Myrtle rust is a particularly damaging and invasive form of rust, affecting species in the myrtle family, including Eucalyptus species in New Zealand. We’ve collated this document: Potential impact of myrtle rust on Eucalyptus species in New Zealand, to help New Zealand growers and plant producers understand how this fungal disease impacts Eucalyptus, how the disease develops, and how best to reduce its spread.

You are welcome to contact us for guidance. Please do not submit suspected samples of myrtle rust to Scion’s Forest health diagnostics service.

Instead, if you think you see symptoms of myrtle rust, please follow the guidance on myrtlerust.org.nz.

Key points

Pathogen origin and strains

• The myrtle rust pathogen (A. psidii) originates from South and Central America. Multiple genetic groups or strains of the pathogen are associated with different hosts in South America – including strains that are associated with Eucalyptus.
• The pandemic strain of myrtle rust present in New Zealand and Australia, and a different strain present in South Africa, both have a wide host range, but are not associated with a particular group of myrtles.

Disease impacts

• Myrtle rust has yet to cause significant impact on planted Eucalyptus in New Zealand and Australia. The disease has not been reported on Eucalyptus in the field or in nurseries in South Africa.
• In parts of South America, the disease is common and severe, primarily affecting Eucalyptus in nurseries and young plantings. It rarely causes mortality of Eucalyptus trees but can have a significant impact on growth and yield.
• Current impacts in New Zealand could change if a new strain is introduced, or if the pathogen evolves.
• Surveillance for myrtle rust in New Zealand is currently reliant on reports from the public. Targeted systematic monitoring in nurseries and young plantations is recommended from November – March, particularly in regions with a suitable climate for the disease.

Factors influencing disease development

• Disease development depends on several factors including:
  - Host genotype (resistance/susceptibility)
  - Pathogen genotype/strain
  - Inoculum load (spore concentrations)
  - Environmental conditions
• Environmental conditions have a key influence on disease development, with favourable conditions being warm and humid. Myrtle rust epidemics in New Zealand on susceptible hosts follow a seasonal cycle, building up from October, peaking over the summer months and declining from April as New Zealand moves into winter.
• Climatically favourable regions for myrtle rust in New Zealand include much of the North Island and top of the South Island.

Disease management

• Good hygiene practices, such as cleaning tools, vehicles, machinery, and clothing exposed to myrtle rust spores will reduce the risk of spreading the disease.
• The three main management practices for the disease in Brazil are breeding for resistance, fungicide application and planting in regions that are unfavourable for the pathogen.
• We do not know the resistance/susceptibility of Eucalyptus genotypes being deployed in New Zealand – they would need to be screened against the pandemic strain.
• Deployment of different clones/genotypes in the same stand is advised to minimise the selective pressure on the pathogen to evolve greater pathogenicity towards a particular genotype.
• Fungicide application in New Zealand is likely to be most effective in nurseries and may not be cost effective in the field. The most effective fungicides known to control A. psidii are those containing Triazoles and Strobilurins, particularly in combination.
• Nursery and forest managers can access current information about climatic risk for disease and management protocols, including information about fungicides using the New Zealand Plant Producers (NZPPI) online disease management platform.