INHIBITION OF PYTHIUM-ULTIMUM IN ROOTS AND GROWTH SUBSTRATE OF MYCORRHIZAL TAGETES-PATULA COLONIZED WITH GLOMUS INTRARADICES

Interactions between the vesicular arbuscular mycorrhizal (VAM) fungus Glomus intraradices and Pythium ultimum (Pu) were studied on Tagetes patula in relation to phosphorus (P) fertilization and delay between t he VAM and pathogen inoculations. Two T patch were sown in each pot 31 days apart. The VAM fungus was inoculated at first sowing and Pu was added 5 days after the second sowing. VAM fungus acid Pu did not affec t plant biomass. Vesicles and arbuscules of the VAM fungus were found in 1 to 46% of root length from the highest to the lowest P level. Pu did not affect VAM colonization. Root length bearing hyphal swellings of Pu was unrelated to P nutrition, but was reduced by VAM colonizatio n. Root length bearing fungal structures was doubled by inoculation wi th Pu alone, but unaffected by Pu inoculation in VAM roots, suggesting that pathogen infection was lower in VAM roots. Number of Pu propagul es in soil was 10 times tower in VAM systems than in controls and was not affected by P level. Therefore, the observed inhibition of Pu was not related to improved P nutrition of VAM plants. As root colonizatio n by P was reduced in the young plants that were not initially mycorrh izal but sown in colonized soil, the reduction of Pa by the VAM fungus could be attributable either to a rapid stimulation of disease resist ance mechanisms when the plants were contacted by the VAM fungus or to interaction between the two fungi in soil.