Submitted to: Crop Protection Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2010
Publication Date: 6/1/2010
Citation: Yakabe, L., Parker, S., Kluepfel, D.A. 2010. Effect of pre-plant soil fumigants on Agrobacterium tumefaciens, pythiaceous species, and subsequent soil recolonization by A. tumefaciens. Crop Protection. 29:583-590. Interpretive Summary:
Technical Abstract: Paradox (Juglans hindsii x J. regia), the dominant rootstock used in the California walnut industry, is susceptible to crown gall, caused by Agrobacterium tumefaciens. In practice, soil fumigation has been a common preplant management strategy for crown gall, but even an industry standard, methyl bromide (MeBr), results in inconsistent disease control. To examine MeBr efficacy and identify potential alternatives, combinations of 1,3-dichloropropene (1,3-D), chloropicrin, iodomethane, dazomet, and metam-sodium were examined. Except for 1,3-D alone, all treatments reduced A. tumefaciens and Phytophthora cactorum populations below detection limits. MeBr eliminated A. tumefaciens populations in buried gall tissue, but a combination of 1,3-D and chloropicrin (TC35) did not. An additional 280 kg/ha of chloropicrin in addition to TC35 was sufficient to eliminate A. tumefaciens populations in buried gall tissue. Of the treatments tested, TC35 was the best alternative to MeBr given its efficacy on soil populations of A. tumefaciens and P. cactorum and potential suppressiveness to soil recolonization by A. tumefaciens. MeBr reduced general aerobic bacterial populations below detection limits producing a temporary biological vacuum. A. tumefaciens reintroduced in soils treated with MeBr and TC35 reached significantly higher populations than in nonfumigated soil, but the population in TC35 treated soil was 100-fold lower than MeBr treated soil 110 d after reintroduction. Increased colonization rates and the high subsequent soil populations could be a mechanism underlying the observed inconsistent crown gall control by MeBr.