Quantitative genetic and QTL evidence for independent genetic control of resistance to myrtle rust and native pathogens in eucalypts

Myrtle rust, Austropuccinia psidii (G. Winter) Beenken, is a pathogen of global significance. Native to South America it is now rapidly spreading worldwide. The pathogen has an unusually broad host range within the Myrtaceae, a predominantly southern hemisphere plant family with many economically important species, including the eucalypts. We here report two studies on genetic variation in susceptibility to myrtle rust in the eucalypt group (Eucalyptus and Corymbia). The first study compares susceptibility of Corymbia citriodora subsp. variagata and Eucalypus globulus to the exotic myrtle rust and native pathogens Quambalaria shoot blight and Teratosphaeria leaf diseases. There was significant additive genetic variation within both host species for susceptibility to the native and also the exotic pathogens. However, in both host species susceptibility to A. psidii was not genetically correlated with susceptibility to their respective native pathogen, providing support for pathogen specific rather than general mechanisms of resistance. Further support for independent genetic control of resistance to myrtle rust and native pathogens comes from our second study, which performed QTL analysis for these pathogens in Corymbia and Eucalyptus. Ten QTL were discovered for rust resistance across both genera, although only one QTL co-location was found across the two genera. Many QTL were also discovered for native pathogen resistance throughout the genome, but again, there were few instances of co-location either with QTL for other native pathogens or the exotic rust. This study highlights candidate genes for future research and demonstrates the complex, genome-wide architecture underlying variation in pathogen resistance in eucalypts. Together, these studies provide evidence for strong genetic control and marked variation in susceptibility within and between the eucalypt species examined, and demonstrate the value of detailed screening for susceptibility in order to predict and manage the impacts of myrtle rust.