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Diversity and Mefenoxam Sensitivity of Phytophthora spp. Associated with the Ornamental Horticulture Industry in the Southeastern United States

January 2013 , Volume 97 , Number  1
Pages  86 - 92

H. A. Olson, Hampton Roads Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, Virginia Beach 23455; S. N. Jeffers, School of Agricultural, Forest, and Environmental Sciences, Clemson University, Clemson, SC 29634; K. L. Ivors, Mountain Horticultural Crops Research and Extension Center, North Carolina State University, Mills River 28759; K. C. Steddom, Texas AgriLife Research and Extension Center, Texas A&M University, Overton 75684; J. L. Williams-Woodward, Department of Plant Pathology, University of Georgia, Athens 30602; M. T. Mmbaga, Otis Floyd Nursery Research Center, Tennessee State University, McMinnville 37110; D. M. Benson, Department of Plant Pathology, North Carolina State University, Raleigh 27695; and C. X. Hong, Hampton Roads Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, Virginia Beach 23455

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Accepted for publication 25 July 2012.

Phytophthora isolates associated with ornamental plants or recovered from irrigation water in six states in the southeastern United States (Georgia, North Carolina, South Carolina, Tennessee, Texas, and Virginia) were identified and screened for sensitivity to mefenoxam. Isolates from forest and suburban streams in Georgia and Virginia were included for comparison. A new in vitro assay, utilizing 48-well tissue culture plates, was used to screen for mefenoxam sensitivity; this assay allowed high throughput of isolates and used less material than the traditional petri plate assay. In total, 1,483 Phytophthora isolates were evaluated, and 27 species were identified with Phytophthora nicotianae, P. hydropathica, and P. gonapodyides, the most abundant species associated with plants, irrigation water, and streams, respectively. Only 6% of isolates associated with plants and 9% from irrigation water were insensitive to mefenoxam at 100 μg a.i./ml. Approximately 78% of insensitive isolates associated with plants were P. nicotianae, and most of these (67%) came from herbaceous annual plants. Most of the insensitive isolates recovered from irrigation water were P. gonapodyides, P. hydropathica, P. megasperma, and P. pini, and 83% of the insensitive isolates from streams were P. gonapodyides. Overall, this study suggests that mefenoxam should continue to be a valuable tool in the management of Phytophthora diseases affecting ornamental plants in the southeastern United States.

© 2013 The American Phytopathological Society