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The effects of rain on different stages of powdery mildew development were investigated quantitatively for Erysiphe graminis on barley, Erysiphe pisi on pea and Sphaerotheca pannosa on rose. Water drops 4 mm in diameter released from a height of 2 5 m onto target leaves reduced the numbers of conidia on the impacted surfaces and inhibited subsequent mildew development, the effects declining as the interval between inoculation and impaction increased. Simulated rainfall applied after inoculation reduced the numbers of conidia on leaves and inhibited mildew development, the effect declining as the interval between inoculation and treatment lengthened. Simulated rainfall applied to infected plants reduced their capacity to inoculate other plants, the effect declining as the interval between treatment and use as inoculum lengthened. Controlled exposure of infected peas and roses to natural rain reduced the numbers of conidia on leaves and inhibited subsequent mildew development, especially on upper leaf surfaces.

Samples of Brassica oleracea seed from Victoria, were tested for the presence of seed-borne Alternaria brassicicola and Alternaria brassicae. A. brassicicola was detected in 26 of 44 samples tested but A. brassicae was not detected in any. Between 24 and 37% of seed was infected, with 4-8% of infection found in the embryo tissues. Inoculation of seed with A. brassicicola resulted in loss of vigour in germinated seedlings, followed by death. The fungus retained its viability and pathogenicity on seed stored for up to 12 months. This investigation indicates that a high proportion of commercially available brassica seed are contaminated with A. brassicicola and may therefore be a primary source of disease for brassica crops in Australia.

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