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Project description

Microbial analysis of walnut replant suppression. (05AW002)
Program UC IPM competitive research grants program
J.O. Becker, Nematology, UC Riverside
J. Borneman, Plant Pathology, UC Riverside
Host/habitat Walnuts
Pest Unspecified Pathogens and Diseases
Disciplines Plant Pathology, Soil Microbiology, Nematology
Air and Water Quality
Start year (duration)  2005 (One Year)
Objectives Identify the microorganisms that cause replant suppression of walnuts.

Isolate the identified microorganisms and confirm their suppressive activity.

Walnut replant suppression produces a dramatic reduction in plant growth with subsequent yield reductions and a shortened production life. As the causes of the suppression are unknown, management strategies have focused on broad-spectrum soil fumigants, which are also the largest contributors to volatile organic compound emissions. This project will endeavor to identify the microorganisms that cause walnut replant suppression by applying a novel population ecology strategy in combination with an array-based microbial identification method. Identifying the causal organisms of walnut replant suppression will lead to the development of targeted biological, cultural, or chemical management strategies.
Final report A greenhouse assay for walnut replant suppression was not satisfactory because the performance of the seedlings varied significantly among and within the soil treatments. Various trials with two replant soils from walnut orchard locations in Fresno County that were exposed to three different preplant physical or chemical treatments did not reveal any significant differences in seedling growth regardless of the treatment. In general, the difficulty to obtain uniform seedlings and the very slow growth of the delicate seedlings in the greenhouse environment made walnut a poor choice for such an assay system. Consequently, we redirected our research to Prunus replant suppression that has similar disease characteristics as walnut. Three replant soils were collected from Prunus orchard sites in Fresno and Merced counties. Experiments involved creating soils with various levels of Prunus replant suppressiveness through a series of differential heat, biocidaL, and fumigation treatments. These soils were then planted with peach seedlings. Initially, test plants (cv. Nemaguard) were derived from rooted cuttings that varied considerably in size, vigor, and root development. Clonally derived plants, however, provided vastly superior seedlings of same age, compact size, and with vigorous root systems. While increased plant vigor and growth are often recorded after soil pasteurization or fumigation, replant suppression is genus specific. Consequently, citrus seedlings were included in the tests as a non-Prunus control to help to distinguish replant suppression from nonspecific effects. At various times, the plants were removed from the pots, assessed, and samples of the soils and roots were collected. The soil treatments affected replant suppression, as indicated by improved Prunus growth compared to the nontreated check.
At the time of this reporting, the research has not been started because walnut seedlings will not be available before mid-January 2006.

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