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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOLOGY AND EPIDEMIOLOGY OF EMERGING PLANT PATHOGENIC OOMYCETES
2011 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.


3.Progress Report
Understanding the soil phase of P. ramorum is integral to preventing spread of the pathogen. To better understand the soil phase, an assay was used to quantify inoculum in runoff from infected plants. This assay is versatile, allowing many types of investigations to be undertaken, including determining the effect of temperature and root age on root infection and inoculum production. Here it was used to determine the relative susceptibility of various eastern riparian plant species, an important task given the repeated introduction of P. ramorum into eastern watersheds. We tested 50 plant species for susceptibility and inoculum production from roots and determined that most plants tested were not very susceptible to the pathogen or did not produce large amounts of inoculum. The assay also proved useful in determining the effect of herbicides on inoculum production from roots of treated plants, a necessary task since herbicides are being used to kill infected trees as part of Oregon's eradication effort. Because it has proved difficult to eradicate P. ramorum from infested nurseries, it is essential to find methods to disinfest nursery media. Due to the proximity of most nurseries to residential areas, use of fumigants such as methyl bromide is difficult, and other safe and effective methods must be found. Two biological control agents and a cultural control method were studied using our assay for quantifying inoculum from infected roots. This research relates to sub-objective #1A of the project.

Chlamydospores of P. ramorum are thought to be a persistent form of the organism, allowing it to survive in soil. Understanding the behavior of chlamydospores will help in the cleanup of infested nurseries. In this performance period, we initiated studies to investigate the effects of plant exudates and pH on chlamydospore germination. Phytophthora kernoviae is a pathogen that is not found presently in the US; however, there is concern of its introduction. We compared sporulation ability of P. kernoviae with that of other common Phytophthora species. Sporangia production of P. kernoviae on Rhododendron, Magnolia tripetala, Liriodendron tulipifera, and Kalmia latifolia was higher than that of P. cactorum and P. syringae. Oospore production on these same hosts was similar or lower for P. kernoviae than that of the other two Phytophthora species. Our findings demonstrate that P. kernoviae, once introduced, could spread rapidly due to its prolific sporulation ability. This research relates to sub-objective #1Ca of the project.


4.Accomplishments
1. Determined the relative susceptability of 50 riparian plant species to infection by Sudden Oak Death determined. As the Sudden Oak Death pathogen, P. ramorum, is found in more and more eastern watersheds, it is necessary to determine what native plant species might be susceptible to root infection by the pathogen, and whether such species might become significant sources of inoculum. Using an assay to quantify inoculum from plants inoculated with P. ramorum, ARS researchers at Ft. Detrick, Maryland, were able to test 50 species for susceptibility and inoculum production from roots. We determined that most plants tested were not very susceptible to the pathogen or did not produce great amounts of inoculum from roots, but that certain genera would be good candidates for special scrutiny, including dogwoods, and Viburnum sp. This information will be useful for regulatory agencies in developing nursery scouting protocols and for the Forest Service in performing perimeter surveys of infested nurseries.

2. Remediation of Phytophthora ramorum-infested soil using biocontrol with Trichoderma asperellum. Methods are badly needed to remediate nursery soils that are infested with the Sudden Oak Death pathogen, Phytophthora ramorum. Laboratory tests conducted by ARS researchers at Ft. Detrick, Maryland, have demonstrated that the biological control agent, Trichoderma asperellum, can reduce P. ramorum soil populations to non-detectable levels within 2 to 4 weeks. We investigated whether T. asperellum is able to reduce P. ramorum soil populations in a nursery setting. T. asperellum was selected and developed into a formulated product that could be applied to larger areas of soil and a research plot was set-up at the Dominican University of California under natural environmental conditions that simulated a commercial plant nursery. Our results show that P. ramorum declined faster in infested microplots treated with the formulated T. asperellum isolate than in non-treated control plots and plots treated with commercially-available biological control products. T. asperellum thus has the potential to be developed into a commercially available product for control of P. ramorum.


Review Publications
Widmer, T.L. 2010. Whole plant inoculations of Viburnum species and cultivars testing for susceptibility to Phytophthora ramorum. Journal of Environmental Horticulture. 28:197-202.

Li, P., Feng, B., Wang, H., Tooley, P.W., Zhang, X. 2011. Isolation of nine Phytophthora capsici pectin methylesterase genes which are differentially expressed in various plant species. Journal of Basic Microbiology. 51:61-70.

Last Modified: 11/23/2014
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