VASCULAR WILT DISEASES IN THE HOME GARDEN

Home gardeners should be aware that there are several agents that can cause wilting of their vegetables or flowers. The most common type of wilting occurs just after transplanting into dry soil, especially on a hot day. This is the result of a root system that is not yet in close contact with water in the soil. Similarly, plants that are well established may wilt after a prolonged dry spell; giving the plants some water can reverse this.

Infections by bacteria and fungi can also cause wilting symptoms. Vascular wilt pathogens infect the vascular (water conducting) system of the plant. The pathogen or the toxins produced by the pathogen can be moved throughout the entire plant along with the plant's sap. A pathogen invading the roots may cause disease symptoms at the top of the plant.

FUNGAL WILT DISEASES

Wilt diseases in the home garden caused by fungi are in either of the genera Fusarium or Verticillium.

Fusarium wilts can affect many plants (hosts) but primarily occur in annual vegetables and flowers. These include:

• Solanaceous crops (tomato, potato, pepper, eggplant)
• Crucifers (radish, cauliflower, cabbage)
• Cucurbits (cucumber, squash, melon)
• Beans and peas
• Strawberries
• Herbaceous ornamentals (carnation, chrysanthemum, gladiolus, crocus, lily, daffodil, tulip)

The most common of these are wilts of tomato and cucurbits.

Verticillium wilts can affect over 200 species of plants; in the home garden, solanaceous plants are particularly susceptible, but strawberries and raspberries may also become infected.

All vascular wilts, regardless of the pathogen, have some symptoms in common. The leaves of infected plants or of parts of infected plants lose turgidity, turn lighter green to greenish-yellow, droop and wilt, turn yellow then brown, and die. In cross sections of infected stems, discoloured brown areas appear as a complete or interrupted ring of discoloured vascular tissues.

Control

Control of these diseases is attained primarily through the use of disease-resistant or disease-tolerant cultivars. Those cultivars that have resistance to Verticillium wilt and Fusarium wilt will be listed in seed catalogues with a "VF" after the name of the cultivar. Verticillium wilt can be avoided somewhat by not planting susceptible crops where solanaceous crops were previously grown. Since wilt pathogens reside in the soil, fungicide sprays are not effective. Soil drenches with fungicides also are not very effective.

BACTERIAL WILT DISEASES

Wilt diseases in the home garden may also be caused by bacterial pathogens. These belong primarily to three genera: Clavibacter, Erwinia, and Xanthomonas.

Bacterial Wilt and Canker of Tomato

Bacterial wilt and canker of tomato, incited by Clavibacter michiganense has symptoms that include single leaflets that curl upward and inward followed by whole leaves shrinking, drying and turning brown. Symptoms may be restricted to one side of the plant. Also, spotting may occur during wet periods on leaves, stem and fruit. Cankers, which are often hard to see, may occur on stems and leaf veins. Later, cracks may appear in these cankers from which slimy masses of bacteria will ooze in wet or humid weather. The pith of the infected stems is often "chambered" with cavities. Symptoms on the fruit are a "bird's eye" type spot (small, white spot with a raised, tan centre). This bacterium does not survive in the soil (unless in old tomato stems which have not completely broken down).

Control

Use seed that is bacteria free, eliminate infected crop residue, and use long crop rotations. (By specifying "HWT" on your seed order, you indicate that you want your seed hot-water-treated to reduce pathogens.)

Bacterial Wilt of Cucurbits

Bacterial wilt of cucurbits, caused by Erwinia tracheiphila is most severe on cucumbers and muskmelon, and is less severe on squash and pumpkin. Watermelon is resistant. The first symptoms of bacterial wilt are wilting of one or more leaves of a vine, followed by wilting of all leaves and the collapse of all vines of the infected plant. This will occur over several days. Wilted leaves and stems shrink and dry up.

Control

The pathogen is spread by the striped and spotted cucumber beetles and survives between crops within the gut of these beetles. Therefore, the means of controlling the disease is to control the cucumber beetles. For insecticides that control the beetles, check Ontario Ministry of Agriculture and Food (OMAF) Publication 64, "The Gardener’s Handbook". This publication can be ordered from your local OMAF office or through the OMAF web site.

Bacterial Wilt of Corn

Bacterial wilt of corn caused by Erwinia stewartii rarely occurs in Ontario. Where it does, it is more of a problem on sweet corn than field corn. The symptoms appear as rapid wilting of the plant, which is either killed or remains stunted and produces no ears. Leaves of infected plants show long, green streaks that have irregular or wavy margins. Vascular bundles are filled with bacteria, which ooze out on the cut surface as yellowish masses. The bacteria may also infect the corn kernels.

Control

The bacteria overwinter inside the corn flea beetle and are spread by the beetle. Control is achieved by the use of resistant varieties, spraying the plants early in the season with insecticides to kill the corn flea beetles, providing the plants with higher levels of calcium and potassium, and limiting nitrogen application.

Black Rot of Crucifers

Black rot of crucifers caused by Xanthomonas campestris first appears as V-shaped yellowed spots at the margins of the leaves. These progress toward the midrib of the leaf while some of the veins on the affected area turn black. Eventually, the leaf turns brown and dies, and the pathogen spreads to the rest of the plant. Thereafter, chlorotic areas may appear anywhere on the leaves, which eventually fall off.

Control

These bacteria overwinter in plant debris and on or in seed. Control is achieved by the use of bacteria-free or hot-water-treated seed and planting disease free transplants into soil in which cruciferous crops showed no incidence of black rot for the previous two or three years.

OTHER CAUSES OF WILT

Fungi, such as Sclerotinia and Botrytis, which attack and girdle the stem, also can cause wilting. Nematodes (microscopic worms which feed on or in plant roots) or root rot fungi, both of which reduce the ability of roots to absorb water, can cause wilt symptoms. Boring insects may break the continuity of the vascular system resulting in wilting. Finally, toxins produced by walnut trees may cause wilting of many garden plants, especially tomatoes.

J.Louter/R.T. Wukasch

PDCF-120 (Revised 2003/11/07)