Identification of QTLs associated with citrus resistance toPhytophthora gummosis

Citrus gummosis, caused byPhytophthora spp., is an important citrus disease in Brazil. Almost all citrus rootstock varieties are susceptible to it to some degree, whereas resistance is present inPoncirus trifoliata, a closely related species. The objective of this study was to detect QTLs linked to citrusPhytophthora gummosis resistance. Eighty individuals of the F1 progeny, obtained by controlled crosses between Sunki mandarinCitrus sunki (susceptible) andPoncirus trifoliata cv. Rubidoux (resistant), were evaluated. Resistance toPhytophthora parasitica was evaluated by inoculating stems of young plants with a disc of fungal mycelia and measuring lesion lengths a month later. Two QTLs linked to gummosis resistance were detected in linkage groups 1 and 5 of theP. trifoliata map, and one QTL in linkage group 2 of theC. sunki map. The phenotypic variation explained by individual QTLs was 14% forC. sunki and ranged from 16 to 24% forP. trifoliata. The low character heritability (h 2 = 18.7%) and the detection of more than one QTL associated with citrusPhytophthora gummosis resistance showed that inheritance of the resistance is quantitative.