Evidence for different QTL underlying the immune and hypersensitive responses of Eucalyptus globulus to the rust pathogen Puccinia psidii

The rust <em>Puccinia psidii</em> infects many species in the family Myrtaceae. Native to South America, the pathogen has recently entered Australia which has a rich Myrtaceous flora, including trees of the ecologically and economically important genus <em>Eucalyptus</em>. We studied the genetic basis of variation in rust resistance in <em>Eucalyptus globulus</em>, the main plantation eucalypt in Australia. Quantitative trait loci (QTL) analysis was undertaken using 218 genotypes of an outcross F₂ mapping family, phenotyped by controlled inoculation of their open pollinated progeny with the strain of <em>P. psidii</em> found in Australia. QTL analyses were conducted using a binary classification of individuals with no symptoms (immune) versus those with disease symptoms, and in a separate analysis dividing plants with disease symptoms into those exhibiting the hypersensitive response versus those with more severe symptoms. Four QTL were identified, two influencing whether a plant exhibited symptoms (<em>Ppr2</em> and <em>Ppr3</em>), and two influencing the presence or absence of a hypersensitive reaction (<em>Ppr4</em> and <em>Ppr5</em>). These QTL mapped to four different linkage groups, none of which overlap with <em>Ppr1</em>, the major QTL previously identified for rust resistance in <em>Eucalyptus grandis</em>. Candidate genes within the QTL regions are presented and possible mechanisms discussed. Together with past findings, our results suggest that <em>P. psidii</em> resistance in eucalypts is quantitative in nature and influenced by the complex interaction of multiple loci of variable effect.