Category Archives: Plant Disease Pointers

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.

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.


July 2021 – Sometimes the Best Medicine is No Medicine at All

Pill IconAt this time of the year, plant diseases are in full swing.  When you see diseases in your garden, a natural tendency is to want to do something to make things better.  In some instances however, ignoring the problem and doing nothing can be your best course of action.  Here are my picks for plant diseases where turning a blind eye (at least right now) will not significantly harm your plants and will make your life easier and less stressful.

Powdery mildews

On most plants, powdery mildews don’t do much damage.  Ignore them now and concentrate on good fall clean up to reduce problems with these diseases next year.  For plants such as phlox, bee balms, cucumbers and pumpkins, powdery mildews can be more problematic.  You may want to consider growing powdery mildew-resistant varieties of these plants in the future.

Tar spot

This disease of maples can be visually alarming but is another disease that typically has little impact on overall plant health.  Just be sure to collect infected leaves in the fall and burn (where allowed), bury or hot compost them to help reduce problems with the disease next year.

Cedar-apple rust

Seeing bright yellow or orange spots on your crabapple leaves?  If so, you’re likely seeing this disease.  Spraying fungicides to control cedar-apple is a total waste.  Save your time, money and energy and prune out the galls on junipers caused by the disease (and the source of the spores that infect your crabapples) or alternatively, just remove nearby junipers

Remember. . . Before choosing a control strategy, know thy plant disease.

Need more information?

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

June 2021: A Change in the Weather: Modifying Garden Microclimate to Improve Plant Health

Windy Cloud IconAs plants in your garden grow and fill in,  wet and humid conditions will arise that are perfect for diseases to develop.  Follow the tips below to prevent diseases in your garden by opening up air flow, reducing humidity and keeping leaves dry.

Leave plenty of space between new transplants

Think ahead to how big plants will be when full-size, and leave enough space so that foliage on adjacent plants won’t overlap when plants are fully grown.

Divide perennials

Plants like peonies and daylilies grow in clumps that can get quite large, and leaf disease problems tend to increase with clump size.  So, divide clumps to yield smaller plants that trap less moist air.  Dividing and replanting will also allow you to correct existing spacing problems.

Thin plants judiciously as they get big

Remove enough leaves and stems to promote good air flow, but not so many that the plants look thin and lanky.

Weed, weed, weed

Ornamentals aren’t the only plants that trap moist air; weeds can do this, too.  Removing weeds routinely can reduce the need to thin the ornamentals that you really want in your garden.

Water from below

Even though Mother Nature supplies water from above in the form of rain, when you need to water, apply water to the soil, rather than over the tops of plants.  Keep leaves dry whenever possible.

Modifying the environment in your garden can help prevent diseases like powdery mildew, shown here on phlox.
Modifying the environment in your garden can help prevent diseases like powdery mildew, shown here on phlox.

With just a little effort, you can create a microclimate in your garden that is less favorable for plant diseases and end up with more vibrant, beautiful, and healthy plants.

Need more information?

For more information on specific plant diseases and their management, check out the fact sheet section of the UW-Madison PDDC website (

May 2021: Smart Shopping: Dodging Disease When Purchasing Plants

Potted Plant IconMay is a prime time to visit your local greenhouse, nursery or garden center to buy annuals, perennials and vegetables for your home garden.  Unfortunately, these plants can be carriers of plant disease-causing organisms.  Here are some pointers on what to look for when buying plants.

Select plants that are vigorously growing, but aren’t overly leggy

Stunted plants often have diseases (e.g., root rots or viral diseases).  Leggy plants may be healthy, but often have growth that is wimpy and easily injured.  Injured tissue can provide entry points for plant pathogens.

Avoid plants with brown leaf spots

Dead spots on leaves can indicate fungal or bacterial infections.  The pathogens involved continue to reproduce in these areas and can spread on the plant, and potentially to other plants.

Avoid plants with odd leaf coloring

Some plants have variegated foliage.  That’s normal.  However, if you see plants with unexpected blotchy light and dark-colored leaves, yellow lines or yellow ring patterns, avoid these.  They may be infected with a virus.

Avoid plants with fuzzy growth on the leaves

Such growth typically indicates a fungal or water mold infection.  For example, white growth on upper and lower leaf surfaces can indicate a powdery mildew problem; white, gray or purplish growth on the undersides of leaves is typical of a downy mildew

Choose plants with healthy, white roots

Examine plant roots whenever possible.  If roots are brown or otherwise discolored, root rots or some sort of physical root injury (e.g., heat injury) may be an issue.

Avoid plants with blotchy leaf color, an indication of a viral infection.
Avoid plants with blotchy leaf color, an indication of a viral infection.

With just a little care, you can buy healthy, pathogen-free plants that will provide months, if not years, of gardening enjoyment.

Need more information?

For more information on plant diseases and their management, check out the fact sheet section of the UW-Madison PDDC website (

April 2021: Water, Water, Everywhere, but Never, Ever on the Leaves

Thundercloud with Rain IconApril conjures up visions of spring rain showers, eventually leading to May flowers.  While Mother Nature waters plants using an overhead sprinkling system (i.e., rain), from plant disease standpoint, this type of watering is less than optimal.

What’s the problem with watering from above?  

Overhead watering wets leaves creating what plant pathologists call a “leaf wetness period,” a time when a thin layer of water coats the leaf.  This layer of water is exactly what most plant diseases-causing fungi require for their spores to germinate and infect.  The longer the leaf wetness period, the more likely leaf diseases will be a problem.

Wet Leaves. Photo by Diana Alfuth
Wet Leaves. Photo by Diana Alfuth

What do I do to prevent watering issues? 

You can’t prevent rain, but during dry periods when you need to water, don’t simulate rain by using a sprinkler.  Instead, use a soaker or drip hose that directs water into the soil rather than onto leaves.  Water at low pressure so that any sprays from these hoses are minimized.  When it does rain, promote rapid drying of leaves by spacing plants as far apart as possible in new plantings, and thinning existing beds to increase spacing.  Wider plant spacing increases air flow, promotes more rapid drying of leaves (when leaves do get wet), and shortens leaf wetness periods, making it more difficult for plant pathogens to get a foothold.  Don’t overwater either.  Excessive soil moisture eventually increases humidity around plants, which slows leaf drying and lengthens leaf wetness periods.

By taking just a little extra care in how you water, you can have a big impact on the health of the plants in your garden.

Need more information?

For more information on leaf diseases, check out the fact sheet section of the UW-Madison PDDC website (

Photo courtesy of Diana Alfuth.

March 2021: Dampening Damping-Off: Tips on Seed Starting to Avoid Disease

Germinating Seed IconAfter a long, cold winter, it’s time to start growing plants from seeds for the upcoming growing season.  Damping-off is a common disease that can prevent seed-starting success.  Here are tips that can help prevent damping-off from being a problem.

Damping Off
Damping Off

Buy high quality seed from a reputable source

High quality seeds are less likely to carry damping-off organisms.

Use pasteurized soil

Pasteurized soil has been steam treated to kill pathogens.

Use clean pots/containers

Store new pots or flats in sealed plastic bags to prevent possible pathogen contamination prior to use.  When reusing pots, soak them in 10% bleach (1 part of a disinfecting bleach, 9 parts water) for 30 minutes, then rinse well to remove bleach residues.

Plant seeds at the proper depth

This will promote quick germination and rapid growth of seedlings out of the early stages of growth when they are most susceptible to damping-off.

Start seeds at higher temperatures

This will again help plants grow out of their susceptible phase quickly.  Consider using a seed-starting heat pad (available at your local garden center), particularly if you start seeds in a colder part of your home (e.g., a basement).

Don’t overwater

Damping-off organisms are more active in wet soils.  Water enough to keep seedlings alive, but keep plants a bit on the dry side to slow development of damping-off pathogens.

Seed starting can be a fun way to start the gardening season.  With just a little extra effort, you can prevent damping-off from dampening your gardening efforts.

Need more information?

For more information on damping-off and its management, check out University of Wisconsin Garden Facts XHT1124 (Damping-Off), available at or

February 2021: Bypassing Plant Pathogens: Promoting Tree and Shrub Health Through Proper Pruning

Pruning in the winter can reduce the risk of disease-causing organisms infecting trees and shrubs through pruning cuts.
Pruning in the winter can reduce the risk of disease-causing organisms infecting trees and shrubs through pruning cuts.

Although it doesn’t seem like the optimal time to be gardening, February is actually a great time to be out pruning your trees and shrubs to make them more structurally sound and aesthetically pleasing.

Why prune now? 

Whenever you prune, you create wounds that potentially can serve as entry points for disease-causing fungi and bacteria.  If you prune in the spring and summer (when it’s warmer and often wetter), these organisms are very active and more likely land on fresh pruning cuts and infect.  When the weather is colder and drier (as it tends to be in February in Wisconsin), disease-causing organisms are much less active and the chances of them infecting though pruning cuts is much reduced.

How do I go about pruning? 

Check out University of Wisconsin Garden Facts XHT1013 (Pruning Evergreens), XHT1014 (Pruning Deciduous Trees) and XHT1015 (Pruning Deciduous Shrubs) for pointers on how to prune.  Prune only when it’s dry, and decontaminate pruning tools between cuts (or at a minimum between each tree or shrub) by treating them with 70% alcohol (e.g., rubbing alcohol right out of the bottle, spray disinfectants containing ~70% alcohol) or (in a pinch) 10% bleach.  Decontaminating tools kills off disease-causing organisms that you might pick up as you prune.  Once done pruning, if you’ve used bleach, be sure to thoroughly rinse your tools, and oil them to prevent them from rusting.

By pruning regularly and taking a few simple precautions as you do, you will end up with trees that are beautiful, structurally sound and healthy.

January 2021: Saying Bye-Bye to Boxwood Blight: Proper Disposable of Holiday Wreaths

Holiday wreaths containing boxwood sprigs can be a source of the fungus that causes boxwood blight. (Photo courtesy of Steven Shimek, Minnesota Department of Agriculture)
Holiday wreaths containing boxwood sprigs can be a source of the fungus that causes boxwood blight. (Photo courtesy of Steven Shimek, Minnesota Department of Agriculture)

As the holiday season wraps up and you begin to put away your decorations for the year, be aware that certain perishable holiday items need to be disposed of properly.  In particular, be careful of how you dispose of holiday wreaths and other garlands that contain boxwood sprigs.

Why the concern? 

Boxwood-containing holiday decorations have been found to be a potential source for the fungus that causes boxwood blight.  This disease has been devastating boxwood plantings in the East for nearly a decade and was recently (starting in 2018) found in Wisconsin in both nursery stock and in home landscapes plantings.  While documented cases of boxwood blight in Wisconsin have been relatively few, it’s important to do everything possible to reduce spread of the boxwood blight fungus.  Boxwood is THE most valuable woody ornamental produced and used in landscapes in the US.

What can I do? 

When disposing of your holiday wreaths and garlands, check to see if they have boxwood sprigs.  If you are unsure, assume that they do.  Dispose of any suspect materials by burning them if that is allowed in your local municipality.  If burning is not possible, bury the materials at least two feet underground (most likely impractical in Wisconsin in January) or double bag the materials in sturdy garbage bags and have them landfilled.

Have questions?

For more information on boxwood blight and its management, check out University of Wisconsin Garden Facts XHT1265 (Boxwood Blight), available at or