California grows more than 91% of fresh strawberries in the United States. Critical to this success has been management of soilborne diseases using pre-plant soil fumigation with methyl bromide. However, international regulations require a phase out of methyl bromide, soon to be completed. Reduced availability of methyl bromide has coincided with increased incidence of soilborne diseases affecting strawberry production, including Verticillium wilt, caused by Verticillium dahliae, and Black Root Rot (BRR). BRR is caused by a complex of soilborne pathogens, including Pythium ultimum, that form lesions on tertiary roots, which are critical to nutrient and water uptake. Consequently, non-chemical alternatives for sustainable management of soilborne diseases and highly productive plants are urgently needed.
Crop rotation with legumes can contribute to plant productivity and disease management by fixing nitrogen and providing a non-host interval during which the pathogen can die by natural attrition. However, rotation crops that appear to be non-hosts because they show no symptoms of disease may nevertheless support development of the pathogen and thus negate the benefit of crop rotation. One objective of this research was to evaluate systemic colonization of ten legume cover crops V. dahliae under field conditions and the extent to which plant residue supports development of V. dahliae microsclerotia (Chapter 1). This included seven cool season legumes: broad 'Windsor' bean, bell bean, field pea, hairy vetch, common vetch, purple vetch and 'Lana' woolypod vetch, and three warm season legumes: sesbania, sunn hemp and black-eyed pea. Frequency of systemic infection at ten weeks ranged from 5% (woolypod vetch) to 23% (field pea) and at the end of the trial ranged from 0% (purple vetch) to 23% (hairy vetch). The trend for mean density of microsclerotia in residue at ten weeks ranged from 0 CFU/g residue (hairy vetch) to 583 CFU/g residue (field pea) and at the end of the trial ranged from 63 CFU/g residue (broad bean) to 1096 CFU/g residue (field pea). In most cases, frequency of infection and formation of microsclerotia in plant residue was higher by the end of the trial than at ten weeks. Thus, in fields infested with V. dahliae, growers should avoid rotation with the evaluated legumes to avoid increasing soil inoculum levels.
Compost can contribute to plant productivity and disease management by improving soil structure and fertility, and providing the necessary factors to shift soils from disease conducive to suppressive. This study was undertaken to evaluate four composts that are available to California strawberry growers: manure compost, spent mushroom compost, vermicompost and yard trimmings compost. The objective was to evaluate the effect on production parameters including soil quality and fertility, and plant growth and yield. Manure and mushroom compost significantly increased soil electrical conductivity, which reached levels of 9.9±1.7dS/m and 7.3±0.8dS/m, respectively. Manure, yard trimmings and mushroom composts shifted soil pH closer to optimal levels for up to 7 months in 4 to 5 of the trials. Mushroom compost had the greatest effect on soil nitrate, with up to 32 mg/kg nitrate higher than the non-amended soil.
Another potential benefit of compost is suppression of soilborne pathogens, which can result from changes in the composition and activity of the soil microbiota. It was an objective of this study to determine if four commercially available composts influence infection of strawberry roots by V. dahliae and P. ultimum. The results showed a significant reduction in V. dahliae root infections in some compost amended soils but results were not consistent across trials. The effect of compost amendments on seedling disease caused by P. ultimum was a reduction in disease incidence by 38-43% compared to the non-amended soil.
The industry-wide shift in strawberry production generates a tremendous need for knowledge transfer and grower support. Accordingly an additional objective of this research was to solicit industry perspectives on the status of soilborne disease management. Results identified crop rotation as the most important tool in the absence of fumigation as, reported by 46% of respondents. When given a choice of thirteen management tools, crop rotation also had the highest ranking by respondents as a practice always used/recommended.
Numerous soilborne disease management tools, like crop rotation and compost, sustain high yields and reduce disease incidence, but vary in status of adoption. At a pivotal time when land is still productive but pathogens are becoming more widespread, a regional plan for maintaining pathogen-free soil has an opportunity to emerge as the foundation for a sustainable industry in the post-fumigation era. (Abstract shortened by UMI.)
|Advisor:||Gordon, Thomas R.|
|Commitee:||Davis, Michael, Kluepfel, Daniel|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 77/02(E), Dissertation Abstracts International|
|Subjects:||Agriculture, Plant sciences, Plant Pathology|
|Keywords:||Compost, Crop rotation, Methyl bromide, Soil fumigation, Soilborne diseases, Strawberry|
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