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United States Department of Agriculture

Agricultural Research Service


Location: Horticultural Crops Research

2011 Annual Report

1a. Objectives (from AD-416)
1.Describe the pathogen biology of exotic, emerging, re-emerging, and invasive plant pathogens affecting horticultural crops. 2.Characterize host ranges and levels of resistance of hosts to exotic, emerging, reemerging,and invasive plant pathogens affecting horticultural crops. 3.Apply knowledge of biology, ecology, and epidemiology to the development of improved integrated disease management approaches.

1b. Approach (from AD-416)
This research will be accomplished through a multifaceted approach integrating the disciplines molecular biology, genomics, population ecology, epidemiology, meteorology, climatology, and microbiology. Initially the project will deal with two recently introduced pathogens, Phytophthora ramorum and Phragmidium violaceum, as well as other Phytophthora, powdery mildew and Botrytis diseases of various horticultural crops. This research project will utilize: novel techniques for rapid and accurate assessment of pathogen presence and abundance in the field; quantitative information on distribution of clones, migration of new strains, degree and rates of out-crossing, and sources of resistance to the introduced pathogens P. ramorum and P. violaceum; elucidation of important genetic traits that impact disease development; and increased understanding of factors influencing disease epidemics that will be used to generate improved disease forecasting models.

3. Progress Report
Substantial progress has been made on several project plan objectives in National Program 303. Under objective 1A, we demonstrated that delaying initiation fungicide application for grape powdery mildew until inoculum is detected in the vineyard air results in 2.3 fewer fungicide applications without causing an increase in disease development. Under objective, 1B we described the evolution and population structure of Phytophthora ramorum documenting migration of novel clones in the United States and worldwide. Our analysis suggests that there were at least 4 global migrations of P. ramorum. We documented that the EU1 clonal lineage migrated from Europe to either British Columbia or Washington. An interactive, searchable database documents emergence of new P. ramorum outbreaks and reports genotype and placement into clonal lineage. We also demonstrated that the American population of Phragmidium violaceum is most closely related to the European population and did not derive from Australia. Under objective 1C progress has been made in terms of documenting the existence of two distinct small RNA classes describing previously unknown genetic mechanisms in the genus Phytophthora. We have characterized biogenesis effectors involved in processing of small RRNAs. As a result of laboratories studies conducted in pursuit of Objective 1D, we developed modifications to the Gubler/Thomas model for grape powdery mildew that were field tested in small plots in both Davis, CA and Corvallis, OR in collaboration with Doug Gubler. Under objective 2, variation in resistance in viburnum and lilac to P. ramorum has been characterized and scientists continue to study the diversity of Phytophthora spp. in nursery environments as a function of plant genotype, season, and cultural practice. All milestones are in progress and most have been fully or substantially met. Research on the development of interpolated weather data systems to aid in disease management decisions demonstrated that interpolated weather was as accurate as weather data collected on site for running disease forecasting models.

4. Accomplishments
1. Phytophthora andina is a species that emerged by hybridization. Recently a new species of Phytophthora, namely P. andina, was described that was identical to the potato late blight pathogen, P. infestans, in the sequence data yet showed a distinctly different host range. We tested the hypothesis that this pathogen emerged via hybridization of two Phytophthora species. ARS scientists in Corvallis, OR, in collaboration with scientists at the University of Florida clearly demonstrated that P. andina is a species derived from hybrid parents P. infestans and an unknown Phytophthora species. This work provides critical insights into mechanism whereby novel plant pathogens emerge.

2. Discovery and description of the new plant pathogen species Phytophthora obscura. Recently, a novel pathogen unknown to science was discovered simultaneously in Germany and Oregon. ARS scientists in Corvallis, OR, in collaboration with a scientist from the Julius Kuhn Institute in Germany described a new Phytophthora pathogen found in Oregon nurseries on kalmia and in Europe associated with horse chestnut. A detailed morphological and phylogenetic analysis provided clear evidence that this is a new species belonging into a newly described subclade 8d of the genus Phytophthora. Knowing the identity of a plant pathogen is critical first step in developing targeted disease management strategies.

Review Publications
Grunwald, N.J., Martin, F.N., Larsen, M.M., Sullivan, C.M., Press, C.M., Coffey, M.D., Hansen Everett, M., Parke, J.L. 2011. a sequence-based Phytophthora identification tool. Plant Disease. 95(3):337-342.

Goss, E.M., Larsen, M.M., Vercauteren, A., Werres, S., Heungens, K., Grunwald, N.J. 2011. Phytophthora ramorum detections in Canada: evidence for migration within North America and from Europe. Phytopathology. 101:166-171.

Cardenas, M., Grajales, A., Sierra, R., Rojas, A., Garavito, M., Lozano, G., Gonzalez-Almario, A., Vargas, A., Marin, M., Fermin, G., Lagos, L.E., Grunwald, N.J., Bernal, A., Salazar, C., Restrepo, S. 2011. Genetic diversity of Phytophthora infestans in the Northern-Andean region. BMC Evolutionary Biology. 12:23.

Wallace, P., Arey, B., Mahaffee, W.F. 2011. Subsurface examination of a foliar biofilm using scanning electron- and focused-ion-beam microscopy. Micron. 42:579-585.

Last Modified: 2/23/2016
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