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United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Disease Management and Characterization of Important Phytophthora Species in Floral Crops

Location: Biological Integrated Pest Management Unit

2013 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 fitness of mefenoxam-insensitive isolates of P. nicotianae to survive in greenhouse crop production was tested by introducing an insensitive isolate (either S0845 or S0846) 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. In a previous experiment, mefenoxam insensitive isolates S0845 and S0846 were recovered from 98% and 90% of the infected snapdragon plants in the respective treatments. In a second experiment, the percentage of mefenoxam insensitive isolates was only 56% (isolate S0845) and 75% (isolate S0846) after three replantings. At the fourth replanting a mefenoxam drench was applied, but severe disease developed within 14 days. The experiments suggest that mefenoxam insensitivity does not impose a fitness penalty to P. nicotianae. Previously, 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. In a second experiment, survival of P. nicotanae 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 regardless of mefenoxam isolate status. In the wet environment, the pathogen survived better under the greenhouse bench where infected disks stayed moister than on the wire-mesh bench but even so survival was almost nil after 27 days regardless of mefenoxam isolate status.

A colonized rice grain inoculum of P. tropicalis was used to inoculate a number of potential new bedding plant species to determine the host range of this emerging pathogen. Plant species and cultivars inoculated in several different experiments included bacopa (Bacopa cordata ‘Bahia White Night’), bidens (Bidens ferulifolium ‘Bidy Gonzales’), cuphea (Cuphea hyssoppifolia ‘Cuphea White’), diascia (Diascia barberae ‘Genta Antique Red’), lobelia (Lobelia sp. ‘Bella Acqua’), nemesia (Nemesia sp. ‘Nemo Ruby’), plectranthus (Plectranthus coleoides ‘Cerveza ‘n Lime’), garden phlox (Phlox maculata ‘Phloxy Lady Purple Sky’), Calocephalus brownii compact, Heliotrope arborescens ‘Nagano’ Alternanthera ficoidea ‘Red Carpet’ Artemisia ‘Parum d’Ethiopia’ Lamiastrum galeobdolom ‘Herman’s Pride’, Sanvitalia ‘Sunvy Super Gold’, Portulaca ‘Cupcake Grape Jelly’, Torenia ‘Purple Moon’, Helichrysum petiolare ‘Silver’, and Lysimachia nummularia ‘Goldilocks.’ Only diascia and nemesia developed symptoms of Phytophthora root rot when inoculated with P. tropicalis. Thus with the exception of these two genera, P, tropicalis should not be a constraint to production of these potential bedding plants.

Publications: Olson, H. A., Benson, D. M. 2013. Host specificity and variations in aggressiveness of North Carolina isolates of Phytophthora cryptogea and P. drechsleri in greenhouse ornamental plants. Plant Dis. 97:74-80. Olson, H. A., Jeffers, S. N., Ivors, K. L., Steddom, K. C., Williams-Woodward, J. L., Mmbaga, M. T., Benson, D. M., Hong, C. X. 2013. Diversity and mefenoxam sensitivity of Phytophthora spp. associated with the ornamental horticulture industry in the southeastern United States. Plant Dis. 97:86-92. Parker, K. C., Benson, D. M. 2013. Efficacy of selected fungicides for control of Pythium root rot on poinsettia, 2012. Plant Dis. Man. Reps. 7:OT004.


Last Modified: 10/21/2014
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