Variation in susceptibility to Phytophthora cinnamomi infection within the genus Lambertia

Abstract

Variation in susceptibility to Phytophthora cinnamomi Rands infection within the genus Lambertia Sm. was evaluated by soil and stem inoculation. Disease-progress curves following soil inoculation were analysed with the three logistic-model parameters of upper asymptote (K max), lag time (t 1/2K ) and intrinsic rate of increase (r), which were combined into a mortality score. Resistant Lambertia taxa had the lowest K max, longest t 1/2K and slowest r. These parameters change to greatest K max, shortest t 1/2K and fastest r for susceptible Lambertia taxa. Mortality score following soil inoculation was significantly positively correlated with lesion score determined by stem inoculation. The plot of the scores ranked the Lambertia taxa in relation to P. cinnamomi susceptibility on the resistance–susceptibility continuum and prioritised taxa in relation to the threat posed by the pathogen. The highest mortality and lesion scores for the threatened rare and endangered taxa L. orbifolia subsp. orbifolia, L. fairallii and L. rariflora subsp. lutea suggest a high risk of extinction from P. cinnamomi infestation. Furthermore, rare and endangered taxa L. orbifolia subsp. Scott River Plains, L. echinata subsp. occidentalis and L. echinata subsp. echinata that have high mortality and moderate lesion scores are also likely to be at high risk of extinction from P. cinnamomi infestation. Among common taxa with restricted geographic distribution, L. ericifolia has high mortality and lesion scores, suggesting a high risk of localised extinction and loss of genetic diversity in P. cinnamomi disease centres. Positioning taxa on the P. cinnamomi resistance–susceptibility continuum needs to be incorporated into extinction-risk methodology so as to prioritise flora for conservation actions according to hazard from the pathogen.