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

Abstract

Resistance to the pandemic strain of Austropuccinia psidii was identified in New Zealand provenance Leptospermum scoparium, Kunzea robusta, and K. linearis plants. Only 1 Metrosideros excelsa-resistant plant was found (of the 570 tested) and no resistant plants of either Lophomyrtus bullata or L. obcordata were found. Three types of resistance were identified in Leptospermum scoparium. 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 K. robusta and K. linearis plants as well as branch tip dieback that developed on almost 50% of the plants. L. scoparium, K. robusta, and K. linearis 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 Kunzea spp. resulted in the development of two new myrtle rust disease severity assessment scales. Significant seed family and provenance effects were found in L. scoparium, K. robusta, and K. linearis: some families produced significantly more plants with leaf, stem, and (in Kunzea 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.

Chasing myrtle rust in New Zealand: host range and distribution over the first year after invasion

“…Despite a number of plants being surveyed until May 2018, only one plant was found to be infected in nature. Artifical inoculation studies have, however, shown that the species is susceptible (Smith et al 2020) which suggests that physiological or environmental factors in the field may be limiting the myrtle rust infections in mānuka plantations, as reported for other Myrtaceae hosts in Australia (Makinson 2018).…”

“…Infection and expression of disease symptoms requires the combination of pathogen, susceptible host and conducive environment, and the absence of detection may be due to the lack of inoculum exposure or suitable environmental conditions. Resistance screening on Kunzea robusta has shown that this species is susceptible to A. psidii although the population tested did have more resistance than some of the other New Zealand Myrtaceae species in which genetic resistance was either low or non-existent (Smith et al 2020). Lophomyrtus bullata and hybrids with L. obcordata were the most frequently detected infected plant species and both resistance screening and Fig.…”

“…Numbers in the coloured circles indicate the total number of infected plants found in each affected region during the three different time periods (May to September 2017; October 2017 to February 2018; and March to June 2018). The date underneath the coloured circles indicates the first detection of A. psidii in the region field surveillance have indicated these to be highly susceptible (Smith et al 2020;Soewarto et al 2019b). The discovery of A. psidii on A. sellowiana in New Zealand was surprising as this species was considered resistant to the pandemic biotype based on the lack of natural infection in countries where the host is grown, and the inability of the pathogen to infect A. sellowiana when artificially inoculated (Morin et al 2012;Soewarto et al 2019a).…”

Monitoring Austropuccinia psidii (myrtle rust) on New Zealand Myrtaceae in native forest

“…Artificial inoculation of Lophomyrtus spp. with A. psidii has also shown the species is highly susceptible and thus far, there is no indication of any resistance in this species (Smith et al 2020). This highlights the need to focus on conservation and germplasm storage of Lophomyrtus spp.…”

“…Nothing is known about the genetic resistance of L. scoparium or Metrosideros spp. in the two sites monitored, although artificial testing of L. scoparium against A. psidii has shown this species is susceptible and there are multiple forms of resistance in the population (Smith et al 2020). During the incursion response there was a high number of myrtle rust detections on Metrosideros spp., but the majority of these detections were on Metrosideros excelsa and hybrids, and not on species of climbing rātā (Beresford et al 2019;Toome-Heller et al 2020).…”