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United States Department of Agriculture

Agricultural Research Service


Location: Horticultural Crops Research

2012 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 in 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:
This research project (Project 5358-22000-034-00D Exotic, Emerging, Re-Emerging and Invasive Plant Diseases of Horticultural Crops) was terminated on 6/18/2012 and continued as 5358-22000-039-00D Exotic and Emerging Plant Diseases of Horticultural Crops. Under objective 1A, we demonstrated and transferred technology for the detection of airborne inoculum for grape powdery mildew and the use of this information to time fungicide applications. The research resulted in the savings of 2.3 fungicide applications on average over the past 5 years in commercial vineyards. We are extending this research by developing methods for growers to perform their own molecular detection assays. Under objective 1B, we described the migration patterns of Phytophthora ramorum. We demonstrated that the EU1 clonal lineage emerged via migration from Europe to the Pacific Northwestern United States and Canada. 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. In objective 1D, we examined the impact of leaf wetness and temperature on the epidemiology of blackberry rust, grape powdery mildew, and ramorum blight. These data were then used to develop or refine disease forecasting models or best management practices. For blackberry rust, we demonstrated that most growing seasons will not be conducive to disease development. For grape powdery mildew, modifications to the Gubler/Thomas infection risk model were developed and tested in Oregon and California in collaboration with the research scientists. These results indicate that our model rules for the impact of temperatures above 30C (86F)reduce pesticide usage without impacting disease development, thus further reducing fungicide usage. 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. Under objective 3, we identified the errors associated with weather data interpolation and demonstrated that these errors have minimal impact on disease management decisions for grape powdery mildew. The larger collaborative group has demonstrated similar results for multiple pathosystems.

4. Accomplishments
1. Developed and demonstrated over a two year period that grower performed Loop Mediated Isothermal Amplification can be used to detect grape powdery mildew. This technology significantly reduces the barriers of capital cost and technical skill to using inoculum detection for initiating fungicide application, thus making this approach available to even small acreage growers. This technology is currently being transferred to two commercial entities as well as made directly available to grape growers in Oregon, Washington, and California. The reduction in fungicide will have significant economic savings and enhance environmental quality by reducing impact on water shed, carbon footprint, and air quality.

Review Publications
Vercauteren, A., Larsen, M.M., Goss, E.M., Grunwald, N.J., Maes, M., Heungens, K. 2011. Identification of new polymorphic microsatellite markers in the NA1 and NA2 lineages of Phytophthora ramorum. Mycologia. 103:1245-1249.

Cardenas, M.E., Tabima, J., Fry, W.E., Grunwald, N.J., Bernal, A.J., Restrepo, S.O. 2012. Defining species boundaries in the genus Phytophthora: the case of Phytophthora andina. A response to “Phytophthora andina sp. nov., a newly identified heterothallic pathogen of solanaceous hosts in the Andean highlands." Plant Pathology. 61(2):215-220.

Grunwald, N.J., Garbelotto, M., Goss, E.M., Heungens, K., Prospero, S. 2012. Emergence of the sudden oak death pathogen Phytophthora ramorum. Trends in Microbiology. 20:131-138.

Pfender, W.F., Gent, D.H., Mahaffee, W.F., Coop, L.B., Fox, A. 2011. Decision Aids for Multiple-Decision Disease Management as Affected by Weather Input Errors. Phytopathology. 101:644-653.

Pfender, W.F., Gent, D.H., Mahaffee, W.F. 2012. Sensitivity of disease management decision aids to temperature input errors associated with out-of-canopy and reduced time-resolution measurements. Plant Disease. 96:726-736.

Last Modified: 2/23/2016
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