Quantitative trait loci controlling Phytophthora cactorum resistance in the cultivated octoploid strawberry (Fragaria × ananassa)

Abstract

The cultivated strawberry, <i>Fragaria × ananassa</i> (<i>Fragaria</i> spp.) is the most economically important global soft fruit. <i>Phytophthora cactorum</i>, a water-borne oomycete causes economic losses in strawberry production globally. A bi-parental cross of octoploid cultivated strawberry segregating for resistance to <i>P</i>. <i>cactorum</i>, the causative agent of crown rot disease, was screened using artificial inoculation. Multiple putative resistance quantitative trait loci (QTL) were identified and mapped. Three major effect QTL (<i>FaRPc6C</i>, <i>FaRPc6D</i> and <i>FaRPc7D</i>) explained 37% of the variation observed. There were no epistatic interactions detected between the three major QTLs. Testing a subset of the mapping population progeny against a range of <i>P. cactorum</i> isolates revealed no significant interaction (<i>p</i> = 0.0593). However, some lines showed higher susceptibility than predicted, indicating that additional undetected factors may affect the expression of some quantitative resistance loci. Using historic crown rot disease score data from strawberry accessions, a preliminary genome-wide association study (GWAS) of 114 individuals revealed an additional locus associated with resistance to <i>P</i>. <i>cactorum</i>. Mining of the <i>Fragaria vesca</i> Hawaii 4 v1.1 genome revealed candidate resistance genes in the QTL regions.