Interactions between Plasmopara helianthi, Glomus mosseae and two plant activators in sunflower plants

Interactions between Plasmopara helianthi, Glomus mosseae and two plant act ivators DL-beta -amino-n-butyric acid (BABA) and CGA 245704 (acibenzolar-S- methyl (BTH)) in sunflower plants susceptible to downy mildew were studied in four experiments using different methods of treatment and pathogen inocu lation. Both chemicals were applied as soil drenches and foliar sprays, whe reas P. helianthi infection was obtained by root and cotyledon inoculations of the seedlings. Soil drenches at the rates of 50 and 100 mg kg(-1) soil of BABA and BTH given 1 and 3 days before P. helianthi inoculation, respect ively to mycorrhizal plants, provided moderate protection against the patho gen (about 50-55%). Morphological changes and decrease in mycorrhizal colon ization in roots of BTH-treated plants and in BTH-treated mycorrhizal plant s were also observed. Delay in the emergence and reduction of the root syst ems were more evident at the highest concentration but decreased with time. These effects were absent with the BABA treatment. Foliar spray treatment of BABA and BTH, applied at 4000 and 200 mug ml(-1), respectively (1 day post-inoculation) to mycorrhizal plants provided good protection (about 80%) against P. helianthi foliar infections. No effects o n mycorrhizal colonization or on root systems were observed. In vitro tests on the effect of the compounds on the mycorrhizal fungus sho wed that the germination of G. mosseae sporocarps increased with BABA treat ment whereas it was greatly inhibited by BTH treatment.