All posts by ddlang

January 2022: Plant Disease Diagnostics Clinic Services for 2022

Microscope IconAs we start 2022, COVID-19 continues to inform how the UW Plant Disease Diagnostics Clinic (PDDC) provides services to the public.  Clinic personnel continue to try 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.  Given that the COVID-19 situation is ever evolving, there will likely be changes in these guidelines as the year progresses.  Be sure to check this document routinely for updates.

PLANT DISEASE DIAGNOSTICS:  In 2020, I started offering formal digital diagnoses, and that service will continue in 2022.  I really encourage you to submit digital photos of your plant disease problems before submitting physical samples.  Use the online form at  If I can see enough of what’s going on in your photos, I will provide you a diagnosis with management recommendations and charge the $20 digital diagnosis fee.  If I cannot make a definitive diagnosis, I will not charge the fee, but I will use the photos to help me provide you with suggestions on what sort of physical sample to submit as a follow-up.  Note that if I charge the digital fee and you end up submitting a follow-up physical sample, the digital fee will be credited towards any lab fees for your physical sample.

You can submit physical samples for diagnosis either by mail or in person.  For increased safety, I suggest submitting by mail.  If you opt to drop off a sample in person, PDDC staff will not be available for an in-person consultation.  While I am committed to providing personalized service to my clients, continuing issues with COVID-19 suggest that limiting person-to-person contact is prudent.  Also note that wearing a mask is required in all UW-Madison buildings at this time.

The PDDC provides a sample drop off location 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 your 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 fill it out ahead of time to speed up the submission process.  Please print neatly and make sure to include your complete mailing address, your phone number and your email address.  At this time, I am emailing virtually all reports and invoices.  If you do not have email however, I definitely can send out a hard copy of your report and invoice.

PRESENTATIONS:  I was hoping to start providing in-person PDDC presentations in 2022, but with COVID-19 cases increasing at this time, I have cancelled the few in-person talks that I already had scheduled in early 2022 and will be limiting presentations to virtual presentations (e.g., via Zoom) until further notice.  If you are interested in having me provide a presentation, 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.

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 revised and rebranded UW Plant Disease Facts (formerly the plant disease-related titles of the University of Wisconsin Garden Facts).  I am also working on converting some of my in-person talks [Fundamentals of Plant Diseases and The Science (and Art) of Plant Disease Diagnosis] into online training modules with voiceovers.  As these new resources become available, I will announce their availability via Twitter, Facebook (@UWPDDC), or via my clinic listserv, UWPDDCLearn (email me to subscribe to this).

CONSULTATIONS:  As always, if you have questions about plant diseases, feel free to contact me by phone at (608) 262-2863 or email at

Stay safe everyone and all the best for 2022!

Thyronectria Canker

What is Thyronectria canker?  Thyronectria canker is a common fungal disease of honeylocust (Gleditsia triacanthos), occurring wherever this tree is grown.  Black locust (Robinia pseudoacacia) and Kentucky coffeetree (Gymnocladus dioicus) have also been reported to be susceptible to the disease.  Thyronectria canker is non-lethal and typically causes relatively minor damage to affected trees.

Thyronectria canker leads to sunken areas on honeylocust branches that are peppered with black spots.
Thyronectria canker leads to sunken areas on honeylocust branches that are peppered with black spots.

What does Thyronectria canker look like?  Thyronectria canker causes branch dieback.  Affected branches are typically relatively small in diameter (approximately one inch or less), although larger branches can become infected.  On smaller branches, distinct sunken areas (cankers) form at the point of infection.  Within the sunken area, small dark-brown to black spots are typically visible.

Where does Thyronectria come from?  Thyronectria canker is caused by the fungus Thyronectria austoamericana (sometimes written Thyronectria austo-americana).  The spots visible within the sunken areas on infected branches are clusters of fruiting bodies (i.e., reproductive structures) of the fungus.  These structures produce multicelled, light- to medium-brown spores that, when released, can blow to susceptible trees, leading to new infections.  Long periods of wet weather are favorable for infection to occur.

How do I save a tree with Thryronectria canker?  Remove infected branches by pruning four to six inches below obviously infected areas on branches.  Be sure to prune only when it is dry, and be sure to disinfest pruning tools between cuts by treating them for at least 30 seconds with 70% alcohol (e.g., rubbing alcohol straight out of the bottle), spray disinfectants (as long as they contain 60 to 70% alcohol) or 10% bleach.  If you use bleach, be sure to thoroughly rinse and oil tools after you are done pruning to prevent rusting.  Dispose of branches by burning (where allowed) or burying them.  DO NOT use fungicides for control of this disease.

How do I avoid problems with Thyronectria canker in the future?  Make sure your honeylocust tree is properly fertilized and watered to reduce stress and promote vigorous growth.  Fertilize your honeylocust only if you have soil and foliage nutrient tests that indicate nutrient deficiencies that need to be corrected.  The UW Soil and Forage Lab ( can assist with testing.  An established honeylocust tree (i.e., a tree that has been planted for three or more years) requires approximately one inch of water per week from the time it buds out in the spring, through the summer and into the fall up until it starts to turn its normal fall color.  When there is insufficient rain, water at the drip line of the tree (i.e., the edge of where the branches extend), or more extensively in the root zone if possible, using a drip or soaker hose.  Remove grass out to the drip line of the tree, and mulch this area with a high quality mulch (e.g., shredded oak bark mulch, one of the cedar mulches).  Use one to two inches of mulch if you have a heavier (e.g., clay) soil and three to four inches if you have a lighter (e.g., sandy) soil.  Keep mulch four inches from the trunk of the tree.  Finally, consider routine maintenance pruning by a certified arborist (see to thin the canopy of your honeylocust tree.  Thinning will provide better airflow through the tree, promoting more rapid drying of branches and leaves, thus leading to a less favorable environment for infections to occur.

For more information on Thyronectria canker:  Contact your county Extension agent.

Iris Severe Mosaic

What is iris severe mosaic?  Iris severe mosaic (also called yellow latent disease or gray disease) is a potentially severe viral disease that can adversely affect both bulb and rhizome-forming irises, as well as crocuses.  German bearded irises are particularly susceptible to the disease.  Commercially produced irises and crocuses affected by iris severe mosaic cannot be sold.  Thus, iris severe mosaic can have potentially significant economic consequences for iris and crocus producers.

Pale green and yellow stripes on iris leaves are typical symptoms of iris severe mosaic.

What does iris severe mosaic look like?  Symptoms of iris severe mosaic can occur on any plant part.  Leaves, particularly middle or outermost leaves, may have pale green to yellow stripes.  Younger, interior leaves often do not exhibit symptoms.  Flowers may develop blotchy color (a symptom known as color break).  Overall, affected plants may be stunted, producing smaller than normal flowers, and smaller bulbs, rhizomes or corms.  Symptoms tend to be more severe when temperatures are cooler.  At higher temperatures, symptoms are less obvious.  Similarly, iris and crocus plants grown indoors tend to develop more severe symptoms than those grown outdoors.  In some situations, plants with iris severe mosaic may not show any symptoms.

Where does iris severe mosaic come from?  Iris severe mosaic is caused by Iris severe mosaic virus (ISMV), a virus transmitted primarily by aphids, specifically the potato aphid (Macrosiphum euphorbiae) and the green peach or peach-potato aphid (Myzus persicae).  These aphids acquire the virus from infected plants and subsequently transmit the virus to non-infected plants as they feed.  ISMV can also be spread as infected plants are divided to produce additional plants.

Tools (e.g., pruning tools, knives, etc.) used when working with infected plants can become contaminated with sap containing ISMV and can serve as another means of spreading the virus to healthy plants.

How do I save plants with severe iris mosaic?  Most types of iris can tolerate low levels of ISMV.  However, infected plants remain infected indefinitely and cannot be treated in any way to eliminate the virus.  Therefore, you should dig up and either bury or burn affected plants as soon as you observe symptoms.  This will help limit the spread of the virus.

How do I avoid problems with iris severe mosaic in the future?  When possible, plant Siberian iris (Iris sibirica) as this species is resistant to ISMV.  Take care when planting German bearded iris (Iris germanica).  This type of iris is very popular (and incredibly beautiful) but tends to show more severe symptoms of iris severe mosaic.  When purchasing iris plants, buy only from reputable producers who have an ISMV management plan.  Such a plan should include careful monitoring of stock plants for iris severe mosaic symptoms, diligent removal and destruction of infected plants, routine removal of weeds in production areas to eliminate plants that can serve as reservoirs for aphids, and applications of insecticides to control aphid populations.

When dividing iris plants, decontaminate tools routinely by treating them for a minimum of one minute with:

  • 75 tablespoons Alconox® (a lab detergent) plus 2.5 tablespoons sodium lauryl sulfate (SLS), also known as sodium dodecyl sulfate (SDS), in one gallon of water, or
  • 14 dry ounces of trisodium phosphate (where allowed by state or local ordinance) in one gallon of water.

These ingredients can be ordered on the internet.  If you decide to use SLS (SDS), be sure to wear gloves, safety goggles and a dust mask, and mix the solution in a well-ventilated area as SLS (SDS) is a known skin and eye irritant.  Once treated, rinse items with sufficient water to remove any residues.

For more information on severe iris mosaic:  Contact your county Extension agent.

July 2021: Summer Doldrums – Wilted Tomatoes in the Garden

Tomato IconI have recently received a slew of questions about wilted tomatoes in home gardens.  Here are the top five reasons that tomatoes can wilt based on samples that I have received in my clinic over the years.

Walnut toxicityOne of the lessons that I have learned after doing plant disease diagnostics for over 20 years is that when a home gardener consults me about wilting tomatoes, the first question I should ask is, “Do you have a walnut tree near your vegetable garden?”  More times than not, the answer is “Yes” and the walnut tree is the cause of the problem.  Black walnuts produce toxins (exuded by roots and produced in leaves and fruits) that adversally affect a wide range of plants,  Tomatoes are particularly sensitive and are often die from the exposure.  Anytime that tomatoes are grown in the root zone of a walnut tree (which extends three to five times the height of the tree from the trunk), problems can arise.  Cutting down walnut trees will not solve the problem in the short term, because roots from the cut tree can continue to exude toxins for 15 to 20 years.  Often the best recourse when walnut trees are present in a landscape is to grow tomatoes in raised beds or in pots to keep tomato roots as far above walnut roots as possible.

Drought stress:  In 2021, lack of rain has been a potential cause for wilting in tomatoes and virtually every other plant.  Most established plants require about one inch of water per week.  When rain is insufficient (as it has been in much of Wisconsin this year), it’s important to apply supplemental water to plants with a soaker or drip hose.  Proper watering can not only prevent wilting in tomatoes, but it can also help improve calcium uptake and reduce problems with blossom end rot.  Using an inch or two of a high quality mulch (my favorites are shredded oak bark mulch and red cedar mulch) around plants can help retain moisture and lessen wilting issues.  Mulching around tomatoes also helps reduce movement of spores (produced in bits of old tomato debris in the soil) of the fungi that cause Septoria leaf spot and early blight.

Bacterial canker:  The bacterium that causes this disease (Clavibacter michiganensis subsp. michiganensis – THERE’S a mouthful) is seedborne, so gardeners typically introduce this pathogen into their gardens on contaminated tomato seeds or transplants.  Plants initially look healthy, but the bacterium eventualy colonizes, discolors and disrupts the water-conducting (vascular) tissue inside the plant, leading to wilting.  Infections can lead to long, somewhat subtle cracks in stems and ultimately less subtle open wounds (i.e., cankers) in stems near the soil line.  Another telltale symptom of the disease can be ghostly-white spots with a darker center (called bird’s-eye spots) on tomato fruits.  Removal and destruction of infected plants, and rotation away from susceptible vegetables (e.g., tomatoes and peppers) for several years in the affected area of a garden are typical management strategies.

Verticillium wiltMany gardeners are familiar with this disease in the context of the death and destruction it brings to woody trees and shrubs.  However, Verticillium, the cause of Verticillium wilt, is an equal opportunity destroyer and can kill a wide range of herbaceous plants as well, including popular vegetables such as solanacoues crops (e.g., tomato, potato, eggplant, pepper) and vine crops (e.g., cucumber, squash, pumpkin).  This fungus is routinely found in the soil and can build up over time if susceptible vegetable crops are grown over and over again in an area where the fungus is located.  Verticillium infects through the roots and colonizes and plugs a tomato’s (or other plant’s) water-conducting tissue, leading to wilting.  Discoloration of a tomato plant’s vascular tissue is a typical symptom of this disease, but stem cracks and cankers are notRotation can be useful as a control strategy for Verticillium wilt, although it is less effective than for bacterial canker because of the wider host range for Verticillium (including many weeds).  For tomatoes, use of resistant varieties can also be useful.  To identify resistant varieties, look for a “V” after the variety name on a tomato seed packet or in the variety description in your favorite seed catalog.

Fusarium wilt.  This disease is very similar to Verticillium wilt except for the fungus involved.  For Fusarium wilt, Fusarium oxysporum f. sp. lycopersici is the culprit.  Fusarium oxysporum is a large fungal species with many special forms (that’s what “f. sp.” stands for), each one adapted to infect a specific host plant or a very small range of host plants (e.g., vine crops).  Fusarium oxysporum f. sp. lycopersici is specific to tomatoes and will not infect other vegetable crops.  If you ever encounter this disease, rotation away from tomatoes in the affected area for several years should work well as a management strategy.  In addition, you can use resistant tomato varieties.  Look for one or more ‘F’s” after the variety name.

As you can imagine, figuring out the exact reason your tomatoes are wilting can be challenging, particularly if there is disease involved.  For help with proper diagnosis of tomato wilts (and other plant problems in general), contact the PDDC at or (608) 262-2863.

To find out more about the clinic and its activities, check out the PDDC website.  To keep up-to-date about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

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

Plant Disease Pointers - July 2021 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 spotThis 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 rustSeeing 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.

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: Fired Up About Fire Blight

FireIconforJune 2021 Monthly ColumnIt‘s the time of year where I am once again getting questions about apple and crabapple trees with dead branches.  Often, in these situations, clients assume that their trees are suffering from fire blight.  This bacterial disease has received a lot of press over the years and can be a serious problem.  However, fire blight is definitely not the only reason that branches on apples and crabapples die.

There are myriad of fungal diseases that can lead to branch dieback.  In particular, fungal canker diseases can be an issue.  I often find fungi like Cytospora, Phomopsis and Sphaeropsis in dead apple or crabapple branches.  These fungi, like the fire blight bacterium, locally infect and girdle branches (thus leading to branch death), often in a somewhat random pattern in a tree.  Sphaeropsis is particularly common.  This fungus not only infects branches but can also infect fruits (causing black rot) and leaves (causing frogeye leaf spot).  More systemic fungal diseases like root and crown rot, Armillaria root disease and black root rot (dead man’s fingers) can also lead to branch dieback.  The pathogens involved in these diseases infect and disrupt root and trunk function, preventing proper water movement from roots to branches.  This lack of water leads to branch death, often over a fairly substantial portion of the tree canopy.

Environmental stresses can also lead to branch death in apples and crabapples.  Drought can lead to branch dieback symptoms similar to those caused by the systemic diseases I described above.  Cold injury can also be a contributing factor.  Growing a non-hardy apple or crabapple variety often leads to dieback issues.  Even on hardy varieties, branch dieback can occur if cold snaps occur in the spring right as or just after trees leaf out.  Lack of snow cover (which insulates soil) coupled with extremely cold winter temperatures can lead to physical injury to roots, which in turn limits water uptake, leading to branch dieback.

So, with all of these potential causes of branch dieback on apples and crabapples, how can you tell if your tree is suffering from fire blight?  The answer is, “It’s not easy!”  People often claim that fire blight leads to a branch with a shepherd’s crook (a downward bend at the branch tip).  However, after years of seeing dead and dying apple and crabapple branches, I just don’t consider this a reliable symptom for diagnosing fire blight.  To me, a shepherd’s crook just indicates that the branch didn’t get enough water and wilted.  That could be due to any of the causes I outlined above.  And conversely, I have seen cases of fire blight where branches don’t have a shepherd’s crook.  What I tend to look for as I’m attempting to diagnose fire blight is oozy material (a combination of sap and bacterial cells) that seeps from affected branch.  I also look for some indication that the infection may have started where flowers were attached.  I look for this latter indicator because trees are often inoculated with the fire blight bacterium by bees that carry the bacterium and drop it off in the flowers as they pollinate.  Even when I see these symptoms, I will only diagnose fire blight if I have evidence that the fire blight bacterium is present.  There are dipstick serological tests (these use the same technology as home pregnancy kits) that I use to confirm the presence of the fire blight bacterium.  If I don’t find evidence of the bacterium, I look for other possible causes of the branch dieback.

So, why do I really need to know if branch dieback is really due to fire blight?  It all comes down to management.  If fire blight is the cause, I recommend very aggressive pruning (roughly 12 inches below where there are obvious symptoms).  The fire blight bacterium can move rapidly down a branch under the bark, so you want to make sure to prune down far enough to remove all of the bacterium.  Fungal pathogens tend to move less rapidly, so you can get by with pruning roughly six inches below where there are obvious symptoms.  If the problem is a root disease of some kind, pruning will not resolve the problem.  Fungicide treatments to the roots may be needed in some instances, or there may be ways of reducing tree stress that slow down the progression of these types of diseases.

It all comes down to the fact that if you don’t know what the underlying problem is with your tree, it is unlikely that you will be able to fix the problem.  So, get a proper diagnosis and then tailor your management strategy to the specific problem(s) you are facing.  Without a proper diagnosis, you can spend a lot of time, effort and money, and not improve the health of your trees one bit.

For help with proper diagnosis of plant problems, contact the PDDC at or (608) 262-2863.  To find out more about the clinic and its activities, check out the PDDC website.  To keep up-to-date about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

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.

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.

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.

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

Vegetable Disease Quick Reference

Septoria Leaf Spot Septoria Leaf Spot and Early Blight
Host:   Tomato
Pathogens:   Septoria lycopersicia and Alternaria solani
Signs/Symptoms:  Spotting and eventual total collapse of leaves working from the bottom of the plant up
For more information see:        UW Garden Facts D0100/D0046
Late Blight Late Blight
Hosts:   Tomato, potato
Pathogen:   Phytophthora infestans
Signs/Symptoms:  Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death
For more information see:        UW Garden Facts D0068
Blossom End Rot Blossom End Rot
Host:   Tomato, pepper, eggplant, cucumber, squash
Cause:   Calcium deficiency
Signs/Symptoms:  Decayed areas on the bottom sides of vegetable fruits
For more information see:        UW Garden Facts D0022
Powdery Mildew Powdery Mildew
Hosts:    Any vegetable, particularly vine crops, peas
Pathogens:    Miscellaneous powdery mildew fungi
Signs/Symptoms:  Powdery white growth on leaves
For more information see:        UW Garden Facts D0086
Common Corn Smut Common Corn Smut
Hosts:   Corn
Pathogen:   Ustilago maydis
Signs/Symptoms:  Pasty white masses on corn ears eventually decomposing into a brown powder
For more information see:        UW Garden Facts D0031
Black Rot Black Rot
Hosts:   Crucifers (e.g., cabbage, broccoli, cauliflower)
Pathogen:   Xanthomonas campestris pv. campetris
Signs/Symptoms:  V-shaped yellow/dead areas on leaves progressing into plant deterioration and death
For more information see:        UW Garden Facts D0019
Potato Scab Potato Scab
Hosts:    Potato, carrot, beet, other root crops
Pathogen:  Streptomyces scabies
Signs/Symptoms:  Brown, rough, scab-like areas on tubers and roots
For more information see:        UW Garden Facts D0083
Verticillium Wilt Verticillium Wilt
Host:   Tomato, pepper, eggplant, potato, vine crops
Pathogen:  Verticillium spp.
Signs/Symptoms:  Leaf yellowing and wilting of plants followed by eventual plant death
For more information see:        UW Garden Facts D0122
Aster Yellows Aster Yellows
Hosts:    Carrot
Pathogens:    Aster yellows phytoplasma
Signs/Symptoms:  Yellow/orange/purple leaves, stunted roots with tufts of white hairy roots
For more information see:        UW Garden Facts D0007
Bacterial Wilt Bacterial Wilt
Hosts:   Vine crops
Pathogen:   Erwinia tracheiphila
Signs/Symptoms:  Sectional wilting and eventual death of plants after cucumber beetle feeding
For more information see:        UW Garden Facts D0014
Basil Downy Mildew Basil Downy Mildew
Host:   Basil
Pathogen:   Peronospora belbahrii
Signs/Symptoms:  Downward-cupped, yellow leaves with purple-gray fuzz on leaf undersurfaces
For more information see:        UW Garden Facts D0015

For more information on conifer diseases:  See or contact your county Extension agent.

May 2021: Rampant, Ravaging Rusts

May 2021 Column IconI really love this time of the year as plants emerge once again after their long winter naps.  I also love the fact that I now get to start looking once again for some of my favorite plant diseases.  At the top of the list at this time of the year are the rust diseases.  Rusts encompass a large group of fungal diseases, where the fungi produce brightly-colored (yellow to orange to bown) spores.  Each rust fungus has a very specific host range.  The following are a few of the rusts (one on a broad-leafed tree, one on a conifer and one on an herbaceous plant) that I have recently seen either through the clinic or in my own yard.

Crown Rust Buckthorn
Crown Rust on Buckthorn

Crown rust:  Crown rust is classic rust of grass species such as turfgrass and oats.  If you’ve ever walked through your lawn and gotten orange shoes, you’ve encountered this disease.  The fungus that causes crown rust is Puccinia coronata, which has several variants adapted to infect specific grass hostsThe disease and pathogen names come from the look of the resting spores of the fungus.  These spores have spikes that give them the appearance of a crown.  At this time of the year, I don’t see crown rust on grass or oats (that comes later in the growing season), but I see it on a second host (called the alternate host) of the fungus, buckthorn.  Buckthorn is actually required by the crown rust fungus to complete its life cycle.  Puccinia coronata causes yellow-orange, powdery patches on the buckthorn leaves and green stems, and I actually use crown rust as an ID feature for buckthorn.  If I see seedlings that I think are buckthorn, but I’m not quite sure, I look for the characteristic orange patches of crown rust to confirm.  And if you need another reason get rid of buckthorn, in addition to this plant being incredibly invasive, here it is.  If you remove buckthorn, you will prevent the crown rust fungus from completing its life cycle and reduce the severity of the disease on turf and oats.

Weirs Cushion Rust
Weirs Cushion Rust on Spruce

Weir’s cushion rustThis is rust disease of spruce that I see infrequently, but I just received a sample of it this past week in the clinic.  I was over the moon!  (Yes, I know I’m weird and lead a very sheltered life.)  The fungus that causes this disease is Ceropsora weirii (formerly Chrysomyxa weirii), a single-host rust fungus that only requires spruce to complete its life cycle.  Infection leads to yellow banding on one-year-old needles.  Within these bands in the spring (typically April or May), fruiting bodies (i.e., reproductive structures) of Ceropsora weirii form yielding orangish spores that blow to newly emerging needles where the fungus infects.  The fruiting bodies are easily visible with a hand lens or even with the naked eye.  Eventually the infected3 needles brown and drop off the tree.  Management of Weir’s cushion rust can be a challenge and typically involves use of fungicides to protect newly emerging needles.

Mayapple Rust

Mayapple rust:  I’d like to give a shout-out to Brenda Dahlfors, Master Gardener Program Coordinator with University of Illinois Extension for sending me photos of this cool rust.  The fungus involved here is another species of Puccinia, Puccinia podophylli.  This is another single-host rust, where the fungus that only infects mayapples.  The disease is most visible in the spring when bright orange, powdery patches develop on the undersides of leaves.  On the upper leaf surfaces above these patches, you will see yellow spots/discolored areas.  The orange spores produced by the pathogen reinfect mayapple plants, causing additional disease.  The bright orange patches tend to fade to a duller brown as they age and convert to producing brown overwintering spores.  These overwintering spores germinate in the spring to produce yet another type of spore that causes the initial infections in the spring.  Careful removal of infected leaves and plant debris (burn, bury or hot compost this material) combined with fungicide sprays where appropriate is the typical management strategy for this disease.

These are just a few of the cool rust diseases that you may encounter as you are out and about.  Watch for these and other rusts, and enjoy them when you find them.  There are the most visually colorful and attractive diseases that I see.  For additional information on the PDDC and its activities, check out the PDDC website.  To learn about new PDDC education materials and programs, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at and ask to be added to the PDDC’s listserv (UWPDDCLearn).

Plant Diseases to Watch For in 2021

Septoria Leaf Spot Septoria Leaf Spot and Early Blight
Host:  Tomato
Pathogens:   Septoria lycopersici and Alternaria solani
Signs/Symptoms:  Spotting and eventual total collapse of leaves working from the bottom of the plant up
For more information see:       UW Plant Disease Facts D0100/46
Late Blight Late Blight
Hosts:  Tomato, potato
Pathogen:   Phytophthora infestans
Signs/Symptoms:  Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death
For more information see:       UW Plant Disease Facts D0068
Septoria Leaf Spot of Lilac Septoria Leaf Spot of Lilac
Host:  Lilac
Pathogen:   Septoria sp.
Signs/Symptoms:  Dead spots on leaves, potentially leading to complete leaf browning
Rhizosphaera Needle Cast Rhizosphaera Needle Cast
Hosts:  Colorado blue spruce, other spruces
Pathogen:   Rhizosphaera kalkhoffii
Signs/Symptoms:  Browning/purpling of interior needles of lower branches, followed by needle drop
For more information see:       UW Plant Disease Facts D0093
Gymnosporangium Rusts
Hosts:  Juniper, apple, crabapple, hawthorn, quince
Pathogen:   Gymnosporangium spp.
Signs/Symptoms:  Brown blobs with orange gelatinous masses (juniper); yellow/orange leaf spots (other hosts)
For more information see:       UW Plant Disease Facts D0058
Elderberry Rust
Hosts:  Elderberry
Pathogen:   Puccinia sambuci
Signs/Symptoms:  Light yellow, powdery growths on branches
For more information see:       UW Plant Disease Facts D0049
Scab Scab (Apple and Pear)
Hosts:   Apple, crabapple, pear, mountain-ash
Pathogens:   Venturia inaequalis, Venturia pirina
Signs/Symptoms:  Feathery-edged spots on leaves and fruits often leading to leaf loss and tree defoliation
For more information see:       UW Plant Disease Facts D0004
Bur Oak Blight
Host:   Bur oak
Pathogen:   Tubakia iowensis
Signs/Symptoms:  Wedge-shaped dead areas on leaves leading to dead leaves that stay attached to trees
Powdery Mildew
Hosts:   Herbaceous and woody ornamentals, fruits, vegetables, turf
Pathogens:   Miscellaneous powdery mildew fungi
Signs/Symptoms:  Powdery white growth on leaves
For more information see:       UW Plant Disease Facts D0084/86/87
Diplodia Shoot Blight and Canker
Hosts:  Austrian pine, other pines
Pathogen:   Diplodia spp.
Signs/Symptoms:  Dieback of branch tips with dead needles showing uneven lengths
For more information see:       UW Plant Disease Facts D0042
Boxwood Blight
Host:  Boxwood
Pathogen:   Calonectria pseudonaviculata
Signs/Symptoms:  Circular, brown leaf spots followed by leaf drop and shrub death
For more information see:       UW Plant Disease Facts D0023

For more information on plant diseases to watch for:  See or contact your county Extension agent.