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INVESTIGATOR: DeWalt, S.

PERFORMING INSTITUTION:
CLEMSON UNIVERSITY
CLEMSON, SOUTH CAROLINA 29634

GENOTYPIC SPECIFICITY IN PLANT-HERBIVORE INTERACTIONS AND ITS APPLICATION TO MANAGEMENT OF AN INVASIVE WEED

NON-TECHNICAL SUMMARY: Optimal matching of host and pathogen genotypes may improve the efficacy of pathogens released as biological control agents against invasive weeds of agricultural weeds and managed woodlands. Depending on the degree of local adaptation of plants to their pathogens, foreign exploration for weed biological control agents may need to be focused on native provenances of particular genotypes of the invasive plant (i.e. the location from where the weed originated), specifically directed away from the native provenance, or conducted wherever feasible. We are investigating the potential for optimizing future biological control of a globally significant weed using genetic surveys and host specificity trials. Clidemia hirta is recognized as one of the world's worst weeds and is among the gravest invasive threats to tropical island ecosystems and biodiversity. This invasive plant has been a target for biocontrol for decades, but agents released to date have not provided sufficient control. The geographic origins of invasive populations have never been pinpointed, so the potential for developing more effective biocontrol agents optimally adapted to the invasive genotypes is unknown. Close matching of invasive plant genotypes with coevolved natural enemies from their native range is generally thought to be important for successful biocontrol, but the effectiveness of deliberately exact genetic-matching is debated. This is important from a practical point of view because genetic surveys of invasive plants across their native ranges require time and effort, which might be used otherwise in discovery and testing of new biocontrol agents. By better understanding the benefits to be gained from genetic-matching of invasive plants and natural enemies, we will improve our efficiency and success in developing biocontrol.We are testing the utility of this approach with a particularly promising candidate agent, the newly described nematode Ditylenchus gallaeformans. This gall-forming natural enemy has been observed to severely impact growth and reproduction of various species within the plant family Melastomataceae in its native range. We have already conducted extensive genetic surveys across the native range of C. hirta and are doing so with the nematode. We will then compare nematode virulence and plant resistance for a variety of plant-nematode genotype combinations. Besides providing a test of the value of careful genotype-matching to biocontrol research in general, this project is expected to identify optimal sources of a highly effective agent for long-term management of this weed, which is of major importance in forests and agricultural lands of Hawaii, American Samoa, Palau, and other islands and ecosystems across the Pacific region. In addition, this project will serve as a test-case of control between a nematode and melastome plant and will be used to determine how control of other invasive plants in Hawaii and elsewhere may be achieved.

OBJECTIVES: The goal of this project is to address how important coevolution is between natural enemies and their plant hosts in determining resistance and tolerance of plants. Depending on the degree of local adaptation of plants to their herbivores and pathogens (i.e. natural enemies), foreign exploration for weed biological control agents may need to be focused on native provenances of particular genotypes of the invasive plant, specifically directed away from the native provenance, or conducted wherever feasible. We will address this question of coevolution by examining a tropical woody plant and a nematode species, which is a common natural enemy that is being pursued as a biocontrol agent. In addition to supporting development of this promising biocontrol agent against a highly invasive plant, this project will evaluate the general question of whether determining the genetic structure of invasive species and their proposed biocontrol agents in the native range is a necessary first step for developing a biological control program.The supporting objectives of the project are the following:1) quantify distribution of genetic variation in the host plant species and nematode across their native ranges,2) assess whether there are differences in pathogenicity and virulence of the different nematode genotypes on different host genotypes, and3) measure the benefit in terms of greater pathogenicity gained from optimal matching of plant and nematode genotypes.