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Title: Detection, quantification and vegetative compatibility of Verticillium dahliae in potato and mint production soils in the Columbia Basin of Oregon and Washington

Author
item Omer, Medani
item JOHNSON, D - WASHINGTON STATE UNIV
item DOUHAN, L - OREGON STATE UNIVERSITY
item HAMM, P - OREGON STATE UNIVERSITY
item ROWE, R - OHIO STATE UNIVERSITY

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2008
Publication Date: 7/20/2008
Citation: Omer, M.A., Johnson, D.A., Douhan, L.I., Hamm, P.B., Rowe, R.C. 2008. Detection, quantification and vegetative compatibility of Verticillium dahliae in potato and mint production soils in the Columbia Basin of Oregon and Washington. Plant Disease. 92:1127-1131.

Interpretive Summary: Verticillium dahliae is a widespread soil-borne fungal plant pathogen that causes wilt disease on many important crops. Verticillium dahliae survives in soil in the form of tiny black structures called microsclerotia, which form in the decaying tissues of infected host crops. Detecting the presence and populations of microsclerotia in soils targeted for potato and mint production is important in disease risk assessment. Strains of Verticillium dahliae are identified using a procedure called vegetative compatibility analysis. In this analysis, strains that have the same characteristics are classified in the same vegetative compatibility group (VCG). This study was undertaken to determine the prevalence and population densities of Verticillium dahliae in commercial production soils in the Columbia Basin of Washington and Oregon prior to planting crops of either potatoes or mint and to determine which vegetative compatibility groups (VCGs) of the pathogen were present. V. dahliae was recovered from soil collected from 77 of the 87 sampled fields that were intended for potato production, and from 41 of the 51 sampled fields that were intended for mint production. Nearly all of the 96 strains of Verticillium dahliae from fields intended for potato production belonged to vegetative compatibility group (VCG) 4, with 86% in VCG 4A, 12% in VCG 4B, and 2% in VCG 2B. Among the 60 strains obtained from fields intended for mint production, 77% were in VCG 4A, 13% in VCG 4B, and 10% in VCG 2B. Since VCG 4A strains of V. dahliae are widespread and numerous, particularly following potato production, but cause only mild to moderate symptoms in mint, this strain is unlikely to seriously endanger subsequent plantings of mint. However, planting potatoes in a field recently used to produce mint may pose a significant risk to the potato crop if high populations of the VCG 4A strain (highly aggressive to potato) predominate. Pre-plant assessment of soil populations of Verticillium dahliae without regard to the relative populations of various strains present in a particular sample may lead to information not fully useful in integrated pest management (IPM) systems.

Technical Abstract: Soil samples from 87 fields intended for potato production in the Columbia Basin of Washington and Oregon and 51 fields intended for mint production in Washington were assayed on a semi-selective medium to quantify populations of V. dahliae. The pathogen was isolated from 77 (89%) of the fields intended for potato production and 41 (80%) of the fields intended for mint production. Population densities ranged from 0 to 169 propagules/g of air-dried soil in fields intended for potato production and 0 to 75 propagules/g of air-dried soil in fields intended for mint production. Isolates of V. dahliae were recovered from soil assay plates and pure cultures were prepared to yield a collection of isolates for vegetative compatibility analysis. Nearly all of the 96 isolates of V. dahliae from fields intended for potato production belonged to vegetative compatibility group (VCG) 4, with 86% in VCG 4A, 12% in VCG 4B, and 2% in VCG 2B. Among the 60 isolates obtained from fields intended for mint production, 77% were in VCG 4A, 13% in VCG 4B, and 10% in VCG 2B. Since VCG 4A isolates of V. dahliae cause only mild to moderate symptoms in mint, prior production of potato in a field is not likely to seriously endanger subsequent plantings of mint if that is the pathotype present. However, planting potatoes in a field recently used to produce mint may pose a significant risk to the potato crop if high populations of the VCG 4A pathotype (highly aggressive to potato) predominate. Pre-plant assessment of soil populations of V. dahliae without regard to the relative populations of various pathotypes present in a particular sample may lead to information not fully useful in IPM systems.