Evaluating the Effects of Antagonistic Interactions on Pathogen Inhibition by Streptomyces Isolates from a Disease Suppressive Soil

Abstract

Diseases of plants threaten global food security and increase the cost of producing food and fiber. Synthetic pesticides have proven effective at suppressing many plant diseases but can require repeated applications at significant expense and cause harm to humans and the environment. Disease suppressive soils often support naturally-occurring soil microbial communities that inhibit plant pathogens and could be used to develop biocontrol or other plant disease management methods. However, high frequencies of resource competition and antagonistic interactions among naturally-occurring pathogen-suppressive populations represent a challenge for reproducing effective microbial disease suppression in agricultural settings. This work sought to further understanding of how the complex network of interactions that occur within disease suppressive soil microbial communities influences pathogen suppression by evaluating pairwise interactions of community members in vitro. Specifically, I characterized inhibition and nutrient competition among community members and their relationships to pathogen inhibition. Among a random collection of 75 Streptomyces isolates from the rhizosphere soil of potato plants grown in a naturally-occurring scab-suppressive soil in Grand Rapids, MN, 34 isolates were able to inhibit pathogenic Streptomyces scabies strain S87. I hypothesized that isolates would have decreased pathogen inhibition when grown in vitro with an inhibitory partner isolate relative to when grown alone. Similarly, isolates were hypothesized to have decreased pathogen inhibition when grown with highly nutrient competitive partners relative to when grown alone. However, when pathogen-inhibiting isolates were grown in pairs there were no consistent effects of partner inhibition or nutrient competition on pathogen inhibition. These results suggest that antagonistic and resource competitive interactions, while potentially important to the long-term establishment of disease suppressive soil microbiomes, may have limited effects on direct inhibition of pathogens. Moreover, this work suggests that successful biological control of plant diseases may not be limited solely to non-antagonistic inoculant mixtures.