Isolation of mutants exhibiting altered resistance to Sclerotinia sclerotiorum from small M-2 populations of an oilseed rape (Brassica napus) variety

Two small M-2 populations, consisting of 39 and 50 plants, respectively, ob tained by EMS-mutagenesis of an inbred line derived from oilseed rape cv. L inetta, were screened for altered leaf response to artificial inoculation w ith Sclerotinia sclerotiorum. In both experiments, the M-2 population exhib ited greater variation and a lower mean infection value than the parental p opulation; individuals in the most resistant class were obtained only from the M-2 population. Parent-progeny analysis of disease response scores reve aled significant regressions only for the mutagenised population, with narr ow-sense heritabilities of 0.75-0.83, compared to 0.14-0.22 for the parenta l population. When larger populations (approximately 600 individuals per po pulation) were screened, similar results were obtained. Mutants with signif icantly greater resistance than the most resistant 'Linetta' line were obta ined at frequencies of 1.7% (from an M-2 population size of 593) to 5.1% (n = 39). The altered leaf response to Sclerotinia in selected mutant lines w as positively correlated with stem response to artificial inoculation. Deta iled analysis of one mutant (HH-1), with significantly higher Sclerotinia r esistance than the parent, demonstrated that HH-1 was more resistant to art ificial stem inoculation than four commercial varieties tested, including c v. Briol, which is reported to exhibit high levels of resistance in the fie ld. Field trials in moderately- and heavily-infested soils showed that HH-1 exhibited significantly greater resistance to natural infection than 'Line tta', with percentage plant deaths of 5.3% (compared to 22.4% in the parent al population) and 13.6% (47.3%) under moderate and high inoculum pressure, respectively. The seed yield of HH-1 was significantly higher than that of the parent population under a heavy Sclerotinia infestation; in the absenc e of Sclerotinia, the yield difference between the two populations was not significant. The implications of these results are discussed in respect of a re-evaluation of the efficacy of mutagenesis for the isolation of agronom ically valuable micro-mutants.