivalent of a home pregnancy test.  Suspect samples must be forwarded to USDA APHIS labs for a final confirmation of race/biovar.

A major problem with Ralstonia wilt is that plants can be contaminated with the bacterium without showing symptoms.  Eventually, in susceptible hosts like geranium, the bacterium colonizes the plant’s water-conducting tissue and blocks water movement, leading to leaf wilting and yellowing.  Sometimes, only part of the plant will wilt at first, but eventually the disease is lethal.  For more on this disease, check out our Ralstonia wilt pest alert.

If you believe you are seeing ANY of the diseases described in this article, please contact me IMMEDIATELY at (608) 262-2863 or  We will need to make arrangements for appropriate testing.  And also, as always, feel free to follow me on Twitter or Facebook (@UWPDDC) to receive updates on these and other diseases.

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

April 2020: What Do I Have to Do to Get My Picture Took?

Camera IconThe impact of COVID-19 on the Plant Disease Diagnostics Clinic (PDDC) and the services that the clinic provides has continued to evolve.  As of March 28, 2020, and until further notice, the PDDC is no longer accepting physical samples for diagnoses, and clinic staff members are restricted to working from home.  Nonetheless, I and the other staff at the PDDC are committed to the providing the best possible services that we can to our clients given these restrictions.

While submitting physical samples is not possible, the PDDC is still accpting digital photos for diagnosis.  In this month’s PDDC web article, I would like to provide pointers on what sorts of pictures you should take to optimze the possibility I will be able to make as accurate a diagnosis as possible, options for getting the photos to the PDDC and what you can expect after your submission.


  • Take LOTS of pictures. Err on the side of taking too many photos.  The more photos you send me, the more likely I will see something that will lead me to an accurate diagnosis.
  • Take a variety of pictures. These should include:
    • Landscape shots. These sorts of photos show how your diseased plant is situated in your yard relative to other plants, buildings, driveways, sidewalks, etc.  They can often provide clues on environmental factors that may be contributing to the disease problem you are seeing.
    • Whole plant shots. These photos will show the distribution of symptoms on the plant.  Are the symptoms in just one area?  Are they scattered throughout the plant?  Is the entire plant affected?
    • Close up shots. Take pictures of affected leaves (both tops and bottoms), branches, roots, fruits or any other affected plant part.  I need to look for symptoms (e.g., leaf spots, cankers, discolorations, growth distortions, etc.), as well as signs of pathogens (e.g., fungal sporulation) that can help me with my diagnosis.
  • Take high quality pictures. This means taking:
    • High resolution photos. The higher the resolution, the better I will be able to increase the size of the picture and still see lots of detail.  The more detail I can see, the more likely I will be able to figure out what’s going on.
    • Crisp, non-fuzzy photos. If a picture is fuzzy, I won’t be able to see much or tell you much.


  • This is typically the easiest way to submit photos.  Use for emailing.
  • The PDDC website. You will now find links on the PPDC website (on the main page, “Sample Collection and Submission” page and “Service and Fees” page) to a “Digital Diagnosis” form.  Fill out the form, upload your photos and click on “Submit”.
  • Text message. If email or submission via the PDDC website are not options for you, feel free to call me at (608) 262-2863.  After we talk, if you need to submit photos, I can provide you a cell phone number where you can send your photos via text message.


  • A prompt response. I try to respond to phone and email inquiries within 24 hours.
  • The best diagnosis that I can provide. I have always maintained that looking at photos is not the best way to diagnose plant diseases.  That said, I will provide you with my best interpretation of what may be happening to your plants based on photos that you provide.
  • Management recommendations. Where possible, I will provide suggestions on how you can mitigate the problem that you are seeing and prevent it from happening in the future.
  • Any follow-up you may need. My door (well, actually my email and phone at this point) is always open if you need additional consultations after I provide my diagnosis.

It is my commitment to provide you with the best possible service under our current circumstances.  Please do not hesitate to contact me at (608) 262-2863 or if you think I might be able to help you.  Also check out the PPDC website for online resources (e.g., University of Wisconsin Garden Facts, Wisconsin Disease Almanac, monthly web articles).  And feel free to follow my clinic updates on Twitter or Facebook (@UWPDDC).

Be safe and stay healthy everyone! 

P.S.:  Bonus points if you know the origin of the title of this month’s article.

March 2020: Plant Disease Diagnostics in the Time of COVID-19

LAST UPDATE:  June 18, 2021

Over the past week, reports of COVID-19 in Wisconsin have increased dramatically.
To help keep PDDC customers and staff safe, I am making changes to the way the PDDC delivers services.
Below is a list of what will change, and what will remain the same.
I will update this list as UW-Madison and PDDC policies change.

Update 6/18/2021


The PDDC continues to accept walk-in samples delivered to the shelving unit in the hallway outside the clinic (Rm. 183 Russell Labs).  However, clinic staff will not be available to interact with clients.  Anyone entering Russell Labs who has not been vaccinated for COVID-19 is required to wear a mask and practice social distancing (6 ft. between people).  For additional details on the procedure for submitting digital samples online and physical samples (either in-person or via mail), please see the 12/28/20 update below.

Update 4/30/2021


Starting May 10, 2021, the PDDC will once again be accepting walk-in samples delivered to the shelving unit in the hallway outside the clinic (Rm. 183 Russell Labs).  However, due to continued concerns about COVID-19, clinic staff will not be available to interact with clients.  Also, anyone entering Russell Labs is required to wear a mask and practice social distancing (6 ft. between people).  For additional details on the procedure for submitting digital samples online and physical samples (either in-person or via mail), please see the 12/28/20 update below.

Update 12/28/2020


As we enter into 2021, COVID-19 continues to be of concern.  Therefore, physical sample submissions to the PDDC will continue to follow the same protocol and prioritization as outlined in my update of April 24, 2020 (see below).   The only changes for the new year are that the PDDC will now charge a $20 fee for digital diagnostics, and there will be a $5 increase in the base fee for physical samples.  For a complete fee schedule, click here.  To submit photos for a digital diagnosis, click here.  To access the PDDC submission form to be included with physical samples, click here.  

Update 4/24/2020


The PDDC has been authorized to begin accepting physical samples  again.  However, clinic staffing and hours will be limited, and the number of samples that the clinic will be able to accommodate will be severely restricted. 

Before submitting a sample, you must contact the PDDC using the form on this page , by emailing or by calling (608) 262-2863.  If you use the clinic online form or email the clinic, please include photos whenever possible.  Based on this initial contact, clinic staff will determine if a digital diagnosis is possible or if a physical sample submission is needed.

Even with prescreening, the PDDC will likely not be able to accommodate all samples that would normally require submission of a physical samples.  As needed, physical sample submissions will be prioritized as follows:

  • Commercial production food and agriculture-related samples (e.g., vegetables, fruits, field and forage crops);
  • Commercial/homeowner samples of regulatory importance (e.g., late blight, boxwood blight);
  • Commercial production, non-food samples (e.g., nursery, greenhouse samples);
  • Homeowner food samples (e.g., vegetables, fruits);
  • Commercial/homeowner non-production, non-food samples (e.g., trees, shrubs, herbaceous ornamentals).

Certain regulatory samples (e.g., sudden oak death/Ramorum blight) will be redirected to the Plant Industry Lab at WI DATCP.

Digital diagnoses will continue to be provided free of charge.   Normal clinic fees will apply to any physical samples submitted to the PDDC.

Update 3/28/2020
Until further notice, the PDDC will not be accepting any physical samples for diagnosis.  Customers are welcome to continue submitting questions and/or digital photos to the PDDC by submitting the form on this page.   

Update 3/18/2020

CALS administration has closed the Russell Labs building to visitors. PDDC customers may only submit samples by mail.

Plant Disease Diagnostics:

  • We will still accept samples for diagnosis.
  • You can still submit your diseased plants by mail.
  • The building is locked, so you can no longer drop off samples in person.
  • I am discontinuing in-person consultations about samples.
  • I will continue to mail out hard copies of reports and invoices.  If you prefer I email your results, please note this on your submission form.


  • All in-person PDDC presentations are cancelled until further notice.
  • Instead, I can provide presentations on a number of topics via remote learning.
    • Click here to learn more about the topics offered.
    • Call me (608-262-2863) or email me ( to learn more about possible remote learning options (e.g. teleconferencing/ videoconferencing, etc.).

Online Resources:


  • In-person consultations are discontinued until further notice.
  • Phone (608-262-2863) and email ( channels are still open.  Digital photos showing the disease problems are welcome, please attach them to your email.  I may not be able to make a formal diagnosis from your photos, but I will do my best.

Most of All…

Stay safe everyone!  I wish you well in the challenging days ahead!!