All posts by hudelson

January 2023: Plant Disease Diagnostics Clinic Services for 2023

Turquoise Microscope Icon2023 has arrived, and I’m already thinking ahead to the deluge of samples that are likely to arrive at the UW Plant Disease Diagnostics Clinic (PDDC) this summer.  PDDC personnel continue their efforts to balance providing high quality services to the public with keeping clinic staff and clients healthy and safe.  The following are current guidelines for submitting samples and tapping into the PDDC’s outreach programs and resources.  Our world is ever evolving, and there will likely be changes in how the PDDC functions as 2023 unfolds.  Be sure to check this document routinely for updates.

Plant Disease Diagnostics

The PDDC continues to offer digital diagnoses, and I really encourage you to submit digital photos of your plant disease problem before submitting a physical sample.  Use the PDDC online form for photo submissions.  In some situations, I will be able to provide a diagnosis and management recommendations based on your photos.  There is a $20 fee for this service.  If I cannot make a diagnosis from your photos, I will not charge the fee, but I will use the photos to provide you suggestions on what sort of physical sample to submit as a follow-up.  Note that if I charge a digital fee and you end up submitting a follow-up physical sample, the digital fee will be credited towards any lab fees for the physical sample.

You can submit physical diagnostic samples either by mail or in person.  If you opt to drop off a sample in person, PDDC staff will not likely be available for in-person consultations.  While I am committed to providing personalized service to my clients whenever possible, the PDDC is short-staffed at this time, and PDDC staff members need to concentrate their efforts on sample processing in order to keep up with incoming samples and provide timely diagnoses.

The PDDC provides an area for physical sample drop off in the hall outside the clinic.  Check for the large, black, wood shelving unit in the alcove just down the hall from the clinic door.  You can drop your sample(s) there.  Be sure to fill out a submission form to include with each sample.  You can find forms in the wall pocket to the left of the clinic door.  The form is also available online if you would like to print and fill it out ahead of time to speed up the submission process.  Please print neatly on the form and make sure to include your complete mailing address (with zip code), your phone number and your email address.  If you are a commercial client, please include the name of the business that you work for.  At this time, I am emailing virtually all reports and invoices.  If you do not have email however, just let me know, and I definitely can and will send out a hard copy of your report and invoice.


I will be providing more in-person plant disease presentations in 2023.  However, due to the cost of traveling (both in terms of time and monetary expense), I will continue to encourage clients to consider virtual presentations (e.g., via Zoom) whenever possible and practical.  If you are interested in having me provide a presentation for a group that you are affiliated with, click here to check out a list of topics that I routinely speak on.  Email me at or call me at (608) 262-2863 to discuss what talk might be best suited for your audience and to schedule a date and time.

In addition in 2023, I will be sponsoring monthly Zoom presentations on a range of plant disease topics.  Click here to check out these presentations and sign up if you are interested.

Online Resources

Via the PDDC website, I will continue to provide weekly updates of my PDDC diagnoses (the Wisconsin Disease Almanac), as well as web articles (hopefully monthly, but frequency will depend on clinic sample volume).  Also check out the PDDC website for the UW Plant Disease Facts (formerly the plant disease-related titles of the University of Wisconsin Garden Facts).

I will be recording the monthly PDDC plant disease talks (mentioned above) and will eventually be working these into online playlists (with full transcripts to fulfill UW-Madison accessibility requirements) that I will make available on my nascent YouTube channel (@UWPDDC).  My Fundamentals of Plant Diseases presentation is currently available as a playlist on my YouTube channel.

Outreach teasers

I am currently working on several new outreach projects.

Limerick Book

First is my book, Limerickettsia:  A Plant Pathologist’s Book of Verse.  This is a book of 52 plant disease-related limericks with supplemental prose discussions of the diseases and original artwork.  I am in the final throes of completing this and getting it printed.  April 3, 2023 update:  Now complete!!  Click here for details.

Plant Disease Medallions

I am also working on the UW Plant Disease Facts Medallions project.  For each of the 130 fact sheets in the UW Plant Disease Facts series, I have created electronic medallions and quizzes.  Eventually, you will be able to read one of the fact sheets and take the corresponding quiz.  If you get a score of 100%, you will be emailed the corresponding medallion.  Collect them all!  Hopefully, this project will be launched to the public this winter or early spring.  April 3, 2023 update:  Now complete!!  Click here for details.

Building Block Plant Diseases

Finally, I am developing building block kits (akin to LEGO kits) of common plant diseases (with supplemental educational materials).  I have designed models for blossom end rot, powdery mildew, downy mildew of grape, apple scab, silver leaf, Armillaria root disease, and (most recently) common smut of corn.  I am now working on getting the blocks for the kits 3-D printed so that I will have prototypes available for view by this spring.  October 27, 2023 update:  The blossom end rot building block kit is set to debut in early December at the Monona Public Library courtesy of the UW-Madison Department of Plant Pathology’s graduate student outreach group, What’s Eating My Plants (WEMP).

Need information on the PDDC? 

As new PDDC resources become available, I will announce their availability via Twitter (@UWPDDC) and Facebook (@UWPDDC), or via my clinic listserv, UWPDDCLearn (email me to subscribe to this).  In addition, you can always contact me by phone at (608) 262-2863 or email at

Have a great 2023 everyone!

December 2022: A Plant Pathologist’s Holiday Shopping List

Present Icon‘Tis the Season to buy holiday gifts.  I often have a difficult time coming up with ideas for what to buy for friends and family who seem to have everything.  Similarly, I’m a frustrating person to buy for; I just have too much stuff.  If you’re trying to come up with that perfect gift, and socks, a scarf or a necktie won’t do, consider a nifty plant disease-related gift that may put a smile on a loved one’s face.  I’m not talking about a greeting card or t-shirt with a plant disease design or pithy plant disease quote but actual diseased plant materials that you can gift to a friend.

Artisan wooden bowls

There are a couple of variations on this gift idea.  If you know of someone who really loves cherry wood (me, me!!), then consider finding a local woodworker who makes bowls from burls that form on the trunks of cherry trees.  These masses of wood (galls) are often the result of a fungal infection by Apiosporina morbosa, the black knot pathogen.  When infections occur on smaller branches, I refer to this disease affectionately as poop-on-a-stick, and the galls are not of much use.  However, larger black knot trunk galls are highly prized for their amazing grain patterns and can be used to make distinctive one-of-a-kind bowls.

The other types of bowls (or other wooden art objects) that are produced from diseased trees are those exhibiting spalting.  These wooden objects are produced from trees that are suffering from wood rot (typically white rot).  These rots are fungal in nature.  As the fungus colonizes the wood and begins to degrade it, dark lines can form at the interface between healthy and diseased wood.  If caught at the right time (before substantial degradation has occurred), the diseased wood can be shaped in to stunning artisan pieces with the decorative line patterns intact.

Walking sticks

Do you have a hiker friend who needs a gift?  If so, then consider a diamond willow walking stick.  Diamond willow is not a type of willow tree, but the result of certain species of willows being infected by canker-causing fungi (in particular the fungus Valsa).  Infection leads the formation of diamond-shaped sunken areas (cankers) on affected branches.  These sunken areas exhibit a distinctive light and dark (often reddish) contrast between healthy and diseased tissue.  The diamond shapes and color combination make the infected branches highly prized for making not only walking sticks but also furniture.


For a less expensive disease-related gift (the wood gifts described above can be quite pricey), consider giving a friend or loved one a poinsettia.  “What’s diseased about a poinsettia?” you may ask.  Well, the poinsettias that you buy at your local greenhouse or floral shop are infected with phytoplasmas.  Phytoplasmas are bacteria-like organisms that colonizes a plant’s food-conducting tissue.  Poinsettias, in their native habitat, are tall (we’re talking 10 ft. or more) and lanky.  The presence of phytoplasmas stunts infected poinsettia plants and makes these plants produce extra branches, thus yielding the beautiful, compact, bushy poinsettias that we have come to love.


For the gourmand on your shopping list, a can of huitlacoche (alternatively called cuitlacoche) might be the perfect stocking stuffer.  Huitlacoche is the polite name for common smut of corn.  This is a fungal disease where infection occurs through corn silks as the plants flower.  The fungus causes corn kernels to enlarge into huge, pasty gray masses (galls again).  Eventually, the interior gall tissue converts into powdery spores.  But, if you catch the galls when they are still solid and fleshy, you can use them for cooking.  Huitlacoche is a common ingredient in traditional Mexican cuisine, and many high-end restaurants now offer it on their menus.  While fresh huitlacoche is always a better choice, canned huitlacoche is readily available in specialty food stores, as well as online.


If you have questions about any of the disease gifts discussed above, or need additional ideas for plant pathology related gardening gifts, feel free to contact the PDDC by email at or by phone at (608) 262-2863.  Also, feel free to check out the PDDC website ( for additional details on plant diseases and their management, as well sample submission.  You can follow the PDDC on Twitter and Facebook (@UWPDDC) or email me to subscribe to the PDDC listserv, UWPDDCLearn, to receive updates on clinic services and educational materials.

Happy Holidays and good luck with your shopping!


Links in this article lead to sites that are for illustrative purposes only and are not an endorsement of any particular vendor or item for sale.

Update 12/9/22 

Here’s another plant disease gift idea I stumbled across (although there are some caveats with this one). . .

Oud perfumes and colognes 

Oud (also spelled oudh) refers to a dark, fragrant resin that is produced in the heartwood of agarwood trees (Aquilaria spp.) in response to infection by the fungus Phialophora parasitica.  Only a small percentage of agarwood trees (maybe 2%) become infected.  The combination of sap and fungus has a scent that evokes leather, saffron and smoke.  Unfortunately, oud has historically has been so valued (for a variety of purposes in addition to perfume production) that agarwood trees have been overharvested to the point where many species are endangered.  There is a move afoot to more sustainably produce oud by cultivating agarwood trees and artificially inoculating them.  So, if you decide that an oud perfume is your gift of choice, I suggest making sure the oud therein is from a sustainable source.

Update 12/14/22:  

And Santa’s plant pathology gift list grows longer (courtesy of Patty McManus, the former UW-Madison fruit pathologist) . . .

Noble rot wines 

Have a wine connoisseur who needs a holiday gift?  Then, consider noble rot wines.  These wines are produced from grapes that have been infected by Botrytis cinerea.  This fungus, if growing conditions are wet for extended periods of time, can devastate a grape crop, rotting the fruits on the vine.  However, if there is only a short wet period (which promotes infection), followed by drier conditions, the fungus causes a dehydration of the grapes without destructive decay.  This raisining of the grapes causes a concentration of sugars, and wines produced from noble rot grapes tend to be sweeter wines.  The presence of Botrytis cinerea also appears to add to the flavor profile of the wine, giving noble rot wines subtle hints of honey, beeswax and/or ginger.  Common noble rot wines include (but are not limited to) Tokaji (from Hungary/Slovania), Sauternes (from France) and Beerenauslese (from Germany/Austria).


March 2022: Checking Out the New Gym (or Flexing Your Muscles on a New Wisconsin Disease)

Barbell IconI’ve been around long enough at the PDDC (25 years next summer) that most of the diseases that I see at the clinic are old friends.  Rhizosphaera needle cast, oak wilt, Verticillium wilt and many others are part of the cast of plant disease characters that I see in Wisconsin every year.  It’s relatively rare that I see brand new diseases.  In recent years, boxwood blight, Neopestalotiopsis leaf spot and fruit rot of strawberry and zonate leaf spot (a disease that had been on my bucket list for years) are new diseases that have crossed my doorstep.  In 2022, I’m watching for yet another new disease:  Japanese apple rust.

Japanese apple rust is a type of Gymnosporangium rust caused by the fungus Gymnosporangium yamadae.  This fungus is native to Asia (specifically China, Korea and Japan) and was first detected in the United States in Delaware and Pennsylvania in 2008 (with a first formal published report in 2009).  In 2021, Sam Fieweger of the Wisconsin Department of Agriculture, Trade and Consumer Protection (WI DATCP) Plant Industry Bureau Lab confirmed the first detection of the disease in Wisconsin.  The disease was identified on a crabapple tree in a nursery in southeast Wisconsin.

Like other Gymnosporangium rusts, Japanese apple rust is an altnernating rust.  The causal fungus requires two different host plants to complete its life cycle.  It spends half of its life on a wide range of apples/crabapples (Malus spp.) and the other half on Chinese juniper (Juniperus chinensis, Japanese garden juniper (Juniperus chinensis var. procumbens) and Sargent juniper (Juniperus chinensis var. sargentii).

Japanese apple rust on juniper.  Photo courtesy of Nancy Gregory, University of Delaware.
Japanese apple rust on juniper.  Photo courtesy of Nancy Gregory, University of Delaware.

Typical symptoms of Japanese apple rust are not radically different than those of other Gymnosporangium rusts.  On juniper hosts, the disease leads to the formation of relatively small branch galls that ooze orange gelatinous arms/masses in the spring.  These galls/masses look (at least to the casual observer) not that different from those produced by cedar-apple rust (CAR) and (particularly) cedar-hawthorn rust (CHR), two common Wisconsin Gymnosporangium rusts.  On apple/crabapple hosts, symptoms of Japanese apple rust are again similar to those of CAR and CHR:  large, brightly-colored leaf spots.  In the cases of CAR and CHR, the spots are yellow to orange.  In the case of Japanese apple rust, the spots are fuchsia to red.  On the undersurface of the leaves where the spots are located, short spiny structures eventually form.  These are reproductive structures that produce spores that reinfect the juniper hosts.

Japanese apple rust on crabapple.  Photo courtesy of Nancy Gregory, University of Delaware.
Japanese apple rust on crabapple.  Photo courtesy of Nancy Gregory, University of Delaware.

The presence of Japanese apple rust in Wisconsin puts a wrench in the recommendations that I typically make for managing Gymnosporangium rusts.  Chinese junipers are not great hosts for CAR, CHR and cedar-quince rust (the other common Gymnosporangium rust in Wisconsin), so I have historically recommended these junipers for clients who are desperate to grow junipers and apples, crabapples and hawthorns near one another.  If (more likely when) Japanese apple rust becomes more prevalent, Chinese junipers will no longer be a good choice to grow alongside apples and crabapples.  In the future, management of Gymnosporangium rusts will likely have to rely on not growing junipers and apples, crabapples and hawthorns anywhere near each other.

Because Gymnosporangium yamadae is not native to the US, Japanese apple rust is of regulatory importance in Wisconsin (and also elsewhere in the US).  There is interest in determining how widespread the disease may be in the state.  For that reason, I will be providing free diagnostics (as long as I am financially able – the testing is a bit pricey) for suspect Japanese apple rust samples.  The easiest hosts to verify the disease on will likely be apples or crabapples.  So, please watch for those fuchsia to red spots.  If you see anything suspicious, please submit digital photos via my online digital diagnostics form.  Let me know in the “Message” field that you suspect Japanese apple rust, and there will be no fee.  If your photos look suspicious, I’ll request a follow-up physical sample for additional lab testing.

Have questions?

If you have questions about Japanese apple rust or submitting a Japanese rust sample (or need advice about plant diseases in general), feel free to contact the PDDC by phone at (608) 262-2863 or email at  Also check out the PDDC website ( for additional details on plant diseases and sample submission.  Feel free to follow the PDDC on Twitter and Facebook (@UWPDDC) or email me to subscribe to the PDDC listserv, UWPDDCLearn, to receive updates on clinic services and educational materials.

Good luck with your new workout!


March 2022: To Reuse Pots or Not Reuse Pots – That is the Question

Clay Pot IconI have recently seen comments and discussions on social media about reusing pots from greenhouse or nursery-purchased plants.  In some instances, there has been discussion of returning pots to nurseries and greenhouses for businesses to reuse.

I’m definitely all for reusing pots whenever possible.  It’s an environmentally sound thing to do.  However, keep in mind that you need to be cautious when reusing pots as they potentially can be sources of disease-causing organisms.  These pathogens may be carryovers from plants that you originally purchased in the pots, or they may be pathogens that the pots picked up later as the pots have be stored in your garden or elsewhere on your property.  In particular, damping-off and root rot pathogens can be found in many garden soils, so anytime pots come in contact with garden soil (and it doesn’t have to be a lot), they can potentially become contaminated.  And, it’s not just pathogens that can be an issue.  Insect pests and (gulp!) jumping worms could also potentially be carried via contaminated soil on recycled pots.

So, if you have a collection of old pots and want to reuse them (or even more importantly want to give them to your local nursery or greenhouse), take some precautions prior to reuse or donation.

Clean your pots thoroughly with soapy water

to remove any bits of soil clinging to the surfaces of the pots that might be harboring pathogens. An added bonus of using an initial cleaning with soapy water is that this should help deactivate plant viruses that might be hanging out on your pots.

Soak your pots in a 1.0-1.5% sodium hypochlorite  solution for 20-30 minutes

Sodium hypochlorite is the active ingredient in disinfecting bleach.  This treatment will help kill pathogens that remain on the surfaces of the pots.  Be cautious when using bleach to avoid contact with skin, eyes and clothing.

Thoroughly rinse the pots to remove bleach residues

that can be toxic to the new plants that you are trying to grow in the pots.

Note that the treatment outlined above may not be successful in all situations.  Bleach treatments tend to work better on clay or ceramic pots but tend to be less successful for plastic pots.  Be that as it may, seriously consider decontaminating you pots prior to reuse or donation to local businesses.  This will help limit, as much as possible, plant pathogen carryover and spread.

Need more information?

For more information on plant diseases and their management, check out the UW Plant Disease Diagnostics Clinic website ( or contact PDDC staff at or (608) 262-2863.

February 2022: ‘Tis the Season – A Tree Pruning Redux

Hand Pruners IconI often talk about pruning as a method for disease management.  Pruning branches with canker diseases (e.g., Nectria canker, Cytospora canker, golden canker) can be critical in keeping diseases in check and preventing pathogen spread.  Pruning healthy trees and shrubs is also important to promote proper growth, reduce the risk of structural failure during extreme weather (e.g., high winds), and, in some instances, enhance flowering.

When pruning your trees and shrubs, consider the following:

Match your pruning technique to the specific tree or shrub you are pruning

Specific trees and shrubs have particular pruning needs, and you need to choose the right pruning technique for the trees and shrubs in your landscape.  Check out the University of Wisconsin Garden Facts Pruning Deciduous Trees, Pruning Deciduous Shrubs, and Pruning Evergreens for details.

Prune in the winter, when possible

From a disease standpoint, pruning in the winter is typically optimal.  Disease-causing organisms and insects that carry them are not active, meaning there is less of a chance that infections will occur through pruning wounds.  Are there exceptions?  Absolutely!  Some research indicates that pruning honeylocust in the summer can help reduce the incidence of Nectria canker.  Also, pruning spring-flowering shrubs in the winter reduces flowering, so pruning these plants right after bloom is a better option.

Prune when it’s dry

If you prune during the growing season, prune when there’s a stretch of several dry days.  Wet weather is a better environment for disease-causing organisms to infect.

Prune to minimize wound size

When removing large tree branches, be sure prune just outside the branch collar (i.e., the slightly swollen area where the branch attaches to the trunk).  This will produce a smaller wound than cutting the branch flush with the trunk.  Similarly, cut smaller branches perpendicular to the long axis of the branch (rather than at an angle).  A smaller wound provides a smaller “target” that plant pathogens have to land on and infect.  A smaller wound also means less time for a tree to produce protective tissue that grows over a pruning cut.

Avoid painting pruning cuts

Paint wounds (and do it immediately) only when pruning oaks and elms during the growing season.  This reduces the risk of transmission of the oak wilt and Dutch elm disease fungi.  Painting pruning cuts on other types of trees can slow development of the protective tissue described above.

Decontaminate, decontaminate, decontaminate

Optimally, decontaminate tools between every cut.  Check out my January Plant Disease Pointers for details on what to use and how long to treat.  Consider using two sets of pruning tools, one that you decontaminate, while you prune with the other.  If decontaminating between every cut is not feasible, decontaminate as often as possible and definitely between plants.

And with that. . . Happy pruning!

Need more information?

For more information on plant diseases and their management, check out the UW Plant Disease Diagnostics Clinic website (, or contact PDDC staff at or (608) 262-2863.

February 2022: Optimizing Your Diagnosis at the PDDC

Diseased Leaves Branches and Fruits IconHere at the Plant Disease Diagnostics Clinic (PDDC), we strive to provide the best possible diagnoses of plant disease problems.  PDDC staff use a range of techniques to do this including visual and microscopic examination of plant tissue, incubation of tissue in moist chambers to coax fungal pathogens to sporulate, culturing techniques to grow pathogens from tissue, serological tests to detect proteins specific to certain pathogens, and DNA/RNA (i.e., genetic material) detection techniques.  The PDDC’s ability to provide a high quality diagnoses using these techniques however, is dependent on receiving a high quality sample to work with.

Here are some pointers on how to provide an optimal sample so that PDDC staff can provide an accurate, timely diagnosis.

Submit a sample when you first see a problem

Diagnosing plant diseases as early as possible allows for greater flexibility in management of diseases once they are identified.  So, contact PDDC staff as soon as you see a problem.  That said, disease-causing organisms may not produce structures that we need to see to make a diagnosis (e.g., fruiting bodies, spores) early in disease development.  We may need to work with you and have you repeatedly sample and submit materials over time to accurately diagnosis your plant disease problem.

Consider submitting photos prior to a physical sample

Sometimes, we can provide a reasonably accurate diagnosis based on photos (tar spot anyone?).  If not, photos can provide useful information on the sort of physical sample that you can send to the PDDC for a more detailed analysis.  You can use the online form at to submit photos.  If for some reason that form doesn’t work for you, feel free to send photos to the clinic email address (  If we are able to provide a diagnosis from your photos, there will be a $20 digital diagnosis fee.  If we make a preliminary diagnosis from the photos but suggest a follow-up physical sample submission for additional testing, the $20 digital fee will be credited towards any lab fees for your follow-up sample.

When submitting a physical sample, make sure you provide the appropriate plant part

If you have any questions about what to submit, contact the PDDC for advice.  Submitting the wrong plant part can lead to an inaccurate or delayed diagnosis.

For herbaceous plants (e.g., herbaceous ornamentals, vegetables), sending in an entire plant may be the best course of action.  In some situations, symptoms that you see on the leaves of plants are an indication of a problem in the root system.  With trees and shrubs, submitting an entire plant is not practical.  For these plants, submitting subsets of leaves/needles, branches and/or roots will likely be more appropriate.

Whatever you end up sending, send A LOT.  Often, we need to perform multiple tests to diagnose a problem.  We don’t want to run short on tissue.  For leaves, send half a dozen to a dozen (or more) showing a range of symptoms.  For branches, send three or more symptomatic branches (with attached leaves or needles where appropriate).  For roots, send a large handful of the small, fibrous roots.

In particular, appropriate branch selection can be critical for accurately diagnosing vascular wilt diseases such as Dutch elm disease, oak wilt and Verticillium wilt.  For these diseases, select branches that have recently wilted/died back.  DO NOT submit branches that easily snap off.  These branches have been dead too long and cannot be accurately tested for vascular wilt pathogens.  If you suspect Verticillium wilt, select symptomatic branches from as low on the tree as possible as the pathogen that causes this disease infects through the roots.  Choose branches that are roughly one inch in diameter.  Larger diameter branches (particularly from oak trees) tend to have thick bark that is difficult to remove without contaminating underlying tissue where vascular wilt pathogens reside.  This contamination will interfere with growing pathogens from branch tissue and can also interfere with DNA-based tests for pathogen detection.  Sometimes, clients like to send in trunk sections for testing for vascular wilts.  These sorts of samples can work, but only if the trunk slices are no more that about one inch thick.  We need to be able to easily pop off the bark from these slices with minimal contamination of the wood underneath.

Submit your sample as quickly as possible to the PDDC

If possible, collect samples just before you mail them or drop them off in person.  If there is going to be a delay in submission, keep samples as cool as possible.  High temperatures can kill certain pathogens and can degrade herbaceous plant tissues, leading to what we not-so-affectionately refer to as “slime in a bag”.  Degraded samples make diagnostic testing more difficult, if not impossible.  Mail samples by overnight mail when possible.  If mailing via regular mail, please mail early in the week (Monday or Tuesday) so that samples do not sit around in a mail facility over a weekend.  Click here for details on how to package samples to make sure they arrive at the PDDC in good shape.

Help us, help you

Providing us with a high quality sample can go a long way in us providing you with a high quality diagnosis.  Let’s work together to make this happen.  If you have questions about submitting a sample (or about plant diseases in general), feel free to contact the PDDC by phone at (608) 262-2863 or email at  Also check out the PDDC website ( for additional details on sample submission.  Feel free to follow the PDDC on Twitter and Facebook (@UWPDDC) or email me to subscribe to the PDDC listserv, UWPDDCLearn, to receive updates on clinic services and educational materials.

Good luck and happy plant disease sleuthing!

January 2022: Decontaminating Your Way to Healthier Plants

Bleach Bottle IconWhen discussing plant diseases and their management, I often emphasize decontaminating pots, gardening tools and work surfaces as one part of a successful disease management strategy.  To kill fungi and bacteria (and to a lesser extent viruses) I often recommend treating items for at least 30 seconds with alcohol, commercial disinfectants, or bleach.

When using alcohol, a 70% solution is optimal for decontamination.  I use ethanol for decontaminating items in my lab, but rubbing (i.e., isopropyl) alcohol is a better option for home gardeners, as it is readily available at a local drugstore or grocery store.  Rubbing alcohol is formulated as a 70% solution and can be used straight out of the bottle.

Commercial disinfectants come in many forms.  I look for products that contain alcohol (you may see ethanol listed as an ingredient) and where the percentage of all active ingredients is as close to 70% as possible.  If you opt to use a spray disinfectant, spray items until they drip and then allow them to air dry.

Bleach is probably the most challenging material to use for decontamination.  Back when I started at the PDDC (almost 25 years ago), most bleach that you could buy at the grocery store was formulated to be 5.25% sodium hypochlorite (the active ingredient), and you just had to make a 10% solution (i.e., 1 part bleach and 9 parts water) to yield an approximate 0.5% solution that is optimal for decontaminating items.  Nowadays, the bleach that you buy can contain very different concentrations of sodium hypochlorite (e.g., 1.8%, 5.25%, 7.5%, 8.25%), and you have to be more careful how you mix up your dilute bleach solution.

Here are some easy instructions on how to dilute the bleach that you buy (assuming it contains 1% or more sodium hypochlorite) to properly to yield a solution that is 0.5% sodium hypochlorite (or slightly higher):

  • Find the percentage of sodium hypochlorite in your bleach in the ingredient list;
  • Multiple that number by two (2);
  • Round the resulting number down to the nearest whole number;
  • Subtract one (1) from that rounded number.
  • This final number is the number of parts of water that you need to mix with one (1) part of your bleach to yield the diluted solution you need for decontaminating items.

When you use bleach on metal tools (alcohol is really a better option for treating metal items), be sure to rinse tools thoroughly with water after treating (to remove bleach residues) and then oil them.  Bleach corrodes metal and will cause metal tools to rust if not used properly.  Also use bleach carefully to prevent contact with your skin and clothing.  Bleach can cause skin irritation/burns and eat holes in your clothes, if not used properly.

Now go forth and decontaminate!

Need more information?

For more information on plant diseases and their management, check out the UW Plant Disease Diagnostics Clinic website ( or contact PDDC staff at or (608) 262-2863.


Root Rots on Houseplants

Wilting of poinsettia associated with Pythium root rot.
Wilting of poinsettia associated with Pythium root rot.

What is root rot? Root rot is a general term that describes any disease where the pathogen (causal organism) causes the deterioration of a plant’s root system. Most plants are susceptible to root rots, including both woody and herbaceous ornamentals. Root rots can be chronic diseases or, more commonly, are acute and can lead to the death of the plant.

How do you know if your plant has a root rot? Homeowners often become aware of root rots when they note that a plant is wilted, even though the soil is wet. Plants with root rots are also often stunted, and may have leaves with a yellow or red color, symptoms that suggest a nutrient deficiency. Careful examination of the root systems of these plants reveals roots that are soft and brown. These roots may have a bad odor.

Where does root rot come from? A large number of soil-borne fungi cause root rots. Pythium spp., Phytophthora spp., Rhizoctonia solani, and Fusarium spp. are the most common root rot fungi. These fungi have wide host ranges, and thus can cause root rots on a wide variety of plants. Most root rot fungi prefer wet soil conditions and some, such as Pythium and Phytophthora produce spores that can survive for long periods in soil or plant debris.

How do I save a plant with root rot? Often the best and most cost effective way of dealing with a plant with root rot is to throw it out. If you decide to keep a plant with root rot, REDUCE SOIL MOISTURE! Provide enough water to fulfill the plant’s growth needs and prevent drought stress, but DO NOT over-water.

Throw out plastic pots if plants grown in them have suffered from a root rot.
Throw out plastic pots if plants grown in them have suffered from a root rot.

We DO NOT recommend use of chemical fungicides for control of root rots on houseplants because of the limited availability of products for use by homeowners, and because those products that are available tend to be expensive.

How do I avoid problems with root rots? First, buy plants from a reputable source and make sure they are root rot-free prior to purchase. Second, replant your houseplants properly. Use a pot with drainage holes, but DO NOT put rocks or gravel at the bottom of the pot. The presence of rocks or gravel can actually inhibit drainage. Use a pasteurized commercial potting mix, NOT soil from your garden. Garden soils often contain root rot fungi. Add organic material (e.g., peat moss) to heavy potting mixes to increase drainage. Third, minimize potential contamination of your plants with root rot fungi. DO NOT reuse potting mix from your houseplants, or water that has drained from your plants, as both potentially can contain root rot fungi. After working with plants with root rot problems, disinfest tools, working surfaces and clay pots with a 10% bleach or detergent solution, or alcohol. DO NOT reuse plastic pots as they are often difficult to disinfest adequately. Finally and most importantly, moderate plant moisture. Provide enough water to fulfill your plants’ needs for growth and prevent drought stress, but DO NOT over-water. In particular, DO NOT allow plants to sit in drainage water. REMEMBER, root rot fungi grow and reproduce best in wet soils.

For more information on root rots: Contact your county Extension agent.


There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.
There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.

What are lichens? Lichens are organisms that arise from mutually beneficial interactions between certain filamentous fungi, algae and yeasts.  The filamentous fungi provide the physical structures of the lichens, as well as protection for the algae and yeasts.  The algae produce food for the fungi and yeasts via photosynthesis.  The yeasts are thought to produce compounds to fend off disease-causing organisms and insect pests.

What do lichens look like? Lichens come in four basic growth forms. Crustose lichens are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens are leaf-like and composed of flat sheets of tissue that are not tightly bound together. Squamulose lichens are composed of scale-like parts. Fruticose lichens are composed of free-standing branching tubes.

Where do lichens come from? Lichens are everywhere. There are an estimated 13,500 to 17,000 species of lichens, and lichens can be found growing in tropical, temperate and polar regions throughout the world. Lichens will grow on almost any surface that is stable and reasonably well-lit. In temperate regions, lichens can often be found growing on the bark of trees or old fence posts. Others lichens grow in less hospitable places, such as bare rock surfaces or old headstones in graveyards, where they aid in the breakdown of rocks and the formation of soil.

There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.
There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.

How do I save a tree with lichens? DO NOT PANIC! Lichens do not harm trees; they are not pathogens or parasites, and do not cause disease. Lichens are self-reliant, with the algal component of the lichen producing food for the organism via photosynthesis. Lichens absorb water and minerals from rainwater and the atmosphere, and because of this, they are extremely sensitive to air pollution. As a result, the presence or absence of certain lichen species can be used as an indicator of levels of atmospheric pollutants. Information on the abundance and species of lichens growing in an area can give a good indication of the local air quality.

For more information on lichens: Contact your county Extension agent.

Deciduous Tree Leaf Disease Quick Reference

Anthracnose Anthracnose
Hosts:  Most trees, commonly ash, maple and oak
Pathogens:  Gloeosporium spp. as well as other fungi
Signs/Symptoms:  Blotchy dead areas on leaves
For more information see:  UW Plant Disease Facts D0002
Purple-bordered Leaf Spot Purple-Bordered Leaf Spot
Host:  Amur, Japanese, red, silver and sugar maple
Pathogen:  Phyllosticta minima
Signs/Symptoms:  Discrete, circular leaf spots with purple borders
For more information see:  UW Plant Disease Facts D0089
 Tubakia Leaf Spot Tubakia (Actinopelte) Leaf Spot
Hosts:  Oak
Pathogen:  Tubakia spp. (Actinopelte spp.)
Signs/Symptoms:  Discrete circular, or irregular blotchy dead areas on leaves
For more information see:  UW Plant Disease Facts D0118
 Apple Scab Apple Scab
Hosts:  Apple, crabapple, pear, mountain-ash
Pathogen:  Venturia inaequalis, V. pirina
Signs/Symptoms:  Circular, black leaf spots with feathery edges; eventual leaf loss
For more information see:  UW Plant Disease Facts D0004
 Gymnosporangium Rusts Gymnosporangium Rusts
Hosts:  Apple, crabapple, hawthorn
Pathogens:  Gymnosporangium spp.
Signs/Symptoms:  Bright yellow-orange, circular leaf spots
For more information see:  UW Plant Disease Facts D0058
 Powdery Mildew Powdery Mildew
Hosts:  Most deciduous trees
Pathogens:  Several genera of powdery mildew fungi
Signs/Symptoms:  Uniform/blotchy powdery white areas on upper and lower leaf surfaces
For more information see:  UW Plant Disease Facts D0087
 Downy Leaf Spot Downy Leaf Spot
Hosts:  Hickory, walnut
Pathogen:  Microstroma juglandis
Signs/Symptoms:  Discrete powdery white areas on lower leaf surfaces
 Clorosis Chlorosis
Hosts:  Oak, red maple
Cause:   Iron or manganese deficiency, often induced by high soil pH
Signs/Symptoms:  Yellow leaves with dark green veins
For more information see:  UW Plant Disease Facts D0030
 Scorch Scorch
Hosts:  Most deciduous trees
Cause:   Water stress induced by drought, high soil salt content, or other water-limiting factors
Signs/Symptoms:  Dead tissue on leaf margins
 Tatters Tatters
Hosts:  Most deciduous trees, but commonly oak
Cause:  Possible early season cold injury
Signs/Symptoms:  Lacy, tattered-looking leaves
For more information see:  UW Plant Disease Facts D0111

For more information on deciduous tree leaf diseases:  See or contact your County Extension agent.