All posts by hudelson

January 2022: Decontaminating Your Way to Healthier Plants

January 27, 2022 hudelson

When discussing plant diseases and their management, I often emphasize decontaminating pots, gardening tools and work surfaces as one part of a successful disease management strategy. To kill fungi and bacteria (and to a lesser extent viruses) I often recommend treating items for at least 30 seconds with alcohol, commercial disinfectants, or bleach.

When using alcohol, a 70% solution is optimal for decontamination. I use ethanol for decontaminating items in my lab, but rubbing (i.e., isopropyl) alcohol is a better option for home gardeners, as it is readily available at a local drugstore or grocery store. Rubbing alcohol is formulated as a 70% solution and can be used straight out of the bottle.

Commercial disinfectants come in many forms. I look for products that contain alcohol (you may see ethanol listed as an ingredient) and where the percentage of all active ingredients is as close to 70% as possible. If you opt to use a spray disinfectant, spray items until they drip and then allow them to air dry.

Bleach is probably the most challenging material to use for decontamination. Back when I started at the PDDC (almost 25 years ago), most bleach that you could buy at the grocery store was formulated to be 5.25% sodium hypochlorite (the active ingredient), and you just had to make a 10% solution (i.e., 1 part bleach and 9 parts water) to yield an approximate 0.5% solution that is optimal for decontaminating items. Nowadays, the bleach that you buy can contain very different concentrations of sodium hypochlorite (e.g., 1.8%, 5.25%, 7.5%, 8.25%), and you have to be more careful how you mix up your dilute bleach solution.

Here are some easy instructions on how to dilute the bleach that you buy (assuming it contains 1% or more sodium hypochlorite) to properly to yield a solution that is 0.5% sodium hypochlorite (or slightly higher):

Find the percentage of sodium hypochlorite in your bleach in the ingredient list;
Multiple that number by two (2);
Round the resulting number down to the nearest whole number;
Subtract one (1) from that rounded number.
This final number is the number of parts of water that you need to mix with one (1) part of your bleach to yield the diluted solution you need for decontaminating items.

When you use bleach on metal tools (alcohol is really a better option for treating metal items), be sure to rinse tools thoroughly with water after treating (to remove bleach residues) and then oil them. Bleach corrodes metal and will cause metal tools to rust if not used properly. Also use bleach carefully to prevent contact with your skin and clothing. Bleach can cause skin irritation/burns and eat holes in your clothes, if not used properly.

Now go forth and decontaminate!

For more information on plant diseases and their management, check out the UW Plant Disease Diagnostics Clinic website (https://pddc.wisc.edu/) or contact PDDC staff at pddc@wisc.edu or (608) 262-2863.

Disease - Herbaceous Ornamental, Fact Sheet

Root Rots on Houseplants

May 29, 2021 hudelson

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.

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.

Disease - Broad Leafed Woody Ornamental, Disease - Needled Woody Ornamental, Fact Sheet, Misc

Lichens

May 28, 2021 hudelson

What are lichens? Lichens are organisms that arise from mutually beneficial interactions between certain fungi and algae. The fungi provide the physical structures of the lichens, as well as protection for the algae. The algae, in turn, produce food for the fungi via photosynthesis.

What do lichens look like? Lichens come in four basic growth forms. Crustose lichens are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens are leaf-like and composed of flat sheets of tissue that are not tightly bound together. Squamulose lichens are composed of scale-like parts. Fruticose lichens are composed of free-standing branching tubes.

Where do lichens come from? Lichens are everywhere. There are an estimated 13,500 to 17,000 species of lichens, and lichens can be found growing in tropical, temperate and polar regions throughout the world. Lichens will grow on almost any surface that is stable and reasonably well-lit. In temperate regions, lichens can often be found growing on the bark of trees or old fence posts. Others lichens grow in less hospitable places, such as bare rock surfaces or old headstones in graveyards, where they aid in the breakdown of rocks and the formation of soil.

How do I save a tree with lichens? DO NOT PANIC! Lichens do not harm trees; they are not pathogens or parasites, and do not cause disease. Lichens are self-reliant, with the algal component of the lichen producing food for the organism via photosynthesis. Lichens absorb water and minerals from rainwater and the atmosphere, and because of this, they are extremely sensitive to air pollution. As a result, the presence or absence of certain lichen species can be used as an indicator of levels of atmospheric pollutants. Information on the abundance and species of lichens growing in an area can give a good indication of the local air quality.

For more information on lichens: Contact your county Extension agent.

Disease - Broad Leafed Woody Ornamental, Fact Sheet, Quick Reference Guide

Deciduous Tree Leaf Disease Quick Reference

April 30, 2021 hudelson

	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 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 (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 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 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 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 Hosts: Hickory, walnut Pathogen: Microstroma juglandis Signs/Symptoms: Discrete powdery white areas on lower leaf surfaces
	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 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 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.

Disease - Needled Woody Ornamental, Fact Sheet, Quick Reference Guide

Conifer Disease Quick Reference

April 30, 2021 hudelson

	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 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 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 Hosts: Spruces Pathogen: Unknown (possibly Setomelannoma holmii) Signs/Symptoms: Needle loss and dieback at or near branch tips
	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 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 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 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 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 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 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.

Disease - Fruit Crop, Fact Sheet

Peach Leaf Curl

April 6, 2021 hudelson

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.

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.

Disease - Needled Woody Ornamental, Fact Sheet

Diplodia Shoot Blight and Canker

March 8, 2021 hudelson

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.

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.

Disease - Needled Woody Ornamental, Fact Sheet

Rhizosphaera Needle Cast

March 1, 2021 hudelson

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.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet, Pest Alerts

Ralstonia Wilt – Pest Alert

May 1, 2020 hudelson

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

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].

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Southern Blight

January 6, 2019 hudelson

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. rolfsii. A. 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.