Profile of Pythium spp. in certified organic fields for vegetable production in central Washington
A. C. ALCALA (1), T. C. Paulitz (2), L. D. Porter (3), L. J. du Toit (1)
(1) Washington State University Mount Vernon NWREC, Mount Vernon, WA, U.S.A.; (2) USDA-ARS Root Disease and Biological Control Research Unit, Pullman, WA, U.S.A.; (3) USDA-ARS, Prosser, WA, U.S.A.
Damping-off is a common disease of many crops, including vegetables. The disease is exacerbated in organic production by the lack of highly effective organic seed treatments. Pythium is the major pathogen typically associated with damping-off in early season plantings in temperate regions, when soil and irrigation water are cold. The objective of this study was to identify Pythium species associated with damping-off in organic vegetable production in the semi-arid Columbia Basin of central Washington, particularly in pea and sweet corn crops. In October 2009, soil samples were collected from 37 certified organic fields with a history of pea and/or sweet corn production. Pythium spp. were baited from the soils using grass leaves and root baiting with pea and sweet corn seeds. Approximately 300 isolates were obtained and identified to species by sequencing the internal transcribed spacer (ITS) region of the rDNA. Nineteen Pythium species were identified, with P. irregulare, P. torulosum, and P. ultimum the most prevalent. Pathogenicity of isolates of each species was tested on pea planted into inoculated soil under cool and wet conditions in a growth chamber. At least six species (P. abappressorium, P. dissotocum, P. irregulare, P. sylvaticum, P. ultimum, and P. violae) caused damping-off, with differences in aggressiveness detected among isolates of each species. Inoculum levels of these species in the sampled soils will be quantified using real-time PCR assays.
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