Factors Affecting Biological Control of Pythium Ultimum on Alfalfa using Seed Treatment with Gliocladium Virens, Trichoderma Harzianum and T. Hamatum

Gliocladium virens Miller et at. applied as a seed treatment controlled alfalfa (Medicago sativa L. ‘Travois’) damping-off in natural soils artificially infested with high levels of Pythium ultimum Trow, but Trichoderma hamatum (Bon.) Bain and Trichoderma harzianum Ragai were not effective at relatively high soil pH levels. Biocontrol with G. virens was nearly as effective as metalaxyl seed treatment, and better than Captain in most environments. Biocontrol was optimum at sail temperatures between 15 and 30 C, soil moistures of 15% to 30%, soil pH of 6 to 8 and when seeds had been treated with conidial concentrations of 109 spores/ml or greater. In a parallel study of steamed soils infested with P. ultimum, biocontrol was most effective between 25 and 35 C. at relatively low soil moistures, and as pH levels of 6 to 8. Amendment of the biocontrol spore suspension with bleached chitin did not affect the ability of the antagonists to protect alfalfa against P. ultimum, while unbleached chitin consistently destroyed effective biocontrol. G. virens seed treatment also effectively controlled damping-off in field tests. In vitro, 30 C and lower pH levels were optimum conditions for growth and sporulation of all three biocontrols