INHIBITION OF FUSARIUM-OXYSPORUM F-SP DIANTHI IN THE NON-VAM SPECIES DIANTHUS-CARYOPHYLLUS BY COCULTURE WITH TAGETES-PATULA COMPANION PLANTS COLONIZED BY GLOMUS INTRARADICES

The effect of the mycorrhizal fungus Glomus intraradices on disease de velopment caused by Fusarium oxysporum f.sp. dianthi in the nonmycorrh izal species Dianthus caryophyllus was studied by co-culture of carnat ion plants with the mycorrhizal species Tagetes patula. Presence of VA M T. patula plants more than doubled the survival of D. caryophyllus, significantly reduced the disease symptoms, and decreased F. o. dianth i propagules by 4:1 in soil. Non-VAM T. patula plants had no effect. D ianthus caryophyllus shoot biomass was reduced by F, o. dianthi in non -VAM controls but was not affected in presence of G. intraradices. Glo mus intraradices alone has no effect on T. patula or D. caryophyllus s hoot biomass. Dianthus caryophyllus mineral shoot content was not modi fied by G. intraradices. In absence of T. patula, G. intraradices did not colonize D. caryophyllus, whereas in its presence, 14-20% of the c arnation root length contained abundant vesicles and hypha but very ra rely arbuscules. The presence of G. intraradices clearly reduced the d isease caused by F. o. dianthi in D. caryophyllus. Reduction in diseas e severity was associated with reduced F. o. dianthi propagule number in the substrate and was clearly unrelated to plant nutrition. Our res ults may be explained either by the induction of D. caryophyllus disea se resistance mechanisms by the mycorrhizal fungus or by direct or ind irect microbial interactions in the soil.