Survival of Detached Sporangia ofPeronospora destructorandPeronospora tabacina

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Abstract
Detached sporangia of P. destructor were exposed for various lengths of time to different laboratory combinations of temperature and relative humidity (RH); temperatures of 10, 25, and 35.degree. C were used in combination with 33, 53, 76 and 95% RH. Germination indicated survival. For all RH tested, P. destructor sporangia survived best at 10.degree. C and poorest at 35.degree. C. For all temperatures tested, sporangia survived poorest at 33% RH, and differences in survival between 53 and 76% RH generally were not significant. Sporangia of P. destructor and P. tabacina were also exposed outdoors on cloudy or sunny days before being put into standard conditions to germinate. Of temperature, RH and sunlight, sunlight was the most important factor in determining sporangia survival. On cloudy days, the average germination of detached sporangia of P. destructor was reduced from 83 to 68% in 6 h, whereas on clear, sunny days germination was reduced from 46 to 0% in 6 h. The effect of solar irradiation on the germination of sporangia of P. tabacina was similar to that for sporangia of P. destructor.