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First Report of Bacterial Blight of Pennycress Caused by Xanthomonas campestris in Wisconsin
Basnet, P; Butchacas, J; Witherell, R; Ebeling-Koning, L; Tauscheck, D A; Hudelson, B; Jacobs, J M; Ellison, S. Plant Disease, 2024.
Milk
White cold drink

 

March 2023: UW Plant Disease Facts Medallions – Collect Them All!!

Medallion Project IconI absolutely loathe exercising.  That said, I try to force myself to do at least some sort of cardiovascular workout every day in an attempt to fend off the weight gain that seems to come with age (and my inability to stop eating everything bad for me in sight).  One thing that (sort of) makes my workouts tolerable is a fitness app that I have downloaded onto my cell phone that allows me to track my steps, minutes of exercise, calories burned, and other sundry exercise-related statistics.  As I achieve goals that I have set for myself, the app plies me with cool-looking electronic medallions (and other bright and shiny visuals) to reward me for my efforts.

How on earth does this have anything to do with plant diseases?  Well, about a year ago, I was contemplating how to entice people to learn more about plant diseases, and how to make it more fun in the process.  I already had plenty of plant disease-related materials to learn from in the form of the UW Plant Disease Facts fact sheets.  There are 130 titles in that series waiting to be consumed and digested.  But, what might encourage people to partake and learn?  My fitness app, with its rewards system, came to mind.

Thus was born the UW Plant Disease Facts Medallion Project.  For each of the 130 facts sheets in the UW Plant Disease Facts series, I have created a unique electronic medallion.  In addition, for each of the 130 fact sheets, I have written a brief quiz.  Earning the medallions is really simple.  Read a fact sheet.  Then, pick the corresponding quiz, and answer the questions.  If you get all of the answers right, you are rewarded the corresponding medallion.  The medallions are automatically emailed to you, and you can save them to your computer or other electronic device.  I am hoping that earning the medallions will make learning about diseases not only educational but fun as well.

The medallions will be rolled out in stages, just to make sure the launch runs smoothly and to efficiently manage any bumps that come up along the way.  The first set of medallions will be those (roughly 30) that have to do with fruit crop diseases.  These will launch this month.  Additional sets will be made available throughout 2023.  Hopefully all 130 medallions will be available by the end of the year.  And then, who will be the first person to earn them all?

Go to the UW Plant Disease Facts Medallion Project overview page to get started earning your medallions.

I want to give a HUGE shout out to Dixie Lang, who does IT and web support for my clinic, for all of her efforts in making this project possible.  She’s the person who figured out how to set up the quizzes online and automate the delivery of the medallions.  Also, many thanks to subscribers to my clinic listserv (UWPDDCLearn).  Late last year, I sent out a request through the listserv for help in proofing the quizzes, and many subscribers helped with this.  In particular, my colleagues Diana Alfuth (Extension Pierce and St. Croix Counties) and Lisa Johnson (Extension Dane County) spent a lot of time reviewing quizzes.

If you have any questions about the Medallion Project, don’t hesitate to contact me.  You can reach me at pddc@wisc.edu or (608) 262-2863.  Happy learning!!

Plant Problems to Watch for in 2023

Lipstick Rust Lipstick Rust
Host:   Chinese juniper, apple, crabapple
Pathogen:   Gymnosporangium yamadae
Signs/Symptoms:   Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)
Boxwood Blight Boxwood Blight
Host:   Boxwood
Pathogens:   Calonectria pseudonaviculata
Signs/Symptoms:   Circular, brown leaf spots followed by leaf drop and shrub death
For more information see:   UW Plant Disease Facts D0023
Late Blight Late Blight
Host:   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 and Early Blight 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
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
Verticillium Wilt Vascular Discoloration Verticillium Wilt
Hosts:   Woody and herbaceous ornamentals, vegetables
Pathogens:   Verticillium sp.
Signs/Symptoms:   Wilting, branch dieback, plant death
For more information see: UW Plant Disease Facts D0121/D0122
Powdery Mildew Powdery Mildew
Hosts:   Herbaceous and woody ornamentals, fruit, 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
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
Diplodia Shoot Blight and Canker Diplodia Shoot Blight and Canker
Hosts:   Austrian pine, other pines
Pathogen:   Diplodia spp.
Signs/Symptoms:   Dieback of brand tips with dead needles showing uneven lengths
For more information see:   UW Plant Disease Facts D0042
Chlorosis Chlorosis
Hosts:    Pin oak, red maple, birch, azalea, white pine, blueberry
Pathogen:   None
Signs/Symptoms:   Yellow leaves with dark green veins
For more information see:   UW Plant Disease Facts D0030
Improper Planting Improper Planting
Hosts:   Woody trees and shrubs
Pathogen:   None
Signs/Symptoms:   No root flare at the soil line, girdling roots, frost cracks, canopy thinning, early fall color, branch dieback, tree/shrub decline and death

For more information on plant problems to watch for:

See https://pddc.wisc.edu/ or contact your county Extension agent.

Plant Problems to Watch for in 2022

Boxwood Blight Boxwood Blight
Host:   Boxwood
Pathogens:   Calonectria pseudonaviculata
Signs/Symptoms:   Circular, brown leaf spots followed by leaf drop and shrub death
For more information see:   UW Plant Disease Facts D0023
Lipstick Rust Lipstick Rust
Host:   Chinese juniper, apple, crabapple
Pathogen:   Gymnosporangium yamadae
Signs/Symptoms:   Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)
Late Blight Late Blight
Host:   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 and Early Blight 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
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
Wood Rots Wood Rots
Hosts:   Woody trees and shrubs
Pathogens:   Miscellaneous wood rot fungi
Signs/Symptoms:   Shelf-like growths on trunks and branches
Canker Diseases Canker Diseases
Hosts:    Woody trees and shrubs
Pathogens:   Miscellaneous canker fungi
Signs/Symptoms:   Sunken areas on trunks/branches
For more information see:   UW Plant Disease Facts D0027, D0037, D0042, D0055, D0074, D0114
Virus Diseases Virus Diseases
Hosts:   All plants, particularly herbaceous ornamentals
Pathogen:   Miscellaneous plant viruses
Signs/Symptoms:   Blotchy leaf color, growth distortions
For more information see:   UW Plant Disease Facts D0036, D0063, D0067, D0115, D0116, D0130
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
Chlorosis Chlorosis
Hosts:    Pin oak, red maple, birch, azalea, white pine, blueberry
Pathogen:   None
Signs/Symptoms:   Yellow leaves with dark green veins
For more information see:   UW Plant Disease Facts D0084
Improper Planting Improper Planting
Hosts:   Woody trees and shrubs
Pathogen:   None
Signs/Symptoms:   No root flare at the soil line, girdling roots, frost cracks, canopy thinning, early fall color, branch dieback, tree/shrub decline and death

For more information on plant problems to watch for:

See https://pddc.wisc.edu/ or contact your county Extension agent.

Boxwood Blight – Pest Alert

What is boxwood blight? 

Boxwood blight (also known as box blight and boxwood leaf drop) is a devastating disease of boxwood (Buxus spp.) that can cause leaf loss and eventual death of affected shrubs.  Boxwood shrubs are commonly grown as hedges and as individual plants in home landscapes and public gardens.  Boxwood blight can affect any type of boxwood (Buxus spp.) including European or common boxwood (Buxus sempervirens), Korean littleleaf boxwood (B. sinica var. insularis), and Japanese littleleaf boxwood (B. microphylla var. japonica).  In addition, the disease has been reported on Japanese and Allegheny pachysandra (Pachysandra terminalis and Pachysandra procumbens respectively), two common groundcovers.  Boxwood blight has been found in Europe and New Zealand, and was first confirmed in the U.S. in 2011.  The disease was first detected in Wisconsin (in Kenosha County) in 2018.  It has subsequently been found in Dane, Door, Milwaukee, Ozaukee, Portage, and Walworth Counties. 

Boxwood blight can cause severe leaf loss and eventual death of boxwood shrubs. (Photo courtesy of David Clement, University of Maryland Extension)
Boxwood blight can cause severe leaf loss and eventual death of boxwood shrubs. (Photo courtesy of David Clement, University of Maryland Extension)

What does boxwood blight look like? 

Initially, brown spots appear on the leaves.  The spots eventually enlarge and merge together.  Infected leaves turn brown and fall off.  Boxwood blight can cause total leaf loss on a shrub within days of the first onset of symptoms.  Dark brown to black sunken areas can also form anywhere on the stems, leading to branch dieback  Boxwood blight often kills plants shortly after all of the leaves drop.  Damage from winter burn (see UW Plant Disease Facts D0127, Winter Burn), dog urine and other diseases such as Volutella blight may look superficially similar to symptoms of boxwood blight. 

Where does boxwood blight come from? 

Boxwood blight is caused by the fungus Calonectria pseudonaviculata (sometimes referred to as Cylindrocladium pseudonaviculatum or Cylindrocladium buxicola) which thrives in humid, warm conditions.  The fungus is typically introduced into any area on nursery plants that are infected, but not showing symptoms.  Holiday wreaths containing boxwood sprigs have also been documented as a source of the boxwood blight fungus.  Once the fungus has been introduced into the landscape, spores can be easily spread by splashing water (e.g., rain or sprinklers), wind or contaminated gardening tools (e.g., pruners, shovels, gloves).  The boxwood blight fungus can survive and produce spores in dead boxwood leaves and branches (including those that have fallen onto the ground) for several years.

How can I save a plant with boxwood blight? 

Because boxwood blight is new to Wisconsin and relatively rare, eradicating the causal fungus may be possible.  Therefore, if you find boxwood blight, remove and destroy any affected shrubs.  Currently, free testing for boxwood blight is available through the UW-Madison Plant Disease Diagnostics Clinic (https://pddc.wisc.edu/).  Plants (roots and all) confirmed to have boxwood blight, as well as any leaves or branches that have fallen from these plants, should be removed and destroyed by burning (where allowed by local ordinance), deep burying (at least two feet deep) or double bagging (in plastic garbage bags), then landfilling.  DO NOT compost any parts of infected shrubs.  Thoroughly decontaminate any tools used in the removal process by treating them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol or certain spray disinfectants) or (as a last resort) in 10% bleach.  If you use bleach, be sure to thoroughly rinse and oil tools after pruning to prevent rusting.

How can I avoid problems with boxwood blight in the future?  

Consider using shrubs other than boxwood in your landscape.  If you decide to use boxwood, choose boxwood blight resistant varieties where possible.  Common boxwood variety ‘Katerberg’ (North Star®); littleleaf boxwood varieties ‘Compacta’, ‘John Baldwin’, ‘Little Missy’, and ‘Northern Emerald’; Japanese littleleaf boxwood varieties ‘Eseles’ (Wedding Ring®), ‘Green Beauty’, ‘Gregem’ (Baby Gem™), ‘Jim Stauffer’, ‘Peergold’ (Golden Dream™), ‘SB108’ (NewGen Independence®), ‘SB300’ (NewGen Freedom®), and ‘Winter Gem’; Korean littleleaf boxwood varieties  ‘Franklin’s Gem’, ‘Nana’, ‘Pincushion’, ‘Wee Willie’, ‘Winter Beauty’, and ‘Wintergreen’; and hybrid boxwood varieties ‘Green Gem’ and ‘Karzgreen (Green Ice®) have been documented to be resistant to boxwood blight and are rated as hardy in at least one of the USDA hardiness zones (4, 5 or 6) found in Wisconsin.  Always buy boxwood shrubs from local, reputable suppliers who have thoroughly inspected boxwood plants for evidence of boxwood blight.  

Leaf spots typical of boxwood blight on boxwood sprigs in a holiday wreath. (Photo courtesy Purdue PPDL)
Leaf spots typical of boxwood blight on boxwood sprigs in a holiday wreath. (Photo courtesy Purdue PPDL)

Isolate new boxwood shrubs from established boxwoods for several weeks before planting, as boxwood blight symptoms not become apparent until weeks after purchase.  DO NOT plant boxwoods in areas where boxwood blight has been a problem in the past, as the fungus can survive in boxwood debris (e.g., leaves and branches) for several years.  When planting, space boxwood plants far enough apart from each other, as well as other shrubs, so that branches on adjacent shrubs do not overlap.  This will increase air flow between plants and promote a drier environment that will be less favorable for boxwood blight development.  Avoid watering plants with sprinklers or overhead with hoses; instead use a soaker or drip hose.  This will limit splash of spores from plant to plant and also promote a drier environment that is less favorable for disease. 

Be cautious when buying holiday wreaths or other garlands.  Avoid holiday decorations that contain boxwood, whenever possible.  If you are unsure whether a wreath that you have purchased contains boxwood, assume that it does, and dispose of it appropriately by burning, deep burying or double bagging and landfilling as described above.  Be careful to collect and dispose of any leaves or branches that may have fallen from wreaths as well.  Make sure that no potentially contaminated materials end up near boxwood shrubs in your yard.  Under NO circumstances should you attempt to compost any suspected boxwood materials. 

Once boxwood blight has been reported near your location, you may want to consider using preventative fungicide treatments for management.  Fungicides containing chlorothalonil (alone or in combination with thiophanate-methyl or tebuconazole), fludioxonil, metconazole, and tebuconazole (as a stand-alone product) have been shown to provide good control of boxwood blight if applied prior to the development of any symptoms.  These fungicides will not cure existing disease.  If you decide to use fungicides, you will need to treat every seven to 14 days throughout the growing season.  DO NOT use fludioxonil, metconazole, or tebuconazole as the sole active ingredient for all treatments.  If you decide to use one of these active ingredients, alternate its use with at least one of the other active ingredients listed above (except DO NOT alternate metconazole and tebuconazole as these products are chemically related).  Alternating active ingredients will help minimize problems with fungicide-resistant strains of the boxwood blight fungus.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible. 

Finally, routinely (e.g., weekly) check boxwood plants for boxwood blight.  Immediately remove any symptomatic plants and fallen leaves and branches, and dispose of them as described above. 

For more information on boxwood blight: 

Contact your county Extension agent.

January 2022: Plant Disease Diagnostics Clinic Services for 2022

Turquoise 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 https://pddc.wisc.edu/digital-diagnosis/.  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 pddc@wisc.edu 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 pddc@wisc.edu.

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 (https://uwlab.soils.wisc.edu/) 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 https://www.waa-isa.org/) 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.
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 toxicity

One 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 wilt

Many 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.

Need help or more information?

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 pddc@wisc.edu 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 pddc@wisc.edu and ask to be added to the PDDC’s listserv (UWPDDCLearn).

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 https://pddc.wisc.edu/search-fact-sheets/ or contact the PDDC at pddc@wisc.edu or (608) 262-2863.