Myrtle rust fungus (Austropuccinia psidii) was first reported in New Zealand in 2017 and has already established itself in warmer northern regions. Native species such as ramarama and rōhutu are particularly vulnerable to the disease, with young, expanding foliage showing the first signs of infection.
The Myrtle Rust Process model, developed for risk prediction when the rust first arrived in New Zealand, was based on data published overseas and had not been verified for the local climate and species. To accurately model the spread of myrtle rust it is important to understand how long plants can remain asymptomatic after infection; how long it takes before new spores are able to be produced (latent period) and how old leaves are when they lose their susceptibility to infection (ontogenic resistance).
This study, led by scientists from Plant & Food Research, and conducted in Brisbane and Auckland, explored the influence of regional temperatures on the development of myrtle rust.
The study found that resistance to infection developed when emerging leaves became fully expanded, with only the top three to four leaf pairs on new flush foliage susceptible to myrtle rust. Modelling the data based on weather conditions in Brisbane, Auckland, Kerikeri and Motueka demonstrated that optimum temperatures are between 22 and 28°C and that rust development stops below 10°C.
The updated data suggests that in warmer areas, such as Northland, rust will continue to develop throughout winter, while in the north of the South Island, rust will remain latent and possibly symptomless over winter, with the disease reasserting itself in spring. Understanding regional differences and the resistance of mature foliage will help in the management of myrtle rust in New Zealand native species in the future.
Funding this study was provided by the New Zealand Ministry for Primary Industries, Myrtle Rust Research Programme 18608.
Source: Plant & Food Research