Independent QTL underlie resistance to the native pathogen Quambalaria pitereka and the exotic pathogen Austropuccinia psidii in Corymbia

Abstract

Fungal diseases such as the exotic myrtle rust (Austropuccinia psidii), and the native Quambalaria shoot blight (QSB; caused by Quambalaria species including Q. pitereka), constitute a significant threat to both native forests and Corymbia plantations in Australia and overseas. We here use quantitative trait loci (QTL) analysis to understand the genetic architecture of resistance to these pathogens in C. torelliana and C. citriodora subsp. variegata. QTL analysis was undertaken using 360 genotypes from two F1 crosses of C. torelliana × C. citriodora subsp. variegata, phenotyped by controlled inoculation with the strain of A. psidii present in Australia and independent inoculations with two strains of Q. pitereka (QSB1 & QSB2). A total of 22 QTL were identified, six for rust and 16 for QSB. The QTL for resistance to A. psidii and Q. pitereka in these pedigrees were independent from one another since they were generally in different parts of the genome, with only one case of co-location (QTL peak location within ± 2 MB). The QTL for the different QSB strains all mapped to discrete locations. The QTL for QSB were generally of a greater effect size than those for A. psidii. Several co-locations with QTL for resistance to rust and other fungal pathogens found in another eucalypt, Eucalyptus globulus, were detected and the implications of this observation are discussed.