Eggplant Integrated Pest Management AN ECOLOGICAL GUIDE

__________________________________________________________________________________Major Diseases of EggplantPlants may be infected at any age by the fungi that cause Fusarium wilt and Verticillium wilt.Symptoms of this disease are leaf yellowing of the lower leaves. The yellowing progresses upward fromthe base of the plant. Wilting or yellowing may occur on only one side of a leaf or a branch, or on oneside of the plant. Often, the area between leaf veins turns yellow first, resulting in V-shaped areas (typicalfor Verticillium, but may also occur for Fusarium). Yellow leaves wilt noticeably before they die. Separateshoots, and later entire plants, finally wilt permanently and die. In fields, the affected leaves may dry upbefore wilting is detected. Woody stem tissue often becomes brown in affected stems. This discolorationcan be detected by cutting affected stems diagonally. The brown discoloration may extend into the rootsand lower part of the stem.Young plants appear normal, but become stunted as they develop. Often no symptoms are seen until theplant is bearing heavily or a dry period occurs. Wilting may occur at midday, when sunlight is bright andtemperature is high. Infected plants may survive the season but are stunted and both yield and fruits maybe small depending on severity of attack.Source and spreadBoth Fusarium and Verticillium wilt are caused by soil-borne fungi that can persist in the soil for manyyears. The fungi produce a very strong type of spores that can survive indefinitely in most fields. Survivalis aided by weeds which are susceptible to the fungus. The fungus can be introduced and spread withsoil that is attached to transplants. Within each cropping season, most disease originates from inoculumsurviving from previous cropping seasons.Wilt disease can also be spread with seed. Long-distance dissemination probably occurs on seed.When fungal spores germinate close to roots of a susceptible plant, the fungus penetrated the fine roothairs of the root system. Penetration is enhanced by root wounds. The fungus grows inside the root andeventually reaches the vessels. It may move slightly inside the plant vessels but is largely confined. Insusceptible varieties the fungus moves through the vessels from the roots both to and through the stem.The vessel are blocked by the fungus and water cannot (or very limited) be transported from the roots tothe leaves and above ground plant parts. These wilt as a result.Role of environmental factorsEnvironmental conditions that affect disease development include temperature, moisture and soil pH.The optimum air and soil temperature for disease development is about 28 o C. No disease develops if thesoil temperature is too cool (15 o C and below) or too warm (35 o C and above), though differences betweenthe fungi species may be found. Generally, Verticillium wilt is retarded by the higher temperatures thatfavor Fusarium wilt. That is why in tropical lowland areas, usually Fusarium wilt is found.When the soil temperature favors disease development, root infection can occur. Root infection can beextensive and the fungus grows up into the lower portion of the stem. If the air temperature is too cool fordisease development, the plants grow well without external symptoms of the disease. Once thetemperature rises, the wilting process may develop quickly.Importance - plant compensation - physiological impactThe wilt fungi usually enter the plant through young roots and then grow into and up the watertransporting vessels of the roots and stem. As the vessels are plugged and collapse, the water supply tothe leaves is blocked. With a limited water supply, leaves begin to wilt on sunny days and recover atnight. Wilting may first appear in the top of the plant or in the lower leaves. The process may continueuntil the entire plant is wilted, stunted, or dead. Plants may recover somewhat but are usually weak, andproduce fruit of low quality.When disease development is stopped or inhibited due to unfavorable environmental conditions (e.g.when the soil is very dry) the plant may form additional shoots to compensate for some of the wiltedshoots. However, the conditions unfavorable for the disease are usually also unfavorable for vigorousplant development. Once infected, the plant usually cannot recover completely.Natural enemies/antagonistsWhen it comes to biocontrol, the different fungi usually have different antagonists (natural enemies).Most antagonistic fungi are very specific in host selection. However, the antagonist Trichoderma sp. hasgiven good results in control of many soil-borne pathogens, including Fusarium and Verticillium.Trichoderma is available commercially or from extension agencies in many countries in Asia.Fusarium can be controlled by a non-pathogenic strain of Fusarium oxysporum. See box below.150Eggplant Ecological Guide