Resistance in oilseed rape ( Brassica napus) and Indian mustard ( Brassica juncea) to a mixture of Pseudocercosporella capsellae isolates from Western Australia

Abstract

Thirty-five genotypes, including 32 varieties of Brassica napus and 3 varieties of Brassica juncea were screened under both controlled environmental and field conditions for resistance to a mixture of 5 Western Australian isolates of the white leaf spot pathogen Pseudocercosporella capsellae, 3 were derived from B. juncea and 2 from B. napus. A range of varietal reactions occurred among the genotypes tested in response to inoculation with P. capsellae, ranking from highly susceptible to resistant. The highly resistant genotypes under controlled environment conditions included B. napus Bravo TT and Thunder TT with disease severity scores of 2.1 and 3.8 (on a scale of 0–10), respectively. In comparison, under the same conditions, the most susceptible genotypes were Trilogy, B. juncea-963, Surpass 404 CL, Westar, B. juncea-98, Stubby and B. juncea-102, all with disease severity scores >6.5. The most highly resistant varieties under field conditions were B. napus Bravo TT [area under disease progress curve (AUDPC) value in relation to the percentage of leaves diseased = 56] followed by Thunder TT (AUDPC value = 66.5). Genotypes Trilogy, B. juncea-963 and Surpass 404 CL were the most susceptible, with the highest AUDPC value of ≥108.5 in relation to the percentage of leaves diseased and the also in relation to degree of leaf collapse (0–10 scale, 0 = nil collapse, and 10 = >90% of leaves collapsed). In the field, there was strong overall correlation between AUDPC in relation to the percentage of leaves diseased, with AUDPC in relation to the leaf area affected ( r = 0.60, P < 0.001, n = 35), with the extent of leaf collapse ( r = 0.87, P < 0.001, n = 35) and with mean leaf size ( r = −0.72, P < 0.001, n = 35), suggesting that different characters assessed could be used to distinguish varietal resistances. There was also a high correlation between the two times of assessment under controlled environment conditions ( r = 0.86, P < 0.001, n = 35), suggesting that either time could be used. Overall, there was a highly significant correlation between the responses in field with those in the controlled environment. AUDPC in relation to the percentage of leaves diseased in the field study was significantly correlated with lesion size at both 14 and 21 days in the controlled environment study ( r = 0.80, P < 0.001, n = 35 and r = 0.82, P < 0.001, n = 35, respectively). The results indicate that, in general, cotyledon inoculations using a conidial suspension under controlled environment conditions could be used to identify resistant lines rather than having to rely upon more expensive and time-consuming field trails. As there was a significant negative correlation of mean leaf size with all other disease traits measured in the field, it is possible that breeders could use estimates of mean leaf size (cm 2) as a guide to the potential level of resistance to P. capsellae in breeding lines.