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

Authors: Kremer, Robert; SOUISSI, THOURAYA - Tunisian National Institute Of Agronomy

Submitted to: Australasian Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2013
Publication Date: 7/15/2013
Citation: Kremer, R.J., Souissi, T. 2013. Phytotoxicity assessment for potential biological control of leafy spurge by soilborne microorganisms. Australasian Plant Pathology. 42(4):441-447.

Interpretive Summary: Invasive plant species are non-native plants that have been introduced into and established in North America. These plants, introduced by early settlers incidentally as contaminating weed seeds or intentionally brought in for agricultural or horticultural uses, greatly reduce agricultural productivity, deteriorate natural habitats, and displace native and endangered plants. A very aggressive and invasive weed is leafy spurge, which infests about 3 million acres of rangeland in the northern Great Plains. It reduces productivity of grazing lands by more than 50% and, in natural areas, crowds out native or desirable plant species and deteriorates wildlife habitat. Leafy spurge cannot be controlled effectively with herbicides, thus land managers must use other methods including biological control. The biological control strategy for leafy spurge has relied primarily on use specific insects that damage the extensive root system. We surveyed numerous leafy spurge plants collected from states in the northern Great Plains and in Canada to obtain specific fungal and bacterial pathogens that could be used additional biological control agents to weaken or kill the weed. Using laboratory tests with in which the collected microorganisms were checked for infection of leafy spurge plant cuttings, we showed that fungi and bacteria inhibited leafy spurge growth, suggesting their potential as biological control agents. We suggest that the microorganisms selected to specifically suppress leafy spurge growth could be applied as a biological preparation or combined other pathogens and insects to greatly improve the effectiveness of biological control and reduce the impact and spread of leafy spurge. Our results have important implications for scientists, extension personnel, and land managers involved in leafy spurge management. The greater efficacy of biological control gained by using multiple organisms may reduce time required to decrease leafy spurge invasions and restore productivity and biological diversity of infested areas.

Technical 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 assure 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. Preliminary lettuce seedling bioassays indicated that 62 and 54% of rhizosphere and 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 preliminary bioassays, were screened directly on leafy spurge cuttings. Culture filtrates of 40% of fungi caused complete chlorosis and leaf wilting. The most effective fungi originated from leafy spurge adventitious shoots. Only intact cells of bacteria were detrimental to leafy spurge, indicating that host-bacterial contact was required for pathogenicity. Results of the survey suggest that leafy spurge rhizospheres and adventitious shoots are good sources of potential biological control microorganisms, which should be considered for inclusion in comprehensive management programs for leafy spurge.