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Figure 1. Potato with dry rot. |
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Fusarium dry rot, caused by the fungus Fusarium solani, is mainly a problem of potatoes in storage or on seed pieces. The fungus usually enters the tuber through a mechanical wound or other. Infected tubers first may show small surface blemishes or lesions. These lesions are light tan or brown and are slightly depressed. The skin over the lesion wrinkles, sometimes in concentric ring patterns as the dead tissue dies out. Decay progresses towards the center of the tuber. The decayed tissue is brown to black and often has a pink to white fungal growth interspersed with the dead tissue. At low temperatures, the decayed tissue remains dry and firm; at higher temperatures the decay may be moist. Rotted tubers will eventually completely rot and mummify. In some cases, secondary organisms, including soft rot bacteria, invade the diseased tissue, resulting in a rapid collapse of the tuber. Infected seed pieces will rot before emergence of sprouts, resulting in skips in the field. Occasionally, small, slowly growing plants will emerge from infected seed pieces, but they will produce few marketable tubers. DISEASE CYCLEThe fungus can survive for many years in soil, but the major inoculum source is seed borne. The fungus survives in soil on the seed piece or in wounds on the tuber. The fungus cannot penetrate the tuber directly; it requires some opening or wound for infection. Recently harvested potatoes are relatively resistant to this disease. Susceptibility to dry rot increases with long storage periods. Dry rot develops most rapidly at a high relative humidity and at moderate temperatures (55-65 F). CONTROLThere are differences in susceptibility of potato cultivars, although none are immune to the problem. Since the pathogen requires wounds for entrance and colonization of the host, it is important to handle tubers with care during harvest and storage. All equipment and storage facilities should be thoroughly cleaned before harvest. Provide a curing period after harvest by holding tubers at moderate temperatures (50-55 F) and a high humidity of 90% for two weeks (except for those potatoes for chipping or immediate sale). Seed tubers can be treated with a fungicide before planting. Dust formulations of captan may be used. The addition of streptomycin at the presently registered concentration of 100 ppm to fungicide dusts may be of some value for eliminating disease-causing bacteria on the surfaces of the seed piece, although results in other states have been erratic.
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Web updated 9/01/06 |