ENVIRONMENTAL-INFLUENCES ON THE DEVELOPMENT OF PUCCINIA-HELIANTHI ON SUNFLOWER

Nine epidemics of rust, caused by Puccinia helianthi, on sunflower (He lianthus annuus) were analyzed concurrently. Although the epidemics oc curred over a period of 3 years (1991 to 1993) and in three growing re gions, the apparent infection rates were relatively uniform (average r ate +/- standard error = 0.237 +/- 0.007). The possibility of using th is rate to predict disease progress at an early stage of an epidemic w as evaluated in four fields during 1994. In general, the extrapolated apparent infection rate was very close to the observed. To understand the biological basis for relatively uniform rates of disease increase, effects of environmental parameters on components of the life cycle o f P. helianthi were examined. Urediniospore germination occurred at te mperatures of 4 to 20 degrees C, lasted 4 to 6 h, and required 6 to 10 h of leaf wetness. At 15 to 25 degrees C, the time from inoculation u ntil the appearance of the first pustule was 8 to 10 days. The minimum , optimum, and maximum temperatures for infection were 4, 10 to 24, an d 30 degrees C, respectively Sporulation occurred over a wide range of temperatures (4 to 39 degrees C); the optimum was between 20 and 35 d egrees C. On the basis of these findings, it was concluded that during the period of rust epidemics in Israel (May through June), the enviro nment is rarely nonconducive to P. helianthi development.