Some Effects of Ultraviolet Radiation on the Pathogenicity of Botrytis fabae, Uromyces fabae and Erysiphe graminis

Abstract

Ultraviolet irradiation of spores of 3 leaf-infecting fungi, Botrytis fabae, Uromyces fabae (causes of "chocolate spot" and rust of broad beans, respectively) and Erysiphe graminis (cause of barley powdery mildew), decreased their pathogenicity, as assessed by counts of local lesions or pustules. The infectivity of B. fabae was lost more rapidly than the ability to form colonies on agar; with E. graminis infectivity was lost more rapidly than the ability to germinate. UV radiation damage to spores of all 3 fungi was mitigated by exposure to daylight after irradiation. The extent of such photoreactivation of B. fabae was the same whether the spores were on the host plant or in vitro. UV irradiation of leaves before inoculation decreased the number of pustules of E. graminis on barley, had no effect on the pustule number caused by U. fabae and increased the number of lesions caused by B. fabae on broad beans. Rubbing leaves with Celite before inoculation also increased the number of B. fabae lesions. Retaining UV-irradiated broad bean plants in day-light or darkness after inoculation with non-irradiated spores of B. fabae did not significantly alter the lesion number. In contrast, more pustules of E. graminis developed on UV-irradiated barley leaves kept in daylight than in darkness.