2010 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.
Our research falls within Component 2 of the National Program 303 Action Plan, and particularly to Problem Statement 2C relating to Population Dynamics, Spread, and Epidemiology of Pathogens. Under Subobjective 1A, Hypothesis 1.A.1., we determined the susceptibility of sprouted oak acorns to Phytophthora ramorum. Sprouted acorns of four different Eastern US oak species were exposed to P. ramorum concentrations of 1, 10, 100, and 1000 zoospores per ml for 24 hours. Results showed that a concentration of 1 zoospore per ml was sufficient for some infection to occur on all species. These results aid in understanding the epidemiology of P. ramorum and the data will increase the accuracy of models that examine the potential spread of this pathogen in the Eastern US. We have also examined the susceptibility of seedlings of 4 species of Eastern oak, finding that infected roots can produce inoculum, sometimes at rates equal to that of the control, Viburnum tinus. Such information is important in evaluating the risk P. ramorum poses to Eastern US oak species. Finally, in experiments exposing healthy plants of V. tinus to plants with infected roots systems under Hypothesis 1.A.2, we also demonstrated plant root-to-root spread under flooded conditions. This is the first documentation of such spread and opens up a new area of inquiry and regulatory concern within P. ramorum epidemiology. Under Hypothesis 1.C.a.5, oospores of emerging pathogen Phytophthora kernoviae were embedded in plastic screens, buried in moist sand stored at different temperatures, and tested for viability over time. Oospores were subjected to high temperatures to determine the upper temperature range for their survival. After 1 year, oospores in sand were viable at all temperatures tested up to 30oC. Oospores were no longer viable when exposed for 6 hours at 50oC or 24 hours at 40oC. This research demonstrates the long-term survival potential of P. kernoviae oospores and reveals temperature treatments that will kill oospores for use by regulators. The P. kernoviae research further supports Component 2 of the National Program 303 Action Plan which states that increased knowledge of pathogen survival is needed to develop methods for reducing the spread of disease, both domestically and internationally, and to develop integrated management practices. In further work under Subobjective 1A, chlamydospores of P. ramorum were used to infest field soil at varying densities. Chlamydospore recovery was determined by baiting with rhododendron leaf discs and dilution plating both when soil infestation was performed and following 30 days storage at 4°C, as recommended in the soil and growing medium sampling protocol recommended by USDA-APHIS. Following 30 days of infested soil storage at 4 C, P. ramorum was detected at significantly higher levels than at time 0, providing experimental evidence for the APHIS recommendations. Our research provides the benefits outlined in National Program 303 Action Plan Component 2, in that understanding these processes can lead to the development of methods that prevent disease or reduce its economic threat.
Roots of many plant species support production of spores of Phytophthora ramorum. P. ramorum is a highly destructive pathogen that is spread within forests and nurseries via dispersal of asexual spores. ARS scientists at the Foreign Disease-Weed Science Research laboratory at Ft. Detrick, Maryland evaluated 46 plant species of economic or ecological importance to the Eastern US for their ability to support root infection and production of P. ramorum spores. Counts from tested plant species were compared to those obtained from a positive control, Viburnum tinus and it was found that many species produce high numbers of P. ramorum spores. As a newly recognized means of spore production and dispersal for this destructive pathogen, this information will be valuable for risk evaluation and regulatory decision-making.
Phytophthora ramorum-infested soil may be remediated by Trichoderma spp. Novel methods of control of P. ramorum in nursery production areas are badly needed to prevent serious losses currently suffered by growers in infested counties. ARS scientists at the Foreign Disease-Weed Science Research laboratory at Ft. Detrick, Maryland examined the potential sixteen Trichoderma isolates to parasitize P. ramorum propagules in soil. Nine isolates showing promise for control were selected for further laboratory tests and added to P. ramorum-infested soil. Three isolates reduced P. ramorum populations to non-detectable limits after 2 to 6 weeks. This research demonstrates that biological control has the potential to be effective in remediating P. ramorum-infested soil and the control agent has the potential to be developed into a commercial product.
5.Significant Activities that Support Special Target Populations
Scientists in our group have participated in an activity targeting historically under-served stakeholders. In the summer of 2010, our research group hosted a student from Southern University, an 1890 School (Historically Black Land Grant Institution), for a portion of his PhD thesis work in Urban Forestry using the sudden oak death pathogen, Phytophthora ramorum. This was the second year of the project in which the student continued studies to screen southern tree species for susceptibility to P. ramorum using our facilities and under our tutelage.
Widmer, T.L. 2010. Differentiating Phytophthora spp. isolated from leaves and stems of rhododendron plants. Plant Health Progress. doi: 10.1094/PHP-2010-0317-01-RS.