INTERACTIONS BETWEEN THE SOILBORNE ROOT PATHOGEN PHYTOPHTHORA-NICOTIANAE VAR PARASITICA AND THE ARBUSCULAR MYCORRHIZAL FUNGUS GLOMUS-MOSSEAE IN TOMATO PLANTS

In order to study the influence of Arbuscular Mycorrhiza (AM) on the d evelopment of root rot infection, tomato plants were raised with or wi thout Glomus mosseae and/or Phytophthora nicotianae var. parasitica in a sand culture system. All plants were fed with a nutrient solution c ontaining one of two phosphorus (P) levels, 32 mu M (I P) or 96 mu M ( II P), to test the consequence of enhanced P nutrition by the AM fungu s on disease dynamics. Mycorrhizal plants had a similar development to that of control plants. Treatment with Phytophthora nicotianae var. p arasitica resulted in a visible reduction in plant weight and in a wid espread root necrosis in plants without mycorrhiza. The presence of th e AM fungus decreased both weight reduction and root necrosis. The per centage reduction of adventitious root necrosis and of necrotic root a pices ranged between 63 and 89%. The enhancement of P nutrition increa sed plant development, but did not appreciably decrease disease spread . In our system, mycorrhiza increased plant resistance to P. nicotiana e var. parasitica infection. Although a contribution of P nutrition by mycorrhiza cannot be excluded, other mechanisms appear to play a cruc ial role.