Interactions between Plasmopara helianthi and arbuscular mycorrhizal fungi in sunflower seedlings susceptible and resistant to downy mildew

Interactions between Glomus mosseae and Plasmopara helianthi in sunflower plants susceptible and resistant to downy mildew were studied in three pot experiments. A reduction of the systemic infection by the pathogen was observed in all experiments and was influenced by the quantity of the mycorrhizal inoculum. Mycorrhizal plants preinoculated with G.mosseae at the highest rate of inoculum showed a well-develpoed mycorrhizal root colonization and a decrease in root infection bt the pathogen. Root and shoot dry weights of mycorrhizal and P.helianthi infected plants were significantly higherthan those of P.helianthi infected ones; similar results were also obtained for root and shoot lengths. Systemic infection and sporulation were lower in mycorrhizal and P.helianthi infected plants compared to P.helianthi infected ones. Histochemical reactions of incompatibility (production of phenolics, accumulation of lignin) performed in roots of susceptible and resistant plants showed that no defence responses towards the pathogen were induced by micorrhizal infection. The mechanisms by which mycorrhizal fungus can reduce disease incidence and pathogen infection are discussed.