Bypassing of a polygenic Microcyclus ulei resistance in rubber tree, analyzed by QTL detection.

Genetic resistance components of the Hevea brasiliensis x H. benthamiana RO 38 cultivar to Microcyclus ulei disease were investigated by inoculating isolates which succeeded in partially or completely infecting genotypes of a mapping population. Progeny of a cross between RO 38 and a susceptible cultivar was inoculated under controlled conditions with three isolates and scored for two resistance traits. Interval mapping and a nonparametric test were used to detect resistance quantitative trait loci (QTLs). Eight significant QTLs were detected, all of them inherited from the interspecific parent. Among these QTLs, only one contributed to the partial resistance against a highly pathogenic isolate, and no QTL was detected for resistance against the most pathogenic isolate. As an unexpected result, a single isolate can thus completely bypass this polygenic resistance. This complex situation, where no clear relationship can be established between number of resistance factors and qualitative vs quantitative or partial vs complete resistance, is discussed and suggestions as to the detection of new and sustainable resistance sources are proposed.