The genetics of blackleg [Leptosphaeria maculans (Desm.) Ces. et De Not.] resistance in rapeseed (Brassica napus L.)

The genetic control of adult-plant blackleg [Leptosphaeria maculans (Desm.) Ces. et De Not.] resistance in rapeseed (Brassica napus L.) was studied in the F₂ and first-backcross populations of the cross Maluka (blackleg-resistant) x Niklas (highly susceptible). A L. maculans isolate possessing high levels of host specificity (MB2) was used in all inoculations. Resistance/susceptibility was evaluated using three separate measures of crown-canker size, i.e. the percentage of crown girdled (%G), external lesion length (E) and internal lesion area (%II). Disease severity scores for the F₂ and first-backcross populations based on E and %II gave discontinuous distributions, indicating major-gene control for these measures of resistance; but those for %G were continuous, indicating quantitative genetic control for this measure. Chi-square tests performed on the (poorly-defined) resistance classes, based on E, in the F₂ and first-backcross populations indicated the likelihood for resistance being governed by a single, incompletely dominant major gene. Although the distributions of the F₂ and first-backcross populations, based on%II, were clearly discontinuous, the observed segregation ratios for resistance and susceptibility did not fit any of the numerous Mendelian ratios which were considered. Differences in inheritance of resistance according to the assessment method and blackleg isolate used, were discussed.     

The detection and molecular mapping of a major gene for non-specific adult-plant disease resistance against stripe rust (Puccinia striiformis) in wheat

A major gene determining non-specific adult-plant disease resistance against stripe rust (Puccinia striiformis) designated Yrns-B1 was mapped by using a cross between ’Lgst.79–74’ (resistant) and ’Winzi’ (susceptible). Analyzing F₃ lines of two consecutive experimental years contrary modes of inheritance were observed due to the intermediate character of the gene and the difference in the disease pressure during the seasons. Using the disease scoring data of both experimental years independently two maps were constructed detecting Yrns-B1 20.5 and 21.7 cM, respectively, proximal to the wheat microsatellite (WMS) marker Xgwm493 on the short arm of chromosome 3BS. The genetic relationships to other major genes or to quantitative trait loci controlling adult plant disease resistance against rusts in wheat are discussed. Received: 27 May 1999 / Accepted: 28 September 1999