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Title: Pythium species associated with damping-off of pea in certified organic fields in the Columbia Basin of central Washington

Author
item ALCALA, ANA VIDA - Washington State University
item Paulitz, Timothy
item SCHROEDER, KURTIS - Washington State University
item Porter, Lyndon
item DERIE, M - Washington State University
item DU TOIT, LINDSEY - Washington State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2015
Publication Date: 5/1/2016
Citation: Alcala, A.C., Paulitz, T.C., Schroeder, K., Porter, L., Derie, M.L., Du Toit, L.J. 2016. Pythium species associated with damping-off of pea in certified organic fields in the Columbia Basin of central Washington. Plant Disease. 100:916-925.

Interpretive Summary: A survey of Pythum species was conducted in irrigated organic processing pea fields in the Columbia Basin of Washington in 2009. 16 species were identified by sequencing and were tested for pathogenicy in the greenhouse. Eight were pathogenic on peas, causing damping-off. The most predominant pathogenic species were P. ultimum, P. irregulare complex and P. abappressorium. Using quantitative PCR, P. ultimum was detected in all fields ranging from 14 to 332 propagules per gram. P. abappressorium was found in 78% of the fields, but at much lower populations. P irregulare complex was found in 57% of the fields. Damping-off of peas may be associated with multiple Pythium species in cool, wet conditions of early spring.

Technical Abstract: A survey of Pythium species in organic vegetable production areas of the semi-arid Columbia Basin of central Washington was carried out in the fall of 2009 to identify species associated with damping-off during early spring planting. Isolates (n = 305) baited from soil sampled from 37 certified organic fields were identified to 16 Pythium species by sequencing the internal transcribed spacer region of ribosomal DNA. Representative isolates of each of the 16 species were tested for pathogenicity on pea in growth chamber conditions that mimicked cool (xx and xxoC by day and night, respectively) early spring conditions. Isolates of eight of the Pythium species were pathogenic on pea, causing damping-off. Isolates within and among these species displayed a range in virulence on pea. The three predominant pathogenic species, P. abappressorium, P. irregulare complex, and P. ultimum, were quantified in soil sampled from the 37 organic fields using real time PCR assays, with P. ultimum was detected in all 37 fields, followed by P. abappressorium (78% of the fields), and the P. irregulare complex (57%). A soil standard curve was developed for each of the three species, and regression analysis used to determine the relationship between DNA concentration and the concentration of propagules in soil. P. ultimum ranged from 14 to 332 CFU/g soil in the 37 fields, the P. irregulare complex ranged from 25 to 228 CFU/g soil, and P. abappressorium DNA was significantly below the quantifiable limit. In summary, P. ultimum was the most prevalent pathogenic species on pea detected in certified organic fields in the semi-arid Columbia Basin of central Washington, but multiple species appear to be associated with damping-off in cool and wet early spring planting conditions.