A newly identified locus controls complete resistance to Microcyclus ulei in the Fx2784 rubber clone

Using cultivars which are genetically resistant to South American leaf blight (SALB) caused by the fungus Microcyclus ulei is the only way to plant rubber trees in disease-affected areas. Numerous field observations led to the hypothesis that the resistance of the cultivar Fx2784 to SALB is likely to be monogenic. In this study, we investigated this hypothesis by examining the distribution of the trait in a cross between the resistant cultivar and a susceptible one. The individuals resulting from this cross were planted in field trials in French Guiana and Brazil. The resistance of all the trees was assessed by field observations. Bulk segregant analysis (BSA) using microsatellite markers was performed in French Guiana to determine which markers were genetically linked to resistance, and the results were validated by field observations in Brazil. In both locations, a 1:1 segregation of the resistance trait was observed, thus reinforcing the monogenic hypothesis. BSA showed tight linkage between resistance and the microsatellite markers located in linkage group 2 in the Hevea genome and enabled to pinpoint the resistance locus. The location was confirmed by observations on the trees planted in Brazil. This result should facilitate the use of Fx2784 resistance in future breeding programs for SALB resistance. This is the third major locus conferring resistance to SALB identified in rubber tree (Hevea spp.). These three loci are genetically independent, a favorable situation for genetic improvement of SALB resistance.