Sporangium Discharge byPeronospora destructor:Influence of Humidity, Red-Infrared Radiation, and Vibration

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Abstract
The effects of relative humidity (RH), red-IR radiation and vibration on sproangium discharge from onion leaves infected with P. destructor were analyzed under precisely controlled conditions. Airflow (0.5 m/s) and temperature (17-18.degree. C) were kept constant. Many sporangia were discharged as the RH was reduced; reductions below .apprx. 59% RH were most effective. Increasing the atmospheric RH from a lower level to a higher level, or to saturation, also triggered sporangium liberation, although the numbers of sporangia released were always less than for comparable reductions of RH. No sporangia were liberated in darkness when the air was saturated (RH 100%). Brief exposures to IR (eg, 1 min) induced sporangium discharge with maximum release occurring at reduced RH. When the air was saturated, only a few spores were liberated by irradiation. Continuous exposure to IR during complete RH cycles (i.e., lowering and then raising RH) enhanced total spore release compared to similar RH changes in darkness. Vibration of onion leaves also triggered sporangium discharge. The number of spores liberated by vibration was influenced by both RH and IR. Greatest discharge occurred when leaves were vibrated while exposed to IR at reduced RH; far fewer sporangia were released when leaves exposed to IR were vibrated in a saturated airstream. No sporangia were liberated from vibrated leaves placed in darkness in a saturated airstream.