WILT

common symptom of plant disease resulting from a water loss in leaves and stems. Affected parts lose their turgidity and droop. Specific wilt diseasescaused by a variety of fungi, bacteria, and virusesare easily confused with root and crown rots, stem cankers, insect injuries, drought or excess water, soil compaction, and other noninfectious problems. Verticillium wilt is a very destructive fungal disease in cool climates. It affects several hundred species of trees, shrubs, vines, flowers, house plants, vegetables, fruits, field crops, and weeds. The causal agent is the soil-inhabiting fungus Verticillium albo-atrum (V. dahliae). In hot weather, the leaves on one or more branches turn dull green to yellow, wilt, and wither, often from the base upward. Annuals and young trees are often stunted and usually die; perennials may die branch by branch over a period of from several weeks to years or apparently recover. The sapwood (just beneath the bark) of wilted branches, when cut lengthwise, usually shows dark streaks. Infections occur through roots and wounds. Control can be obtained by growing disease-free and resistant or immune plants, rotating with highly resistant or immune plants for five years or longer, destroying infected plants and susceptible weeds, fertilizing and watering to encourage vigour, removing wilted branches on trees and shrubs (sterilizing tools between cuts), and avoiding the wounding of roots or stems when planting or cultivating. Soil known to harbour Verticillium can be fumigated or heat-treated to kill the disease organisms. Oak wilt, caused by the fungus Ceratocystis fagacearum, is a serious disease in the eastern half of the United States. All oaks (Quercus) are susceptible, as are Chinese, European, and American chestnuts, tan oak (Lithocarpus densiflorus), and bush chinquapin. Trees in the red- and black-oak group usually die within several weeks during late spring and summer. Their leaves turn pale or dull green, look water-soaked, curl upward, and often become yellowed or bronzed from the margins inward; the upper branches are usually affected first. White and bur oaks commonly die back slowly over several years. Their leaves usually wilt, turn light brown, curl, and cling to their stems. The disease is spread from tree to tree by natural underground root grafts, sap- and fungus-feeding insects, and possibly by squirrels. Control measures include prompt removal of diseased trees, injecting Vapam (sodium methyldithiocarbamate) into the soil midway between healthy and recently infected trees, avoidance of injuring or pruning of trees from budbreak to midsummer, and painting wounds promptly with tree-wound dressing. Bacterial wilt, caused by numerous species of the genera Corynebacterium, Erwinia, Pseudomonas, and Xanthomonas, induces stunting, wilting, and withering, starting usually with younger leaves. Stems, which often shrivel and wither, show discoloured water-conducting tissue. A bacterial ooze is often evident when infected stems are cut and squeezed. Rapidly expanding, dark green, water-soaked areas or streaks may develop first in leaves. Bacterial wilt may be managed by growing resistant varieties; planting disease-free materials in well-drained, fertile soil that is clean or sterilized; observing stringent sanitation including weed- and insect-control measures; and rotating susceptible crops. Spotted wilt, caused by a virus, is transmitted by the larvae of several species of insect called thrips. Plants commonly are stunted and bunchy. Brown, purplish, pale green, red, yellow, or white rings (often zoned) and spots form in leaves, flowers, and fruit; long streaks may develop in petioles and stems. Leaves are distorted, sometimes mottled, and may turn yellow or bronze. Tops may wilt and wither; fruit is often rough and distorted. Control includes growing resistant varieties and disease-free stock, spraying or dusting during thrips infestations, promptly destroying infected plants and crop debris after harvest, and observing stringent weed control. See also Fusarium wilt; Panama disease; Dutch elm disease.