A monogenic dominant inheritance of resistance to Berkeleyomyces rouxiae and Berkeleyomyces basicola in lettuce (Lactuca sativa) cultivar ‘La Brillante’

Black root rot, caused by two Berkeleyomyces species, is a major disease of lettuce in tropical and subtropical areas. The most promising strategy for management of these polyphagous and aggressive pathogens is the deployment of genetic resistance. The heirloom cultivar ‘La Brillante’ displayed resistant reaction against a broad range of isolates of both Berkeleyomyces species ( B. basicola and B. rouxiae ). Inheritance studies investigating the resistance of ‘La Brillante’ to both pathogens were conducted separately employing F ₂ populations derived from crosses with the highly susceptible cultivar ‘Elisa’. The hybrid origin of individual F ₁ plants was assessed via RAPD marker genotyping. Two bona-fide F ₁ plants were then self-fertilized to obtain two independent F ₂ segregating populations. The contrasting parents and F ₂ seedlings were then separately inoculated with B. rouxiae (247 seedlings) and B. basicola (166 seedlings). Chi-square test was employed to assess the goodness of fit of the observed segregation patterns against each pathogen to predicted Mendelian ratios. All ‘Elisa’ plants displayed highly susceptible reactions to both pathogens, whereas all ‘La Brillante’ plants were resistant even under harsh inoculation conditions, indicating high levels of phenotypic penetrance of this trait. The F ₂ populations inoculated with either B. basicola or B. rouxiae displayed a good fit to 3:1 resistant/susceptible segregation ratios, indicating monogenic dominant resistance to both pathogens. Additional experiments are necessary to verify whether the resistances to B. rouxiae and B. basicola are controlled by either the same gene/locus or distinct genes. This simple monogenic genetic resistance control of B. basicola and B. rouxiae will facilitate the incorporation of these traits into elite lettuce breeding lines and cultivars.