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United States Department of Agriculture

Agricultural Research Service

Related Topics


Location: Emerging Pests and Pathogens Research

2012 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:
The objective of this research is to reduce crop losses to diseases caused by major species of Phytophthora attacking floral crops: P. nicotianae, P. drechsleri, P. cryptogea, and P. tropicalis on key crops for each pathogen. Milestone 1. Determination of fungicide and biopesticide efficacy in various rotation schemes in greenhouse trials. New fungicides and biopesticides evaluated for Phytophthora in ornamental crops. Several fungicides and biopesticides were evaluated for control of Phytophthora root rot of Gerber daisy caused by P. cryptogea, a frequently encountered pathogen in floriculture. In greenhouse trials at NC State University in Raleigh, researchers applied biopesticides 3 to 5 days before inoculation with P. cryptogea, while fungicides were applied at the time of inoculation. Efficacy of the treatments was assessed by plant growth and root rot ratings at the end of experiments. Phosphite salt fungicides, biopesticides, and strobilurin fungicides were ineffective. Over three years of trials, fluopicolide, fenamidone, and ametoctradin + dimethomorph were consistently effective in disease control when applied as drenches on a 14-day schedule while cyazofamid and dimethomorph always had root rot rating less than in the control. Because the effective fungicides have different modes of action, growers have valuable rotation options for managing Phytophthora root rot and avoiding pathogen resistance in Phytophthora populations. Milestone 2. Estimate of extent for multiple resistance of mefenoxam insensitive isolates to fungicides in other FRAC groups. Isolates of P. cryptogea, P. drechsleri, P. nicotianae, and P. tropicalis collected in previous surveys of North Carolina floriculture facilities were screened for multiple resistance to fungicides that are phosphorus acid generators. Vital (34.3 percent phosphorous acid equivalent; FRAC 33) was incorporated into V8 agar at a final concentration of 500 ppm of H3PO3 and then added to microwells in a 48-well plate. After 2 days mycelia growth in each microwell was rated on a 0-3 scale where 0 was no growth, 1 was a few hyphal strands emerging from agar disk of the Phytophthora spp., 2 was mycelium covering 50% or microwell agar surface, and 3 was mycelium covering entire microwell agar surface. There were two runs for each isolate tested with three observations per run. Four isolates of P. cryptogea from Dusty Miller and gerbera daisy were all sensitive to Vital (rating 0). Nine isolates of P. drechsleri, one from fuchsia and the rest from gerbera daisy, were sensitive to Vital at 500 ppm (rating 0). Of these 9 isolates, all were insensitive to mefenoxam in previous trials. Twenty-two isolates of P. nicotianae from calibrachoa, Dusty miller, Euphorbia ‘Bonfire’, fuchsia, petunia and verbena were all sensitive to Vital (ratings 1 or less). Of the 22 P. nicotianae isolates, 17 were intermediate or insensitive to mefenoxam in previous trials. Four isolates of P. tropicalis from gloxinia, pothos, and verbena were all sensitive to Vital (rating 0), while two of these were intermediate in sensitivity to mefenoxam. Multiple resistance to a phosphorus acid generating fungicide was not detected in isolates of P. drechsleri and P. nicotianae that were insensitive to mefenoxam. Milestone 3. Development of survival and fitness estimates for Phytophthora inoculum in the greenhouse. The fitness of mefenoxam-insensitive isolates of P. nicotianae to survive in greenhouse crop production was tested by introducing an insensitive isolate (either SO845 or SO846) at one end of a planted tray of snapdragon opposite a mefenoxam-sensitive isolate (S1103) of the pathogen at the other end of the tray. After Phytophthora root rot had developed the root system was cut into short 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 the third replanting when disease incidence was near 100% root isolations were made for Phytophthora from each individual snapdragon. Recovered isolates were tested for mefenoxam sensitivity at 100 ppm in a microwell assay method. Mefenoxam insensitive isolates SO845 and SO846 were recovered from 98 percent and 90 percent of the infected snapdragon plants in the respective treatments. Apparently insensitivity to mefenoxam does not impose a fitness penalty on survival and infection, although the trial needs to be repeated. Survival of mefenoxam insensitive and sensitive isolates of P. nicotianae in colonized crop debris left behind in the greenhouse after harvest was tested by placing colonized vinca leaf disks in different zones in the greenhouse. Mesh bags containing colonized leaf disks were placed on and under greenhouse benches with and without watering to simulate survival of the pathogen. Survival of P. nicotianae only occurred in leaf disks placed in wet environments on or under the greenhouse bench, whereas P. nicotianae could not be recovered from leaf disks in a dry environment after a week. In the wet environment where the pathogen survived, leaf disks decomposed rapidly such that after a month no tissue was left to sample. Impact: Since the floriculture industry is faced with Phytophthora populations resistant to the most commonly used fungicide, several alternative Oomycete fungicides were proven effective in disease control thus providing alternative fungicides to avoid disease loss in floriculture crops across the United States. Understanding the genetic makeup of Phytophthora populations in ornamental crops can help growers target specific control practices that have the most direct effect on disease control. Phytophthora cryptogea and P. drechsleri: two pathogens of ornamental crops. Even though Phytophthora cryptogea and P. drechsleri have been described for over 90 years in ornamental crops, identifying these two species has been problematic because they are very similar. Researchers at NC State University in Raleigh, studied the evolutionary history of these two species with isolates previously collected from floriculture crops in North Carolina commercial greenhouses by use of coalescent- and parsimony based analyses, two methods based on the DNA sequence of an isolate. The results suggested that P. cryptogea and P. drechsleri are sister species that shared genetic recombination events in the past as they continue to diverge today. On a practical basis the results demonstrated the recurrence of the same genotype of P. drechsleri in one production facility over multiple years suggesting that carry over of the pathogen was more important than introduction of the pathogen as a cause of epidemics. In this situation, a grower would focus on sanitation practices to eliminate carry over of the pathogen to prevent disease. Publications: Benson, D. M., and Parker, K. C. 2011. Efficacy of fungicides and biopesticides for management of Phytophthora crown and root rot of Gerber daisy. Online. Plant Health Progress doi:10.1094/PHP-2011-0512-01-RS. Olson, H.A., Carbone, I., and Benson, D.M. 2011. Phylogenetic history of Phytophthora cryptogea and Phytophthora drechsleri isolates from floriculture crops in North Carolina greenhouses. Phytopathology 101:1373-1384.

4. Accomplishments

Last Modified: 07/27/2017
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