Components of adult plant resistance to powdery mildew (Erysiphe graminis f. sp. avenae) in oats

The adult plant reaction to mildew (Erysiphe graminis f. sp. avenae) of four oat genotypes was studied. A negative relationship existed in these genotypes between length of latent period and the percentage leaf area with mildew. The very susceptible CI 4877 had the shortest latent period and the largest area of leaf infected, while the highly resistant Cc 4761 had the longest latent period and the least leaf area covered with mildew. The same association also occurred between leaves within a genotype, particularly those genotypes with adult plant resistance, i.e., Maldwyn and Cc 4761. The more recently developed upper leaves showed the longest incubation period and least percentage area affected compared with lower leaves. A visual score of pustule sporulation indicated that CI 4877, Milford and Maldwyn had similar well-sporulating foci, but in Cc 4761 the pustules showed significantly less sporulation. Estimates of the relative number of spores produced per day over a 15 day period reflected the level of resistance. Thus the most susceptible genotype CI 4877 released over 50 times more spores than Cc 4761, while Milford and Maldwyn respectively produced 9 and 4.5 times more spores than the highly resistant Cc 4761. The longer latent period of Cc 4761 was reflected in the delay this genotype showed in reaching its relatively low peak production of spores. The possible use of the latent period as a factor when selecting for adult plant resistance in early growth stages is discussed.