Identification and genetics of resistance to cercospora leaf spot (Cercospora zonata) in faba bean (Vicia faba)

The fungal disease cercospora leaf spot CLS (Cercospora zonata) has affected major faba bean (Vicia faba) production regions in southern Australian in the last several years. This study offers the first report of sources of resistance to CLS in faba bean and describes techniques to evaluate resistance to C. zonata in faba bean genotypes within a controlled environment. The method was rapid (43 days), repeatable (R(2) > 0.74) and demonstrated positive correlations (R(2) > 0.45-0.80) to data collected from field disease nurseries under naturally established CLS epiphytotics. All faba bean cultivars currently adopted by the Australian industry were found to be susceptible to CLS and defoliation was found to be an important component of disease expression. Genetic analysis of segregation patterns in F(2) derived F(3) families of 1322/2*Farah (resistant*susceptible) showed the mode of inheritance of resistance to C. zonata was monogenic dominant. F(3) families were shown to segregate in the ratio of 1:2:1 for homozygous resistant: heterozygous: homozygous susceptible (chi(2)(2) = 2.78; P > 0.05) and individual plants within heterozygous F(3) families segregated in the ratio of 3:1 for resistant: susceptible responses (chi(2)(1)= 2.93; P > 0.05). Monogenic dominant inheritance also explained the change in frequency of resistant and susceptible plants within a population of cv. Cairo following one generation of self-pollination (chi(2) = 0.88, 0.3 < P < 0.5). The sources of resistance identified in this study are being used to transfer CLS resistance to adapted faba bean genotypes for future cultivar releases to the southern Australian industry.