|Samac, Deborah - Debby|
Submitted to: Biological Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/6/2001
Publication Date: 3/1/2002
Citation: Xiao, K., Samac, D.A., Kinkel, L.L. 2002. Biological control of Phytophthora root rots on alfalfa and soybean with Streptomyces. Biological Control. 23:285-295. Interpretive Summary: Root rots of alfalfa and soybean caused by the pathogen Phytophthora are highly destructive on these two crops throughout the world. Root rots can significantly reduce plant establishment and crop yields and are difficult to control. Plant resistance is the major control method for both crops. None of the available alfalfa varieties are immune to the disease and under rsevere conditions resistant varieties can be highly diseased. In soybean, resistance to specific races of the pathogen has resulted in new races and disease on previously resistant soybean varieties. We investigated use of Streptomyces, a common soil bacterium that produces a variety of antibiotics, as a component of an integrated biological control strategy for root rots of alfalfa and soybean. Of the 53 isolates tested, 66% inhibited growth of the alfalfa pathogen and 74% inhibited growth of the soybean pathogen in culture plates. When plants were grown in sterile vermiculite and treated with Streptomyces and a single isolate of the pathogen, the number of healthy plants increased dramatically and the symptoms on diseased plants were less severe. In the absence of the pathogen, several isolates of Streptomyces increased the size and weight of alfalfa seedlings compared to untreated plants. When alfalfa treated with Streptomyces was grown in field soil with a high level of the pathogen, the number of healthy plants and the forage yield was much greater than for the untreated plants. The next phase of the work is to test combinations of streptomyces isolates with resistant plant varieties and various cultural practices to develop an integrated management scheme for root rots. This research will contribute to recommendations that will result in higher crop yields with fewer inputs and lower production costs.
Technical Abstract: A collection of 53 antibiotic-producing Streptomyces isolated from soils from Minnesota, Nebraska, and Washington were evaluated for their ability to inhibit plant pathogenic Phytophthora medicaginis and Phytophthora sojae in vitro. Eight isolates having the greatest pathogen-inhibitory capabilities were subsequently tested for their ability to control Phytophthora root rots on alfalfa and soybean in sterilized vermiculite an naturally infested field soil. The Streptomyces isolates tested significantly reduced root rot severity in alfalfa and soybean caused by P. medicaginis and P. sojae, respectively (p<0.05). On alfalfa, isolates varied in their effect on plant disease severity, percent dead plants and plant biomass in the presence of the pathogen. The same eight isolates of Streptomyces were also tested for inhibitory activities against each other, and against three strains of Bradyrhizobium japonicum, and two strains of Sinorhizobium meliloti isolated from soybean and alfalfa, respectively. Streptomyces isolates clustered into two major compatibility groups: isolates within the same group were noninhibitory towards one another in vitro. The compatibility groups corresponded with groupings obtained based upon inhibition of B. japonicum and S. meliloti strains.