Biological control of alfalfa seedling diseases by a disease-suppressive strain of Streptomyces was evaluated in vitro, in controlled environments, and in the field. Streptomyces strain 93 inhibited the growth of soil-borne pathogens causing seed rot and seedling damping-off in vitro but did not affect the growth of the symbiont Rhizobium meliloti. Treating seeds of a susceptible alfalfa variety with Streptomyces spores inhibited the development of Pythium damping-off in a rolled paper towel assay. Control of Phytophthora root rot was achieved by infesting soil with Streptomyces at 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 of Streptomyces alone 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 and Streptomyces resulted 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 of Streptomyces and 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 utilizing Streptomyces to control alfalfa seedling diseases.
- Medicago sativa
- Phytophthora medicaginis
- Phytophthora root rot
- Pythium ultimum
- disease-suppressive Streptomyces