Phytophthora cinnamomi is a destructive pathogen that causes dieback and death in many plant species in Australian native ecosystems. Susceptibility varies widely between related taxa, although the reasons for this variability are poorly understood. In glasshouse studies we confirmed field observations that Lambertia formosa, a New South Wales shrub, is less susceptible to P. cinnamomi than the related Western Australian species, L. inermis. Following inoculation, L. inermis roots are heavily colonised by the pathogen, leading to dieback and high mortality rates. Pathogen restriction in L. formosa correlates with more rapid and intense release of superoxide at the penetration site, and the activation of the phenylpropanoid pathway, than in L. inermis root tissues. Potassium phosphonate reduces symptom severity in inoculated L. formosa, and reduces both mortality and symptom severity in L. inermis. Phosphonate-induced protection in both species is associated with increased superoxide release 8 h after inoculation, and increased phenylalanine ammonia lyase activity 24 h after inoculation.
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