Biocontrol of the pathogen Phytophthora parasitica by arbuscular mycorrhizal fungi is a consequence of effects on infection loci

The impact of colonization by the arbuscular mycorrhizal fungus (AMF) Glomu s mosseae on tomato root necrosis caused by the soil-borne pathogen Phytoph thora parasitica was investigated. Studies in situ permitted infection loci to be identified and the effects of the AMF on numbers to be elucidated. E ffects were significant and, 7 and 16 days after inoculation with zoospores of the pathogen, roots of plants colonized by the AMF had 39% and 30%, res pectively, fewer infection loci than those that were not. Concurrent studie s of the rate of spread of necrosis within roots showed no changes caused b y the AMF. At harvest, 26 days following inoculation with the pathogen, 61% of roots of noncolonized plants were necrotic compared with only 31% in AM F-colonized plants. It is concluded that effects on numbers of infection lo ci are one mechanism via which AMF achieve biocontrol of this pathogen in t omato. Measures of the effects of the AMF on root system architecture sugge st that no significant changes occur and are thus not the reasons for the r eduction in infection loci. The implications of these data for agricultural practice and biocontrol research are discussed.