New Phytophthora species in Western Australia: Pathogenicity and control by phosphite in vitro and in planta

Rajah Belhaj

A number of new Phytophthora species to Western Australia, have been isolated from dying native species in southwest Western Australia (SWWA). However, little is known about their host range or whether the pathogenic species can be controlled by phosphite. The effect of phosphite on 23 Phytophthora species was investigated in vitro. On solid medium, EC50 values were 5 to >160 μg/ml phosphite, whilst in liquid medium buffered using 0.03 M MES (2-(N-morpholino) ethanesulfonic acid), EC50 values ranged from 30 to >900 μg/ml. Nineteen of 23 species displayed a much higher level of tolerance in buffered liquid medium to phosphite than in solid, or unbuffered liquid medium. Assessment of phosphite tolerance was more accurate using liquid, rather than solid medium. Casuarina obesa, Banksia littoralis, B. occidentalis, B. grandis, Lambetia inermis, Corymbia calophylla, and Eucalyptus marginata were screened in the glasshouse as possible susceptible hosts by inoculating soil with 22 Phytophthora species and assessing plant growth after 6 weeks. P. niederhauserii had a wide host range similar to P. cinnamomi. Other species that killed one or more hosts were P. elongata, P. boodjera, P. moyootj, P. constricta and P. rosacearum. No Phytophthora species tested killed C. calophylla. To test the effectiveness of phosphite as a control agent, Eucalyptus marginata, B. occidentalis, B. littoralis and L. inermis were sprayed with 0.5% phosphite seven days before soil inoculation with ten Phytophthora species. No phosphite-treated plants died, but in unprotected controls P. cinnamomi and P. neiderhauserii killed at least one host plant of all species, while plants of E. marginata were also killed by P.constricta, P. boodjera, P. elongata, and P. multivora. For P. constricta, P. boodjera, P. multivora, P. rosacearum and P. arenaria, for one or more host species, the reduction of shoot or root growth caused by the pathogen was not eliminated by spraying plants with phosphite. There was no relationship between phosphite tolerance in vitro (as determined by EC50) and the response to phosphite in planta: for example, P. gibbosa was highly tolerant in vitro but controlled by phosphite in planta. In L. inermis subsp. inermis all species of Phytophthora reduced root mass even in plants sprayed with phosphite. Phosphite prevented lesion development in E. marginata and B. occidentalis after underbark inoculation of with Phytophthora cinnamomi and P. niederhauserii.

New Phytophthora species in Western Australia: Pathogenicity and control by phosphite in vitro and in planta

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