Genotype × Isolate Interactions for Tan Spot Resistance Screening in Mungbean

Tan spot, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff), is an important bacterial disease of mungbean (Vigna radiata) in Australia. A key question for resistance screening is whether the relative resistance or susceptibility of mungbean genotypes depends on the isolate used. We evaluated ten Australian Cff isolates in two studies comprising 30 diverse mungbean genotypes (Study 1) and 12 breeding lines and varieties (Study 2), with two glasshouse trials per study. Primary analyses excluded the non-pathogenic control Cff089. Host genotype, isolate and genotype × isolate interaction affected disease severity in both studies (p < 0.001). Isolate and genotype × isolate interaction accounted for 17.3% and 25.3% of total variance in Study 1, and 17.1% and 53.1% in Study 2, respectively. The rank order of the nine pathogenic isolates was not consistent between studies; only two retained the same rank (Spearman ρ = −0.08, p = 0.83). Because the genotype panels did not overlap, this cross-study ranking comparison is descriptive. Genotype AGG325972MUNG had the lowest estimated disease severity in Study 1 (62.1) and showed low mean severity across the eight pathogenic isolates for which observations were available. These results show that isolate choice can alter the apparent resistance of germplasm and therefore influence selection decisions. Screening with multiple characterised Cff isolates should provide a more reliable basis for selecting mungbean material for tan spot resistance.