Instituto de Investigaciones Agropecuarias y Forestales, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Mexico 58240
United States Department of Agriculture-Agricultural Research Service, Prosser, WA 99350
Campo Experimental Valle de Mexico, CIR-CENTRO INIFAP, Chapingo, Mexico 56230
Department of Plant Pathology, Cornell University, Ithaca, NY 14853
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Accepted for publication 14 August 2003.
Survival and infectivity of oospores in soils naturally infested with P. infestans oospores were studied in central Mexico. Sporangia were selectively eliminated from soil samples to determine infectivity attributable to the presence of oospores. Selective elimination of sporangia was achieved by two cycles of wetting and drying the soil. Oospore concentration, viability, and infectivity varied among soils collected during the winter fallow in different locations of central Mexico. In some soils, oospores were infective regardless of the time at which they were collected during the winter fallow. However, oospore viability and infectivity decreased following 2 years of intercropping. The number of stem lesions and initial disease severity were significantly higher in soils with moderate (20 to 39 oospores g-1 soil) oospore infestation compared with soils with low (0 to 19 oospores g-1 soil) infestation. Our study confirms that oospores can survive winter fallow and serve as a source of primary inoculum in the central highlands of Mexico. Oospore survival appeared lower in the Toluca Valley soil, which may be an indication of soil suppressiveness.
potato late blight
© 2004 The American Phytopathological Society