Impact and disease centre characteristics of Phytophthora cinnamomi infestations of Banksia woodlands on the Swan Coastal Plain, Western Australia

Phytophthora cinnamomi Rands was isolated from either dead plants or soil at 46 disease centres in Banksia woodland at national parks and reserves on the Swan Coastal Plain. Phytophthora cryptogea Pethybridge & Lafferty was also isolated from roots of dead Acacia pulchella R.Br. in one disease centre infected with P. cinnamomi. Dead plants were infected with Armillaria luteobubalina Watling & Kile in four disease centres on the Spearwood Dune System, and these centres were excluded from further analysis. Phytophthora cinnamomi diseased areas ranged from 0.01 to 30 ha in size (mean 1.6 +/- s.e. 0.7 ha). The total area infested for the 46 disease centres was 71.5 ha. Impact of P. cinnamomi was high in 17% of disease centres and low in 11% of disease centres. Age of plant death was a mixture of old and recent in 85% of disease centres. Mainly old deaths occurred in only 4% of disease centres. The proportion of species dying in infested areas varied between 10-64% (mean 28 +/- s.e. 2%) and was positively correlated with impact type. It was found that infestation decreased species number; on average, there were seven fewer species in infested compared to non-infested areas. Four plant species associated with moist sandy sites tended to occur more frequently in centres of high impact than by chance alone. Occurrence of P. cinnamomi was related to soil association with soils of 60% of the disease centres belonging to the Bassendean or Southern River associations of the Bassendean Dune System. Sixteen percent of disease centres occurred in the Cannington, Guildford and Serpentine River associations of the Pinjarra Plain. No disease centres of P. cinnamomi were found on soils of the Spearwood and Quindalup Dune Systems. A water table was found within 3 m of the soil surface in 48% of the centres. Disturbance was associated with all disease centres. Firebreaks were associated with 72% of disease centres. Banksia woodland remnants on the Bassendean Dune System and the Pinjarra Plain are highly vulnerable to infection by P. cinnamomi and their conservation requires control of existing infestations and protection from introduction of the pathogen.