Differential sensitivity to dryness of conidia of exserohilum turcicum on corn leaves and artificial media

Interrupting a 9-h dew period applied to corn (Zea mays) plants inoculated with Exserohilum turcicum by a 24-h dry period (25° C, 40-60% RH) beginning 0.5 or I h after inoculation resulted in 83 and 97% less infection, respectively, compared to plants inoculated and kept constantly wet for 9 h. Inoculating corn plants with conidia that had been germinated on Millipore membranes or on 2% water agar for 1 h and then dried for 24 h resulted in only 31% less infection compared to corn plants inoculated with nondried conidia. Conidia germinated for 1 h on corn leaves and dried lost the capacity to develop infection structures. These results imply that consecutive dew periods, each shorter than 5 h (minimal dew period required to establish infection), do not accumulate to produce infection. These findings can explain why, in Israel, epidemics do not develop until mid August when dew lasts at least 6 h each night.