Location: Foreign Disease-Weed Science Research
2018 Annual Report
Objectives
Objective 1: Develop immuno-diagnostic assays for detection of Phytophthora ramorum and other emerging Phytophthora species in host plants as well as nursery irrigation and aquatic environmental samples. [NP303, C1, PS1]
Subobjective 1A: Develop a molecular detection assay for Phytophthora ramorum based on cell wall-specific proteins.
Objective 2: Develop formulations of antagonistic microorganisms including Trichoderma spp. that can be used as a management tool to reduce soil and leaf populations of Phytophthora ramorum and other emerging Phytophthora species. [NP303, C2, PS2]
Subobjective 2A. Investigate the use of biocontrol agents against selected Phytophthora species, seek to enhance their effectiveness, and identify new biocontrol agents.
Objective 3: Characterize the biology and epidemiology of emerging plant diseases caused by oomycetes such as Plasmopara obducens (Impatiens downy mildew) as the basis for improved disease management strategies. [NP303, C2, PS2C]
Subobjective 3A: Determine key characteristics of epidemiology of emerging oomycete plant pathogens.
Subobjective 3B: Determine the risk potential posed by exotic oomycete species to US agricultural plant species.
Approach
Using specialized containment facilities, we will obtain data in key research areas to assist in detecting and managing emerging oomycete pathogens including Phytophthora ramorum, P. kernoviae, and Plasmopara obducens. For pathogen detection, our approach is to develop enzyme-linked immunosorbent and lateral flow device immunoassays for detection of P. ramorum in plant products and nursery irrigation and runoff samples. We will also continue to develop formulations of antagonistic microorganisms including Trichoderma sp. that can be used as a management tool to reduce soil and leaf populations of Phytophthora ramorum and other emerging Phytophthora species, and will seek to identify new biocontrol agents. To elucidate key characteristics of epidemiology of emerging oomycete pathogens, we will use a variety of experimental approaches in specialized laboratory and greenhouse facilities. We will determine the nature of systemic infection of Impatiens sp. by Plasmopara obducens, and whether the pathogen is seed-borne. We will also define parameters for sporulation and survival of P. ramorum on a key host plant species, and improve Phytophthora kernoviae detection in soil using plant leaves as bait. Understanding key features of biology, epidemiology, and detection will contribute to development of improved management practices and recommendations. Our results will also provide a scientific basis for development of nursery industry best management practices aimed at minimizing disease outbreaks and enhancing interstate commerce.
Progress Report
Progress was made under all three of our objectives, all of which fall under National Program 303 Component 1, Problem Statement 1: Diagnostics, Etiology, Genomics and Systematics of Plant Disease and Associated Microbes and Component 2, Problem Statement 2C: Ecology and Epidemiology of Plant Diseases. Under Objective 1, Sub-objective 1A of our project plan, we applied a proteomic approach to identify proteins secreted by Phytophthora ramorum. From these, we selected extracellular protein candidates to serve as antigens for generation of Phytophthora genus-level antibodies to use in the development of immunoassays to detect P. ramorum in nursery irrigation and surface water samples. To date, rabbit polyclonal antibodies have been generated and are in testing for specificity and sensitivity in the laboratory and at the National Ornamental Research Site at Dominican University of Califormia, in San Rafael, California. We cooperated with scientists from USDA, APHIS, CPHST (Center for Plant Health Science and Technology) and PathSensors, Inc. to formally validate a diagnostic biosensor instrument capable of detecting Phytophthora spp. at the genus-level in infected leaf samples. Under Objective 3, Subobjective 3A, we determined that the temperature at which Phytophthora ramorum sporangia are produced had a large influence on subsequent production of motile zoospores. Sporangia produced at 20°C showed greater zoospore numbers compared with sporangia produced at 15°C. Greatest numbers of zoospores over both temperatures were produced between 8°C and 20°C. Also under Objective 3, Subobjective 3A, we made significant progress in understanding the life cycle of Impatiens downy mildew (IDM), a newly emerging disease causing significant losses to the ornamentals industry. Thick-walled resting spores (oospores) were used to infect germinating seed of Impatiens walleriana, demonstrating that oospore infection was possible. Systemic infection (infection inside the plant) was observed in Impatiens walleriana plants inoculated with the pathogen on the roots. Pods from infected plants were found to be frequently infected with mycelium and oospores of the pathogen. The pathogen was also found to overwinter in infected debris. Finally, we performed a study that demonstrated that detached leaves of Rhododendron spp. were the best baits to use for soil detection of Phytophthora kernoviae, a foreign tree and shrub pathogen of quarantine significance. Using rhododendron leaf baits, P. kernoviae was detected at levels as low as 1 oospore per cubic centimeter of soil. Five different Rhododendron spp. or cultivars were tested and showed similar levels of sensitivity in the baiting asay. Other leaf baits, including four species of Viburnum, one Camellia sp., Kalmia latifolia, and Magnolia stellata, varied in the detection limits but most consistently were able to detect minimum levels at 10 oospores per cm3 soil. We also determined that low soil pH (<4) inhibited germination of P. kernoviae oospores and therefore prevented detection by the leaf bait assay. These results will substantially improve our detection ability for P. kernoviae should it enter the U.S.
Accomplishments
1. Progress in defining the life cycle of the Impatiens Downy Mildew (IDM) pathogen. Impatiens is the most popular bedding plant in America, with sales estimated at more than $115 million in 2014. The first major outbreaks of the damaging disease Impatiens downy mildew in the U.S. were observed in 2011 and by 2014, downy mildew had been reported damaging Impatiens from 34 states. It is very important to understand the life cycle of the pathogen (Plasmopara obducens) in order to give good control recommendations to growers and gardeners. Our work has shown that oospores (thick-walled resting structures) require only one mating type to be formed, that cold-conditioning leads to oospore germination, that conditioned oospores can infect Impatiens plants, and that oospores in infested debris that has overwintered outdoors can infect germinating Impatiens seeds. We also showed that infected plants can produce pathogen-infested seeds that can spread the disease when planted. Our work has provided new knowledge of pathogen biology that strongly suggests that IDM in addition to being wind-borne, can also overwinter in infested garden beds. Our findings have resulted in the implementation of new management guidelines for growers, extension agents and homeowners to prevent the serious losses posed by this disease.
Review Publications
Widmer, T.L., Johnson-Brousseau, S., Kosta, K., Ghosh, S., Schweigkofler, W., Sharma, S., Suslow, K. 2017. Remediation of Phytophthora ramorum-infested soil with Trichoderma asperellum isolate 04-22 under ornamental nursery conditions. Biological Control. 118:67-73.
Widmer, T.L., Mcmahon, M.B., Frederick, R.D. 2018. Phytophthora stricta isolated from Rhododendron maximum in Pennsylvania. Plant Disease. 102:827.
Salgado-Salazar, C., Shishkoff, N., Daughtrey, M.L., Palmer, C., Crouch, J. 2018. Downy mildew: a serious disease threat to rose health worldwide. Plant Disease. https://doi.org/10.1094/PDIS-12-17-1968-FE.
Tooley, P.W., Browning, M.E. 2018. Sporangia production by Phytophthora ramorum on Rhododendron 'Cunningham's White' at different relative humidities. Phytopathology. 108:721-729. https://doi.org/10.1094/PHYTO-10-17-0338-R.
