Suppression of the biocontrol agent Trichoderma harzianum by mycelium of the arbuscular mycorrhizal fungus Glomus intraradices in root-free soil

Trichoderma harzianum is an effective biocontrol agent against several fung al soilborne plant pathogens. However, possible adverse effects of this fun gus on arbuscular mycorrhizal fungi might be a drawback in its use in plant protection. The objective of the present work was to examine the interacti on between Glomus intraradices and T. harzianum in soil. The use of a compa rtmented growth system with root-free soil compartments enabled us to study fungal interactions without the interfering effects of roots. Growth of th e fungi was monitored by measuring hyphal length and population densities, while specific fatty acid signatures were used as indicators of living fung al biomass, Hyphal P-33 transport and beta-glucuronidase (GUS) activity wer e used to monitor activity of G. intraradices and a GUS-transformed strain of T. harzianum, respectively. iis growth and metabolism of T. harzianum ar e requirements for antagonism, the impact of wheat bran, added as an organi c nutrient source for T. harzianum, was investigated. The presence of T, ha rzianum in root-free soil reduced root colonization by G, intraradices, The external hyphal length density of G. intraradices was reduced by the prese nce of T. harzianum in combination with wheat bran, but the living hyphal b iomass, measured as the content of a membrane fatty acid, was not reduced. Hyphal P-33 transport by G, intraradices also was not affected by T, harzia num. This suggests that T. harzianum exploited the dead mycelium but not th e living biomass of G. intraradices, The presence of external mycelium of G . intraradices suppressed T. harzianum population development and GUS activ ity, Stimulation of the hyphal biomass of G, intraradices by organic amendm ent suggests that nutrient competition is a likely means of interaction. In conclusion, it seemed that growth of and phosphorus uptake by the external mycelium of G, intraradices were not affected by the antagonistic fungus T , harzianum; in contrast, T, harzianum was adversely affected by G, intrara dices.