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United States Department of Agriculture

Agricultural Research Service

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Location: Crops Pathology and Genetics Research

2013 Annual Report

1a. Objectives (from AD-416):
1. Assess incidence and statewide distribution of soilborne pathogens emerging in California strawberry fields grown without MB. 2. Facilitate evaluations of treatments or strategies for control of soilborne pathogens emerging in California strawberry fields grown without MB. 3. Contribute summaries of project findings to PAW-MBA educational outreach efforts.

1b. Approach (from AD-416):
Objective 1. The investigators will assess incidence and statewide distribution of the emerging pathogens by performing diagnostics on diseased strawberry plant specimens from fruiting fields and nurseries treated with MB alternatives. The samples will be collected from growers, pest control advisors, and UC Farm Advisors. A database will be used to document sample information, i.e., crop history, pre- and post-plant treatments pertaining to soilborne disease, disease symptoms, geographical location, etc. Pathogens will be detected and identified using culture-based and culture-independent (i.e., PCR-based) diagnostics, as appropriate. Objective 2. The investigators will facilitate evaluations of treatments or strategies for managing the emerging soilborne pathogens by partnering with general horticulturists, plant breeders, and other plant pathologists. For example, the investigators may provide pathology support (i.e., pathogen isolates, inoculum, diagnostic plant pathogen assays, etc.) to strawberry plant breeders evaluating genetic resistance to the emerging pathogens, and they may contribute diagnostic expertise that will be critical for field assessment of soil or plant treatments (i.e., alternative fumigants, fungicides, non-fumigant treatments) designed to manage the emerging pathogens. Further development of accurate soil assays for M. phaseolina and F. oxysporum f. sp. fragariae also may be undertaken. Objective 3. The investigators will work collaboratively to prepare annual analyses and summaries of the assessments described above (Objectives 1, 2). The information will be prepared and submitted by the investigators for inclusion on the PAW-MBA website and for presentation in county-based UC Extension meetings and newsletters. Field demonstrations will be held for selected trials, as appropriate. The investigators will collaborate with UC Farm Advisors in outreach efforts designed to foster awareness and optimal management of the emerging pathogens.

3. Progress Report:
This project is related to objective 3 of the inhouse parent project, "Identify key variables affecting the efficacy of methyl bromide (MB) alternatives and demonstrate management and predictive use of the variables to optimize performance of the alternatives". Surveys were conducted in FY2013 in commercial strawberry fields with plant decline in coastal districts of California. Results of the survey indicated that charcoal rot, caused by Macrophomina phaseolina, is continuing to emerge as a serious pathogen of strawberry. It was first detected in California in 2005, and is now found in Ventura, Santa Barbara, Monterey, Santa Cruz, and Santa Clara Counties. Moreover, finds in new fields document intensification of the disease, particularly in central and southern coastal production areas. Macrophomina phaseolina therefore appears to be the most important soilborne pathogen currently affecting strawberry production in California. Fusarium wilt of strawberry was originally limited to Ventura County but was later also identified in Monterey County. In the past year, the occurrence of Fusarium wilt was confirmed in both San Luis Obispo and Santa Barbara counties. In nearly all cases where either Fusarium wilt or charcoal rot have been found, standard pre-plant, flat fumigation with methyl bromide plus chloropicrin has not been used for several years. Ten cultivars and 26 breeding lines (= 36 genotypes) were tested for susceptibility to Fusarium wilt by immersing roots of runner plants in an aqueous suspension of 5 x 106 pathogen spores per ml. Of the cultivars tested, Monterey and Benicia were the most severely affected by Fusarium wilt, with resistance scores of 2.5 and 2.9, respectively (on 1-5 scale with 1 corresponding to a plant killed by Fusarium wilt and 5 corresponding to a plant with no symptoms of disease). As observed in previous tests, San Andreas and Portola were highly resistant, and both had scores of 5.0, which means that plants remained disease-free throughout the season. Ventana has appeared resistant in previous tests but in the 2013 test this cultivar had an average resistance score of 3.6, indicating that some damage resulted from the inoculations. Among breeding lines evaluated, scores ranged from 1.1 to 5.0, with an overall mean of 3.9. Sixty-two percent of breeding lines had resistance scores of 4.3 or higher. Experiments were conducted to test the effects of the fungicide Topsin M (Cerexagri-Nisso LLC), and the biological products, Serenade Soil (Bayer) and Tenet (SipcamAdvan) on development of Fusarium wilt. All three products were tested at three inoculum levels (0, 10,000 and 25,000 colony forming units of F. oxysporum f. sp. fragariae per gram of potting mix), and two pH levels (pH 6 and 7) in a factorial design. The application of Topsin M as a root-dip did not have a significant effect on severity of Fusarium wilt. This is consistent with the result of previous field studies in which this fungicide was applied during the season through a drip system. The lack of efficacy is probably due to a relatively short half-life of the active ingredient in soil and the fact that new root growth is not protected. Serenade and Tenet were both applied as root-dips and as a soil drench just prior to planting. Neither treatment had any significant effect on the progression of Fusarium wilt. Nine cultivars and three breeding lines were tested for susceptibility to disease caused by Macrophomina phaseolina. Plants were inoculated by dipping roots in a suspension of sclerotia prepared by blending fully colonized plated of potato dextrose agar in sterile water. The results revealed significant differences in susceptibility among the genotypes that were tested. A range in resistance scores of 2.0 to 4.5 resulted, implying that variation in susceptibility exists with the UC breeding population. We can now use this procedure to determine if the variation that we have observed is heritable and if it corresponds to similar variation in susceptibility under field conditions. A trial with an expanded set of genotypes is under way this year and results to date are consistent with last year’s findings. Our first attempt to assess the host range of Macrophomina phaseolina isolates from strawberry failed to document the susceptibility of other crops regarded as hosts to this pathogen. This could be an indication that the strawberry pathogen shows some degree of host-specificity, which could mean that rotations with these crops will not increase the risk of disease to a strawberry crop that follows in the same field. However, it is also possible that the inoculation method we used and/or the conditions under which plants were maintained were not conducive to disease development on the crops being tested. One factor that may be important is stress. Charcoal rot on many crops is closely associated with environmental stress, such as a lack of water. With this in mind, we are re-testing the same crops while subjecting them to stress induced by high temperature and by withholding water.

4. Accomplishments

Last Modified: 06/25/2017
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