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United States Department of Agriculture

Agricultural Research Service

Related Topics


Location: Emerging Pests and Pathogens Research

2011 Annual Report

1a. Objectives (from AD-416)
A. Phytophthora diseases have become a major problem in floral crops throughout the United States in recent years due to movement of infected plant material between greenhouse production facilities and the ability of the pathogen to become established in production facilities once introduced. Surveys of production facilities have identified P. nicotianae as the most commonly occurring Phytophthora sp. on a number of different crops but P. drechsleri, P. cryptogea and P. tropicalis also occur. A high percentage of isolates of P. nicotianae and P. drechsleri are resistant to the most commonly used fungicide, mefenoxam. Phytophthora tropicalis, a recently described species in Hawaii, is becoming more prevalent on many different floral crops across the entire U.S. Floral crop growers need disease management information and tools to reduce or eliminate losses due to Phytophthora diseases. Evaluation of new fungicide chemistry and biopesticides along with novel rotation schemes will be made in greenhouse trials. Cross-resistance of mefenoxam-insensitive isolates of P. nicotianae and P. drechsleri toward new fungicides chemistries will be tested. The persistence of mefenoxam-insensitive and sensitive isolates will be measured in simulated greenhouse productions systems including re-used irrigation water. Aggressiveness of P. tropicalis will be assessed in pathogenicity trials with the reported new hosts of this pathogen. Applications of fungicides for control of plant diseases are potentially disruptive of insect biocontrol programs that include fungus-based biopesticides (mycoinsecticides). An additional objective is to enhance integration of Phytophthora disease management practices with biologically-based IPM programs being developed for thrips, whiteflies, and other insect pests.

1b. Approach (from AD-416)
New management products in rotations with biopesticides to reduce environmental impact will be evaluated against the major species of Phytophthora attacking floral crops: P. nicotianae, P. drechsleri, P. cryptogea, and P. tropicalis on key crops for each pathogen. Greenhouse trials under drip irrigation and uniform disease pressure will allow comparison of different fungicide chemistries and rotation schemes. In vitro screening of new fungicides in different FRAC groups will be tested for cross-resistance with selected mefenoxam-insensitive isolates of P. nicotainae and P. drechsleri from floral crops in a microtiter plate assay. Putative insensitive isolates will be evaluated in greenhouse trials with appropriate floral crops to determine if fungicides in other FRAC groups are still effective in control. Simulated survival trials in our University greenhouse will test the ability of fungicide-sensitive and fungicide-insensitive isolates to survive on bench, floor, and tool surfaces between cropping cycles. P. tropicalis will be investigated in greenhouse pathogenicity trials for variation in aggressiveness toward the host of pathogen isolation compared to aggressiveness on other known floral crop hosts of this Phytophthora species. Laboratory assays and small-scale greenhouse tests conducted by collaborating USDA-ARS researchers in Ithaca, NY will assess compatibilities among fungicides used for Phytophthora control and beneficial fungi used for insect pest management. Project results will be distributed to growers and crop protection specialists through workshops, field days, and publications in print and electronic formats.

3. Progress Report
Milestone 1. Determine fungicide and biopesticide efficacy in various rotation schemes in greenhouse trials. Phytophthora surveys in production facilities in North Carolina over the past 10 years have shown that P. nicotianae, P. drechsleri, P. cryptogea, and P. tropicalis (in descending order of frequency) are the important Phytophthora species attacking floriculture crops. Almost 70% of the P. nicotianae isolates, all P. drechsleri isolates and over 70% of the P. tropicalis isolates were insensitive to the most widely used Oomycete fungicide, mefenoxam (Subdue MAXX). Isolates of a Phytophthora species insensitive to mefenoxam have a high risk of developing cross-resistance to related fungicides. Therefore, newer Oomycete fungicides (with different modes of action) must be evaluated, so that rotation schemes can be recommended to growers with mefenoxam-resistance problems. Two trials with Gerbera daisy in a peat moss-based soilless mix were established with two isolates of P. drechsleri insensitive to mefenoxam. Drenches of several promising Oomycete fungicides were made following inoculation. Both trials ran 6 wk with three applications of fungicides. In both trials, all mefenoxam-treated plants were killed by P. drechsleri, confirming insensitivity of these isolates. Oomycete fungicides tested but failing to control the disease included fluoxastrobin, fosetyl-Al, cyazofamid, dimethomorph, and a combination product, boscalid plus pyraclostrobin. Disease was controlled by fluopicolide, fenamidone, mandipropamid, and a combination product, ametoctradin plus dimethomorph. Additional trials will be established to determine if these effective products have activity against other Phytophthora species with mefenoxam insensitivity. Milestone 2. Estimate extent for cross-resistance of mefenoxam-insensitive isolates to other fungicides. An in vitro protocol is under development to evaluate a sub-set of mefenoxam-insensitive isolates of P. nicotianae, P. drechsleri, and P. tropicalis for cross-resistance to phosphorous acid fungicides. Milestone 3. Develop survival curves for Phytophthora spp. in greenhouse inoculum carry-over trials. The fitness of mefenoxam-insensitive isolates of P. nicotianae to survive in greenhouse crop production was tested by introducing an insensitive isolate at one end of a planted tray of snapdragon opposite a mefenoxam-sensitive isolate of the pathogen at the other end of the tray. After Phytophthora root rot had developed, the top of each plant was removed and the root system was cut up into centimeter-long segments and re-mixed with the soilless mix in the tray. The tray was then replanted with healthy snapdragon seedlings and disease allowed to develop naturally. After a third replanting, root isolations were made for Phytophthora from each individual snapdragon. Recovered isolates will be tested for mefenoxam sensitivity and the proportion of insensitive isolates assessed to determine fitness of insensitive isolates to survive in the absence of mefenoxam applications. Project activities have been monitored by the ADODR via e-mail communications.

4. Accomplishments

Last Modified: 10/20/2017
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