Variation within the genus Lambertia in efficacy of low-volume aerial phosphite spray for control of Phytophthora cinnamomi

Soil and stem wound inoculation was used to determine variation in phosphite efficacy for the control of Phytophthora cinnamomi within the genus Lambertia. Disease progress curves following soil inoculation with P. cinnamomi were analysed with the parameters of the logistic model. The asymptote and lag logistic model parameters of disease progress better differentiated between phosphite treatments than the rate parameter. Low-volume phosphite spray significantly reduced the asymptote K-max of % mortality and significantly increased the delay t(1/2K) for all Lambertia taxa soil inoculated. K-max was significantly negatively correlated with delay t(1/2K) and taxa sprayed with 48 kg phosphite/ha had a lower K-max and longer delay t(1/2K) than taxa not sprayed. Phosphite significantly reduced colonisation by P. cinnamomi in stems of 19% of Lambertia taxa tested. The differences in phosphite efficacy between soil and wound inoculation suggests that phosphite may inhibit the infection process more than post-infection stem colonisation. The lack of direct relationships between control of P. cinnamomi and tissue phosphite concentrations may be related to different effective phosphite thresholds between taxa. Time-course studies of in-planta phosphite concentrations in relation to effective control of P. cinnamomi are required in different plant species and environments.