New Zealand provenance Leptospermum scoparium (mānuka) expresses three resistance phenotypes to the pandemic biotype of Austropuccinia psidii, the causal pathogen of myrtle rust

Abstract

Three resistance phenotypes to the pandemic biotype of the invasive fungal biotrophic pathogen Austropuccinia psidii were identified in artificially inoculated New Zealand provenance Leptospermum scoparium (mānuka) plants grown from seed collected from mother trees growing in different locations within New Zealand. The first two resistance phenotypes, putatively a constitutive response and a hypersensitive response, were leaf resistance phenotypes. On the lateral and main stems a putative constitutive stem resistance was also observed in the trials. Mānuka is the first myrtaceous species where consistent infection of stems in trials was observed, resulting in the development of a new disease severity assessment scale. No individual plants demonstrated both leaf resistances: c. 18% of plants showed both stem resistance and one of the leaf resistances; however c. 26% plants showed either a leaf or a stem resistance but not both, and the remainder of the plants tested (56%) were susceptible, with some plants showing extensive symptom expression. Plant and seed family analysis revealed limited genetic linkage between the putative constitutive leaf and stem resistances suggesting two independent resistance mechanisms. The locale from where the seed were collected was important, as the proportion of leaf and/or stem resistance plants grown from the independent seed families varied by seed provenance.