INHERITANCE OF RESISTANCE TO LEPTOSPHAERIA-MACULANS IN BRASSICA-JUNCEA

The inheritance of resistance to Leptosphaeria maculans, the causal ag ent of black leg of crucifers, was studied in Brassica juncea. Three r esistant accessions (UM3021, UM3043, and UM3323) and one susceptible a ccession (UM3132) of B. juncea were crossed in a complete diallel. Par ents, F-1, and F-2 progenies were evaluated for all crosses using both cotyledon and stem inoculation. Cotyledon reaction was evaluated with two isolates of L. maculans, but stem reaction was evaluated with one isolate. Disease reactions observed for individual plants were the sa me for both inoculation methods and for both isolates of the pathogen for cotyledon reaction. No segregation was observed for the crosses be tween resistant accessions (UM3043 x UM3323 and UM3021 x UM3323), but a few susceptible plants were observed in the F-2 progeny of crosses b etween resistant parents (UM3021 x UM3043). This was probably due to h eterozygosity in some parental plants of UM3021. For crosses between t he susceptible parent and resistant parents, F-1 plants for two crosse s were all resistant. For cross UM3132 x UM3021, some susceptible plan ts occurred, which was also suggestive of heterozygosity in UM3021. Al though resistance in F-1 was dominant, for F-2 populations, segregatio n fit either 13:3, 3:1, or 1:3 ratios, indicating that resistance can be either a dominant or recessive trait. F-3 families derived from som e susceptible F-2 plants from crosses UM3021 x UM3132 and UM3043 x UM3 132 were evaluated using the cotyledon inoculation method only. Segreg ation of F-2 plants and F-3 families in crosses involving resistant an d susceptible parents indicated that the resistance to L. maculans in B. juncea is controlled by two nuclear genes with dominant recessive e pistatic gene action.