Relation of weather variables and host factors to incidence of airborne spores of Bipolaris sorokiniana

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Abstract
Seasonal and daily incidence of airborne spores of B. sorokiniana were studied in barley field plots in relation to crop development, disease severity, and weather variables. Sporulation was observed only on dead or senescent barley. The incidence of airborne spores before 8-11 July was low because spore-bearing barley foliage was scarce and weather factors were often unfavorable for sporulation. Large numbers of spores encountered after 8-11 July coincided with rapid disease progress and repeated occurrence of persistent leaf wetness and high temperatures. Cumulative spore counts showed that few spores were dispersed sufficiently early in the growing season to serve as inoculum in the epidemics. Numbers of airborne spores were low at night, but increased abruptly after dawn to peak concentrations during 0900-1700 h. Circumstantial evidence indicated that spore production was promoted by persistent leaf-surface wetness, high relative humidity (RH), and high temperatures (> 15.degree. C), but restricted by cool temperatures. Correlative studies showed that spore release was caused by rapidly declining RH and wind. Spores were dispersed mostly when the leaves were dry and the numbers dispersed correlated very strongly with wind speed. Few spores were dispersed when leaves were wet and RH high.

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