| dc.description.abstract | Black leg, caused by the fungus Leptosphaeria maculans, is a widespread disease of winter canola (Brassica napus) in Oklahoma. Major resistance genes (Rlm) are expressed in seedlings and interact with avirulence genes in L. maculans (AvrLm) in a gene-for-gene manner. Little is known about the avirulence genes and race structure of the pathogen population in the southern Great Plains. Likewise, there is limited information about the presence of resistance genes in cultivars and hybrids grown in the region. The presence of avirulence alleles and the race structure of the L. maculans population were determined using a combination of pathogenicity tests on differential cultivars harboring resistance genes Rlm1 and Rlm2,3 and PCR amplification of avirulence alleles AvrLm1, AvrLm4-7, AvrLm6. Avirulence alleles AvrLm6 and AvrLm4-7 were prevalent (100%) in the local population (N=95), whereas AvrLm1 and AvrLm2,3 presence was 38% and 9%, respectively. Four races (Av1,2,3,6,4-7; Av1,6,4-7; Av2,3,6,4-7; Av6,4-7) were identified in the population. Races Av6,4-7 (56%) and Av1,6,4-7 (35%) were the most predominant and were further characterized for AvrLm4, AvrLm5 and AvrLm6 based on the phenotype interaction on differential cultivars harboring Rlm4 and Rlm5,6. This characterization resulted in a reclassification into three races Av1,6,7,(5); Av1,4,6,(5,7) and Av6,7,(5) which were used to screen 53 winter canola cultivars, hybrids and breeding lines for seedling resistance. Most (62%) entries were susceptible to all three races and lacked specific resistance genes. Several (23%) conventional (non-glyphosate tolerant) cultivars and hybrids were heterogeneous in resistance to one or more races. Glyphosate-tolerant entries currently grown in the region generally lacked major resistance genes, except for DKW46-15 which had heterogeneous resistance from Rlm4 and Rlm7. The hybrids Dimension, Safran, Visby, DK Sensei, and the rapeseed cultivar Rossini (9%), were resistant to all races possibly due to the presence of Rlm6 and/or Rlm7. Unknown resistance was found in 6% of the entries, which suggested the presence of other resistance genes not assessed in this study. Understanding the race structure of the pathogen population will be useful for development of resistance and effective deployment to control black leg in winter canola. There is a need to grow cultivars or hybrids with effective major gene resistance in Oklahoma and surrounding states. | |
