Influence of leaf wetness, temperature, and rain on poplar leaf rust epidemics

Severe epidemics of poplar leaf rust, caused by Melampsoramedusae Thüm. f.sp. deltoidae, developed on eastern cottonwood, Populusdeltoides Bartr., in 1988 and 1989 in a mixed clonal plantation and in a natural stand in western Kentucky despite precipitation deficits and high maximum temperatures during the exponential phase of the epidemic. Minimum temperatures during that period, however, were within the range reported to be optimal for uredospore germination (15–21 °C). Average duration of leaf wetness for the entire epidemic was 10.6 h in 1988, and 11.2 and 11.7 h in 1989 in the natural stand and the plantation, respectively. Maximum infection of artificially inoculated detached leaves of eastern cottonwood occurred after 8 h of continuous leaf wetness. These results suggest that rain deficit and high maximum temperatures are not limiting factors for poplar leaf rust epidemics when night conditions frequently offer optimal temperature and leaf wetness for infection.