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United States Department of Agriculture

Agricultural Research Service


Location: Horticultural Crops Research

2009 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. Formerly 5358-22000-024-00D (2/03). FY06 $59,4000 Program Increase (memo #139). Formerly 5358-22000-030-00D (2/07).

3. Progress Report
We have been field testing the qPCR method for quantifying E. necator inoculum in 13 vineyards and 18 different traps since April 2009. We have developed a method for quantifying inhibition of the qPCR reaction and can use this method to obtain better estimates of inoculum concentration. This is our second year of validation of inoculum detection for timing fungicide applications in commercial vineyards. Results are similar to 2008 where 2-3 fungicide applications were not made with no difference in disease development between reduced application and growers standard fungicide program. We have developed SSR markers for describing the Phragmidium violaceum population and demonstrated that U.S. population is more diverse than the Australian population and is more closely related to the European population. These data indicate that P. violaceum was either introduced in to the United States several years prior to its discovery in 2005, or numerous introductions from different inoculum sources were made. Modifications to the Gubler/Thomas model for infection risk of grape powdery mildew to more accurately reflect the impact of high temperature on disease development are being tested in the field for a second year. Efforts to test a blackberry rust model have been hampered by the lack of favorable weather epidemic development for the past three years; however, the model has correctly indicated that conditions were unfavorable. Based on an analysis of climate data and discussion with stakeholders, this research is being terminated to pursue other research directions that will yield greater benefit. Objective 3 is currently being validated at 5 vineyards locations across the Willamette valley Oregon as well as at 9 other locations in Oregon and Washington by cooperators. We developed a new technique for examining microbial colonization of leaf surfaces. We demonstrated that a focus ion beam can be used to cut samples placed in scanning electron microscope to examine the 3-D structure of microbial biofilms. Substantial progress has been made on several project plan objectives. We described the evolution and population structure of Phytophthora ramorum documenting migration of novel clones in the United States and demonstrated that the three known clonal lineages of P. ramorum evolved in geographic isolation over long evolutionary times predating the emergence of agriculture before they emerged as significant pathogens in the United States. An interactive, searchable database documents emergence of new P. ramorum outbreaks and reports genotype and placement into clonal lineage. 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 continue to determine leaf wetness and temperature conditions under which P. ramorum can infect viburnum and lilac. 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.

4. Accomplishments
1. Development of Blackberry Rust Management Recommendations. Blackberry rust caused by Phragmidium violaceum can cause severe economic damage in several commercial blackberry cultivars. ARS scientists in Corvallis, OR developed new knowledge of the disease epidemiology, pathogen biology, diversity, efficiency and timing of fungicides to develop an economical management system for Blackberry rust. The management program consists of shifting a dormant lime-sulfur application from the winter to 2 to 3 weeks prior to bud break, and a fungicide application prior to bloom if disease monitoring of the crop and adjacent feral blackberries indicates it is warranted. The increased understanding of the pathogen biology indicates that weather conditions are unlikely to be favorable to the development of severe epidemics in most years. These recommendations have significantly reduced the cost of control and reduced the amount of fungicides being used.

2. Population genetic analysis infers migration pathways of Phytophthora ramorum in U.S. nurseries. Phytophthora ramorum is the exotic pathogen that is responsible for sudden oak death in California forests and ramorum blight of common ornamentals. ARS scientists in Corvallis, OR conducted a population analysis to infer migration routes of Phytophthora ramorum in US nurseries. The nursery trade has moved this pathogen from source populations on the West Coast to locations across the United States, thus risking introduction to other native forests. Two eastward migration pathways were revealed, one containing isolates from Connecticut, Oregon, and Washington and the other isolates from California and the remaining states. This work has implications for eradication and management practices of the pathogen in U.S. nurseries.

Review Publications
Peetz, A., Mahaffee, W.F., Gent, D.H. 2009. Effect of temperature on the sporulation and infectivity of Podosphaera macularis on Humulus lupulus. Plant Disease. 93:281-286.

Gent, D.H., Turechek, W., Mahaffee, W.F. 2008. Spatial and Temporal Stability of the Estimated Parameters of the Binary Power Law. Phytopathology. 98:1107-1117.

Mahaffee, W.F., Pethybridge, S.J.,Gent, D.H. editors. 2009. Compendium of hop diseases and pests. St. Paul, MN: APS Press. 93 p.

Mahaffee, W.F., Pethybridge, S. 2009. The genus Humulus. In: Mahaffee, W.F., Pethybridge, S.J., and Gent, D.H. editors. Compendium of Hop Diseases and Pests. St. Paul, MN: APS Press. p. 1-5.

Mahaffee, W.F., Englhard, B. 2009. Gray mold. In: Mahaffee, W.F., Pethybridge, S.J., and Gent, D.H. editors. Compendium of Hop Diseases and Pests. St. Paul, MN: APS Press. p. 24-25.

Mahaffee, W.F., Englhard, B., Gent, D.H. 2009. Powdery mildew. In: Mahaffee, W.F., Pethybridge, S.J., and Gent, D.H. editors. Compendium of Hop Diseases and Pests. St. Paul, MN: APS Press. p. 25-31.

Mahaffee, W.F. 2009. Cone early maturity. In: Mahaffee, W.F., Pethybridge, S.J., and Gent, D.H. editors. Compendium of Hop Diseases and Pests. St. Paul, MN: APS Press. p. 73-74.

Mahaffee, W.F., Pethybridge, S., Gent, D.H. 2009. Nutrient imbalances. In: Mahaffee, W.F., Pethybridge, S.J., and Gent, D.H. editors. Compendium of Hop Diseases and Pests. St. Paul, MN: APS Press. p. 75-77.

Mahaffee, W.F., Pethybridge, S., Gent, D.H. 2009. Injuries caused by environmental factors. In: Mahaffee, W.F., Pethybridge, S.J., and Gent, D.H. editors. Compendium of Hop Diseases and Pests. St. Paul, MN: APS Press. p.77-78.

Gent, D.H., Pethybridge, S., Mahaffee, W.F. 2009. Chemical injury. In: Mahaffee, W.F., Pethybridge, S.J., and Gent, D.H. editors. Compendium of Hop Diseases and Pests. St. Paul, MN: APS Press. p. 78-80.

Grunwald, N.J., Goss, E.M. 2009. Evolution and genetics of the invasive sudden oak death pathogen Phytophthora ramorum. In: Lamour, K. and Kamoun, S. editors. Oomycete Genetics and Genomics: Diversity, Interactions, and Research Tools. Hoboken, NJ. John Wiley and Sons Inc. p.179-196.

Grunwald, N.J., Davis, E.A., Rolfe, K.J. 2009. Efficacy of registered and unregistered fungicides for control of ramorum blight on camellia. Plant Disease Management Reports. 2:OT015.

Grunwald, N.J., Goss, E.M., Press, C.M. 2008. Phytophthora ramorum: A pathogen with a remarkably wide host range causing sudden oak death on oaks and ramorum blight on woody ornamentals. Molecular Plant Pathology. 9(6):729-740.

Grunwald, N.J., Goss, E.M., Ivors, K., Garbelotto, M., Martin, F.N., Prospero, S., Hansen, E., Bonants, P., Hamelin, R., Chastagner, G., Werres, S., Rizzo, D.M., Abad, G., Beales, P., Bilodeau, G.J., Blomquist, C., Brasier, C., Briere, S., Chandelier, A., Davidson, J.M., Denman, S., Elliott, M., Fichtner, E.J., Frankel, S.J., Goheen, E.M., De Gruyter, H., Heungens, K., James, D., Kanaskie, A., Mcwilliams, M., Moralejo, E., Osterbauer, N.K., Palm, M.E., Parke, J.L., Shamoun, S.F., Shishkoff, N., Tooley, P.W., Vettraino, A., Webber, J., Widmer, T.L. 2009. Standardizing the nomenclature for clonal lineages of the sudden oak death pathogen, Phytophthora ramorum. Phytopathology. 99:792-795.

Malvick, D.K., Grunwald, N.J., Dyer, A.T. 2009. Population structure, races, and host range of Aphanomyces euteiches from alfalfa production fields in the central U.S.A. European Journal of Plant Pathology. 123(2):171-182.

Prospero, S., Grunwald, N.J., Winton, L.M., Hansen, E.M. 2009. Migration patterns of the emerging plant pathogen Phytophthora ramorum on the west coast of the United States of America. Phytopathology. 99:739-749.

Elliott, M., Sumampong, G., Varga, A., Shamoun, S.F., James, D., Masri, S., Briere, S.C., Grunwald, N.J. 2009. PCR-RFLP markers identify threelineages of the North American and European populations of Phytophthora ramorum. Forest Pathology. 39:266-278.

Last Modified: 2/23/2016
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