The 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.

What kinds of pictures should I take?  

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.

How do I get my pictures to the PDDC?

Email

This is typically the easiest way to submit photos.  Use pddc@wisc.edu 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.

What can I expect when I submit my photos?

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.

My commitment

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 pddc@wisc.edu 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.

LAST UPDATE:  June 18, 2021

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.

Presentations

• 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 (pddc@wisc.edu) to learn more about possible remote learning options (e.g. teleconferencing/ videoconferencing, etc.).

Online Resources

• The Wisconsin Disease Almanac (weekly updates of my PDDC diagnoses) will continue.
•  Monthly web articles will continue.
• University of Wisconsin Garden Facts (Fact Sheets) – updates and additions will continue.
• New online training modules may be added based on my current presentations.  See the existing IPM Scout School and Master Gardener training modules for examples of the format.
• As I release new training modules, I will announce them via Twitter or Facebook (@UWPDDC), or if you are one of my Extension colleagues, via email.

Consultations

• In-person consultations are discontinued until further notice.
• Phone (608-262-2863) and email (pddc@wisc.edu) 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!!

Last month in my summary of my activities for 2019, I mentioned that I had given 111 talks/presentations/workshops during the course of the year.  This month I’d like to elaborate a bit on the types of outreach presentations that I provide.

Audiences

Quite frankly, I’m willing to talk about plant diseases on any crop/plant other than turf (there are other Extension specialists who handle this crop).  I truly think that plant diseases are INCREDIBLY COOL.  I give talks to both commercial (i.e., agricultural/horticultural grower and consultant) and consumer (i.e., general public) audiences.

Most typically though, I provide education for the general public, often concentrating on teaching about diseases of herbaceous ornamentals, woody ornamentals and vegetables.  I often partner with county Extension staff, Master Gardener Volunteer groups, technical college instructors, professional organizations, public libraries and garden clubs to reach as wide a range of home gardeners (and horticulture professionals) as possible.

Types of Talks

To learn more about the specifics on the types of talks that I do, check out a couple of sections of the Plant Diseases Diagnostics Clinic (PDDC) website:

• 2020 Events calendar (under “Outreach..2020 Events” in the site menu)
• Master Gardener Resources (under “Outreach..Master Gardener Resources” in the site menu)

2020 Events Calendar

On my events calendar, I try to list every talk/presentation/workshop that I give during the year.  The calendar changes constantly as I book new presentations.

The calendar entries includes the titles of my talks, as well as the dates, times and towns where I will be.  If you ever see a presentation in a town near you and are interested in additional details on the program, feel free to contact me and I can provide those details.

Watch the entries as the date approaches.  I typically create handouts for my talks and link to the handouts on my calendar.  You are welcome to view these handouts online or download them to see what I’m talking about and hopefully learn a thing or two, even if you can’t attend the presentation.

Master Gardener Resources

The presentations/programs described on this page are a series of talks and hands-on opportunities that I have put together specifically to address the continuing education needs of Master Gardener Volunteers.  That said, virtually all of these presentations could be of interest to a general public audience.

Many of the talks fall into the “Diseases of” category, where I discuss diseases of broad plant groups such as vegetables, herbaceous ornamentals, evergreens and deciduous trees and shrubs.  Other of these talks cover more specific plants such as hostas and orchids.

Also included on this page are talks that are more conceptual in nature including my The Science (and Art) of Plant Disease Diagnosis and Growing Healthy Plants:  Basics in Plant Disease Management talks.

Some of the talks are just fun topics that I like to get geeky about, such as my Plant Diseases in History presentation.  The newest addition in this latter category is Confessions of a Black Thumb:  Plants That I Have Killed (or at Least Seriously Maimed), a cautionary tale of my personal gardening disasters (and there have been many).

If you are interested in getting down and dirty about plant diseases, I offer my Signs and Symptoms Workshop.  In this hands-on activity, participants have the opportunity to see and work with actual diseased plant specimens.

Finally, I offer tours of the PDDC (located on the UW-Madison campus), and typically combine this tour with tours of the Insect Diagnostic Lab, the Wisconsin Insect Research Collection and/or the Allen Centennial Garden.

Intrigued? 

If you have a group that might be interested in learning more about plant diseases, don’t be bashful about contacting me to explore the possibility.  You can contact me at pddc@wisc.edu.  Book early as my calendar often fills up quickly.  And for addition information on the PDDC, its activities and resources, check out other sections of the PDDC website, or follow the clinic on Twitter or Facebook (@UWPDDC).

Keep warm!  And don’t worry because spring gardening season (with all those uber-cool plant diseases) is coming soon!!

Wow!  It’s 2020 already and I can’t say that I’ve recovered yet from the avalanche that was 2019.

Sample Processing

Clinic staff processed 1506 samples, up roughly 17.5% from 2018.  Samples came from 64 of Wisconsin’s 72 counties, as well as FL, IA, IL, IN, MA, ME, MI, MN, and NY.  Sample numbers were up sharply (despite the PDDC having increased fees in 2019) most likely in part due to having had two seasons back to back that were incredibly wet and thus favorable for disease development.

Molecular Diagnostics

I also believe that the PDDC’s expanding molecular (i.e., DNA-based) diagnostic offerings have helped drive higher clinic sample numbers.  Many of the molecular tests that the PDDC offers are particularly popular and useful for commercial vegetable producers.  Once again, kudos to Sue Lueloff, the PDDC’s Assistant Diagnostician, for stepping up and handling all of the molecular samples.  Sue tested numerous samples for bacterial soft rot pathogens, the oak wilt pathogen and phytoplasmas.  In addition to processing routine samples (included in the total mentioned above), Sue also worked on a survey with the Wisconsin Department of Natural Resources (WI DNR), testing ~100 tree samples from around the state for phytoplasmas.  Testing from this survey will be completed in early 2020.

Sample Highlights

As always, while Sue handled the molecular side of things at the PDDC, I handled the more classical side of the diagnostic process that involved culturing (i.e., growing pathogens from plant tissue) and microscopy.  My highlight, sample-wise, for the year was confirming my first case of boxwood blight.  This disease had been first reported in nurseries in Wisconsin by the Wisconsin Department of Agriculture, Trade and Consumer Protection (WI DATCP) in 2018, but the sample I received at the PDDC was the first instance of the disease popping up in a landscape setting (i.e., in a home garden).  I also spent a fair amount of time dealing with samples suspected of having sudden oak death/Ramorum blight.  This disease has caused wide-spread death of oak trees (as well as other trees, shrubs and herbaceous plants) in California and Oregon.  The first Wisconsin case was confirmed by WI DATCP in 2019.  Thus far, none of the putative sudden oak death samples submitted to the PDDC has tested positive for the disease.

Disease Consulting by Email and Phone

In addition to coordinating efforts with live samples in the PDDC, I continued to provide digital disease diagnostics via email, through the UW-Madison Division of Extension PlantDOC online diagnostic website, and through the Association of Specialty Cut Flower Growers Facebook page.  I also spent a lot of time on the phone answering plant disease-related questions.  All told, I likely had 1500+ exchanges in the process of handling online plant disease inquiries and phone calls.

Outreach

PDDC outreach activities, once again, hit an all-time high in 2019.  I did 111 talks/presentations/workshops in 22 Wisconsin counties (many of these in-person).  My biggest outreach event was again Wisconsin Public Television’s Garden Expo.  I spent three days at the event, gave three talks (two on diseases of trees and shrubs, one on plant disease management) and helped answer questions with Lisa Johnson of Extension Dane County at Larry Meiller’s Garden Talk session.  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 3,435 University of Wisconsin Garden Facts fact sheets, brochures/informational materials and other written materials.  Across all outreach programs in 2019, I interacted with almost 224,000 people.  As always, a big thanks goes out to Larry Meiller for having me on his radio show with its awesome listenership.

Team Effort

What happens at the PDDC is not a solo effort.  I have LOTS of help.  I have 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 keeps the PDDC billing database running and up to snuff), Laurie Ballentine of the Russell Labs support staff (who prints and folds and otherwise produces all of the written handouts I use for my outreach efforts), and finally Alex Mikus (an undergraduate here at the UW-Madison) and Gisele Guzman (a participant in the TOPS program at East High School), my hard-working student hourlies who helped process the bulk of PDDC samples and kept me on my toes.

Onward Ho!

Now, 2020 has arrived and it’s time to gear back up!  “It’s showtime, folks!”  (Brownie points if you can identify the film that quote comes from.)

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 pddc@wisc.edu.

Over the past two weeks, I have received a number of photos of incredibly cool microorganisms that have popped up in people’s gardens or around their homes.  These organisms have all turned out to be non-pathogens (i.e., they don’t cause plant diseases), but they are some of the more extraordinary organisms that I encounter in my job.

Bird’s Nest Fungi

If you’ve recently spread new mulch, you may eventually notice small (maybe ¼ to ½ inch in diameter) cup-like structures forming in groups on the mulch surface.  Closer examination of the cups will reveal egg-shaped structures inside.  Looking at these tiny structures as a whole, you’d swear that some sort of miniature bird has built a series of nests in your flowerbed.  But these mini-“nests” are actually reproductive structures of one of the bird’s nest fungi.  The “eggs” are packets of fungal spores attached to the interior of the cup by a cord-like structure.  When a raindrop hits the nest, the momentum of the falling water catapults an “egg” into the air.  The attached “cord” stretches and eventually breaks.  The broken end is sticky and eventually adheres to some object (e.g., a branch or leaf) as the “egg” flies through the air.  The attached “egg” swings around on the cord (think of a bola) and eventually gloms onto something as well.  As the walls of the catapulted “egg” degrades, the spores inside are released.  Being elevated, the spores are more likely to be picked up in air currents.  What an amazing adaptation that increases the likelihood that the spores are spread over a large distance!

Stinkhorns

Sometimes when people dig in their gardens they encounter large (an inch or larger in diameter), soft, egg-like structures in the soil.  Folks often think these are actual bird or reptile eggs.  However, if you cut the “eggs” through the center along the long axis, you discover inside a mushroom-like structure with a stalk and an oftentimes honeycomb-like cap.  These mushrooms enlarge (eventually causing the “eggs” to “hatch”) and poke up above the soil surface.  These emerging mushrooms often resemble a certain, uh-hem, part of the male anatomy.  I have had a number of amusing conversations over the years as clients have attempted to describe stinkhorns without naming the offending body part.  The name stinkhorn comes from the fact that as they produce spores, these fungi tend to smell pretty rank.  The foul small (akin to rotten meat) attracts certain insects (e.g., flies) that pick up the spores and move them long distances.  Again, this is another amazing dispersal mechanism. 

Slime Molds

Slime molds are not true fungi, but members of a unique group of organisms called the myxomycetes.  These organisms spend part of the lives in an amoeba-like stage (so in a sense are animal like), but when they reproduce, form spores the way true fungi do.

The most common slime mold I encounter is one I affectionately call dog vomit slime mold, but is more commonly called scrambled egg slime mold.  This slime mold really looks like someone’s dog upchucked in your flowerbed.  Watch for this slime mold (usually in the genus Fuligo) after you’ve spread a new layer of mulch on your flowerbeds.

Another slime mold you may encounter is Physarum which is often found on turfgrass.  I remember seeing this slime mold in a lawn walking home from work one day.  I thought that someone had been out on their lawn spray painting some furniture and left gray paint residue on their grass.  On closer inspection however, the gray turned out to be thousands of small, spherical fruiting bodies (i.e., reproductive structures) of the slime mold.  I have also seen this slime mold crawl over and fruit on other plants such as ginseng and tomato.

Perhaps the most dramatic slime mold I see is the chocolate tube slime mold (Stemonitis), which often fruits on the walls of houses.  It can form HUGE masses of what appear to be clusters of miniature cattails.  One of my friends saw this slime mold and claimed it was straight out of Stranger Things (the horror television series I’ve been binge-watching of late).  There’s no accounting for taste!  Sigh.  Slime molds will ever remain cool in my book.

For addition information on the PDDC and its activities and the cool disease samples I get to see, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.

With the arrival of June, the plant disease floodgates have burst and the PDDC has been deluged with plant specimens with a wide range of plant health issues.

Probably the most striking samples showing up at the clinic right now are those that are showing symptoms of winter/cold injury.  I continue see samples from trees and shrubs where the entire plant has died over the winter with no signs of life this spring.  In particular, I have seen (and have also heard about) burning bushes that did not survive the winter.  While my clients have been distraught over this death and destruction, I have been trying to offer a silver lining (shocking, I know for Dr. Death!).  I point out that burning bush is classified as invasive plant in Wisconsin and that the death of these plants provides an opportunity to replace the shrubs with something more exciting and environmentally friendly.

When entire trees and shrubs have not died over the winter, in many cases parts of them have.  In particular, I have seen boxwoods, arborvitaes and other evergreens where branch tips have died and bleached over the winter.  This is a fairly classic symptom of winter burn/winter injury that I see every year.  This symptom might be a consequence of direct cold injury to tissue (a distinct possibility due to the extreme cold temperatures that we experienced in late January) or due to loss of moisture (from lack of sufficient water in the fall and/or exposure to dehydrating winds over the winter).  Slightly more subtle winter injury comes in the form of deciduous trees that leaf out, typically producing small leaves that then collapse and dry up.  I have been seeing this a lot on fruit trees (e.g., apples, pears, plums, cherries).  In these situations, there was likely sufficient internally stored water in the trees to initiate bud break and start leaf expansion, but also enough cold injury to the vascular (i.e., water-conducting) tissue to limit subsequent water movement into the leaves to further expand them and keep them alive.  I expect to see this collapse of leaves continue throughout the summer.  Many folks see this dieback on apples and pears and assume the problem is fire blight, but I have yet to diagnose this disease so far this year (and I’ve been trying very hard to find it).  Management for this type of dieback is simple pruning.  I suggest pruning four to six inches below obviously dead areas on branches.  The best time to prune for most trees and shrubs tends to be in the winter, but if you need to prune during the summer, be sure to prune only when it is dry.  Even though I don’t believe the dieback that I have described above is disease related, just to be safe, I still recommend that you disinfest 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.  I do not recommend using pruning paints except if you are forced to prune an oak tree during the growing season (to prevent transmission of the oak wilt fungus via sap beetles).  You can dispose of branches by taking them to your municipal yard waste recycling center (if you have one available), burning them (where allowed by local ordinance) or burying them.

Another variation on leafing out that I have seen this year that I am attributing to cold injury is where trees leaf out completely, but have smaller than normal leaf size.  In these situation, I am suspicious that there is minor damage to the vascular tissue the trees, but not sufficient to totally prevent water from reaching branch tips.  I have a redbud tree in front of my home showing this symptomology.  The tree typically has large, lush foliage that provides a privacy screen for my front door.  This year the leaves are quite small and I can easily see through the thin canopy.  This tree also had much reduced flowering this year compared to previous years.  I have also seen smaller leaves on a lot of maples (particularly silver maples) this year, but these trees have shown excessive seed production compared to other years.  I call this overflowering/seeding phenomenon the “Oh my gosh, I’m going to die, I’d better reproduce” syndrome.  Over-flowering/seeding is typical for stressed trees.  However, because this year’s flower buds were formed last summer, the underlying stress that led to over-flowering/seeding was not our winter weather, but other stresses that occurred during the growing season of 2018.  For trees with smaller leaves, I suggest making sure they are adequately watered.  I typically recommend that trees and shrubs receive approximately one inch of water per week.  If Mother Nature doesn’t cooperate, then I suggest setting up a soaker or drip hose at the drip line of the tree (i.e., the edge of where the branches extend) and applying whatever additional water is needed.  Affected trees should continue to receive water until they start to turn their normal fall color in the autumn.

And if all of the environmental stress-related issues aren’t enough, I’ve been getting inquiries about (and finally receiving samples of) what appears to be anthracnose on maples.  I expect to eventually see this disease on other trees as well.  Typical symptoms of the disease are brown to black, necrotic (i.e., dead), blotchy areas on leaves.  Our wet spring weather has been very favorable for this disease to develop.  Luckily, anthracnose tends to be a very cosmetic disease and typically causes little long-term damage to trees.  Cultivating the ability to ignore anthracnose symptoms and doing good fall clean-up of the leaves and removing them from your property typically are the best ways to handle this disease.

Phew, what a June!  Now, onward to July.  I have a feeling the deluge is going to continue!

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 pddc@wisc.edu.

P.S.:  Brownie points for those of you who recognize the origin of the title of this month’s article.

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 pddc@wisc.edu.

As 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 pddc@wisc.edu.