Temperature-light sensitivity of infection types expressed by cultivars of Populus deltoides Marsh. to races of Melampsora medusae Thüm.

Infection types were assessed when four cultivars of Populus deltoides Marsh. were inoculated with six races of Melampsora medusae Thüm. and incubated in combinations of light intensity (100, 500 and 1000 μEm^-2s^-1) and temperature (15 and 25°C). Resistance in the cultivars was always at a maximum when the race/cultivar complexes were incubated in the light intensity (1000 μ^-2s^-1)/high temperature (25°C) regime. Inereasing light intensity was generally associated with reduced infection type in the cultivars; the reduction was more pronounced in an incubation temperature of 25 than 15°C. The potential epidemiological significance of the results is discussed.