Trichoderma harzianum is an effective biocontrol agent against several fungal soilborne plant pathogens. However, possible adverse effects of this fungus on arbuscular mycorrhizal fungi might be a drawback in its use in plant protection. The objective of the present work was to examine the interaction between Glomus intraradices and T. harzianum in soil. The use of a compartmented growth system with root-free soil compartments enabled us to study fungal interactions without the interfering effects of roots. Growth of the fungi was monitored by measuring hyphal length and population densities, while specific fatty acid signatures were used as indicators of living fungal biomass. Hyphal 33P transport and β-glucuronidase (GUS) activity were used to monitor activity ofG. intraradices and a GUS-transformed strain of T. harzianum, respectively. As growth and metabolism of T. harzianum are requirements for antagonism, the impact of wheat bran, added as an organic nutrient source for T. harzianum, was investigated. The presence of T. harzianum in root-free soil reduced root colonization by G. intraradices. The external hyphal length density of G. intraradices was reduced by the presence of T. harzianum in combination with wheat bran, but the living hyphal biomass, measured as the content of a membrane fatty acid, was not reduced. Hyphal 33P transport by G. intraradices also was not affected by T. harzianum. This suggests that T. harzianum exploited the dead mycelium but not the living biomass of G. intraradices. The presence of external mycelium of G. intraradices suppressed T. harzianum population development and GUS activity. Stimulation of the hyphal biomass ofG. intraradices by organic amendment suggests that nutrient competition is a likely means of interaction. In conclusion, it seemed that growth of and phosphorus uptake by the external mycelium ofG. intraradices were not affected by the antagonistic fungus T. harzianum; in contrast, T. harzianumwas adversely affected by G. intraradices.

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This study demonstrates that outward growth of mycelium from primary foci through bulk potting mix to roots of adjoining plants can be an important means of spread of damping-off and root rot caused by Pythium ultimum. The use of a rhizobox system, which confines plant roots, enabled us to study the spread of actively growing mycelium between root systems placed at precise distances from each other. In steamed potting mix, hyphae of P. ultimum on average grew 9.6 cm from diseased root tissue compared to 5.3 cm in raw potting mix. The density of mycelium was highest within the first 2 cm from the infected root tissue, decreasing with increasing distances from the roots. Accordingly, the disease on adjacent plants decreased as the distance from infected roots increased. The time required for damping-off of adjacent plants was 3 days slower in raw as compared to steamed potting mix and increased by 2 days for each additional centimeter between the rhizoboxes. The presence of Trichoderma harzianum diminished the production of secondary inoculum and reduced the ability of P. ultimum hyphae to extend through bulk potting mix. In conclusion, the concentration of the primary inoculum, the plant density, the distance separating diseased from healthy roots, the resident microflora, and the presence of an antagonist were shown to be important factors affecting disease spread by mycelial growth.

Ulocladium atrum (isolates 385 and 302) consistently inhibited Botrytis aclada sporulation on dead onion leaf pieces under constant moist conditions and with an interrupted wetness period of 9 h. Clonostachys rosea (isolate 201) was as effective as U. atrum under constant moist conditions, but was ineffective if exposed to a drying period. No sporulation of B. aclada was observed 8 and 12 days after inoculation in the presence of U. atrum 302. C. rosea 201 significantly reduced B. aclada sporulation 8 days, but not 12 days after inoculation. When U. atrum 302 or C. rosea 201 was applied 1 day prior to B. aclada the antagonistic effect was higher compared to when the antagonists were applied on the same day. C. rosea 201 and U. atrum 302 did not obstruct the growth of B. aclada from necrotic onion leaf tips into living tissue, when artificially induced necrotic leaf tips were infested with B. aclada 24 h prior to antagonists. Three days after antagonist application, no symptoms could be observed on the healthy leaf tissue, nor was there sporulation on the necrotic leaf tip. However, B. aclada was immunologically detected 2 cm below the inoculation site. We conclude that under constant moist conditions the antagonists C. rosea 201 and U. atrum 302 cannot stop the progress of B. aclada from necrotic into fresh leaf tissue.