2012 Annual Report
1a.Objectives (from AD-416):
Oomycete plant pathogens cause a wide range of serious diseases of great concern to U.S. agriculture, and some are of recent emergence or reemergence. Obtaining knowledge of the biology, taxonomy, ecology, and pathology of selected diseases caused by members of this group is key to developing improved management practices and pest risk assessments which will allow damage caused by such diseases to be reduced. Our first objective is to determine basic knowledge of the biology, taxonomy, ecology, and pathology of selected Oomycete pathogens as the basis for development of improved control/management strategies and pest risk assessments. Our second objective is to determine the response of selected host species to inoculation with selected Oomycete pathogens to determine susceptibility of plant species of agricultural and commercial importance as well as naturally occurring plant species. Our proposed studies will focus on sudden oak death caused by Phytophthora ramorum and brown stripe downy mildew of maize caused by Sclerophthora rayssiae var. zeae. Attainment of our objectives will benefit U.S. agriculture by contributing new knowledge regarding biology, infectivity, spread potential, and survival ability of destructive Oomycete pathogens thus facilitating the development of improved regulatory, management, and control strategies.
1b.Approach (from AD-416):
Research will be conducted utilizing specialized containment facilities to investigate critical biological factors required to develop improved identification, detection, monitoring, and management strategies for selected Oomycete plant pathogens. We will obtain isolates of selected pathogens and perform containment greenhouse, growth chamber, and laboratory experiments to elucidate their biology, taxonomy, ecology, and pathology. Pathogenicity studies will be performed on selected host plant species to determine levels of susceptibility to emerging Oomycete pathogens.
A two-year study on remediation of Phytophthora ramorum-infested soil under nursery conditions was completed in California. Results showed that an experimental biological control agent consisting of Trichoderma asperellum, grown on wheat bran, was more effective in reducing P. ramorum than two other commercially available biological control products and similarly effective than a chemical control over the 12 week sampling period. Further greenhouse studies demonstrated that this experimental T. asperellum was also effective in reducing mortality of pepper plants caused by soil infestations of P. capsici. A new project was started to examine the role that endophytes might play in protecting plants from P. ramorum infection. Over 600 fungal isolates collected from leaves of rhododendron plants were screened for antagonistic activity towards P. ramorum. Over 100 isolates showed some inhibitory activity and tests have begun to isolate the compound(s) responsible for this inhibition. This may lead to novel compounds that can be used to reduce the spread of P. ramorum. A quantitative assay was developed to measure production of inoculum from roots infected with P. ramorum. This assay was initially used to study epidemiological parameters such as temperature and was then used to determine risk for eastern riparian plant species; currently it is being used to study the effects of herbicides on inoculum production (which are being used to kill infected plants in Oregon) and the effectiveness of fungicides and biological control agents in reducing inoculum levels and root colonization. This research will help in the development of management plans in wild land areas by evaluating different herbicides. It will help in the development of management plans for remediation of infected nurseries by providing methods to remediate nursery potting media and reduce plant-to-plant spread. Nurseries are the means of long-distance spread of this pathogen, and infested nurseries on the East coast are sources of inoculum in steam water. In this performance period, we also initiated studies to investigate the effect of temperatures between 5 and 30 C on germination of chlamydospores of Phytophthora ramorum. Isolates from three different P. ramorum clonal lineages were compared in replicated experiments. The total number of germinated spores was evaluated at 1, 2, 4, 6, and 8 days after plating on selective agar medium. The temperature optimum for growth for all isolates was 20 C, with little or no growth occurring at 5 or 30 C.
Control of sudden oak death. Phytophthora ramorum causes sudden oak death and also seriously impacts the commercial nursery industry due to losses resulting from quarantine issues. The nursery industry badly needs new methods of control of P. ramorum so that infested nurseries can be removed from quarantine status and resume normal production. ARS researchers at Fort Detrick, Maryland demonstrated for the first time in a nursery setting, that the beneficial biocontrol fungus Trichoderma asperellum grown on wheat bran and raked into nursery test-plot soil, can reduce P. ramorum soil populations to non-detectable levels after 6 weeks. California regulatory agents confirmed these results at a commercial nursery, and the nursery was lifted from quarantine status. The new method will have wide applicability in reducing losses to the nursery industry due to P. ramorum, and technology transfer is underway to facilitate development of a commercial formulation of the biocontrol fungus.