Variability in phosphite sensitivity observed within and between seven Phytophthora species

Phosphite is used to control and manage many phytophthora diseases in horticultural systems worldwide and natural ecosystems in Australia, Africa, New Zealand and parts of Northern America and Europe. Phosphite does not kill Phytophthora species, but inhibits growth while also stimulating host defence responses. Phytophthora species differ in their underlying tolerance to phosphite and isolates have been shown to acquire tolerance after prolonged exposure. Intra- and inter-specific variability in phosphite sensitivity is of interest to determine the efficacy and sustainability of phosphite for the treatment of phytophthora diseases, which continue to spread globally. Seven Phytophthora species were tested for their sensitivity to phosphite in vitro in a mycelial growth experiment. Phytophthora agathidicida was the species most sensitive to phosphite, being inhibited by 98.7% on average at the lowest phosphite treatment (15 µg/mL phosphite), followed by P. aleatoria, P. cinnamomi, P. pluvialis, P. multivora, P. kernoviae and P. citricola. Huge intraspecific variability was observed with P. kernoviae, which raises the question of whether diseases caused by P. kernoviae such as phytophthora needle blight of Pinus radiata could be managed effectively with phosphite. Further work is required to determine the phosphite sensitivity of different introduced and native Phytophthora species growing in key hosts and whether tolerance observed in vitro is also expressed in vivo.