AHMAD, S. J., and BAKER, R. 1988. Implications of rhizosphere competence of Trichoderma harzianum. Can. J . Microbiol. I 34: 229-234. Seed treatment with conidia of rhizosphere-competent mutants of Trichoderma harzianum reduced the incidence of preemergence damping-off of barley, cucumber, pea, radish, and tomato induced by Pythium ultimum. Wild-type parents of these mutants were less effective in control. When rhizosphere-competent mutants were applied to seed or when a peat-1 bran preparation was added to soil, the resulting plants produced significantly higher fruit weight and higher dry weights than those treated with rhizosphere-incompetent wild types and controls. Seed treatment with mutants increased the inci-I dence of emergence and resultant plant growth was significantly ( P = 0.05) better than when mutant strains were added to soil in peat-bran. There was, however, no significant ( P = 0.05) difference between the two types of application of the wild types. When cucumber seeds, treated with a T. harzianum rhizosphere-competent mutant (T-95) or its parent wild type (WT), were sown in raw soil kept under constant matric potential with no additional water added, the roots grew 8 I cm in 8 days. Untreated seeds produced roots 7 cm long. Fewer colony-forming units of P. ultimum per milligram were isolated from rhizosphere soil of the T-95 treated seedlings than in the untreated controls and those treated with WT. Pythium i ultimum was not detected in the 8th cm (farthest from seed) root segment of T-95 treated seeds, whereas the last centimetre I of root segment from untreated and WT-treated seeds yielded 3000 colony-forming units/g rhizosphere soil. Seed treatment with rhizosphere-competent mutants of T. harzianum is an effective delivery system to achieve biocontrol and increase I growth response. 1 AHMAD, S. J., et BAKER, R. 1988. Implications of rhizosphere competence of Trichoderma harzianum. Can. J . Microbiol.34 : 229-234. Le traitement de semences avec des mutants de Trichoderma harzianum, mutants competents au niveau des rhizospheres, a reduit l'incidence de la fonte des semis causie par le Pythium ultimum chez l'orge, le concombre, le pois, le radis et la tomate. Sur ce rapport, les parents sauvages de ces mutants furent moins efficaces. Ces mutants competents ont etC utilises soit directement pour le traitement des semences, soit m&lCs a des preparations de son et de tourbe et incorpores dans du I avec les mutants compktents a favorise l'emergence et une croissance significativement supkrieures (P = 0,05) des plants comparativement aux rendements obtenus avec le traitement des sols. Les types sauvages utilises dans ces deux formes de traitement n'ont pas fourni de differences significatives au niveau de P = 0,05. Des graines de concombre ont Cte traitees soit avec le mutant T-95, soit avec les parents sauvages de ce mutant, et ensemencees dans du sol brut maintenu constant quant au potentiel matriciel et sans apport d'eau additionnel; en 8 jours, une croissance racinaire de 8 cm fut obtenue. Par ...