Category Archives: Disease – Herbaceous Ornamental

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

 

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.

Root Rots on Houseplants

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

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

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

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

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

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

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

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

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

Elderberry Rust

What is elderberry rust?  Elderberry rust is a visually striking fungal disease that affects stems, leaves and flowers of plants in the genus Sambucus (i.e., elderberries).  The disease also affects sedges (Carex spp.).  On elderberries grown as ornamentals, as well as on sedges, the disease is primarily a cosmetic problem.  However, if elderberries are grown for fruit, the disease can disrupt flower and fruit formation, thus reducing fruit yield.

An elderberry rust gall on elderberry (left) and leaf spots caused by elderberry rust on sedge (right). Photos courtesy of Jenell Bindl (left) and Michele Warmund, University of Missouri (right)
An elderberry rust gall on elderberry (left) and leaf spots caused by elderberry rust on sedge (right). Photos courtesy of Jenell Bindl (left) and Michele Warmund, University of Missouri (right)

What does elderberry rust look like?  Elderberry rust is most noticeable on elderberries where it causes growth distortions and swellings (i.e., galls) on leaves and stems.  Galls are often very large, bright yellow and powdery from spores produced by the causal fungus.  In extreme cases, galls can resemble banana slugs that have attached themselves to branches.  Infected flowers become thick, swollen and green-tinged rather than white.  Affected plant parts are covered with a network of small (approximately 1/16 inch in diameter) ring-like spots.  These spots are reproductive structures of the rust fungus and produce the powdery spores that coat the galls.

On sedges, elderberry rust causes brownish leaf spots, often with yellow halos.  The spots eventually erupt releasing powdery, rusty-orange spores.

Where does elderberry rust come from?  Elderberry rust is caused by the fungus Puccinia sambuci, also known as Puccinia bolleyana.  The fungus overwinters in sedge debris, and spores produced in this debris blow to elderberry plants in the spring, leading to infection and gall formation.  Spores produced in elderberry galls blow back to sedges, where infection of newly produced leaves (and other plant parts) occurs.  These infections lead to spotting and to the formation of a third type of spore that reinfects sedges causing additional spotting.  Late in the season a fourth type of spore is produced that serves as the overwintering phase of the fungus.  Infection of both elderberries and sedges is favored by wet weather.

How do I save plants with elderberry rust?  Elderberry rust is not a lethal disease on either elderberry or sedge.  When galls form on elderberry, simply prune these out.  This will make elderberry plants more aesthetically pleasing and limit spread of the fungus to sedges.  When pruning, cut branches four to six inches below each gall.  Between cuts, decontaminate pruning tools by treating them for at least 30 seconds with 70% alcohol (e.g., rubbing alcohol straight out of the bottle), a spray disinfectant containing 60-70% active ingredient, or a 10% bleach solution (i.e., one part of a disinfecting bleach and nine parts water).  If you decide to use bleach, be sure to rinse your tools thoroughly after you are done pruning and then oil them to prevent rusting, which can be caused by bleach use.  Dispose of galls by burning (where allowed by local ordinance) or burying them.

How do I avoid problems with elderberry rust in the future?  In landscape settings, the best way to avoid problems with elderberry rust is to remove any sedges that are growing near elderberry plants.  The farther the distance between elderberries and sedges, the less likely that elderberry rust will be an issue.  Also, increase airflow around elderberry plants by thinning them and removing surrounding plants.  Increased airflow will dry plants more rapidly and make the environment less favorable for infection.  DO NOT use a sprinkler to water plants, as that wets leaves and provides a better environment for infections to occur.  Instead, use a soaker or drip hose to apply supplemental water to the soil at the drip lines of the plants (i.e., the edges of where the branches extend).  While fungicides are available for rust control in commercial elderberry production, these products are not recommended for use in home garden settings.

For more information on elderberry rust:  Contact your county Extension agent.

Tobacco Mosaic

Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.
Tobacco mosaic causing a blotchy light and dark coloring (mosaic) of tobacco leaves.

What is tobacco mosaic?  Tobacco mosaic is a common viral disease of worldwide distribution that affects over 200 species of herbaceous and, to a lesser extent, woody plants.  Common hosts include tobacco, solanaceous vegetables (e.g., pepper, tomato) and vining vegetables (e.g., cucumber, melon, squash), as well as a wide range of ornamentals (e.g., begonia, coleus, geranium, impatiens, million bells, petunia).  The disease has its biggest impact on vegetables, where it can reduce yield and affect quality to the point that commercial crops cannot be marketed.

What does tobacco mosaic look like?  Symptoms of tobacco mosaic vary in type and severity depending on the plant infected, plant age, the variant of the virus involved, and environmental conditions.  On leaves, typical symptoms include blotchy light and dark areas (called mosaic); cupping, curling, elongation (strapping), roughening, wrinkling and other growth distortions; and smaller than normal size.  Fruits may have a blotchy color, ripen unevenly, be malformed or have an off flavor.  Entire infected plants are often stunted.  Other viral diseases like cucumber mosaic (see UW Plant Disease Facts D0036, Cucumber Mosaic) can cause symptoms similar to tobacco mosaic.  Often, multiple viral diseases can simultaneously affect a single plant.  Certain herbicide exposures (see UW Plant Disease Facts D0060, Herbicide Damage), nutrient deficiencies or toxicities, high temperature and even insect feeding can also cause similar symptoms.  Proper diagnosis of tobacco mosaic requires lab testing.

Where does tobacco mosaic come from?  Tobacco mosaic is caused by Tobacco mosaic virus (TMV), the first virus ever identified.  Numerous variants (strains) of the virus have subsequently been described.  TMV survives in infected plants (including viable seeds), as well as in debris from these plants.  Plant-based products (most notoriously tobacco products) can harbor the virus.  TMV is very stable and can survive for long periods of time; there are reports of TMV surviving and remaining infectious after 50 years in storage at 40°F.  Because of its stability, TMV can survive on and be picked up from hands, clothing, gardening tools, work surfaces and any other object (e.g., door knobs) that gardeners may handle.

TMV is highly transmissible and is commonly spread by handling infected plants, then healthy plants.  Spread via gardening tools is also very common.  No specific insects spread TMV (the way that aphids spread Cucumber mosaic virus).  However, bees and chewing insects (e.g., grasshoppers) can transmit TMV through casual contact or their feeding as they move from plant to plant.

Leaf growth distortions caused by tobacco mosaic.
Leaf growth distortions caused by tobacco mosaic.

How do I save a plant with tobacco mosaic?  There is no cure for tobacco mosaic.  Once infected, plants remain infected for life, and typically the virus spreads throughout the plant from the point of infection.  Infected plants and any associated debris should be burned (where allowed by local ordinance) or double-bagged and disposed of in a landfill.  DO NOT compost plants with this disease.  Thoroughly decontaminate any items that have come into contact with infected plants or their debris by treating them for a minimum of one minute with:

  • 2.75 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 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.  Also, thoroughly wash your hands with soap and water, and launder any clothing that you wore while disposing of infected plants and debris.

How do I avoid problems with tobacco mosaic virus in the future?  Inspect plants prior to purchase for any symptoms of tobacco mosaic, and DO NOT buy symptomatic plants.  Purchase seed from a reputable supplier that routinely inspects their seed-producing plants for symptoms of viral (and other) diseases.  If you use tobacco products, DO NOT use them around plants.  Also, wash your hands thoroughly with soap and water prior to handling plants, and consider wearing freshly laundered clothing when gardening.  Finally, decontaminate (as described above) any items that might harbor TMV to help prevent spread.  Even if you do not use tobacco products, routine handwashing and decontamination of gardening tools and other items can help prevent tobacco mosaic from being a problem.

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

Ralstonia Wilt – Pest Alert

What is Ralstonia wilt?  Ralstonia wilt (also sometimes known as Southern wilt) is a typically lethal disease that affects over 250 plants in over 40 plant families.  Susceptible greenhouse-grown ornamentals include, but are not limited to, plants in the genera Capsicum, Cosmos, Cyclamen, Dahlia, Fuschsia, Gerbera, Hydrangea, Impatiens, Lantana, Nasturtium and Pelargonium.  Vegetables such as eggplant, pepper, potato and tomato, as well as tobacco, are also susceptible.  Ralstonia wilt was first reported on geraniums (Pelargonium spp.) in Wisconsin in 1999.  In 2020, the disease was reported on Fantasia® ‘Pink Flare’ geraniums in Michigan.  Potentially infected ‘Pink Flare’ geraniums were also distributed to 38 other states including Wisconsin.

Yellowing and wilting characteristic of Ralstonia wilt. Photo courtesy of WI DATCP
Yellowing and wilting characteristic of Ralstonia wilt. Photo courtesy of WI DATCP

What does Ralstonia wilt look like?  Symptoms of Ralstonia wilt in geraniums are similar to those associated with bacterial blight (caused by Xanthomonas campestris pv. pelargonii).  Initially, lower leaves of infected plants yellow and wilt, then die.  Yellowing and death of upper leaves follow.  Symptoms may initially occur on only one side of the plant.  Internally, the water-conducting tissue of the plant browns, and then the entire stem rots from the inside out.  Eventually, infected plants die.

Where does Ralstonia wilt come from?  Ralstonia wilt is caused by the bacterium Ralstonia solanacearum (formerly Pseudomonas solanacearum).  This bacterium is commonly found in tropical, sub-tropical and warm temperate climates, but it is not believed to survive cold temperatures such as those typical of Wisconsin winters.  The bacterium can be moved in symptomless plants or cuttings, or in contaminated soil and plant debris (where the pathogen can remain dormant for many years).  Several subgroups (i.e., races and biovars) of R. solanacearum have been recognized, each with a different host range.  R. solanacearum race 3, biovar 2 is of particular concern because it causes a serious disease of potato called brown rot.  In addition, this race/biovar has been listed as a select agent by the U.S. government and is considered to have potential to be developed as a bioterrorist weapon against U.S. agriculture.

How do I save plants with Ralstonia wilt?  There are no known treatments that will save plants affected by Ralstonia wilt.  If you believe your plants are suffering from this disease, immediately contact your local department of agriculture or county Extension agriculture or horticulture agent to arrange for confirmatory testing.  If you live in Wisconsin, you can contact the UW-Madison Plant Disease Diagnostics Clinic (see below for contact information) for assistance.  If your plants test positive for R. solanacearum race 3, biovar 2 the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) must be notified and this organization will provide guidance on proper disposal of contaminated plants, as well as decontamination of greenhouses or other sites where contaminated plants have been grown.

How do I avoid problems with Ralstonia wilt in the future?  Start by purchasing and growing pathogen-free plant cuttings.  Keep plants from different suppliers physically separated by at least four feet to minimize the risk of cross contamination should a shipment of plants prove to be contaminated.  Because R. solanacearum is easily moved with soil or water, minimize splashing or any other movement of water or soil from plant to plant when watering.  When taking cuttings or trimming plants, be sure to clean cutting tools between cuts using an approved disinfectant.  For a complete list of such products, contact the UW-Madison Plant Disease Diagnostics Clinic (see below for contact information).  Also wear disposable gloves (nitrile are best) when handling plants, and change gloves between working with different geranium varieties.  This will minimize the possibility of moving R. solanacearum by touch.  If gloves are not available, wash your hands frequently and thoroughly (especially between geranium varieties) with lots of soap and water or with an alcohol-based hand sanitizer.  Remove and destroy weeds or weed debris as these can harbor the pathogen.  Finally, do not grow plants in a greenhouse where the disease has occurred unless it has been properly decontaminated.

For more information on Ralstonia wilt or help in diagnosing this problem:  Contact Brian Hudelson, Plant Disease Diagnostics Clinic, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI  53706-1598 [phone: (608) 262-2863, fax: (608) 263-3322, email:  pddc@wisc.edu].

Typhula Blight

Typhula blight causes circular patches of bleached turf that often merge to form larger, irregularly-shaped bleached areas.
Typhula blight causes circular patches of bleached turf that often merge to form larger, irregularly-shaped bleached areas.

What is Typhula blight?  Typhula blight, also known as gray or speckled snow mold, is a fungal disease affecting all cool season turf grasses (e.g., Kentucky bluegrass, creeping bentgrass, tall fescue, fine fescue, perennial ryegrass) grown in areas with prolonged snow cover.  These grasses are widely used in residential lawns and golf courses in Wisconsin and elsewhere in the Midwest.

What does Typhula blight look like?  Typhula blight initially appears as roughly circular patches of bleached or straw-colored turf that can be up to two to three feet in diameter.  When the disease is severe, patches can merge to form larger, irregularly-shaped bleached areas.  Affected turf is often matted and can have a water-soaked appearance.  At the edges of patches, masses of grayish-white fungal threads (called a mycelium) may form.  In addition, tiny (1/64 to 3/16 inch diameter) reddish-brown or black fungal survival structures (called sclerotia) may be present.  Typhula blight looks very similar to Microdochium patch/pink snow mold (see UW Plant Disease Facts D0073, Microdochium Patch), but the Microdochium patch fungus does not produce sclerotia.  

Where does Typhula blight come from?  Typhula blight is caused by two closely related fungi Typhula incarnata and Typhula ishikariensis.  In general, T. incarnata is more common in the southern half of Wisconsin while T. ishikariensis is more common in the northern half of the state.  To distinguish between the two Typhula species, look for sclerotia.  Sclerotia of T. incarnata are reddish-brown and 1/16 to 3/16 inch in diameter.  Sclerotia of T. ishikariensis are black and 1/64 to 1/16 inch in diameter, resembling flakes of pepper.  Typhula blight develops when there is a prolonged period (more than 60 days) when snow covers unfrozen ground and soil temperatures are just above freezing (30 to 40˚F).  Typhula blight typically does not develop if the ground freezes before the first snowfall in the fall/winter, or if snow melts during the middle of winter exposing the ground to colder temperatures.  Damage from Typhula blight is usually not noticed until snow melts in spring.  Applying high nitrogen fertilizers in the fall can increase the risk of Typhula blight developing over the winter.

Sclerotia (survival structures) of Typhula incarnata are small and red or rust-colored.
Sclerotia (survival structures) of Typhula incarnata are small and red or rust-colored.

How do I save a turf with Typhula blight?  Turf with Typhula blight may or may not recover depending on the fungus involved.  Both T. incarnata and T. ishikariensis can infect and kill turf leaves, but only T. ishikariensis kills the plant crown.  Turf infected with T. incarnata typically recovers quickly once normal turf growth resumes in the spring.  Turf infected by T. ishikariensis often suffers from crown injury and death, making it less likely to recover and more likely to require replanting in spring.  Lightly raking infected turf and reseeding in spring can help speed turf recovery.

How do I avoid problems with Typhula blight in the future?  Consider planting less-susceptible turfgrass species (e.g., fine fescues) to limit the impact of Typhula blight.  Also, avoid using fast-release fertilizers in late fall that can predispose turf to Typhula infections.  Finally, continue to mow turf until it goes dormant as this will help prevent excessive turf top growth that is more easily infected by Typhula.

Because turf often recovers naturally from Typhula blight, fungicide treatments are typically NOT warranted for home lawns.  However, fungicide treatments may be needed to prevent severe damage when a lawn has a history of the disease and when predictions for the upcoming winter suggest that snow cover will persist on unfrozen ground for longer then three months.  Many effective fungicides are available for snow mold control in home lawns and include the active ingredients azoxystrobin, propiconazole, pyraclostrobin, and tebuconazole.  For the most up-to-date fungicide recommendations for Typhula blight, please visit the Turf Pest Management Mobile website (https://turfpests.wisc.edu/).  Fungicide applications should be made in October or November to prevent disease in the spring.  If you decide to use fungicides for control, read and follow all label instructions to ensure that you use the product that you select in the safest and most effective manner possible.

For more information on Typhula blight:  Contact the Turfgrass Diagnostic Lab (https://tdl.wisc.edu/) or your county Extension agent.

Southern Blight

The Southern blight fungus produces large numbers of spherical, light tan to dark red resting structures called sclerotia (red arrow).

What is Southern blight?  Southern blight is a lethal fungal disease that is most common in the tropics and subtropics.  This disease causes damage in the southern United States and can even cause problems in temperate locations like Wisconsin during periods of warm, moist weather.  Southern blight has a wide host range, affecting over 500 plant species.  Vegetable and fruit hosts include tomato, pepper, onion, beet, rhubarb, strawberry, lettuce, cucumber, melon, carrot, asparagus and parsley.  Ornamental hosts include aster, black-eyed Susan, dahlia, daylily, gladiolus, hosta, impatiens, peony, petunia, rose, salvia, sedum and viola.  Small woody trees and shrubs can be affected as well.

What does Southern blight look like?  Southern blight initially leads to a water-soaked appearance on lower leaves or water-soaked lesions (spots) on lower stems.  Any plant part that is near or in contact with the soil may become infected.  Infected plants yellow and wilt, often within days of infection, particularly when the weather is moist and warm (80 to 95°F).  Fruit rots, crown rots and root rots are also typical symptoms of the disease.  Thick mats of white fungal threads (called mycelia) may grow from infected tissue, radiating from the plant onto the soil surface.  Sclerotia (small spherical structures that are about the size of mustard seeds) develop on infected tissue and on the soil surface.  Sclerotia range in color from light tan to dark reddish-brown to black.

Where does Southern blight come from?  Southern blight is caused by the fungus Athelia rolfsii (formerly Sclerotium rolfsii), which lives in soil, on plants (including weeds), and in plant debris.  The fungus can be spread through movement of infested soil and plant debris, on infected plants, in contaminated irrigation water, and through use of contaminated tools.  In Wisconsin, A. rolfsii most likely enters gardens on infected nursery stock or infested mulch.  Freezing temperatures will kill A. rolfsii mycelia, but sclerotia can survive temperatures as low as approximately 14ºF.

Southern blight can be a serious disease of vegetables, including tomatoes, leading to wilting and plant death. (Photo courtesy of Gary E. Vallad, University of Florida)

How can I save a plant with Southern blight?  Identify the extent of an infestation based on visible dead/dying plants, fungal mycelia and sclerotia.  Remove all plants (including roots), as well as three inches of soil, from at least 12 inches beyond the infested area.  Start at the edge of the infested area and work toward the center.  Bag all plants and soil and dispose of these materials in a landfill.  Turn any remaining soil in the infested area eight to 12 inches to bury any sclerotia that you may have missed.  This will reduce the length of time that the sclerotia will survive.  Grow non-susceptible plants (e.g., larger woody ornamentals) in the affected area for two to three years to allow time for sclerotia to die naturally.  Fungicides containing azoles (e.g., propiconazole, tebuconazole), fludioxonil, flutolanil, mancozeb, PCNB, strobilurins (e.g., azoxystrobin, fluoxastrobin), thiophanate-methyl, and triadimefon are all labeled for Southern blight control, but may have varying levels of effectiveness.  All of these products will likely be more effective if applied as preventive treatments rather than in an attempt to “cure” existing disease.  If you decide to use fungicides, DO NOT use one active ingredient for all treatments.  Instead, alternate the use of two or more unrelated active ingredients to help minimize problems with fungicide-resistant strains of A. rolfsii.  DO NOT alternate active ingredients that are chemically related (e.g., propiconazole and tebuconazole, or azoxystrobin and fluoxastrobin).  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.

How can I prevent Southern blight in the future?  Inspect new plants for sclerotia and mycelia of A. rolfsii prior to transplanting.  Bag and dispose of diseased plants as described above.  Use high-quality mulches (e.g., shredded oak bark mulch, red cedar mulch) in your garden and avoid any mulches that you suspect might be contaminated with A. rolfsiiA. rolfsii thrives under moist conditions.  Therefore, thin existing gardens or space plants farther apart in new gardens to improve airflow and promote more rapid drying of foliage and soil.

For more information on Southern blight:  Contact your county Extension agent.

Cucumber Mosaic

What is cucumber mosaic?  Cucumber mosaic is a viral disease of worldwide distribution that affects over 1200 plant species.  Hosts include a wide range of fruits, vegetables, herbaceous and woody ornamentals, and weeds.  The disease has perhaps its biggest impact in vegetable production where it can cause significant losses in yield and vegetable quality.

Cumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)
Cucumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)

What does cucumber mosaic look like?  Symptoms of cucumber mosaic can vary widely depending on host species, host variety, and time of infection.  Typical symptoms include stunting of entire plants, mosaic or mottling (i.e., blotchy white, yellow, and light green areas) and ring spots (i.e., ring-like areas of discolored tissue) on leaves and fruits, and a variety of growth distortions such as cupping, puckering and strapping (i.e., elongation and thinning) of leaves as well as warts on fruits.  In extreme situations, parts of an affected plant or even an entire plant may die from the disease.

Where does cucumber mosaic come from?  Cucumber mosaic is caused by Cucumber mosaic virus (CMV) which can overwinter in susceptible biennial or perennial weeds, as well as in perennial agricultural crops (e.g., alfalfa) and perennial herbaceous and woody ornamentals.  Seeds and even pollen from certain host plants can carry the virus, and thus the virus can be spread via these plant parts.  More commonly, CMV is spread by aphids [see the University of Wisconsin Garden Facts XHT1043 (Aphids) for details on these insect pests] which can pick up the virus from infected plants and transmit it to healthy plants as they feed.  Over 80 species of aphids can potentially transmit CMV.  The severity of cucumber mosaic oftentimes depends on the size and activity of aphid populations in an area, as well as on the number infected plants in an area serving as reservoirs for the virus.

Cumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)
Cucumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)

How do I save plants with cucumber mosaic?  There is no known cure for cucumber mosaic.  Infected plants should be removed and destroyed to eliminate the plants as potential reservoirs for the virus (which can subsequently be spread to other nearby healthy plants).  Infected plants can be burned (where allowed by local ordinance), deep buried or hot composted.  Killing infected plants with herbicides can also be an effective management strategy.

How do I avoid problems with cucumber mosaic in the future?  Buy certified, virus-free seeds and plants.  Consider using CMV-resistant varieties of lettuce, spinach, cucurbits (e.g., cucumber, melon and squash) and other vegetables where available.  Seed catalogs often contain information on CMV resistance that can be useful for variety selection.  Remove weed hosts whenever possible around your garden, and mulch vegetable and ornamental gardens to inhibit weed growth.  Consider using floating row covers where possible to prevent aphids from reaching susceptible plants.  DO NOT use insecticides to control aphids because such treatments are unlikely to act fast enough to prevent aphids from transmitting CMV, and may actually stimulate aphids to move and feed more widely, thus leading to increased spread of the virus.

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

Wood Mulch and Tree Health

What are the benefits of wood mulch?  Wood mulch is typically available as chipped wood, or shredded or chunked bark, and can contribute to tree health in many ways.  When high quality, composted mulches are applied two to four inches deep in a ring three to six feet in diameter (or greater) from the trunk of a tree, mulch can help preserve moisture, control weeds, limit damage to the trunk from mowers and string trimmers and moderate the soil temperature.  Use four inches of mulch when soils are light and well-drained, and two inches of mulch on heavier, clay soils.

Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.
Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.

Can wood mulch harm trees?  Use of improperly composted mulches (some-times called “sour mulches”), can lead to tree nutrient deficiencies.  Sour mulches can also produce gases like methane and ammonia that can be toxic to plants.  Foliage on trees surrounded by sour mulches may initially turn yellow, then brown, die and fall off.  If your mulch smells like vinegar, ammonia or sulfur, it is likely a sour mulch and should be removed.  Replace the sour mulch with a high quality, composted mulch and consult with your local Extension agent about testing the soil for nutrient deficiencies.  Fertilize appropriately based on the results of these tests.

Improper application of mulch can also lead to problems.  Piling wood mulch up against the trunk of a tree can keep the bark underneath excessively wet.  This wetness can contribute to bark decay.  In addition, use of thick mulch layers (greater than four inches) can lead to overly wet soils that are favorable for development of root rots (see UW Plant Disease Facts D0094, Root and Crown Rot).  To avoid these problems, make sure mulch is applied at least four inches away from the trunk of a tree and that the mulch layer is the appropriate thickness for the soil type in your landscape (see above).

Does woody mulch harbor or attract insects?  Insects such as earwigs [see UW Bulletin A3640, Controlling Earwigs (available at https://learningstore.extension.wisc.edu/)], centipedes (see University of Wisconsin Garden Facts XHT1113, Centipedes), millipedes (see University of Wisconsin Garden Facts XHT1108, Millipedes) and sowbugs (see University of Wisconsin Garden Facts XHT1110, Sowbugs) can feed on decaying organic matter in mulches.  While these insects are often only nuisances, earwigs can feed on and cause damage to a variety of ornamentals, particularly to flowering plants.  If mulch is used near entrances to a home or around basement windows, these unwanted insects may get inside.  Termites ingest wood and can be attracted to wood mulch, but new termite colonies are not likely to become established due to use of wood mulches.  Typically, termites are not a problem in Wisconsin, and when colonies are found, they occur only in the southern half of the state.

Carpenter ants [see UW Bulletin A3641, Controlling Carpenter Ants (available at https://learningstore.extension.wisc.edu/)] and powderpost beetles (see University of Wisconsin Garden Facts XHT1053, Powderpost Beetles) are unlikely to utilize mulch as a food source because conditions required for their development would not be satisfied by wood mulch.  Carpenter ants do not ingest wood as a food source; instead, they chew non-living wood (in trees or landscape timbers, etc.) to excavate galleries in which they live and raise their young.  Since wood mulch is composed of small wooden pieces, it would not serve as a home.  To avoid potential insect problems, keep mulch as far away from the foundation of your home as possible and seal all holes and crevices that insects might use as entry points.  Also, periodically inspect landscape timbers and the house for termites.

Does woody mulch harbor tree pathogens?  Wood mulch may come from many sources, including trees and shrubs that have died from a wide range of diseases.  To be harmful to your trees, disease-causing organisms (pathogens) would have to survive in mulch and these organisms would have to move from the mulch either directly, or through the soil, to their new host – your tree.  There is currently very little research on this topic.

Elm trees killed by Dutch elm disease (see UW Plant Disease Facts D0045, Dutch Elm Disease), can serve as breeding areas for native and European elm bark beetles.  Bark beetles that breed in logs or firewood from these trees can pick up the fungi that cause Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi) and carry these fungi from tree to tree.  Chipping infected elm trees creates an unfavorable environment for bark beetles yet there is no scientific literature that describes the level of risk of transmitting the Dutch elm disease fungi from wood chips or bark chunks to healthy elms.

Oak trees killed by oak wilt (see UW Plant Disease Facts D0075, Oak Wilt) can be attractive to several sap-feeding beetles that can potentially pick up the oak wilt fungus (Bretziella fagacearum) and move it in the landscape.  This process is affected by moisture and temperature and would likely be disrupted by the chipping and composting process yet there is no scientific literature that describes the level of risk of transmitting the oak wilt disease fungus from wood chips or bark chunks to healthy oaks.

Research at the University of Wisconsin-Madison suggests that wood chip mulches produced from trees suffering from Verticillium wilt (see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs) can serve as a source of the fungus (Verticillium dahliae) that causes the disease.  These studies show that Verticillium can survive for at least one year in mulch and that use of this contaminated mulch can lead to Verticillium wilt in both woody and herbaceous plants.  Therefore use of mulches produced from trees with Verticillium wilt should be avoided.

For more information on wood mulch and tree health:  Contact your county Extension agent.