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Influence of Humidity and Red-Infrared Radiation on Spore Discharge by Drechslera turcica—Additional Evidence. C. M. Leach, Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; Phytopathology 70:192-196. Accepted for publication 5 September 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-192.

The influence of atmospheric humidity and red-infrared radiation on release of conidia from maize leaf lesions was studied in a series of experiments using a special “spore release apparatus.” Temperature and air velocity were kept constant in all experiments. A low air velocity (0.5 m/sec) was selected to minimize the possibility of “wind” removal. Irradiation of sporulating maize leaf lesions with red-infrared radiation (IR) greatly increased spore release at lowered relative humidities (RH), but not when the air was saturated. Numbers of spores liberated from irradiated specimens always greatly exceeded those released in darkness under the same conditions. Even short exposures (1 to 2 min) of IR at constant low relative humidity caused massive release of conidia. A water filter (20 cm deep), used to remove heat, had little effect on spore release. When visible wavelengths were eliminated by filters, the IR alone (> 800 nm) triggered spore release. Experiments confirmed earlier reports that spore release may be initiated by either lowering or raising the atmospheric humidity. When the humidity was lowered and raised in a cyclical manner, a characteristic bimodal pattern of spore liberation occurred. The first major peak of spore release always coincided with humidity reduction and a second peak resulted from the raising of the humidity. The number of spores released by these humidity changes was higher when the changes occurred at fairly low humidities and was less near saturation. Though spore release triggered by humidity changes in darkness was less than in specimens exposed to red-infrared radiation, when the humidity was repeatedly cycled (ie, lowered and raised) in darkness, the number of spores released progressively increased for each successive cycle.