Resistance of New Zealand Provenance <i>Leptospermum scoparium, Kunzea robusta, Kunzea linearis</i>, and <i>Metrosideros excelsa</i> to <i>Austropuccinia psidii</i>

Abstract

Resistance to the pandemic strain of <i>Austropuccinia psidii</i> was identified in New Zealand provenance <i>Leptospermum scoparium</i>, <i>Kunzea robusta</i>, and <i>K. linearis</i> plants. Only 1 <i>Metrosideros excelsa</i>-resistant plant was found (of the 570 tested) and no resistant plants of either <i>Lophomyrtus bullata</i> or <i>L. obcordata</i> were found. Three types of resistance were identified in <i>Leptospermum scoparium</i>. The first two, a putative immune response and a hypersensitive response, are leaf resistance mechanisms found in other myrtaceous species while on the lateral and main stems a putative immune stem resistance was also observed. Both leaf and stem infection were found on <i>K. robusta</i> and <i>K. linearis</i> plants as well as branch tip dieback that developed on almost 50% of the plants. <i>L. scoparium, K. robusta</i>, and <i>K. linearis</i> are the first myrtaceous species where consistent infection of stems has been observed in artificial inoculation trials. This new finding and the first observation of significant branch tip dieback of plants of the two <i>Kunzea</i> spp. resulted in the development of two new myrtle rust disease severity assessment scales. Significant seed family and provenance effects were found in <i>L. scoparium, K. robusta</i>, and <i>K. linearis</i>: some families produced significantly more plants with leaf, stem, and (in <i>Kunzea</i> spp.) branch tip dieback resistance, and provenances provided different percentages of resistant families and plants. The distribution of the disease symptoms on plants from the same seed family, and between plants from different seed families, suggested that the leaf, stem, and branch tip dieback resistances were the result of independent disease resistance mechanisms.