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


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