The Impact of Phytophthora cinnamomi in Tasmania

White waratah turns black when killed by Phytophthora Cinnamomi.

Within Tasmania 181 plant species have so far been recorded as hosts for Phytophthora cinnamomi. There is considerable variation in response to infection by P. cinnamomi among these host species. Some hosts can be resistant, or show no signs of disease, such as Gymnoschoenus sphaerocephalus (buttongrass). At the other extreme species such as Agastachys odorata (white waratah), are rapidly killed and may not regenerate in infected areas.

For many species some plants may survive within the affected population. Species that suffer very high levels of mortality are called highly susceptible. Other species for which only a few individuals in a population will die, or only loss of vigour is apparent, are called slightly susceptible. The rate of symptom development following infection by the fungus may also vary between species.

It is not always clear whether surviving individual plants are an indication that a proportion of the population is resistant, or whether they have simply escaped disease at that particular site.

In Tasmania, P. cinnamomi causes severe disease in the understorey, or the shrub and ground layers of the vegetation. It is considered to be a major threatening process which could lead to loss of plant diversity in Tasmania. P. cinnamomi is not a significant cause of disease in eucalypts in Tasmania, as it is with some eucalypts in Western Australia.

The impact of P. cinnamomi over time in Tasmanian vegetation types is yet to be scientifically documented. However it is possible to observe at least short term structural and floristic changes as a result of P. cinnamomi infection at many locations around Tasmania.

The susceptible species in Tasmania tend to come from the shrub and herbaceous families Dilleniaceae, Epacridaceae, Fabaceae, Proteaceae and Rutaceae. Resistant species generally belong to the grass and sedge families (though there are some notable exceptions ). As a result, grasses or sedges become a more prominent component of the vegetation in P. cinnamomi infested areas.

Structural layers may also be lost from the vegetation where larger shrubs are affected. For example, emergent Banksia marginata (silver banksia) is lost from buttongrass morrland in infested areas.
The ultimate impact on the survival of susceptible species at a site is dependant on many factors including:

• reproductive biology, such as mode of regeneration, time to reach maturity and the type, size and longevity of seed banks.
• the level of mortality in the population
• the relationship between regeneration events (e.g. fire) and disease events.

Buttongrass moorlands in western Tasmania are particularly susceptible to P. cinnamomi. Many thousands of hectares are known to be infested. The near absence of flowers is noticeable in recently diseased areas in the early summer months. This is indicative of the reduction in the number of the more brightly flowering susceptible shrubs within a sedge dominated plant community.

Agastachys odorata (white waratah) shows no resistance to P. cinnamomi and may face local extinction in diseased areas. Many other susceptible species in the moorlands die in a colourful display of autumn like colours but will re-establish from seed later. For example, Sprengelia incarnata (pink swamp heath) shows little resistance to attack by P.cinnamomi, but appears to successfully complete its life cycle, germinating, maturing and seeding before succumbing to disease. However, the size and life span of these plants could be significantly altered in diseased areas.

Heathlands and healthy understorey dry sclerophyll forests are also particularly susceptible. More than half of the plant species present in some heath communities are highly susceptible to P. cinnamomi.

Diseased grass trees, Xanthorrhoea australis

The most easily identified and observed plants affected are the Xanthorrhoea spp (grass trees) which collapse dramatically once infected. Few grass trees survive in infested areas, which may result in changes to the vegetation structure. There are likely to be flow on impacts on some fauna in diseased areas. Studies in Victoria suggest that loss of grass trees may affect the population of a small marsupial Antichinus stuartii. What the impact is on the many invertebrates that live in and around grass trees is unknown. Grass trees have died in large numbers in many areas around Tasmania, including within national parks such as Mount William N.P. and Douglas Apsley N.P.

​At least 39 of Tasmania's threatened plant species are susceptible to P. cinnamomi. Glasshouse tests and in some cases field observation, indicate these species are likely to suffer significant mortality if they are infected. As these plant species have small populations and often highly localised distributions, the introduction of P. cinnamomi would significantly threaten the survival of these species.

Though massive mortality may be witnessed with the initial onslaught of P. cinnamomi into a highly susceptible plant community, many of the affected species appear to continue to coexist with P. cinnamomi on the site in the short to medium term. The degree to which plant species are lost from sites will depend on the capacity for the highly susceptible species that are present to reproduce within the disease regime that establishes itself on the site. This may also be complicated by micro features at sites which provide refuges, locations protected from invasion by P. cinnamomi, for highly susceptible species e.g. rock plate environments.