Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2004
Publication Date: 4/1/2006
Citation: Kremer, R.J., Caesar, A.J., Souissi, T. 2006. Soilborne microorganisms of euphorbia are potential biological control agents of the invasive weed leafy spurge. Applied Soil Ecology. 32(1):27-37. Interpretive Summary: Invasive plant species are non-native plants that have been introduced into and established in the U.S. These plants, entering the U.S. accidentally 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. Invasive plants, or invasive weeds, are very difficult to control and require a combination of control methods to limit their impact and spread. One such 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 using specific insects that damage the extensive root system. We tested whether leafy spurge damage caused by insects could be enhanced with addition of specific fungal and bacterial pathogens. A survey of leafy spurge and closely related plants in their native range in Europe revealed that poorest plant growth was observed when insect feeding on roots was in combination with pathogen infections. We further showed that fungi and bacteria collected from the European specimens selectively inhibited leafy spurge growth, suggesting their potential as biological control agents. We suggest that a multiple agent approach that combines insects and pathogens will greatly improve 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 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 in 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 edaphic factors and host plant conditions with biological control activity; and to screen rhizobacteria isolated from Euphorbia spp. for traits that might contribute to growth suppression of leafy spurge. Greenhouse pathogenicity tests revealed that the most virulent soilborne fungi, Fusarium and Rhizoctonia species, were isolated from insect-damaged roots of Euphorbia spp. High proportions (>50%) of rhizosphere bacteria were classified as deleterious rhizobacteria (DRB) using a callus tissue bioassay. Euphorbia spp. with high DRB numbers coincidentally displayed severe fungal disease symptoms and insect feeding damage. Selected soil properties were not correlated with potential biological control 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 involves plant-associated microorganisms contributing to a synergism with root-damaging insects. Furthermore, the results are valuable for selecting sites for collecting soilborne microorganisms on weeds in their native habitat for subsequent evaluation as biological control agents in similar habitats in their invasive ranges in North America.