Mechanism of Biological Control of Preemergence Damping-off of Pea by Seed Treatment withTrichodermaspp.

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Abstract
Application of conidia of isolates of Trichoderma harzianum or T. koningii to pea seed reduced the incidence of preemergence damping-off of peas induced by a Pythium sp. Germination of sporangia of the Pythium sp. or chlamydospores of Fusarium oxysporum f. sp. cucumerinum in the spermosphere of peas treated with the isolates of Trichoderma was comparable to that of nontreated controls. Addition of asparagine or glucose to the seed did not nullify biological control of preemergence damping-off, suggesting that competition for nutrients (nitrogen and/or carbon) was not involved. Also, several factors precluded mycoparasitic interactions as likely candidates in antagonism. Single interactions were observed infrequently and occurred 24 hr or more after mycelial contact in dual culture. Germination of conidia of Trichoderma spp. required > 10-14 hr of incubation at 26.degree. C, whereas sporangia of the Pythium sp. germinated within 90 min. The seed coat was colonized by Pythium sp. in 10 h and the embryo was infected 24-48 hr after inoculation. Time restraint and lack of extensive mycoparasitism of the Pythium sp. suggest that this mechanism of antagonism was not operative in biological control. However the antagonists produced a factor that resulted in plasmolysis of hyphal tip cells of the Pythium sp. before contact of mycelium in dual culture. Culture filtrates of isolates of Trichoderma spp. inhibited growth of the pathogen in vitro but not of Rhizoctonia solani or T. harzianum. The operation of this toxic factor in the spermosphere provides a likely explanation for the phenomena observed with biological control.