Research reveals new threat to native forests

It could begin among the cool, damp rhododendrons and azaleas of suburban Melbourne – a few fungal spores clinging to leaves of an imported nursery specimen added to one of the many heritage gardens in eastern Melbourne’s Dandenong Ranges.

From there rangers at a nearby state park might notice leaves and branches dying on manna gums, the trees on which koalas feed. Only some time later would the disaster be realised. Across south-eastern Australia, mountain ash trees would develop cankers on their trunks and soon even the strongest of the species – the tallest hardwood trees in the world, which take 70 years to mature – would begin dying.

CRC for National Plant Biosecurity PhD candidate Kylie Ireland says this possible scenario highlights the susceptibility of Australian flora to an incursion of Phytophthora ramorum, the pathogen responsible for sudden oak death disease.

Kylie revealed through her PhD studies that a number of economically valuable Australian species are potentially susceptible to the pathogen.

Should the disease find its way to Australia it could result in “a potential epiphytotic” – an epidemic of the disease.

Eucalypts and the Nothofagus genus (beeches) are the most susceptible species.

The Phytophthora genus is named from the Greek ‘phyton’ (plant) and ‘phthora’ (destruction) and has caused widespread damage in Europe and the United States, affecting wild environments, such as California’s redwood forests, and ornamental nursery species.

It was first documented in the 1990s as simply causing twig blight on rhododendrons in Europe. However, research began in earnest once highly valued old oak trees – “a keystone species”, as Kylie points out – started dying, apparently overnight, hence how it got its common name sudden oak death.

P. ramorum affects plants in three ways: attacking the wood, causing cankers (sudden oak death); attacking the shoots or branches (ramorum dieback); or attacking the foliage (ramorum blight). Asymptomatic infection can also occur in some plants.

Kylie says when P. ramorum affects the leaves of the plant it is not deadly to the host. This is also the only form of the disease that produces spores capable of spreading the infection. It makes foliar-affected plants high-risk carriers of disease.

Kylie’s research was two pronged:

• investigating the susceptibility of Australian species to P. ramorum, through laboratory work in California, and
• climate modelling to identify and target high-risk areas in Australia for early detection surveillance.

The first part of her research showed that many Australian plant species may be susceptible to the pathogen. Of 69 species tested Kylie found all were capable of being infected, with half moderately to severely affected. All the species are important in the nursery, cut-flower and forestry industries, or in native forests.

Highly susceptible foliar hosts identified included conesticks (Isopogon cuneatus), rose cone flower (I. formosus), alpine ash (E. delegatensis), shining gum (E. nitens), karri (E. diversicolor) and tea tree species (E. viminalis and Leptospermum scoparium).

Species capable of producing spores and infecting other plants included willow myrtle or peppermint tree (Agonis flexuosa), red flowering gum (Corymbia ficifolia), spotted gum (C. maculata) and myrtle beech species (E. delegatensis, E. viminalis and I. formosus and Nothofagus cunninghamii). Her research found that E. viminalis, in particular, is capable of high levels of sporulation.

Preliminary results indicate a number of Australian species susceptible to trunk cankers, including mountain ash (E. regnans). The spores of P. ramorum like to travel in water to get to their next host and they prefer cool climates; the mists of the California redwood forests and the moist climate of Melbourne’s Dandenong Ranges are both ideal.

Climate modelling was an important part of Kylie’s research, incorporating information about environmental factors associated with infection and disease development into a model to work out the path P. Ramorum might take across the Australian landscape.

“Risk predictions generated by the models and an understanding of the pathogen’s host range will allow us to target high-risk areas for early detection surveillance,” she said. “It will also help Australian regulators develop appropriate quarantine protocols.”

Kylie said the most likely route of arrival for P. ramorum would be an imported ornamental plant.