The susceptibility of rare and threatened NSW species to the root-rot pathogen Phytophthora cinnamomi: 2. The identification of species requiring protection or further research

Context

The response of most native plant species in New South Wales (NSW) to infection by the oomycete pathogen Phytophthora cinnamomi Rands is unknown, which makes decisions about disease management difficult.

Aims

We aim to improve knowledge about the potential threat from P. cinnamomi by testing a further 32 threatened species for their response to the pathogen and developing a method for prioritising management and susceptibility testing.

Methods

Susceptibility to infection and host response were evaluated in glasshouse experiments where the pathogen was introduced to pots containing the threatened species, and the results were compared with control uninoculated pots. Our prioritisation used modelled habitat suitability for P. cinnamomi, proximity to known P. cinnamomi occurrences, and numbers of plant species populations at least 1 km apart to rank 928 rare and threatened plant species native to NSW for either management or susceptibility testing.

Key results

Phytophthora cinnamomi was re-isolated from the roots of 10 of the 32 species assessed, most of which also showed significant mortality or disease symptoms. Darwinia peduncularis B.G.Briggs, Hibbertia circinata K.L.McDougall & G.T.Wright, Isopogon fletcheri F.Muell., Phebalium speciosum I.Telford, Pultenaea baeuerlenii F.Muell. and Pultenaea parrisiae J.D.Briggs & Crisp were the most severely affected species. The effect of P. cinnamomi is known for only 63 rare and threatened species in NSW. The Greater Sydney region is a hotspot for rare and threatened plant species with a high priority for susceptibility testing.

Conclusions

The prognosis in the wild for rare and threatened plant species affected by P. cinnamomi depends on (1) habitat suitability for the pathogen, with subalpine and arid-zone species unlikely to be affected, (2) the number of unaffected populations, with two severely affected species that occur only on infested sites (Hibbertia circinata and Prostanthera marifolia R.Br.) facing extinction in the near future and (3) climate, with some species (e.g. Pomaderris delicata N.G.Walsh & Coates) apparently affected only in unusually wet years. Further susceptibility testing of rare and threatened species is required. This should be supported by taxonomic studies of genera (e.g. Hibbertia, Pultenaea) commonly affected by the pathogen.

Implications

Many more plant species in NSW are likely to be severely affected by P. cinnamomi than currently known, and may require active management of the disease for their long-term survival.