Experiences with myrtle rust overseas

09 Jul 2021

Experiences with Austropuccinia psidii in Hawaii and the Americas of relevance to the land down under

Presenter: Phil Cannon, Forest Pathologist, US Forest Service, California 

14th July 2021 11:00am

Phil will present a talk in 3 parts:

1: Although Austropuccinia psidii was found in Brazil in 1884 it drew little attention until the 1970s and 80s when a race of this rust began to cause massive infections and defoliation of millions of planted Eucalyptus grandis trees. A staunch effort was undertaken to enable Brazilian Research Institutions to study and cope with the disease. Many of the early (and successful) research efforts focused on breeding for resistance to the fungus and on defining the environmental factors conducive to disease development.

2: A. puccinia had spread wide and far in the Americas largely moving to areas north and east of the Andes and it evolved as it spread. Eventually, the ‘pandemic’ race of this fungus made its way to Hawaii in 2004 where it spread quickly. Hawaii’s climate was conducive to the rust and 32 myrtaceous species in that state were found to be susceptible. Fortunately, the ohia tree (Metrosideros polymorpha), Hawaii’s most important tree species, was not one of them. However, residents wondered if another race could be more aggressive. A large collection of isolates was made in Brazil and their DNA was compared using molecular genetic approaches; 128 distinct isolates were identified. Five of these were tested and three were found to be highly pathogenic on ohia. Hawaii initiated a ban on importation of myrtaceous species. 

3: Although the amount of damage to Myrtaceae species by this rust in Hawaii seemed to drop off substantially for about 10 years, an extremely wet year in 2018 saw massive increases in rust development on several myrtaceous species on several islands. Even the ohia tree was found to be vulnerable during this period when tens of thousands of ohia trees growing in rain-prone locations became defoliated. Given climate change will likely bring some very large swings in the future weather of the tropics, it might also be anticipated that similar explosive changes in rust epidemics could occur more commonly and elsewhere as well.

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