Biological Control of Fungi Causing Alfalfa Seedling Damping-Off with a Disease-Suppressive Strain ofStreptomyces

Biological control of alfalfa seedling diseases by a disease-suppressive strain ofStreptomyceswas evaluatedin vitro,in controlled environments, and in the field.Streptomycesstrain 93 inhibited the growth of soil-borne pathogens causing seed rot and seedling damping-offin vitrobut did not affect the growth of the symbiontRhizobium meliloti.Treating seeds of a susceptible alfalfa variety withStreptomycesspores inhibited the development ofPythiumdamping-off in a rolled paper towel assay. Control of Phytophthora root rot was achieved by infesting soil withStreptomycesat the time of planting under greenhouse conditions. The frequency of healthy plants increased significantly for the susceptible variety and the average disease severity index decreased significantly for both the resistant and susceptible varieties tested. Field experiments were performed in 1993 and 1994 to evaluate the effect ofStreptomycesalone and in combination with the fungicide metalaxyl on disease control and plant performance. In 1993, neither treatment alone improved seedling establishment or disease control over controls. However, the combination of fungicide andStreptomycesresulted in the highest seedling survival, dry matter production, and frequency of healthy plants with no or slight symptoms of Phytophthora root rot. In the 1994 field experiment, plots receiving the combination ofStreptomycesand fungicide were not significantly different from the untreated control in seedling survival or in incidence or severity of root rot. These studies indicate that a potential exists for utilizingStreptomycesto control alfalfa seedling diseases