|Jones, Cecilia - UNIVERSITY OF MINNESOTA|
Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 10, 1996
Publication Date: N/A
Interpretive Summary: A number of soil fungi can cause alfalfa seeds to rot and may induce disease on germinating seedlings. This disease complex can lead to low yielding, nonpersistent fields. There are no alfalfa varieties available with resistance to all of the pathogenic organisms in this complex. Nor do current management practices provide effective control against this set of pathogens. A strain of the soil bacterium Streptomyces was found to inhibit the growth of a number of alfalfa pathogens in an agar culture plate assay. Treating alfalfa seeds with Streptomyces inhibited the development of Pythium damping-off and Phytophthora root rot (PRR) in laboratory tests. Field experiments performed in 1993 showed that treating fields with the fungicide metalaxyl or Streptomyces alone did not improve seedling establishment or disease control, however, the combination of fungicide and Streptomyces resulted in high seedling survival, dry matter production, and frequency of healthy plants. In the 1994 field experiment the combination of Streptomyces and fungicide significantly increased plant populations for the PRR resistant variety. These results indicate that Streptomyces has the potential to control a number of alfalfa pathogens in the seedling disease complex and to augment resistance and fungicide control. Use of such a biocontrol agent could decrease costs in establishing alfalfa fields and increase plant density and plant health.
Technical Abstract: Biocontrol of alfalfa seedling diseases by a pathogen-suppressive strain of Streptomyces was evaluated in vitro, in controlled environments, and in the field. Streptomyces strain 93 inhibited the growth of soilborne 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 ealfalfa variety with Streptomyces spores inhibited the development of Pythium damping-off in a rolled paper towel assay. Control of Phytophthora root rot (PRR) by inoculation of soil with Streptomyces at the time of planting was observed 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. Field experiments were performed in 1993 and 1994 to evaluate the effect of Streptomyces alone and in combination with the fungicide metalaxyl on disease and plant performance. In 1993, neither treatment alone improved seedling establishment or disease control, 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 PRR. In the 1994 field experiment, the combination of Streptomyces and fungicide significantly increased plant populations for the resistant variety, however, there was no significant treatment effect on the susceptible cultivar Vernal. These results indicate that Streptomyces has the potential to control a number of alfalfa pathogens in the seedling disease complex and to augment resistance and fungicide control.