Novel Use of a Pyrenomycetous Mycoparasite for Management of Fusarium Wilt of Watermelon.

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is a destructive disease that limits watermelon production in many areas of the world. The discovery of several pyrenomycetous ascomycetes occurring naturally in association with different formae speciales of F. oxysporum identified these fungi as potential biological control organisms for watermelon wilt. One such mycoparasitic isolate, identified as Sphaerodes retispora var. retispora, was chosen for biological control and ecological trials in the greenhouse. Four different methods to inoculate the mycoparasite were evaluated, three of which utilized the parasite encapsulated into sodium alginate pellets. The other method employed root-dipping plants with mycoparasite ascospore suspensions. Ecological factors also were investigated, including the ability of S. retispora var. retispora to colonize watermelon roots, and its ability to survive in soil over time and reduce propagules of F. oxysporum f. sp. niveum. In the biological control studies, the use of the mycoparasite significantly reduced plant mortality and increased dry weights of watermelon plants after being challenged with F. oxysporum f. sp. niveum, compared with pathogen-inoculated controls. It appears that the incorporation of the parasite into alginate pellets in the planting mix at seeding may be the most practical method for future field evaluations of transplant-grown vegetable crops. In the ecological studies, the mycoparasite was recovered from infested soil after 9 months, but was only isolated from watermelon roots when applied in the presence of F. oxysporum. S. retispora var. retispora had no effect on F. oxysporum f. sp. niveum propagules after being applied to soils in the greenhouse.