Category Archives: Fact Sheet

Deciduous Tree Leaf Disease Quick Reference

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

For more information on deciduous tree leaf diseases:  See https://pddc.wisc.edu/ or contact your County Extension agent.

Conifer Disease Quick Reference

Root and Crown Rot Root and Crown Rots
Hosts:  All conifers
Pathogens:   Assorted root rot fungi/water molds
Signs/Symptoms:  Poor growth, branch dieback, discolored and deteriorated roots
For more information see:       UW Plant Disease Facts D0094
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
Cytospora Canker Cytospora Canker
Hosts:  Colorado blue spruce, other spruces
Pathogen:   Cytospora kunzei
Signs/Symptoms:  Branch dieback with milky-white patches of dried sap on affected branches
For more information see:       UW Plant Disease Facts D0037
Spruce Needle Drop Spruce Needle Drop
Hosts:  Spruces
Pathogen:   Unknown (possibly Setomelannoma holmii)
Signs/Symptoms:  Needle loss and dieback at or near branch tips
Gymnosporangium Rust 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
Phomopsis Tip Blight Phomopsis Tip Blight
Hosts:  Junipers
Pathogen:   Phomopsis juniperovora
Signs/Symptoms:  Browning and dieback of branch tips  in spring and early summer as new growth emerges
For more information see:       UW Plant Disease Facts D0077
Diplodia Shoot Blight and Canker 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
Dothistroma Needle Blight Dothistroma Needle Blight
Hosts:  Austrian pine, other pines
Pathogen:   Dothistroma pini
Signs/Symptoms:  Needle tip browning and death with a distinct break between live and dead tissue
For more information see:       UW Plant Disease Facts D0043
Drought Stress Drought Stress
Hosts:  All conifers
Cause:   Insufficient water
Signs/Symptoms:  Purpling/browning of needles near branch tips or higher up in plants during the summer
Winter Injury Winter Injury/Winter Burn
Hosts:  All conifers, particularly yew and juniper
Cause:   Insufficient water
Signs/Symptoms:  Needle browning/bleaching over winter or in spring as plants come out of dormancy
For more information see:       UW Plant Disease Facts D0127
Herbicide Damage Herbicide Damage
Hosts:  All conifers
Cause:   Exposure to herbicides
Signs/Symptoms:  Twisted or otherwise distorted growth, needle yellowing or browning, plant death
For more information see:       UW Plant Disease Facts D0060

For more information on conifer diseases:  See https://pddc.wisc.edu/ or contact your county Extension agent.

Peach Leaf Curl

What is peach leaf curl?  Peach leaf curl is a common disease of peach and nectarine trees throughout the Midwest and eastern U.S.  Where the disease is severe, tree vigor and fruit quality and yield are reduced.  Peach leaf curl often becomes more prevalent after relatively mild winters, which are more favorable for the survival of the organism that causes the disease.  A related disease, plum pockets, affects plums.

Leaf distortions and discoloration typical of peach leaf curl.
Leaf distortions and discoloration typical of peach leaf curl.

What does peach leaf curl look like?  Diseased leaves are distorted with puckered, thickened, twisted areas that can be light green, yellow, or reddish to purple in color.  Leaves later turn brown and fall from the tree.  Diseased shoots are stunted with small, yellowish leaves, or have leaves arranged in tight whorls (rosettes).  Diseased flowers may abort, leading to reduced fruit set, while diseased fruit are bumpy, reddish in color, and fall prematurely.

Where does peach leaf curl come from?  Peach leaf curl is caused by the fungus Taphrina deformans, which overwinters in bark and bud scales of peach and nectarine trees.  Fungal spores infect leaves and shoots in the spring while leaves are still in the bud and as they just begin to emerge.  Mild (50 to 70°F), wet weather during this period favors infection.  Additional spores form on the surface of diseased tissue, and these spores cause new infections if the weather remains mild and wet.

How do I save trees that have peach leaf curl?  Peach leaf curl is unlikely to kill a peach or nectarine tree on its own.  However, if significant premature leaf drop occurs, trees will be susceptible to drought stress and winter injury.  To help maintain tree vigor, apply water (approximately one inch per week) at the drip lines (i.e., the edges of where the branches extend) of peach and nectarine trees during dry periods.  Also, fertilize trees with nitrogen, but avoid fertilizing after August 1; late season fertilization will prevent trees from hardening off properly before winter, making them prone to winter injury.  Finally, thin fruit if the crop load is heavy.

How do I avoid problems with peach leaf curl in the future?  Because Taphrina deformans survives in bark and bud scales, removing diseased leaves in the fall will not reduce disease.  To prevent serious problems with peach leaf curl, plant resistant or tolerant peach varieties (e.g., ‘Frost’, ‘Indian Free’, ‘Q-1-8’, varieties derived from ‘Redhaven’).  Avoid growing susceptible varieties (e.g., those derived from ‘Redskin’).  In addition, consider applying a single fungicide spray in the fall after leaf drop or in the spring before buds begin to swell to control peach leaf curl (and also plum pockets).  Effective fungicide active ingredients include chlorothalonil, copper (e.g., Bordeaux mixture), and ferbam.  Choose a fungicide that is labeled for use on edible fruit crops, and read and follow all label instructions to ensure that you use the product in the safest and most effective manner possible.

For more information on peach leaf curl:  Contact your county Extension agent.

Diplodia Shoot Blight and Canker

Diplodia shoot blight and canker killing branch tips of Austrian pine.
Diplodia shoot blight and canker killing branch tips of Austrian pine.

What is Diplodia shoot blight and canker?  Diplodia shoot blight and canker (formerly Sphaeropsis shoot blight and canker) is one of the most common fungal diseases of Austrian pine in Wisconsin.  This disease can also affect other pines including red, jack, Scots and mugo pine, as well as other conifers including cedars, cypresses, firs, junipers and spruces.

What does Diplodia shoot blight and canker look like?  Initially, affected branch tips may ooze a large amount of resin.  Eventually, these branch tips brown and die, with dead needles on these branches having varying lengths.  As the disease progresses, sunken or swollen, discolored areas (called cankers) may form on infected twigs.  Diplodia shoot blight and canker can be distinguished from damage from boring insects (where there also may be heavy resin flow) by an absence of any tunneling.

Where does Diplodia shoot blight and canker come from?  Diplodia shoot blight and canker is caused by several fungi in the genus Diplodia.  These fungi include Diplodia sapinea (formerly known as Diplodia pinea and Sphaeropsis sapinea), which has historically been cited as the cause of the disease on Austrian pine.  However, this fungus is only one of several Diplodia species that can cause problems on this host.  Diplodia fungi survive in infected shoots and pinecones where they form small, black fruiting bodies (i.e., reproductive structures) that produce brown-colored spores.

How do I save a tree with Diplodia shoot blight and canker?  Immediately remove and destroy diseased branch tips.  Also, where possible, remove and destroy pinecones that have fallen from infected trees.  Dispose of these materials by burning (where allowed by local ordinance) or burying them.

When pruning, cut branches six to eight inches below the point where they are obviously infected.  Prune only in dry weather.  Between cuts, decontaminate pruning tools by treating them for at least 30 seconds with 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants), or a 10% bleach solution (i.e., one part of a disinfecting bleach and nine parts water).  Decontaminating tools is important to help prevent accidental movement of Diplodia fungi from branch to branch during pruning.  If you decide to use bleach, be sure to rinse your tools thoroughly after pruning and then oil them to prevent rusting.

Needles of different lengths (left) and production of excessive resin on branch tips (right) are typical symptoms of Diplodia shoot blight and canker.
Needles of different lengths (left) and production of excessive resin on branch tips (right) are typical symptoms of Diplodia shoot blight and canker.

How do I avoid problems with Diplodia shoot blight and canker in the future?  Avoid planting Austrian pines; plant other types of evergreens instead.  Minimize any stresses on established Austrian pines.  Water trees adequately, particularly during dry periods.  Established trees should receive approximately one inch of water per week from the time that the ground thaws in the spring, through the summer and into the fall up until the ground freezes or there is a significant snowfall.  New transplants (i.e., conifers planted within three years) require approximately two inches of water per week.  During periods with insufficient rain, apply water at the drip lines of trees (i.e., the edges of where the branches extend) using a drip or soaker hose.

When planting trees, be sure to allow ample space for roots to grow, avoid compacting the soil around trees, and make sure there is adequate soil drainage.  Mulch trees to at least their drip lines with a high-quality mulch (e.g., shredded oak bark mulch, red cedar mulch).  Use one to two inches on heavier (e.g., clay) soils.  Use three to four inches on lighter (e.g., sandy) soils.  Keep mulch approximately four inches from tree trunks.  DO NOT fertilize new transplants, and fertilize established conifers only when a soil fertility test indicates that fertilization is needed.  DO NOT overfertilize, particularly with nitrogen.

Finally, you may want to apply fungicides to help prevent infections.  Apply fungicides only after you have pruned out diseased branches as described above.  Fungicides prevent infections but do not cure existing infections.  Alternate use of fungicides containing thiophanate-methyl and chlorothalonil that are labeled for use on conifers.  Start applications at bud break and continue at 14 day intervals until full shoot elongation.  DO NOT use thiophanate-methyl alone.  Overuse of thiophanate-methyl can potentially select for variants of Diplodia that will no longer be controlled by this active ingredient.  Be sure to read and follow all label instructions of the fungicides that you select to ensure that you use the products in the safest and most effective manner possible.

For more information on Diplodia shoot blight and canker:  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.

Rhizosphaera Needle Cast

Browning of interior spruce needles caused by Rhizosphaera needle blight.
Browning of interior spruce needles caused by Rhizosphaera needle blight.

What is Rhizosphaera needle cast?  Rhizosphaera needle cast is the most common disease of Colorado blue spruce in Wisconsin, making Colorado blue spruce unsightly and unusable in many landscape settings.  The disease also affects other conifers including black, Engelmann, Serbian, Sitka, and white (e.g., Black Hills) spruce; Austrian, mugo and eastern white pine; Douglas-fir, balsam fir and western hemlock.

What does Rhizosphaera needle cast look like?  The first noticeable symptom of Rhizosphaera needle cast is purpling or browning and loss of the innermost needles on lower branches of spruce trees.  Often, the youngest needles at the tips of branches remain healthy.  Rows of small, black spheres form along the length of infected needles and are visible with a 10X hand lens.  These black spheres are fruiting bodies (i.e., reproductive structures) of the fungus that causes the disease and are diagnostic.

Where does Rhizosphaera needle cast come from?  Rhizosphaera needle cast is typically caused by the fungus Rhizosphaera kalkhoffii, although other species of Rhizosphaera can be involved depending on the host.  Infected needles, including those that are still attached to branches and those that have fallen to the ground, produce spores that can be blown or splashed to healthy needles.

How do I save a tree or shrub with Rhizosphaera needle cast?  Consider treating affected trees with fungicides labeled for use on evergreens and containing copper or chlorothalonil.  Treatments will not cure existing infections, but can prevent additional infections.  Apply treatments every three to four weeks starting as new needles emerge each spring.  Continue applications through periods of wet weather.  For fungicide treatments to be effective, thoroughly cover all needles.  This may be difficult on large trees.  Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with Rhizosphaera needle cast in the future?  The easiest way to avoid Rhizosphaera needle cast is to avoid planting Colorado blue spruce.  If you do plant this tree, consider using dwarf varieties, and allow adequate spacing between trees so that branches will not overlap when they are full size.  Dwarf varieties and properly spaced larger spruce varieties will have better air penetration and needles will dry more quickly.  Dry needles are less likely to be infected.  Check existing spruce trees for the disease, and remove and destroy any diseased branches and needles by burning (where allowed by local ordinance), burying or hot composting.

For more information on Rhizosphaera needle cast:  See UW Bulletin A2640, Colorado Blue Spruce and Other Conifers Disorder:  Rhizosphaera Needle Cast (available at https://learningstore.extension.wisc.edu/) or 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.

Tomato Spotted Wilt of Potato

What is tomato spotted wilt?  Tomato spotted wilt is a common viral disease of worldwide distribution that can affect over 1000 plant species.  Economically important hosts include a wide range of vegetables, fruits, field crops, and ornamentals.  Many weeds are also potential hosts.  Tomato spotted wilt is especially important in greenhouse production where it can cause significant economic losses in horticultural and floral crops.  In potato production, tomato spotted wilt is uncommon but of enough concern that routine monitoring for the disease in greenhouse and field settings is warranted.

Leaf spots with concentric rings (left) and stem streaking (right) typical of potato plants due to Tomato spotted wilt. Photos courtesy of G. J. Holmes (left) and Joshua Kunzman (right).
Leaf spots with concentric rings (left) and stem streaking (right) typical of potato plants due to Tomato spotted wilt. Photos courtesy of G. J. Holmes (left) and Joshua Kunzman (right).

What does tomato spotted wilt look like?  Symptoms of tomato spotted wilt vary widely depending on host species, host variety and when during development a plant is infected.  Typical symptoms on leaves include yellow, brown or black ringspots (i.e., ring-like areas of discolored tissue).  On some hosts, the ringspots can form a target-like pattern.  Distorted leaf growth can also be a typical symptom.

On potato leaves, tomato spotted wilt can lead to necrotic (i.e., dead) areas with or without yellow haloes.  These symptoms can resemble symptoms of early blight (see UW Plant Disease Facts D0046, Early Blight).  Black streaks on petioles or stems and branch tip dieback are also common symptoms on potato (as well as other hosts).  Potato tuber symptoms include surface rings or dark patches, and internal discolored patches, rings, spots or flecks.

Where does tomato spotted wilt come from?  Tomato spotted wilt is caused by Tomato spotted wilt virus (TSWV), which is closely related to Impatiens necrotic spot virus (INSV), the cause of impatiens necrotic spot (see UW Plant Disease Facts D0067, Impatiens Necrotic Spot).  TSWV is found in the sap of infected plants and is most commonly spread by thrips (see University of Wisconsin Garden Facts XHT1022, Managing Thrips in Greenhouses).  At least eight thrips species can be involved in TSWV transmission.  Thrips larvae acquire the virus as they feed on infected plants, then transmit the virus as they move to and feed on healthy plants.  Once thrips acquire the virus, they can transmit it for their entire lifespans.  The severity of tomato spotted wilt depends on the size and activity of thrips populations at a location, as well as on the number of infected plants (often weeds, but potentially infected potatoes as well) serving as reservoirs for the virus.  TSWV can also be transmitted mechanically (e.g., on tools used to trim branches or cut potato tubers), but this method of transmission is much less common than transmission by thrips.

How can I save plants with tomato spotted wilt?  There is no known cure for tomato spotted wilt.  Infected plants should be removed and destroyed to eliminate a reservoir for the virus that can subsequently contribute to spread to other plants.  Infected plants can be burned (where allowed by local ordinance), deep buried or hot composted.

Surface and internal tuber symptoms due to Tomato spotted wilt . Photos courtesy of Joshua Kunzman.
Surface and internal tuber symptoms due to Tomato spotted wilt . Photos courtesy of Joshua Kunzman.

How can I avoid problems with tomato spotted wilt in the future?  Prevent introducing TSWV and thrips into your greenhouse by carefully inspecting any new plants for potential problems.  Test suspect plants for TSWV using dipstick tests [available from Agdia, Inc. (www.agdia.com)] or by submitting a sample to the UW-Madison Plant Disease Diagnostics Clinic (pddc.wisc.edu).  Remove and dispose of any infected plants.

To prevent new infections via thrips, remove weeds in and outside of greenhouses to eliminate TSWV and thrips reservoirs.  Place 400-mesh screens on vents to limit thrips movement.  Also limit thrips movement on clothing by avoiding colors (e.g., pink, blue, yellow, white or green) that can attract thrips.  Monitor for thrips using blue and yellow sticky cards placed above plants throughout the greenhouse and near doors and vents.  Use chemical and/or biological control products to control thrips.  See University of Wisconsin Garden Facts XHT1022, Managing Thrips in Greenhouses, for details on what products to use and when to use them.  Multiple applications will likely be necessary because thrips eggs are not killed by insecticides.  If you find plants that you suspect are infected with TSWV, remove and dispose of these plants immediately.

To prevent mechanical transmission of TSWV, be sure to routinely decontaminate any items (e.g., pruners, knives, pots, work surfaces) that come into contact with plants by treating them for a minimum of one minute with a solution of one of the following:

  • 2.75 tablespoons Alconox® (a type of lab detergent) plus 2.5 tablespoons sodium dodecyl sulfate (SDS) [also known as sodium lauryl sulfate (SLS)] in one gallon of water, or
  • 14 dry ounces of trisodium phosphate in one gallon of water.

In field settings, tomato spotted wilt does not appear to be a significant problem on potato (although it can be a significant problem on other crops).  Use of resistant or tolerant potato varieties may reduce yield loses, but unfortunately there is little information on which potato varieties are resistant/tolerant to the disease.  Removing symptomatic plants can reduce potential reservoirs of TSWV, but may not be feasible.  More important in field settings may be to keep weeds under control.  Current research suggests that TSWV does not persist long-term in the field unless there are perennial weeds to serve as TSWV reservoirs.

For more information on tomato spotted wilt:  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.