Category Archives: Monthly Column

May: Heinous Hitchhikers – Purchased Plants as Pathogen Providers

As we get into May, many gardeners begin thinking about buying perennials to replace plants that have died over the winter, or annuals to fill decorative pots and hanging baskets.  Being the optimist that I am (NOT), whenever I’m visiting my local nursery or garden center, I’m always evaluating plants as potential sources of plant pathogens.  For me, having diseased plants can be good (demo plants for classes and workshops, anyone?).  For sane, rationale gardeners however, avoiding potentially diseased plants is a must.  Here on some pointers on what you can do to try to minimize the likelihood that you will bring home unwanted guests as you garden.

  • Buy plants from a reputable business. Most businesses want to sell a good product.  The livelihood of a nursery or greenhouse depends on the quality of the products that it sells.  If a business sells poor plant materials, this reputation will get around and the business will likely not last long.  One way to decide on where to shop is to check with friends or colleagues on where they have purchased high quality plants in the past.  Word of mouth is often the best recommendation for a business.  That said, keep in mind that sometimes even the best, most conscientious plant producers/sellers can have disease problems.  In the past, diseases like Ralstonia wilt and impatiens downy mildew have been serious, and economically devastating diseases, for producers/sellers and no one has been immune to these disease issues.
  • Buy locally, when possible. Locally produced plants are often better adapted to the local climate, which can translate into better survival long term for perennial plants.  For annuals, plants grown in southern regions are often more likely to be exposed to pathogens earlier during production, with more opportunities for infections to occur.  These pathogens can travel north with plants as they are shipped into Wisconsin for sale.  Introductions of the Southern blight fungus and late blight pathogen have occurred in this manner in the past.
  • Avoid plants showing disease symptoms. Look carefully for any abnormalities in plant size, growth, or color that might indicate disease issues.  Common disease symptoms can include necrotic (i.e., dead) areas on leaves that might indicate a fungal, bacterial, nematode, or even viral  Also watch for lightning bolt-like line patterns, ringspots, or just blotchy light and dark patches on leaf tissue.  These symptoms are typical of viral diseases such as tobacco rattle, cucumber mosaic and hosta virus X.  Be sure to pop plants out of their pots to inspect the roots.  Roots should be plentiful and white.  If the roots are few and far between, or even worse, brown, then root rots could be a problem.  If you see abnormalities of any kind, DO NOT buy the symptomatic plants.
  • Avoid plants showing signs of disease-causing organisms. Some types of pathogens, particularly fungi and water molds, can produce spores on plant surfaces that will be visible to the naked eye.  Typical diseases where pathogens might be visible include powdery mildews and downy mildews, including the infamous impatiens downy mildew and basil downy mildew.  If you see any indication of this sort of growth, again DO NOT buy the plants.
  • Avoid plants with insects. Insects can cause damage to plants on their own through their feeding activities, so it is important not to bring home these pests with your plant purchases.  Insect pests can spread to other plants in your garden and cause substantial damage on their own.  From a disease standpoint, insects are important because they are plant pathogen vectors, moving disease-causing organisms from plant to plant as they feed.  There are insects that are known to move fungal, and bacterial pathogens in the environment, but where insects tend to have their biggest impact is through movement of viral and phytoplasma  In particular, aphids and thrips are important in moving viruses such as Cucumber mosaic virus and Impatiens necrotic spot virus from plant to plant.

With a little bit of effort and by using good observational skills, you can minimize the risk of bringing diseased plants into your garden.  However, even if you follow the advice outlined above, purchasing plants is not totally risk-free.  Sometimes plants harbor disease-causing organisms with nary a symptom nor sign in sight.  These pathogens may rear their ugly heads and start to cause problems once you’ve begun growing the plants in your garden.  Even if you dodge the bullet and successfully avoid purchasing infected plants, know that Mother Nature has tricks up her sleeve to bring plant pathogens to you.  So expect at least a little bit of disease, no matter how careful you are.  In the end though, plant diseases tend to be the exception and not the rule, so remember that most of time when you look at your garden, what you will see will be healthy and happy plants.  KEEP ON GARDENING AND LOVE EVERY MINUTE OF IT!

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


April: To Prune, or Not to Prune, That is the Question.

ShearsAs warmer weather has spread through much of Wisconsin, I have talked with more and more gardeners who are chomping at the bit to get into their yards and start the 2019 gardening season.  One of the activities these gardeners are contemplating is the pruning of their trees and shrubs.  They often ask me whether spring is a good time to prune from a plant disease perspective.

The answer is:  It depends.

Overall, I am a proponent of winter pruning.  Typically there is a slight warming at the end of January or at the beginning of February where it’s warm enough to prune and not freeze to death, but not warm enough that disease-causing organisms are likely to be active.  In my mind, pruning during this window reduces the risk of pathogens infecting through pruning wounds.  There can be exceptions to this rule of thumb however.  There is some research that indicates that pruning honey-locust trees in the summer (during hotter, drier periods) can reduce the risk of Nectria canker compared to pruning in the winter.

When pruning in the spring, the trees that I have the most concern about are oak trees.  In particular, I worry about transmission of the oak wilt fungus, Bretziella fagacearum.  This fungus can be moved from tree to tree by sap beetles that become active as temperatures warm.  These insects are attracted to wounds, including those caused by pruning.  Some municipalities restrict pruning of oaks after April 1 in anticipation that temperatures will shortly be warm enough that sap beetles will be active.  Use of a calendar date as a cut off for pruning oaks can be problematic however if spring arrives early.  For that reason, I really ONLY recommend winter pruning for oaks.  If for some reason, you really need to prune an oak at another time of the year, paint over any pruning wounds.  You can use a commercial pruning paint if you like, but left over latex paint (from painting a room in your home) will work just as well.  The paint provides a physical barrier that makes a wound less attractive to sap beetles.  Be sure to paint wounds on oak trees IMMEDIATELY.  There is research that indicates that sap beetles can visit wounds in as little as 10 minutes.

In general, when pruning tress other than oaks in the spring (or any time other than the winter), always be sure to prune when it’s dry and when there is dry weather predicted for several days post pruning.  Dry weather is less favorable for fungal spores (which might land on a pruning cut) to germinate and infect.  Also be sure to prune properly based on the type of tree or shrub.  Laura Jull of the UW-Madison Department of Horticulture has authored several excellent fact sheets on how to prune evergreens, deciduous trees and deciduous shrubs.  Check these out!!  When pruning out diseased branches, prune four to six inches below obviously diseased areas if you suspect a fungal disease and 12 inches below obviously diseased areas if you suspect a bacterial disease.  In the best of all possible worlds, you should decontaminate your pruning tools between every cut to limit possible movement of pathogens via your tools from branch to branch or from tree to tree.  You can use a 30 second dip in 70% alcohol (e.g., rubbing alcohol) or in a commercial disinfectant for this.  Alternatively, you can use a spray disinfectant, spraying your tools until they drip and then allowing them to air dry.  Except in situations where oaks are pruned during the growing season, I do not recommend using paint on pruning cuts.  Paints tend to slow down the formation of callus tissue, the tissue plants produce to naturally cover over wounds.

Finally, avoid what I tend to try to do when I prune, which is to prune off my fingers.  OUCH!!

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

March: Viral Villains – Gruesome Guests for Indoor Gardeners and Greenhouse Growers

Recently, I have seen an increase in conservatory and greenhouse-grown plants arrive at the PDDC.  It’s certainly the time of year that greenhouses gear up their plant production in anticipation of spring sales (assuming that spring is going to arrive this year – I have my doubts).  There are several viral diseases that I routinely see in home and greenhouse-grown plants that, if undetected, can spread easily and pose challenges for both indoor and outdoor gardeners.

CymMV Orchid
CymMV on an Orchid

I recently received several orchid samples from a local conservatory.  As it turned out, several of the plants were infected with Cymbidium mosaic virus (CymMV), an orchid specific virus.  In some orchid species, CymMV causes few, if any symptoms.  In other orchid species, a typical symptom is the appearance of necrotic (i.e., dead) leaf spots, symptoms that in most other plants I would attribute to fungal or bacterial pathogens.  Over the years, I’ve learned that with orchids, testing right away for viral pathogens like CymMV, particularly when there is leaf spotting (and oftentimes even when there isn’t), is a good idea.  Luckily, I have a quick, easy-to-use serological dip stick test (the plant virus equivalent to a home pregnancy test) to test for CymMV. When plants test positive, my recommendation is to throw out the infected plants.  It’s too easy to accidentally move viruses around in plant sap that you get on tools or even your hands when you are trimming leaves or deadheading flowers.  Once infected plants are removed, it’s important to decontaminate items that may have come into contact with the plants.  For details on what to use, check out the recipes in the University of Wisconsin Garden Facts on Hosta virus X (HVX).  HVX is another common plant virus, albeit on hostas rather than orchids.

INSV Begonia
INSV on Begonia

I also recently received a Lysimachia sample with a viral problem that turned out to be a bit more of a challenge to diagnose.  The plants came from a commercial greenhouse.  I noted that the edges of the leaves were dead and also noted damaged areas elsewhere on the leaves.  Some of the damage seemed to be consistent with that due to thrips feeding.  This sent up a red flag, as thrips can carry plant viruses like Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV).  These viruses can infect a wide range of plants, can spread quickly (it doesn’t take a lot of thrips) and can cause significant economic loss.  I used dip stick tests for INSV and TSWV, as well as for two other common plant viruses [Cucumber mosaic virus (CMV) and Tobacco mosaic virus (TMV)], but all of these tests were negative. To double-check that my thoughts about possible thrips damage were reasonable, I showed this sample to PJ Liesch, the UW-Madison/Extension insect diagnostician.  PJ verified the presence of a small number of thrips in the sample, but indicated that the brown leaf edges were not a typical symptom of thrips feeding.  PJ was on vacation when this sample arrived and he wasn’t able to look at the sample for about a week after submission.  By that time, I had noticed that the plants were developing additional symptoms including growth distortions and botchy color.  Everything was pointing to a viral problem.  Given PJ’s verification of thrips, I again tested the sample for INSV and TSWV, and lo and behold, this time the sample tested positive for INSV.  At that point, everything fell into place and I reported back to my client that I thought INSV was the primary issue with the plants, and recommended plant removal and decontamination.  The conflicting results that I got with this sample point out a difficulty in confirming viral pathogens.  Dip stick tests require a certain amount of a virus to be present in a sample to get a positive reaction and the amount of a virus in a plant can vary both in terms of the age of the plant part being tested (old vs. young leaves), as well as how long the plant has been infected.  Testing symptomatic tissue of different ages, as well testing more than once over a period of several days, can be critical in making an accurate diagnosis.

TMV on Tobacco

Finally, the virus that I haven’t yet seen this year (and that I don’t really want to see) is TMV.  This virus has a particularly wide host range and is particularly nasty given how easily it can be moved around.  You can pick up TMV on your fingers as you handle infected plants and transmit the virus by touch as you handle health plants.  TMV is a very stable virus.  It not only can be found in live plants, but can also be found in dead/dried plant tissue, including dried and processed tobacco.  If you smoke or use chewing tobacco, you are at increased risk of picking up this virus and spreading it around.  TMV can also hang around on inert items (e.g., clothing, boxes, work surfaces, tools) and can eventually be moved from these items to plants.  TMV, given its stability and easy transmission, is one of the most problematic and destructive viruses that I know of.  The growth distortions and blotchy color (i.e., mosaic) caused by TMV make infected plants unmarketable.  Destroying infected plants, and carefully and systematically decontaminating anything that has come into contact with infected plants is a must to get this virus under control.  And if you are thinking of quitting smoking or chewing tobacco, and need another reason, this virus is it.

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

February: Wacky Wisconsin Winter Weather

After last week’s sobering, subzero weather, I began to think about all of the ways winter in Wisconsin is designed to make the life of plants, particularly woody ornamentals, difficult.

Snow IconSnow:  Snow can be a mixed blessing.  I like to see a few inches of snow on the ground, because snow actually has an insulating effect.  Without a snow layer, soil temperatures can become quite cold far down into the soil profile and this can lead to cold injury to plant roots.  Such injury can outright kill plants, or alternatively lead to a slow, painful death where plants leaf out, but leaves rapidly dry up and die because there is a lack of a functional root system to take up water to supply to the emerging leaves.

Excessive snow can also be problematic.  In particular, I see situations where white cedars (arborvitaes) become so laden with snow that a variety of problems can arise.  In extreme instances, the weight of the snow may be so heavy that branches will snap.  In other instances, the snow simply causes the branches to bend downward.  This may seem innocuous, but if the snow doesn’t melt away and the plants are weighted for an extended period, branches may not spring back into their normal position and the shrubs end up deformed.

Cold IconCold Temperatures:  Late January’s deep freeze here in Wisconsin likely led to significant plant damage.  Plants have particular ranges of temperatures that they can tolerate (usually described in terms of their USDA Hardiness Zone).  If temperatures drop outside of this optimal range, physical injury to branches and trunks, and even plant death can result.  As I mentioned above, lack of snow cover can make cold injury worse by allowing for additional cold injury to roots.  Another contributing factor can be that many gardeners want to “push the envelope” and grow plants (often exceptionally beautiful trees and shrubs) that are not rated for their hardiness zones, but for warmer environments.  Often these marginally hardy plants will do well for many years, until they experience an extreme winter.  We’ll have to wait to find out how severe this winter’s damage has been until spring arrives.  At that point, we’ll be able to see how many trees and shrubs don’t leaf out or are stone dead.  I’m expecting plants like Japanese maple, magnolia, redbud and many types of fruit trees (particularly peach and apricot) to be hardest hit.

Ice IconIce:  Now that temperatures have warmed a bit, I’m seeing some areas of Wisconsin experiencing “wintry mixes” of precipitation.  Often this means freezing rain, which can coat branches and, depending on the duration of the rain and the specific temperature, lead to thick layers of ice that can be so heavy that they cause branches to break.  Some gardeners, in an effort to prevent this breakage, attempt to knock ice from branches, but this technique runs the risk of damaging overwintering buds.  I have fond memories (read EXTREME sarcasm here) of an ice storm in the mid-1970’s that caused substantial tree damage, paralyzed Madison and left my family without power for five days.  We spent a lot at the mall.

Wind IconWind:  High winds can cause extensive plant dehydration.  I most commonly see this as a problem on evergreen trees and shrubs.  I think of windy conditions, alone or in concert with cold temperatures, as being particularly damaging on Alberta spruce, boxwood and white cedar (arborvitae).  Watch for brown or bleached needles and branch dieback on these plants, particularly as they come out of dormancy in the spring.  This damage in often referred to as winter burn.  In extreme situations, high winds can physically damage and break off branches.

Now that I’ve totally depressed you by considering all of the possible adverse effects of winter weather, let’s try to put a sunny spin on things and think of the upside:  All of the winter death and destruction provides ample opportunities to plant new and exciting trees and shrubs (and even herbaceous plants), and watch these new plants grow and mature.  🙂

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


January: 2018 in Review

Calendar at AngleAs the New Year rolls in, it’s time to reflect on the past year and all that happened at the PDDC.

Clinic staff processed 1282 samples, down roughly 11% from 2017.  These samples came from 62 of Wisconsin’s 72 counties, as well as FL, IA, ID, IL, ME, MI, MN, MO, NM and TX.

While samples numbers were down, the complexity of the samples seemed to be on the rise.  In particular, requests for molecular diagnostics increased this year and I have to give kudos to Sue Lueloff, the PDDC’s Assistant Diagnostician, for stepping up and handling all of these samples.  Sue tested numerous samples for phytoplasmas, an incredibly complex process involving identifying DNA sequences of these bacteria-like organisms.  She confirmed the presence of the cranberry false blossom phytoplasma, a pathogen that hasn’t been documented in WI for many decades.  Sue was also involved in the first detection in Wisconsin of Xanthomonas vasicola pv. vasculorum, the corn bacterial streak pathogen.  Sue also confirmed the presence of Verticillium nonalfalfae, a rather odd species of Verticillium (at least in my experience), in Verbena.  PDDC staff still need to complete Koch’s postulates with this fungus, but if successful, this will be the first report of Verticillium wilt of Verbena.

While Sue handled the molecular side of things at the PDDC, I concentrated on handling the more classical side of the diagnostic process that involved culturing (i.e., growing pathogens from plant tissue) and microscopy.  I spent a lot of time identifying Phyllochora maydis, the corn tar spot fungus (a recent addition to the pantheon of fungal plant pathogens in the state).  I also got to see a new fungus that has been on my personal bucket list for years:  Cristullariella (cause of zonate leaf spot).  This is a rather uncommon disease that I saw in 2018 on both maple and grape.  I also continued to provide digital disease diagnostics via email, through the UW-Extension PlantDOC online diagnostic website, and through the Association of Specialty Cut Flower Growers Facebook page.  I logged roughly 1800 exchanges in the process of handling online plant disease inquiries.

2018 Ed StatsPDDC outreach activities hit an all-time high in 2018.  I did 107 talks/presentations/workshops in 31 Wisconsin counties (virtually all of these in-person visits).  My biggest outreach event was again Wisconsin Public Television’s Garden Expo.  I spent three days at the event, gave two talks on diseases of vegetables and helped answer questions with Lisa Johnson of Dane County UW-Extension at Larry Meiller’s Garden Talk session (always a blast!).  As always, I had a steady stream of visitors to the PDDC booth, talking with and answering questions for folks the entire time.  I distributed 10,300 University of Wisconsin Garden Facts fact sheets (a record), 663 brochures/informational handouts of various kinds and 154 handouts for my talks.  Across all programs in 2018, I interacted with over 230,000 people.  Again, a big thanks goes out to Larry Meiller for having me on his radio show with its awesome listenership.

2018 marked my 20th anniversary at the PDDC.  I really couldn’t have accomplished what I have over those 20 years (and what I continue to accomplish) without the support of a number of people.  I already mentioned Sue Lueloff (molecular diagnostician extraordinaire) above.  Also part of my team are Ann Joy (who does data entry that is instrumental in keeping federal funds through the National Plant Diagnostics Network flowing into the clinic), Dixie Lang (who makes the PDDC website look beautiful and work smoothly), Laurie Ballentine of the Russell Labs support staff (who prints handouts and prints and folds clinic brochures), and finally John Lake (just graduated from the UW-Madison with a degree in Plant Pathology) and Stephanie Salgado (now a senior at James Madison Memorial High School, my alma mater), my superhero student hourlies who processed the bulk of PDDC samples this year and kept me from going insane.  A special congratulations to John and his wife Michelle on a new addition to their family (Daphne Day) who arrived just before Christmas!

I’m not sure what 2019 will bring, but let’s see what happens!  Bring it on!

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: Ho! Ho! Ho! A Plant Health Horror for the Holidays!

As the December holiday season nears, one of the traditions for many folks is to decorate their homes with festive greens.  When selecting wreaths and garlands to using in decorating, keep in mind that you may not only be bringing pine boughs and holly into your home, but also plant pathogens.

Of particular concern has been the boxwood blight fungus (Calonectria pseudonaviculata).  Boxwood blight is a devastating, lethal disease of boxwood, one of our most popular landscape shrubs.  Pachysandra (a common groundcover) is also susceptible to the disease.  Boxwood blight was first identified in the eastern US in 2011 and this past summer was reported for the first time in Wisconsin in a nursery in Kenosha County.  The Wisconsin Department of Agriculture, Trade and Consumer Protection is currently trying to contain and (hopefully) eradicate the infestation.

While the greatest potential for introducing the boxwood blight pathogen into an area is through movement of infected landscape shrubs, there have also been reports in other states (e.g., Indiana) of the pathogen having been found in holiday wreaths festooned with sprigs from infected boxwood shrubs.  These wreaths were produced in North Carolina and then shipped to other states for sale.  The manufacturers unwittingly shipped the boxwood blight pathogen with their wreaths.  A similar scenario could potentially occur in Wisconsin, and if contaminated wreaths are not handled properly, extensive spread of the boxwood blight pathogen could result.

So, what can you do?  First, don’t be afraid to enjoy a holiday wreath; just consider buying one that does not contain boxwood sprigs.  If you are unsure whether a wreath you have already purchased contains boxwood, assume that it does (just to safe) and dispose of it appropriately.  One option is to burn the wreath, if your municipality allows this.  Otherwise, double bag your wreaths in garbage bags, seal them up and place them in your municipal garbage to be landfilled.  Be careful to watch for any leaves or branches that may have fallen from the wreaths and collect these up and dispose of them as well.  In particular, you want to make sure that no potentially contaminated material ends up near boxwood shrubs that you have in your yard.  Under NO circumstances should you attempt to compost any suspect boxwood materials.  Again, if you are uncertain if you are decorating with boxwood, assume that you are and act accordingly.  This is a situation where you can have a huge impact in helping prevent the spread of an economically important disease-causing organism.

If you have questions about boxwood blight (or other plant diseases), feel free to contact the PDDC at (608) 262-2863 or  Also watch for a new University of Wisconsin Garden Facts fact sheet on boxwood blight that should be available on the PDDC website ( early in 2019.  Also feel free to follow the PDDC on Facebook and Twitter @UWPDDC to receive updates on emerging diseases (such as boxwood blight) and their management.

November: Thank Heaven for New Plant Diseases

Turkey on PlatterOne of the traditions of the Thanksgiving season, is to contemplate the past year and express thanks for positive aspects of our lives.  Thinking of this concept in the context of plant diseases, I thought that in this month’s web article, I would discuss new diseases that I saw in the clinic in 2018 that make me thankful for having a job that is always stimulating and never dull.

Cranberry false blossom.  The most seasonally appropriate of the new diseases that I encountered this year was cranberry false blossom.  This is not a new disease of cranberry and was originally first described in the 1920’s in cranberries in Wisconsin.  However, until this year, the disease has not been observed for decades in the state.  Cranberry false blossom is caused by a phytoplasma (i.e., a bacterium-like organism) that is transmitted by the blunt-nosed leafhopper.  Typical symptoms include odd-shaped, discolored and sterile flowers, excessive branching of vines (called brooming) and early fall reddening of foliage.  Sue Lueloff [the Plant Disease Diagnostics Clinic (PDDC) Assistant Diagnostician], working with Patricia McManus (the UW-Madison/Extension fruit pathologist) and Lindsay Wells-Hansen of Ocean Spray, is working to get a better sense of how widespread a problem this disease may be in commercial cranberry bogs in the state.

Bacterial streak of corn

Bacterial streak of corn.  This disease, caused by the bacterium Xanthomonas vasicola pv. vasculorum, was first reported in the US in Nebraska in 2016 and was found in Wisconsin this past summer.  Sue Lueloff was again instrumental in confirming this disease, working with Damon Smith (the UW-Madison/Extension field crop pathologist), as well as with scientists at USDA APHIS.  Typical symptoms of bacterial streak include linear, necrotic (i.e., dead) stripes (typically with a bit of a yellow halo) on affected corn leaves.  The long-term impact of bacterial streak on corn production in Wisconsin is not clear.  Damon and other corn researchers will be monitoring and assessing the disease in the coming years.  Currently USDA APHIS scientists are attempting to complete Koch’s postulates with an isolate of the bacterium that they recovered from Wisconsin corn samples.  If this is successful, publication of a first report of the disease for Wisconsin will follow in a scientific journal (something that always looks good on resume).

Verticillium wilt of VerbenaI am always watching for Verticillium wilt on new host plants, but this find caught me a bit off guard.  A local greenhouse submitted Verbena leaves to my clinic in mid-summer, complaining that branches on affected plants had died back.  As I microscopically examined the leaves, I noted fungal spores and conidiophores (i.e., specialized spore producing fungal threads) that were consistent with those of Verticillium (the fungus that causes Verticillium wilt).  But the conidiophores were odd, looking beefier than those of Verticillium that I had seen in the past.  I was able to grow a pure culture of the organism from the leaves and turned this over to Sue who once again did her molecular diagnostic magic.  She identified the Verticillium as Verticillium nonalfalfae, a species I had never before encountered.  I have plans to try to complete Koch’s postulates with this organism, and if successful, I will be able to publish a first report ever of Verticillium wilt on Verbena. 

Zonate leaf spot of maple

Zonate leaf spot.  This disease, caused by the fungus Grovesinia moricola (formerly Cristulariella moricola), has been on my plant disease bucket list for years, ever since I saw drawings of the microscopic, tree-like reproductive structures of the fungus in one of my plant disease references.  This summer, Brianna Wright, the Marathon County UW-Extension horticulture educator, emailed me photos of maple leaves with circular necrotic spots with concentric rings.  My fungey senses (the plant pathology equivalent of Spiderman’s spidey senses) immediately went off, and I begged Brianna to send me a sample of leaves.  She graciously did, and sure enough, there were the itsy-bitsy tree-like structures characteristic of Grovesinia.  Interestingly, the exact same day Brianna’s maple leaves arrived, I also received a grape leaf sample for another part of Wisconsin with exactly the same pathogen and disease.  Oh, the irony.  It took me 20 years to see this disease for the first time and then I received two samples on the same day.

So there you have it, a sprinkling of the diseases that make me thankful to be a plant disease diagnostician.  That said, as I reread this article, it dawns on me that what I am even more thankful for is having Sue Lueloff as a colleague in my clinic.  Her molecular diagnostic skills have greatly enhanced the services that the Plant Disease Diagnostics Clinic (PDDC) has been able to provide over the past year.  Happy Thanksgiving to her and to all of you!

To learn more about common diseases and disease management, explore the PDDC website ( and in particular, check out the fact sheet section of the website.  Also follow the PDDC on Facebook and Twitter @UWPDDC to receive updates on emerging diseases and their management.

October: Dr. Death’s Halloween

Jack O Lanterns

October hosts my favorite holiday of the year, Halloween.  Call me sentimental, but how can you go wrong (having a moniker like “Dr. Death”) with celebrating a holiday that caters to things that creep and crawl in the night.  Most people think what I enjoy and do professionally is pretty weird and I have to say there’s a part of me that revels in sharing with the public the bizarre (well, I’d say “cool”) things that I get to see every day.  Halloween fits in perfectly with what I readily admit is my somewhat warped and twisted world viewpoint.  What can I say?  I love my job.  And luckily, my clients have embraced (or at least tolerate) my rather eccentric world viewpoint.

In the spirit of the Halloween season, here are my votes for the top three most disgusting and horrifying diseases.

Bacterial Soft RotIf you like slimy, oozy, stinky plant diseases, this is the one for you.  Bacterial soft rot is caused by several bacteria in the genera Pectobacterium, Dickeya, and Clostridium (amongst others).  The characteristic of these diseases is the collapse and liquefication of plant parts.  I most commonly see this disease (but not exclusively) in vegetables like potatoes, carrots, cucurbits (e.g., pumpkins and squash) and cole crops (e.g., broccoli and cabbage).  If you’ve ever had potatoes liquefy in your refrigerator (or worse yet, your cupboard), you’ve experienced the joys of bacterial soft rot.  Soft rot bacteria liquefy plants parts by producing enzymes called pectinases.  These enzyme digest plant pectin, the “glue” that holds plant cells in place and helps give plants their shape.  Once the pectin is gone, the plant structure collapses into a soupy mess.  EWWWWW!!!  Oftentimes this slimified plant tissue has a rather fetid stench.  In particular in my mind, soft-rotted broccoli has the ultimate, vomit-inducing bouquet.  The worst part about working with bacterial soft-rotted materials is that after a while you get used to the smell.  But it seeps into your clothing and when you get on the bus to go home, people move away from you because they think you have a personal hygiene problem.

Armillaria Root DiseaseI most commonly see this disease associated with woody trees and shrubs.  The fungi involved (several species in the genus Armillaria) typically infect through roots (often wounded roots) and colonize up into the trunks under the bark where they form a thin, creamy white fungal masses called a mycelial fans.  Eventually Armillaria produces mushrooms (called honey mushrooms) which help with its reproduction.  The reason I find this disease very creepy is that it produces root-like (or shoestring-like) structures called rhizomorphs.  These grow outward from an infected plant, “looking for” other plants to infect.  I’m anthropomorphizing a bit here, but the fact that Armillaria can grow from tree to tree, is quite disturbing to me.  And even if the trees are removed, the fungus is still in the soil.  Amanda Gevens, the UW-Madison/Extension vegetable pathologist, recently told of her experience with Armillaria when she grew potatoes recently cleared forest land.  As she harvested her potato tuber, she found that they were covered with rhizomorphs.  CREEPY!!!  Armillaria can infect and kill trees (and other plants) over large areas.  There’s actually a super-colony of Armillaria in the Upper Peninsula of Michigan covering 37 acres.  The nearby town of Crystal Falls celebrates “Humongous Fungus Fest” every August in honor of this organism.

Common Corn SmutAs I’ve said before, I LOVE plant diseases, but I have to say this fungal disease of corn really grosses me out.  I think this is because I encountered this disease as a kid when I helped my maternal grandparents on their farm every summer in central Illinois.  I would often run across corn ears with the swollen, tumorous masses (galls) characteristic of common smut.  I don’t know which stage was worse, the early stage where the galls were pasty, zombie gray, or the later stage where the masses converted to powdery mass of spores that crumbled and blew away.  In my adult years, I have come to have a greater appreciation of this disease as the early, fleshy, zombie phase is sold in high end restaurants as a culinary delicacy under its Native American name “huitlacoche”.  But even so, the horrible childhood memories of this disease linger.  SHIVER!!!  And another cautionary tale in the context of smut. . .Be sure when doing internet searches on smut diseases to include the pathogen name (in the case of common corn smut, Ustilago maydis) in your search.  If you search on just “smut”, you will end up at a number of very, um, interesting websites that have NOTHING to do with plant diseases.

So there is my Halloween plant disease hall of fame.  Enjoy.  And once you think you’re grossed out by these beauties, try doing an internet search on “cordyceps, insects and photos”.  You won’t be able to sleep for days.  But that’s an article for PJ Liesch, my colleague in the UW-Madison Insect Diagnostic Lab.  MWAH-HAH-HAH!!!!!

To learn more about common diseases and disease management, explore the Plant Disease Diagnostics Clinic (PDDC) website ( and in particular, check out the fact sheet section of the website.  Also follow the PDDC on Facebook and Twitter @UWPDDC to receive updates on emerging diseases and their management.

September: Water, Water, Neverwhere

WaterThere is a certain irony of me writing an article about watering plants after Madison, WI (where I live) recently received record amounts of rainfall and, as I write this article, is experiencing flooding in many areas of the city.  However, as September arrives, the psychology of many gardeners seems to be that as we enter the fall, plants really don’t need a lot of water.  Nothing could be farther from the truth.  Fall is a critical period for watering, particularly for evergreens, which require sufficient internally stored water to make it through harsh winter conditions without dehydrating and suffering from winter burn.

Wet LeavesEstablished trees, shrubs and herbaceous ornamentals (i.e., plants that have been in the ground three years or more) typically require roughly one inch of inch per week.  New transplants (i.e., plants that have been in the ground less than three years) require slightly more water (on the order of one and a half to two inches per week).  If Mother Nature does not cooperate with natural rain, then gardeners should apply supplemental water with a soaker or drip hose in as much of the root zone of plants as possible.  Often, gardeners have limited ability to water this large of an area (note that the roots of a tree can extend on the order of three to five times the height of the tree from the trunk).  In such situations, I recommend concentrating watering at the drip lines of trees and shrubs (i.e., the edges of where branches extend).  Deciduous trees and shrubs (i.e., those that drop their leaves or needles) should be watered up until they begin to turn their normal fall color.  Evergreens, on the other hand, should be watered up until the ground freezes or there is a significant snowfall.  To get a rough estimate of how much water is being applied, sink empty tuna cans in the soil so the lips of the cans are level with the soil surface.  Place a section of hose over the can and see how much water collects.  When there is an inch of water in the can, then roughly one inch of water has been applied in that area.

MulchAlso consider mulching properly to help keep plants properly hydrated.  Turf is a great competitor with other plants for water, so I typically recommend removing turf from around other plants (particularly trees and shrubs) out to at least their drip lines.  These areas should be mulched with a high quality mulch (my favorites are shredded oak bark mulch and red cedar mulch).  I typically recommend roughly one to two inches of mulch on a heavier, clay soils and a bit more (say, roughly three inches) on a lighter, sandy soil.

Watering CanWith proper watering and mulch, you can help maximize the likelihood that your plants will survive harsh winter conditions and come through healthy, happy and ready to vigorously grow the following growing season.  As always, if you have questions about this topic or about plant diseases and their management in general, feel free to contact me at (608) 262-2863 or

P.S.:  I can’t use the reference “Neverwhere” in the title of my article without plugging the book from which this word originates.  “Neverwhere” is one of my favorite fantasy books, penned by the amazing Neil Gaiman.  Check it out!

August: Free Plant Disease Testing Through the PDDC

While the Plant Disease Diagnostic Clinic (PDDC) typically charges a small fee for processing plant disease samples, there are certain diseases where diagnoses are performed for free.  These diseases typically fall into four categories:

  1. Diseases that have not been documented in Wisconsin, but should they be introduced could cause serious damage, and thus early detection is critical for proper management.  The disease that currently falls into this category is thousand cankers disease of walnut.  This disease has been, for all practical purposes, lethal on black walnut wherever it has been found.  The pathogen involved is a fungus (Geosmithia morbida) that is transported by and introduced into walnut trees by the walnut twig beetle.  Neither the fungus nor the insect has been found to date in Wisconsin, but I am watching carefully for both.  If you see declining walnut trees with yellow leaves and a thin canopy, particularly with small, pin-sized holes on larger diameter branches, get a sample to the PDDC.  Invoke the words “thousand cankers disease” and the diagnosis is free.
  2. Newly introduced diseases of regulatory importance where documenting how widespread these diseases are can be important for eradication and limit of spread.  Boxwood blight falls into this category.  This disease is incredibly destructive to boxwoods (lethal in many cases) and was first documented in southeast Wisconsin in late July of this year.  Typical symptoms include black spots on leaves and stems, progressing to defoliation, dieback and oftentimes shrub death.  Unfortunately, boxwoods tend to be prone to a variety of dieback issues (winter burn being the most common), so I have been telling everyone for several years, “If your boxwood has branch dieback, send a sample to the PDDC, invoke the words ‘boxwood blight’ and get your free diagnosis.”  Submissions of these samples are even more critical now that the disease is in the state to figure out how widely distributed this disease is.
  3. New diseases to that state that aren’t necessarily of serious concern (at least at this point), but should be documented to keep track of their distribution and/or provide samples for researchers.  Corn tar spot falls into this category.  Many of you are likely familiar with tar spot of maple.  This tree disease (caused by the fungi Rhytisma americanum and Rhytisma acerinum) has been common in Wisconsin for years.  However, in 2016, a visually similar disease (caused by the fungus Phyllachora maydis) was found on corn.  At this point, corn tar spot has not been a particularly serious disease, but Diane Plewa (my diagnostic counterpart at the University of Illinois) is studying this disease as part of her PhD research and is interested in obtaining samples of the disease from as many locations as possible.  So, if you see corn tar spot in Wisconsin, fill out a corn tar spot survey form, send the form and your sample to the PDDC for a free initial ID (including your complete mailing address so I can get a report to you) and I will forward the sample to Diane.
  4. Diseases of extreme economic importance that recur every year.  Late blight of tomato and potato falls into this category.  Wisconsin is the third largest grower of potatoes in the US and also has a thriving fresh market tomato industry.  Late blight can be devastating to both (it did cause the Irish potato famine of the 1840’s and 1850’s).  Knowing when the pathogen (Phytophthora infestans) arrives and perhaps even more critically which type(s)/variant(s) of the pathogen is(are) in the state (there are many) are important for choosing appropriate fungicides for control.  Every year I offer free late blight testing, and I forward positive samples to Amanda Gevens, the UW-Madison/Extension vegetable pathologist for typing.  Most of the time late blight is not the problem (typically Septoria leaf spot or early blight), but better to be safe than sorry.  Get those tomato and potato samples in for your free diagnosis.  The first report of late blight for 2018 just came through this week.

Be sure to take advantage of the PDDC’s free services while they last, but also remember that the PDDC needs paying samples as well to help fund the clinic’s operation.  So submit early and often!  As always, if you have questions about plant diseases and their management, or PDDC activities and services, feel free to contact me at (608) 262-2863 or