GENETICS OF VIRULENCE IN LEPTOSPHAERIA-MACULANS (DESM) CES ET DE-NOT,THE CAUSE OF BLACKLEG IN RAPESEED (BRASSICA-NAPUS L)

The genetic basis of virulence of 24 isolates of L. maculans collected from various sites throughout south-eastern and south-western Austral ia were studied using five clone-lines of. napus. The experimental des ign allowed the estimation of the environmental and genetic components of variance using a standard analysis of variance. Virulence of these isolates (as measured by the percentage of stem girdling, %G) on the clone-lines NCII and Tap was found to be most likely controlled by a s mall number of genes; the broad-sense heritabilities were 79.7% and 67 .5% for virulence on NCII and Tap, respectively. The significance of t hese results in relation to the potential of L. maculans in adapting t o new resistant B. napus cultivars is discussed.