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SummaryThrough a backcrossing program two series of oat (Avena sativa L.) lines were created, one early and one midseason, that were isogenetic except for genes that conditioned crown‐rust reaction. The lines were tested under crown rust epiphytotic and rust‐free environments, and three of the resistance genes were found associated with sizable deviations in grain yield under the rust‐free conditions. In both early and midseason genetic backgrounds the crown rust reaction gene from C.I. 8079, an Avena sterilis line, was associated with an increase in grain yield. The crown‐rust reaction gene from C.I. 7171 was associated with an increase in yield in the early genotype, but not in the midseason. The crown rust reaction gene from C.I. 7232, a derived tetraploid, was tested only in the midseason genotype in which it was associated with a 9.6% reduction in grain yield.

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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