Phytotoxicity assessment for potential biological control of leafy spurge by soilborne microorganisms

Abstract

Leafy spurge (Euphorbia esula-virgata), a native of Eurasia, is a serious invasive weed of western grasslands of North America. It is very difficult and cost-prohibitive to control with herbicides; control by insect biological control agents and cultural practices are minimally effective in suppressing vegetative growth and seed production. Current biological control of leafy spurge with pathogens is primarily with mycoherbicides, which require specific environmental conditions and repeated applications to be effective. Alternative biological control approaches using selected microorganisms to attack roots and adventitious shoots may effectively decrease vigor of leafy spurge without environmental manipulations to ensure control efficacy. Our objectives were to survey leafy spurge accessions and their native soils for associated microorganisms, and to assess these microorganisms for potential biological control. A proposed screening protocol, consisting of primary and secondary assays, effectively detected bacteria and fungi with high phytotoxicity toward leafy spurge. Primary lettuce seedling bioassays indicated that 62 % of rhizosphere and 54 % of endorhizal bacteria significantly (P = 0.05) inhibited root growth, causing necrotic lesions. Over 60 % of fungal isolates bioassayed on rice agar significantly inhibited root growth of lettuce seedlings. The most effective microbial isolates, based on primary bioassays, were screened directly on leafy spurge cuttings in secondary assays. Only intact cells of bacteria were detrimental to leafy spurge, indicating that host-bacterial contact was required for pathogenicity. Culture filtrates of 40 % of the test fungi caused complete chlorosis and leaf wilting. The most effective fungi originated from leafy spurge adventitious shoots. Results of the survey suggest that leafy spurge rhizospheres and adventitious shoots are potential sources of biological control microorganisms, which should be considered for inclusion in comprehensive management programs for leafy spurge.