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)
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 Cucumbermosaicvirus (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.
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.
Cumber 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 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.
What is white mold? White mold, also known as Sclerotinia stem rot, is a serious and often lethal fungal disease that affects over 400 species of plants in many plant families. White mold causes severe damage in commercial snap, kidney and lima bean production, as well as commercial sunflower production. White mold also can be a serious problem in home vegetable gardens on tomatoes, potatoes, cucurbits (e.g., cucumber, pumpkin and squash), carrots, lettuce, celery, brassicas (e.g., cabbage, broccoli, and cauliflower), basil, and rosemary. White mold is a common disease on many annual and perennial ornamentals as well.
Stem cankers, cottony mycelia and sclerotia (see arrows) of white mold on snap beans.
What does white mold look like? Symptoms of white mold vary depending on the plant infected. White mold can lead to crown and stem cankers, root rots, wilts, damping-off of seedlings, and blossom and fruit rots. Plants affected by white mold can wilt rapidly due to stem-girdling cankers at or near the soil line. Brown spots can appear on flower petals and buds. Cottony masses of fungal threads (called mycelia) may appear on stems or on nearby soil. Hard, irregularly-shaped masses (called sclerotia) develop within or on the surface of infected plants. The sclerotia are white at first and then turn dark brown or black as they mature.
Where does white mold come from? White mold is caused by several species of the fungus Sclerotinia, most commonly Sclerotiniasclerotiorum. White mold fungi overwinter as sclerotia in dead plant material or in infested soil. Sclerotia can survive in soil for at least 3 years. During periods of cool, wet or humid weather, sclerotia germinate to form either fungal threads (called hyphae) or tiny, mushroom-like spore-producing structures (called apothecia). Apothecia can release millions of spores over a period of several days. Spores are dispersed primarily by wind, but also by rainsplash and insects. Both spores and hyphae can infect plant tissue, with infection often occurring through dead or declining plant parts (e.g., flowers and leaves), and then spreading to healthy tissue. Infections may also occur through healthy plant parts that are growing near or in contact with the soil. In some plants, seeds can be contaminated by sclerotia or mycelia of white mold fungi.
How do I save plants with white mold? White mold is difficult to manage once infections have occurred. Prune four to five inches below obviously disease tissue or remove entire plants if they are severely affected. DO NOT compost these materials because of the risk of spreading white mold fungi via long-lived sclerotia. Burn these materials instead.
White mold can be destructive on ornamentals such as Liatris.
How do I avoid problems with white mold in the future? Check seed for sclerotia and remove these structures if you find them. Make sure your soils are well-drained, and avoid overwatering as well as frequent, light watering. DO NOT overhead water (e.g., with a sprinkler). Instead water deeply early in the day with a drip or soaker hose. Promote good air circulation in your garden so that plants more rapid dry when they do get wet (e.g., from natural rain). Better air circulation can be achieved by choosing cultivars or varieties of plants that have a more open growth form, by spacing plants farther apart, by avoiding excessive use of nitrogen fertilizers (which can promote excessive production of lush, thick foliage), and by keeping weeds under control. Weed control also eliminates potential alternate hosts that white mold fungi can infect. As leaves and flowers or your plants naturally wither and die, remove them, as well as any other plant debris that may harbor white mold fungi. Finally, in beds where severe cases of white mold have occurred, remove and replace infested soil.
For more information on white mold: Contact your county Extension agent.
What is tobacco rattle? Tobacco rattle is a common and potentially serious viral disease that affects a variety of herbaceous ornamentals including, but not limited to, astilbe, bleeding heart, coral bells, daffodil, epimedium, gladiolus, hyacinth, marigold, tulip and vinca. Tobacco rattle can also affect vegetable crops such as beans, beets, peppers, potatoes, and spinach. On potatoes, the disease is referred to as corky ring spot (see University of Wisconsin Garden Facts XHT1186).
Tobacco rattle virus-infected plants often have leaves with yellow line patterns.
What does tobacco rattle look like? Symptoms of tobacco rattle can vary depending on the plant species or variety/cultivar affected, and depending on environmental conditions. Infected plants may exhibit mottling (i.e., blotchy light and dark discoloration of leaf tissue), yellow ring spots or line patterns, localized necrotic lesions (i.e., dead spots), chlorotic (i.e., yellow) spots or streaks, or leaves with notches. Plants that become infected at a young age can also exhibit a variety of leaf and stem deformities. In potato, the disease can also manifest itself as a series of corky arcs or rings in tuber tissue.
Where does tobacco rattle come from? Tobacco rattle is caused by the tobacco rattle virus (TRV) which is spread primarily by stubby-root nematodes, a group of microscopic, worm-like organisms in the genera Trichodorus and Paratrichdorus. These nematodes feed on the roots of infected plants, acquiring TRV, then move to non-infected plants where their subsequent feeding spreads the virus. TRV can be spread mechanically when pruning tools or knives that are used to trim or divide plants become contaminated, and by grafting. TRV can also be found in seeds from infected plants.
How do I save a plant with tobacco rattle? Once plants have become infected with TRV, they remain infected indefinitely. Infected plant cannot be treated in any way to eliminate the virus and should be removed and disposed of by burning, burying or composting.
How do I avoid problems with tobacco rattle in the future? The best way to prevent problems with tobacco rattle is to grow plants that are not susceptible to TRV. Such plants include, but are not limited to, annual phlox, carnation, devil’s trumpet (downy thorn-apple), sweet William, zinnia and zombie cucumber. Carefully inspect susceptible plants for symptoms of tobacco rattle prior to purchase and do not buy symptomatic plants. When pruning or dividing plants, be sure to clean tools between cuts by dipping them for at least 30 seconds in a solution that is a combination of 1% sodium lauryl sulfate (use 10% shampoo as a source of this chemical) and 1% Alconox (an industrial detergent). This will help prevent the mechanical spread of the virus. Control of the nematodes that transmit TRV is not practical in home garden settings.
For more information on tobacco rattle: Contact your county Extension agent.
Rapid wilting and die back of branch tips can be a symptom of ramorum dieback.
What is sudden oak death? Sudden oak death (also called ramorum leaf blight or ramorum dieback) is an oftentimes lethal disease that has caused widespread death of tanoak (Lithocarpusdensiflorus), coast live oak (Quercus agrifolia), California black oak (Quercuskelloggii), and Shreve oak (Quercus parvula var. shrevei) in California. The disease has also been reported in Oregon, as well as in Europe. Currently 60 species of plants have been reported to be susceptible to the disease. Twenty-eight of these plants are confirmed hosts. In addition to the trees listed above, the following trees fall into this group:
bigleaf maple (Acer macrophyllum)
Bodnant viburnum (Viburnum X bodnantense)
‘Brouwer’s Beauty’ pieris (Pieris floribunda X japonica)
California bay laurel (Umbellularia californica)
California buckeye (Aesculus californica)
California coffeeberry (Rhamnus californica)
California honeysuckle (Lonicera hispidula)
canyon live oak (Quercus chrysolepis)
coast redwood (Sequoia sempervirens)
doublefile viburnum (Viburnum plicatum var. tomentosum)
douglas-fir (Pseudotsuga menziesii var. menziesii)
evergreen huckleberry (Vaccinium ovatum)
Formosa firethorn (Pyracantha koidsumii)
‘Forest Flame’ pieris (Pieris formosa X japonica)
Himalaya pieris (Pieris formosa)
Japanese camellia (Camellia japonica)
Japanese pieris (Pieris japonica)
laurustinus (Viburnum tinus)
madrone (Arbutus menziesii)
manzanita (Arctostaphylos manzanita)
rhododendron (Rhododendron spp.)
Sasanqua camellia (Camellia sasanqua)
toyon (Heteromeles arbutifolia)
western starflower (Trientalis latifolia)
witch hazel (Hamamelis virginiana)
An additional 31 species including the following are suspected hosts, although tests to verify their susceptibility have not yet been completed:
Burkwood viburnum (Viburnum X burkwoodii)
California hazelnut (Corylus cornuta)
Camellia X williamsii
cascara (Rhamnus purshiana)
Chinese pieris (Pieris formosa var. forrestii)
common lilac (Syringavulgaris)
David viburnum (Viburnumdavidii)
drooping leucothoe (Leucothoefontanesiana)
European beech (Fagussylvatica)
European cranberrybush viburnum (Viburnumopulus)
European turkey oak (Quercuscerris)
European yew (Taxus baccata)
fragrant viburnum (Viburnum farreri)
grand fir (Abies grandis)
Holm oak (Quercus ilex)
horse-chestnut (Aesculus hippocastanum)
lingonberry (Vacciniumvitis-ideae)
mountain laurel (Kalmialatifolia)
Northern red oak (Quercus rubra)
Pieris formosa var. forrestii X Pieris japonica
poison oak (Toxicodendron diversiloba)
Prague viburnum (Viburnum X pragense)
reticulate camellia (Camelliareticulata)
salmonberry (Rubus spectabilis)
Southern red oak (Quercus falcata)
strawberry tree (Arbutusunedo)
sweet chestnut (Castanea sativa)
Viburnum X carlcephalum X Viburnumutile
Victorian box (Pittosporumundulatum)
wayfaringtree viburnum (Viburnumlantana)
wood rose (Rosagymnocarpa)
At this time, sudden oak death has not been reported in Wisconsin. However, the disease was recently found at a large nursery near Los Angeles, CA that ships plants throughout much of the United States, including Wisconsin. Thus, the disease may be present in Wisconsin, but not yet detected.
Ramorum leaf blight symptoms can mimic those of other leaf spots and blights.
What does sudden oak death look like? Symptoms of sudden oak death vary depending upon the plant species infected. On some hosts, infections occur primarily on leaves leading to light brown leaf spots and blotches. These leaf symptoms may be indistinguishable from other, more common, leaf spots and blights, or may mimic sunburn or leaf scorch symptoms. Twigs and branches that become infected often wilt, forming a “shepherd’s-crook”, and subsequently die back. Infection of tree trunks leads to cankers (i.e., sore-like areas) that produce copious amounts of an amber to black colored ooze. This ooze can dry to form a stained area on the bark. Removing the bark over the affected area will reveal discolored wood beneath that sometimes (but not always) has a black border. Cankers can eventually expand to girdle trunks, thus resulting in the death of the tree or shrub. Trunk infections appear not to extend into the root system of the plant. Once sudden oak death cankers develop, other pathogens may invade the infected areas, accelerating tree or shrub death and complicating the diagnosis of the disease.
Where does sudden oak death come from? Sudden oak death is caused by the fungus Phytophthora ramorum, which was first recognized as a pathogen in 1995. Phytophthora ramorum can be spread over long distances through movement of infected plants or infested plant parts. The fungus can also be moved with contaminated soil (e.g., on vehicle tires, tools, or shoes), or in contaminated water. Once established on plants in a given location, the fungus produces reproductive structures (called sporangia) that can be moved from plant to plant by rain splash, or wind. Phytophthora ramorum has currently been documented as occurring in California, Oregon and Europe, but the recent discovery of the pathogen in a large nursery in California that ships plants throughout the United States raises the possibility that this pathogen may have been spread to other areas.
How do I save a plant with sudden oak death? If you believe you have seen a plant that has sudden oak death, IMMEDIATELY call the Plant Disease Diagnostics Clinic (PDDC) at (608) 262-2863 to make arrangements for an appropriate diagnosis. Because Phytophthoraramorum is a regulated, quarantined pathogen, DO NOT remove the affected plant (or parts thereof) or take the plant from the site where it is located. PDDC staff will make arrangements for sample collection and testing. If your plant tests positive for Phytophthora ramorum, it will be removed and destroyed to help prevent further spread of the pathogen.
How do I avoid problems with sudden oak death in the future? Carefully inspect any new nursery stock upon delivery (or prior to purchase, if possible) for symptoms of sudden oak death. Keep new stock isolated from older stock as long as possible, to minimize possible movement of the pathogen should the disease develop after plants have arrived. If you see any suspect symptoms, alert the PDDC so that arrangements can be made for proper testing for Phytophthora ramorum.
For more information or help in diagnosing sudden oak death: Contact Brian Hudelson, Plant Disease Diagnostic Clinic, Department of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI 53706-1598, phone: (608) 262-2863, fax: (608) 263-2626, email: bdh@plantpath.wisc.edu, see the USDA APHIS sudden oak death website http://www.aphis.usda.gov/ppq/ispm/sod, or contact your County Extension agent.
What is Southern blight? Southern blight is a serious, and typically lethal, fungal disease that is most common in the tropics and subtropics. This disease also can cause damage in the southern U.S., and can even become a problem in temperate states 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, dahlia, daylily, gladiolus, hosta, impatiens, peony, petunia, rose, rudbeckia, salvia, sedum and viola. Small woody ornamentals can be susceptible as well.
The Southern blight fungus produces large numbers of spherical, light tan to dark red resting structures called sclerotia (red arrow).
What does Southern blight look like? Southern blight initially leads to a water-soaked appearance on lower leaves, or a water-soaked lesion (spot) 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, and typically radiate 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.
Where does Southern blight come from? Southern blight is caused by the fungus Sclerotium rolfsii, which lives in soil, on plant debris and on weed hosts. The fungus can be spread through movement of infested soil, infected plants and contaminated irrigation water, and by use of contaminated tools. In Wisconsin, S. rolfsii most likely enters gardens on infected nursery stock or infested mulch. Freezing temperatures will kill S. rolfsii mycelia, while sclerotia can survive at temperatures above approximately 14ºF.
How can I save a plant with Southern blight? Infected plants, as well as soil from six inches beyond an infested area, should be removed. Bury any remaining sclerotia eight to 12 inches to reduce the length of time that they will survive in the soil. Grow non-susceptible plants (e.g., larger woody ornamentals) in the affected area for two to three years following an infestation to allow time for sclerotia to die naturally. Fungicides containing triadimefon may be effective for control of Southern blight. However, these products will likely be more effective if applied as preventive treatments rather than in an attempt to “cure” existing disease.
How can I prevent Southern blight in the future? Inspect new plants prior to transplanting for sclerotia and mycelia, and throw away diseased plants. Avoid mulches that might be contaminated with S. rolfsii. S. rolfsii thrives in moist conditions. Therefore, thin existing gardens or space plants farther apart in new gardens to improve airflow and promote rapid drying of foliage.
For more information on Southern blight: Contact your county Extension agent.
What is powdery mildew? Powdery mildew is a disease that occurs on the above-ground parts (especially the leaves) of many herbaceous ornamental plants, as well as deciduous trees and shrubs, indoor houseplants, and many agricultural crops. Conifers are not affected by this disease.
Plants such as phlox, Monarda and zinnia are very susceptible to powdery mildew.
What does powdery mildew look like? The name of this disease is descriptive. The upper and (less frequently) lower surfaces of leaves, as well as stems of infected plants, have a white, powdery appearance. They look as though someone has sprinkled them with talcum powder or powdered sugar.
Where does powdery mildew come from? Powdery mildew is caused by several closely related fungi that survive in plant debris or on infected plants. These fungi are fairly host specific. The powdery mildew fungus that infects one type of plant (e.g., phlox) is not the same powdery mildew fungus that infects another (e.g., lilac). However, if you see powdery mildew on one plant, then weather conditions, usually high humidity, are favorable for development of the disease on a wide range of plants.
How do I save a plant with powdery mildew? DO NOT panic! For many plants, powdery mildew is a cosmetic, non-lethal disease. For other plants [e.g., phlox, Monarda, zinnia (see University of Wisconsin Garden Facts XHT1175)], powdery mildew can cause severe leaf loss. When a highly valued plant has had severe leaf loss due to powdery mildew for several years, you may want to consider using a fungicide for control. Fungicides containing dinocap, dithiocarbamates, myclobutanil, triadimefon, triforine, sulfur or thiophanate methyl are registered for use against powdery mildew. A combination of baking soda (11∕2 tablespoons) and light weight horticultural (e.g., Sunspray®) oil (3 tablespoons) in water (1 gallon) has also been shown to be effective for powdery mildew control. Most products should be applied every seven to 14 days from bud break until humid weather subsides. Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible. Also consider pretesting any product you decide to use on a small number of leaves or plants before treating a larger area to make sure there are no toxic effects, particularly when treating during warmer weather.
How do I avoid problems with powdery mildew in the future? Consider buying plant varieties that are powdery mildew resistant. This won’t guarantee that your plants will be powdery mildew-free every year, but should result in less severe disease when it occurs. Reduce the humidity around your plants by spacing them further apart to increase air flow. Be sure not to over-water as this can lead to higher air humidity as well. Finally, at the end of the growing season, remove and destroy any infected plant debris as this can serve as a source of spores for the next growing season.
For more information on powdery mildew: See UW-Extension Bulletin A2404 or contact your county Extension agent.
Root-knot nematodes (Meloidogyne spp.) are small, soilborne, worm-like organisms that infect many agricultural and horticultural plants. Root-knot nematodes are found worldwide, and are named for the swellings (called “galls” or “knots”) that they cause in plant roots. Economically-important species of Meloidogyne include M. arenaria, M. hapla, M. incognita, and M. javanica. Of these, M. hapla (commonly known as Northern root-knot nematode) is most likely to be found in Wisconsin soils.
Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.
Appearance: Root-knot nematodes are about one-tenth the size of a pinhead and are typically embedded inside roots. They are impossible to see with the naked eye. Juvenile root-knot nematodes (both males and females), as well as adult males, are vermiform (i.e., worm-shaped) and live in the soil. Adult females are spherical in shape and live inside roots. Both males and females possess a thin, tube-like structure called a stylet that they use for penetrating root tissue.
Symptoms and Effects: When root-knot nematodes enter roots, they release chemicals that cause nearby root cells to enlarge. This leads to the formation of swollen, distorted areas in roots known as galls or knots. The number and size of galls varies depending on plant species and cultivar, and the number of root-knot nematodes in the soil. On some hosts (e.g., grasses) root swelling can be very difficult to detect. Nematode feeding interferes with proper root function (e.g., water and nutrient movement). Thus infected plants may be stunted and wilted, may exhibit discolorations (e.g., yellowing) typical of plants with nutrient deficiencies, and ultimately (in field or vegetable crops) may have lower yields. Because root-knot nematodes tend not to be uniformly distributed in the soil, symptomatic plants often occur in patches and are typically surrounded by plants of normal height and appearance. Environmental factors such as slope, soil type, or soil moisture can cause similar patchy patterns, so identification of a root-knot nematode problem requires examination of symptomatic plants at a lab qualified to perform nematode diagnostics.
Life Cycle: Root-knot nematodes (i.e., M. hapla) are native to Wisconsin and can be spread whenever contaminated soil or infected plants are moved. Root-knot nematodes survive the winter as eggs in the soil. Like insects, root-knot nematodes have several juvenile stages and the nematodes molt (i.e., shed their outer layers) as they grow. The second juvenile stage of root-knot nematode is the most important, because at this stage the nematode seeks out and infects plant roots. Once it has entered a root, a root-knot nematode molts three more times before becoming an adult. A male root-knot nematode is able to move about freely and can leave a root. A female root-knot nematode remains and feeds in a given location within a root. Eventually, a female enlarges to the point where a portion of her body extends to the root surface and this allows her to lay her eggs in the soil. In some hosts, eggs can also be found within the galls.
Control: If you are having a root-knot nematode problem in your garden, consider crop rotation and the use of cover crops as management tools. See University of Wisconsin Garden Facts XHT1210, “Crop Rotation in the Home Vegetable Garden” and XHT1209, “Using Cover Crops and Green Manures in the Home Vegetable Garden” for details. If used properly, these techniques can be effective in reducing the number of root-knot nematodes in the soil. M. hapla, the root-knot nematode species most common in Wisconsin, does not infect corn, wheat, oats or rye, so use of these crops in a rotation or as cover crops often provides great benefit.
Cover crops of French marigolds (Tagetespatula) also have been shown to reduce the number of root-knot nematodes in soil. This common garden ornamental releases a chemical (alpha-terthienyl) that is highly toxic to root-knot nematodes and prevents their eggs from hatching. As an added bonus, root-knot nematodes are not able to develop properly in marigold roots. When using crop rotation or cover crops, proper broadleaf weed control is critical because weeds can provide a place for root-knot nematodes to survive and reproduce.
Finally, consider amending the soil in your garden with organic matter such as compost or leaf mulch. Such amendments tend to increase the diversity of microorganisms in the soil and can encourage the growth of certain soilborne fungi that ensnare and feed on root knot nematodes, and parasitize their eggs.
For more information on root-knot nematode: Contact your county Extension agent.
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.
Wilting of poinsettia associated with Pythium root rot.
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., Rhizoctoniasolani, 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. 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.
What is root rot? Root rot is a general term that describes any disease where the pathogen (causal organism) attacks and leads to 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.
Brown discoloration of roots typical of root rots.
How do you know if your plant has a root rot? Gardeners often become aware of root rot problems when they see above ground symptoms of the disease. Plants with root rot are often stunted or wilted, and may have leaves with a yellow or red color, suggesting a nutrient deficiency. Examination of the roots of these plants reveals tissue that is soft and brown.
Where does root rot come from? A large number of soil-borne fungi cause root rots. Pythium spp., Phytophthora spp., Rhizoctoniasolani, 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.
How do I save a plant with root rot? REDUCE SOIL MOISTURE! Provide enough water to fulfill a plant’s growth needs and prevent drought stress, but DO NOT over-water. Remove excess mulch (greater than four inches) that can lead to overly wet soils. Chemical fungicides (PCNB, mefenoxam, metalaxyl, etridiazole, thiophanate-methyl and propiconazole) and biological control agents (Gliocladium, Streptomyces, and Trichoderma) are labeled for root rot control. However, DO NOT use these products unless you know exactly which root rot pathogen is affecting your plants. Contact your county Extension agent for details on obtaining an accurate root rot diagnosis and for advice on which, if any, fungicides you should consider using.
How do I avoid problems with root rots? Buy plants from a reputable source and make sure they are root rot-free prior to purchase. Establish healthy plants in a well-drained site. Moderate soil moisture. Add organic material (e.g., leaf litter or compost) to heavy soils to increase soil drainage. DO NOT over-water. Provide just enough water to fulfill the plants’ needs for growth and prevent drought stress. Also, DO NOT apply more than three inches of mulch in flowerbeds. REMEMBER, root rot fungi grow and reproduce best in wet soils. Finally, minimize movement of root rot fungi in your garden. DO NOT move soil or plants from areas with root rot problems. DO NOT water plants with water contaminated with soil (and thus potentially with root rot fungi). After working with plants with root rot problems, disinfest tools and footwear with a 10% bleach or detergent solution, or alcohol.
For more information on root rots: Contact your county Extension agent.
What is Ralstonia wilt? Ralstonia wilt (also sometimes known as Southern wilt) is a usually 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 has recently been detected in geraniums (Pelargonium spp.) in Wisconsin.
What does Ralstonia wilt look like? Symptoms of Ralstonia wilt in geraniums are similar to those associated with bacterial blight (caused by Xanthomonascampestris 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. Finally, infected plants die.
Where does Ralstonia wilt come from? Ralstonia wilt is caused by the bacterium Ralstoniasolanacearum (formerly Pseudomonassolanacearum). This bacterium is commonly found in tropical, sub-tropical and warm temperate climates, but 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 or biovars) of R. solanacearum have been recognized, each with a different host range. R. solanacearum race 3, biovar 2 is of particular concern it causes a serious disease of potato called brown rot. In addition, this race/biovar has been listed as a select agent in the Agriculture Bioterrorism Act of 2002 and is considered to have potential to be developed as a bioterrorist weapon.
How do I save plants with Ralstonia wilt? There are no known treatments that will save plants infected with 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 (see below for information on where to submit samples in Wisconsin). If your plants test positive for R. solanacearum race 3, biovar 2 the U.S. Animal and Plant Health Inspection Service (APHIS) must be notified and this organization will provide guidance on proper disposal of contaminated plants and decontamination of greenhouses.
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 4 ft. 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 by dipping them in a 10% bleach solution, ammonia or alcohol (spray disinfectants that contain at least 70% alcohol can also be used). Also wash your hands frequently when handling plants to minimize the possibility of moving the bacterium by touch. 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/Extension, 1630 Linden Drive, Madison, WI 53706-1598 [phone: (608) 262-2863, fax: (608) 263-2626, email: bdh@plantpath.wisc.edu.
