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Funded Project

Funding Program: Regional IPM Grants (S-RIPM)

Project Title: Oomyceticide resistance and Phytophthora disease management for the ornamental horticulture industry

Project Director (PD): Chuanxue Hong [1]

Lead State: VA Lead Organization: Virginia Tech

Cooperating State(s): North Carolina, South Carolina, Texas

Undesignated Funding: $129,906

Start Date: Jul-01-2010 End Date: Jun-30-2012

Summary: This research project with a budget of $129,906 aims to enhance the profitability of nursery and floriculture industry and the aesthetic value of horticultural products in recreational environments and residential/public areas by improving the sustainability of IPM programs. It targets oomyceticides (i.e., fungicides specially-active against oomycetes) which are essential tools for protecting the regions multibillion-dollar ornamental horticulture industry from diseases caused by Phytophthora spp. To effectively address this emerging oomyceticide resistance problem, we have formed a regional working group including cooperators from Florida, Georgia and Tennessee. Our efforts will focus on identifying and containing resistant populations, eliminating ineffective treatments, and preventing sensitive populations from developing resistance. Specifically, lab assays will be performed to determine how widespread and severe mefenoxam- and phosphiteresistance is in the southeast and the association with geographic location, host plant, and substrate; if this resistance originated from isolates in nature before these compounds were introduced to the market; and whether populations resistant to one compound are likely to develop resistance to another. Field/greenhouse trials will be conducted to assess whether mefenoxam-resistance buildup in P. nicotianae is attributed to the fitness and competitiveness of resistant over sensitive populations and repeated exposures; and whether this model system applies to other species. Additional experiments will be conducted to evaluate the risk of major species to develop resistance to seven newly-labeled compounds and those pending labels. We will partner with the industry and use the new knowledge to formulate and implement anti-resistance strategies and sustainable IPM programs for Phytophthora disease management. Objectives: 1. Assess and compare the scope and degree of mefenoxam resistance among species and populations of Phytophthora recovered from nursery and floral crops, associated soil and surrounding forests, irrigation reservoirs and natural waterways in the southern region 2. Test the hypothesis that buildup of mefenoxam resistance in P. nicotianae is attributed to the fitness and competitiveness of resistant over sensitive populations as well as repeated exposures to the chemical in fields. Validate the applicability of this model system to other species of Phytophthora 3. Evaluate phosphite sensitivity in Phytophthora species and investigate the relationship between phosphite and mefenoxamresistant populations 4. Determine the risk of other registered compounds to lose efficacy to those mefenoxamresistant Phytophthora populations 5. Partner with the horticultural industry in formulation and implementation of science-based, antiresistance and Phytophthora diseasemanagement strategies for improved nursery and floral crop health and productivity, and environmental sustainability.

Final Report:

Results From report submitted by the PI to USDA CRIS report system PROGRESS: 2010/07 TO 2011/06 OUTPUTS: Resistance to mefenoxam, a fungicide frequently used for the management of Phytophthora diseases in the ornamental industry, has been identified as a growing concern within the industry. In order to efficiently screen large culture collections, a high-throughput assay using 48-well culture plates was developed to evaluate sensitivity to mefenoxam. Phytophthora isolates were collected from ornamental plants, irrigation water, and natural streams and soil in the southeastern United States. To date, 1,030 isolates of Phytophthora from Georgia (GA), North Carolina (NC), South Carolina (SC), Tennessee (TN), Texas (TX), and Virginia (VA) have been identified to species and screened for sensitivity to 100 ppm of mefenoxam. These isolates belonged to 23 species, and 95 of them were insensitive to mefenoxam. These insensitive isolates were identified from P. citricola, P. drechsleri, P. gonapodyides, P. hydropathica, P. insolita, P. megasperma, P. nicotianae, P. PgChlamydo, and P. pini. The insensitive isolates originated predominantly from ornamental plants and irrigation water; however, nine isolates of P. gonapodyides from GA and one isolate of P. hydropathica and one isolate of P. megasperma from VA collected from streams also were insensitive to mefenoxam. Additional isolates have been screened for mefenoxam insensitivity; however, their identities are yet to be determined. To validate the mefenoxam insensitivity assay, isolates of P. nicotianae collected from annual vinca identified as insensitive and sensitive to mefenoxam were inoculated onto annual vinca plants treated with the label rate of mefenoxam (Subdue Maxx). Only mefenoxam-insensitive isolates caused significant foliar blight on annual vinca plants treated with mefenoxam, supporting the in vitro mefenoxam assay. In many plant pathogens, pesticide resistance reduces the fitness of the organism in the absence of the pesticide. The fitness of mefenoxam-insensitive and mefenoxam-sensitive isolates of P. nicotianae in the absence of mefenoxam was evaluated. After three subsequent plantings of snapdragons into potting mix infested with the isolates, the population structure of this pathogen isolated from the snapdragon roots was comprised of 88 to 100% mefenoxam-insensitive isolates, suggesting that mefenoxam-insensitive isolates are more fit and better able to compete for infection sites than mefenoxam-sensitive isolates. The mefenoxam-screening assay was adapted to evaluate isolates for sensitivity to 500 ppm of phosphorous acid, another extensively used compound for Phytophthora disease management. A lab-generated phosphite-insensitive isolate of P. capsici was obtained for the insensitive control. Initially, 28 mefenoxam-insensitive Phytophthora isolates from VA were screened during the assay development. Of these, three isolates of P. pini were insensitive. Currently, all participating research labs are assessing the phosphorous acid-screening protocol. PARTICIPANTS: PD: Dr. Chuan Hong, Virginia Polytechnic Institute and State University, Virginia Beach, VA 23455, email: chhong2@vt.edu; CoPD: Dr. D. Michael Benson, North Carolina State University, Raleigh, email: mike benson@ncsu.edu; CoPD: Dr. Steven Jeffers, Clemson University, Clemson, SC 29634, email: sjffrs@clemson.edu; CoPD: Dr. Kelly Ivors, North Carolina State University, Mills River, NC 28759, email: kelly ivors@ncsu.edu; CoPD: Dr. Karl Steddom, Texas A&M University, Overton, TX 75684, email: ksteddom@tamu.edu; Investigator: Dr. Heather Olson, Virginia Polytechnic Institute and State University, Virginia Beach, VA 23455, email: heather.a.olson@gmail.com; Cooperator: Dr. Margaret Mmbaga, Tennessee State University, McMinnville, TN 37110, email: mmmbaga@tnstate.edu; Cooperator: Dr. Jean WilliamsWoodward, University of Georgia, Athens, GA 30602, email: jwoodwar@uga.edu. Web conferences involving all project individuals have been held every three months to discuss project results. TARGET AUDIENCES: Extension activities targeting crop consultants and crop producers within the ornamental horticulture industry have focused on emphasizing the potential for mefenoxam resistance and the importance of crop rotation for mitigating resistance development in Phytophthora. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period. IMPACT: 2010/07 TO 2011/06 The ornamental horticulture industry involves extensive plant movement between production facilities within a state and between states. This plant movement allows the spread of Phytophthora throughout the industry. By studying mefenoxam resistance at a regional level, the spread of mefenoxam-resistant isolates of Phytophthora can be reduced. Identifying the scope and degree of mefenoxam resistance in the ornamental horticulture industry has been crucial in directing proper use of this compound for disease management and improving its overall efficacy. It has helped avoid reapplication of this compound where resistant populations have been identified, mitigating crop loss risk while preventing its unnecessary release into the environment. By rotating with new chemistries, crop health risk is lessened and oomyceticide resistance development is slowed. Most importantly, growers better understand the importance of resistance management strategies and avoid repeated use of the same compounds, thus reducing the risk of Phytophthora developing resistance to new chemicals.

Outcomes N/A

Impacts From report submitted by the PI to USDA CRIS report system The ornamental horticulture industry involves extensive plant movement between production facilities within a state and between states. This plant movement allows the spread of Phytophthora throughout the industry. By studying mefenoxam resistance at a regional level, the spread of mefenoxam-resistant isolates of Phytophthora can be reduced. Identifying the scope and degree of mefenoxam resistance in the ornamental horticulture industry has been crucial in directing proper use of this compound for disease management and improving its overall efficacy. It has helped avoid reapplication of this compound where resistant populations have been identified, mitigating crop loss risk while preventing its unnecessary release into the environment. By rotating with new chemistries, crop health risk is lessened and oomyceticide resistance development is slowed. Most importantly, growers better understand the importance of resistance management strategies and avoid repeated use of the same compounds, thus reducing the risk of Phytophthora developing resistance to new chemicals.

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