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Phaeotheca dimorphospora, which was first isolated from elm wood and found to be antagonistic in vitro against the Dutch elm disease pathogen Ophiostoma ulmi, was tested for antifungal activity in vitro against other tree pathogens by a variation of the agar layer technique. Phaeotheca dimorphospora produced antifungal compounds that were strongly inhibitory against a wide range of tree pathogens in addition to O. ulmi, such as Gremmeniella spp., Armillaria spp., Septoria musiva, Verticillium albo-atrum, Cylindrocladium floridanum, Phytophthora sp., Nectria galligena, and Heterobasidion annosum. Under light and interference microscopy, four types of morphological changes were observed in the pathogens tested: swelling of hyphae, production of resting spores such as chlamydospores and of sclerotia, extrusion of cytoplasm from hyphal tips, and bursting and destruction of mycelium. Chloroform-soluble antagonistic compounds were extracted that showed both fungicidal and fungistatic effects on the test organisms. Key words: Phaeotheca dimorphospora, biological control, fungal antagonist, hyphal interactions, antifungal metabolites, tree diseases.

The antagonistic fungus Phaeotheca dimorphospora was tested as a biocontrol agent against the Dutch elm disease pathogens, Ophiostoma ulmi and O. novo‐ulmi, on Ulmus americana seedlings in the glasshouse. Curative inoculation of seedlings with P. dimorphospora had no significant effect on disease development. Conditioning inoculation of seedlings with the antagonist protected them against O. ulmi but not against the more aggressive O. novo‐ulmi. In seedlings challenged with both the antagonist and either one of the pathogens, P. dimorphospora only spread locally around the inoculation point; however, the pathogens spread systemically throughout the whole plant. O. novo‐ulmi survived in the region colonized by P. dimorphospora, whereas O. ulmi did not.

In Quebec, the root rot fungus Armillaria mellea (Vahl: Fr.) Kummer in the broad sense was found to be composed of three intersterile groups or biological species by using mating tests with standard voucher strains. Monosporous cultures of our specimens were compatible with strains of groups I, V, and VI. Groups I and V corresponding to A. ostoyae (Romagn.) Herink and A. sinapina sp.nov., respectively, are described and their occurrence and ecology documented. Morphological characteristics of fruiting bodies and of vegetative isolates can be used to differentiate A. ostoyae, A. sinapina, and A. mellea s.str. Armillaria ostoyae and A. sinapina are mild pathogens or saprotrophs on declining trees or stumps, whereas A. mellea s.str. appears to be an aggressive pathogen.

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