Effects of antibiotic-producing Streptomyces on nodulation and leaf spot in alfalfa

Abstract

The ability of antibiotic-producing streptomycetes to colonize alfalfa ( Medicago sativa L.) plants and influence the activities of a fungal plant pathogen ( Phoma medicaginis var. medicaginis) and a mutualistic symbiont ( Sinorhizobium meliloti) was investigated. Streptomyces strains were introduced around seeds at the time of planting. Hyphal filaments and spore chains were observed by scanning electron microscopy on roots of alfalfa seedlings receiving the streptomycete amendment. Streptomyces strain densities on leaves decreased 10–100-fold over an 8-week period, while densities on roots remained constant over time. The Streptomyces strains also colonized alfalfa root nodules. We then tested the ability of 15 antibiotic-producing strains of Streptomyces to inhibit in vitro growth of Phoma medicaginis var. medicaginis Malbr. & Roum., the causal agent of spring blackstem and leaf spot of alfalfa. The majority of the Streptomyces strains inhibited growth of three diverse strains of P. medicaginis. In a detached leaf assay, one Streptomyces strain decreased leaf spot symptoms caused by P. medicaginis when inoculated onto leaves 24 h before the pathogen. Two Streptomyces strains decreased defoliation caused by P. medicaginis when the streptomycetes were introduced around seeds at the time of planting. We also examined inhibitory activity of Streptomyces strains against 11 strains of S. meliloti. Eight of the 15 Streptomyces strains inhibited in vitro growth of five or more of the S. meliloti strains, while four Streptomyces strains had no effect on growth of any test strains. In a growth chamber assay, two of six Streptomyces strains, when inoculated into the planting mix, significantly reduced plant dry weight compared to the treatment with S. meliloti alone, but did not significantly reduce the number of nodules. These results suggest that careful selection of Streptomyces isolates for use in biological control of plant diseases will limit the potential negative impacts on rhizobia.