Inheritance of resistance to Berkeleyomyces rouxiae and Berkeleyomyces basicola in the lettuce cultivar La Brillante

Abstract

Black root rot, caused by two Berkeleyomyces species ( B. basicola and B. rouxiae ), is a major disease of lettuce ( Lactuca sativa L.) in tropical areas. Nationwide surveys in Brazil indicated similar levels of incidence as well as an overlapping geographical distribution of B. basicola and B. rouxiae across major lettuce-producing areas. The most promising strategy for managing these polyphagous and aggressive pathogens is the deployment of genetic resistance. The heirloom cultivar ‘La Brillante’ displayed resistance to a broad range of isolates of both Berkeleyomyces species. However, to date, there is no information about the genetic control of B. basicola and B. rouxiae . Inheritance studies investigating the resistance of ‘La Brillante’ to isolates of 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 selfed to obtain two independent F 2 segregating populations. The contrasting parental cultivars and F 2 seedlings were then separately inoculated with B. rouxiae (F 2 population size = 247) and B. basicola (F 2 population size = 166). Chi-square test was employed to assess the goodness of fit of the observed segregation patterns against each pathogen to the 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 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 and dominant resistance to both pathogens. Additional experiments are necessary to verify whether the same gene/locus or distinct genes/loci are controlling the resistance to B. rouxiae and B. basicola . Nevertheless, this simple monogenic inheritance will facilitate the incorporation of these resistant traits into elite lettuce breeding lines and cultivars.

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