Soilborne microorganisms of Euphorbia are potential biological control agents of the invasive weed leafy spurge

Leafy spurge (Euphorbia esula-virgata), a native of Eurasia, is a serious invasive weed of grasslands of the northern Great Plains of the U.S. and prairie provinces of Canada. Leafy spurge is very difficult to control with herbicides, insect biological control agents, and other cultural practices. Previous field investigations revealed pathogen–insect interactions on the roots of leafy spurge leading to mortality. In order to exploit this synergistic relationship as an effective biological control strategy, we undertook an exploration of Europe for soilborne fungi and rhizosphere bacteria on Euphorbia spp. growing in a wide variety of soils in different landscapes. All microbial cultures were screened for growth suppressive or disease potential on leafy spurge plants or callus tissue. Study objectives were to determine relationships of some edaphic factors and host plant conditions with biological control activity, and to screen rhizobacteria isolated from Euphorbia spp. for traits that might contribute to suppression of leafy spurge growth. The most virulent soilborne fungal strains of Fusarium and Rhizoctonia species, based on greenhouse pathogenicity tests, were isolated from roots of Euphorbia spp. with insect feeding damage. High proportions (>50%) of rhizobacteria were classified as deleterious rhizobacteria (DRB) using a callus tissue bioassay. Euphorbia spp. at sites with high DRB numbers displayed severe fungal disease symptoms and supported insect infestations. Selected soil properties were not correlated with potential biocontrol activity of microbes on leafy spurge; however, insect presence and disease ratings were associated with incidence of growth-suppressive microbes. Certain physiological traits (i.e., exopolysaccharides and hydrogen cyanide production) were good indicators of deleterious activity of rhizobacteria. Our study illustrates that the most effective condition for inducing disease and subsequent mortality of leafy spurge includes a synergism between plant-associated microorganisms and root-damaging insects. Furthermore, the results are valuable for identifying sites for collecting soilborne microorganisms on weeds in their native range for evaluation as biocontrol agents in their invasive range.