Survival of Phytophthora infestans sporangia exposed to solar radiation

The effect of solar irradiance (SI) on the viability of sporangia of isolat es belonging to two clonal lineages, US-1 and US-8, of Phytophthora infesta ns was assessed. Exposure during a 3-h period on sunny days (SI > 600 W/m(2 )) drastically reduced germination regardless of the time of day of the exp osure. After 1 h of exposure on sunny days, the viability of sporangia decr eased by approximate to 95%, and the effective time necessary to inactivate 95% of the sporangia was 1.1 h. The effective dose to inactivate 95% of th e sporangia on sunny days was 2.6 MJ/m(2). On overcast (SI < 300 W/m(2)) da ys, survival after 3 h was reduced only slightly. Thus, other variables bei ng equal, sporangia will survive hours longer in the atmosphere on cloudy d ays than on sunny days.