Survival of Sporangia of New Clonal Lineages of Phytophthora infestans in Soil Under Semiarid Conditions

The Columbia Basin of south-central Washington and north-central Oregon is a major potato-growing region in North America where more than 65,000 ha of potatoes are grown annually. The environment is semiarid (mean rainfall is between 15 and 22.5 cm), and the potato crop is irrigated mostly by center-pivot systems. Although the Columbia Basin is semiarid, potato production is annually threatened by late blight caused by Phytophthora infestans, a devastating disease of potato throughout North America and the world (8). Late blight develops on potato foliage under conditions when ambient relative humidity is above 90% and temperatures range from 7 to 21°C (19,31). Tubers become infected in the field when zoospores or sporangia of P. infestans are washed from infected foliage and come in contact with tubers (18,19), resulting in infections through buds, lenticels, or wounds (15,36). Beginning in 1991, metalaxyl-resistant genotypes of P. infestans were detected in the Columbia Basin (6) and started to replace the metalaxyl-sensitive US-1 strain that was believed to have dominated the region since 1947 (4,23,24). Previous research on the survival of sporangia of the US-1 genotype of P. infestans under controlled conditions found that sporangia survived for 42 days in vitro in nonsterile soil (36) and 70 to 80 days in vitro in sterile soil (16,20,32,36). Under natural environmental conditions, sporangia of P. infestans in naturally infested soil and artificially infested soil in pots survived for 21 and 32 days, respectively (18,26). However, none of these environmental conditions assessing sporangia survival were under semiarid conditions. Viability of sporangia of new clonal lineages of P. infestans was previously reported in water (29) and when directly exposed to the environment (33) under semiarid growing conditions that prevail in the Columbia Basin, but currently, there is no published information on sporangia viability in soil of the new metalaxyl-resistant genotypes of P. infestans in any potato-growing region throughout the world and particularly in the Columbia Basin. The development of fungicide resistance by plant pathogens on the basis of previous research has often been correlated with a loss in total fitness by the resistant pathogen (7), so determining the survival of metalaxyl-resistant genotypes of P. infestans is important in modeling the epidemiology of the pathogen. Therefore, the purpose of this study was to assess the effects of solar irradiance, soil moisture, and soil type on the survival of sporangia of new metalaxylresistant clonal lineages of P. infestans under semiarid environmental conditions.