A Soil Bioassay for Predicting the Risk of Spinach Fusarium wilt

Abstract

The maritime Pacific Northwest is the only region of the United States suitable for production of spinach seed, a cool-season, daylength-sensitive crop. However, the acidic soils of this region are highly conducive to spinach Fusarium wilt, caused by Fusarium oxysporum f. sp. spinaciae. Rotations of at least 10 to 15 years between spinach seed crops are necessary to reduce the high risk of losses to this disease. The objectives of this study were to develop a greenhouse soil bioassay to assess the relative risk of Fusarium wilt in fields intended for spinach seed production, and to identify soil chemical and physical properties associated with conduciveness to this disease. Preliminary bioassays established a protocol for growing spinach plants in a greenhouse environment and inducing Fusarium wilt symptoms so that the bioassay can be completed in <2 months. Test soils with a range of Fusarium wilt inoculum potentials, and three spinach inbred parent lines (highly susceptible, moderately susceptible, and moderately resistant to Fusarium wilt) were used to evaluate sensitivity of the bioassay to different levels of risk of Fusarium wilt. Then, from 2010 to 2013, spinach seed growers and stakeholders submitted soil samples from 147 fields for evaluation with the bioassay. The fields were each under consideration for planting a spinach seed crop, yet the bioassay revealed a wide range in Fusarium wilt inoculum potential among soil samples. Differences in susceptibility to Fusarium wilt of the three inbred lines were key to detecting differences in wilt risk among soils. Visits to spinach seed crops planted in fields evaluated in the bioassay, as well as test plots of the three inbred lines planted in growers’ seed crops, confirmed the predictive value of the bioassay for Fusarium wilt risk. Correlation analyses for 23 soil properties revealed significant relationships of 15 soil properties with the Fusarium wilt potential of a soil, but the correlations were influenced significantly by susceptibility of the inbred line to Fusarium wilt (13, 10, and 8 soil properties correlated significantly with Fusarium wilt risk for the susceptible, moderate, and partially resistant inbreds, respectively). Multiple regression analyses identified different statistical models for prediction of Fusarium wilt risk depending on the spinach inbred line, but the best fitting model explained <34% of the variability in Fusarium wilt risk among 121 fields evaluated in the soil bioassay. Thus, no model was robust enough to replace the bioassay for the purpose of predicting Fusarium wilt risk.

Development and Evaluation of a TaqMan Real-Time PCR Assay for Fusarium oxysporum f. sp. spinaciae

“…This might have been due to the confounding presence of another soilborne vascular wilt pathogen, Verticillium dahliae, detected in soils from these sites (14,15,24). Furthermore, development of Fusarium wilt in spinach seed crops is strongly influenced by the susceptibility of spinach cultivars or inbred lines (12,14), as well as by crops rotated with spinach, rotation intervals between spinach seed crops, soil properties (22,25), and other factors. Lack of uniformity in distribution of F. oxysporum f. sp.…”

“…Naturally infested soils from spinach production sites and disease severity data. The TaqMan real-time PCR assay was evaluated using soil sampled from Washington spinach seed production sites representing a total of 82 samples collected from fields in northwestern Washington with a history of spinach Fusarium wilt: (i) samples designated "SV" collected from growercooperator fields (n = 9) in Skagit County, WA in 2008, representing a range of 1 to 15 years of rotation out of spinach seed crops (Table 4); (ii) samples (n = 20) collected from replicated plots in a 2008 spinach seed crop trial located in Skagit County, WA ('08SPFWLA') and planted to spinach inbred lines that ranged from highly susceptible to moderately resistant to Fusarium wilt (14; Table 5); (iii) samples (n = 24) collected from plots in a 2009 spinach trial located in a Skagit County field ('09SPFWLA') (24; Table 5); and (iv) soil (n = 29) collected in 2009 and 2010 from fields in Island, Skagit, Snohomish, and Whatcom Counties in northwestern Washington ('10SPFWSB') (25; Table 5), with severity of spinach Fusarium wilt ranging from 0 to 100% (25). In the 2008 and 2009 spinach trials, incidence of plants with wilt symptoms was rated regularly through the season, spinach biomass and seed yield were measured, and the amount of F. oxysporum in the soil in each plot at harvest was quantified by plating soil samples onto Komada's agar medium (14,24).…”

“…In the 2008 and 2009 spinach trials, incidence of plants with wilt symptoms was rated regularly through the season, spinach biomass and seed yield were measured, and the amount of F. oxysporum in the soil in each plot at harvest was quantified by plating soil samples onto Komada's agar medium (14,24). In the bioassay of 29 fields sampled in 2009 and 2010, severity of Fusarium wilt was rated weekly, spinach biomass measured, and F. oxysporum enumerated on Komada's agar medium for each soil sample (25). Negative control soil samples were obtained from a field in the Columbia Basin of central Washington (VSP#08-19) that had never been planted to spinach but was naturally infested with F. oxysporum f. sp.…”

“…Pearson's correlation coefficients were calculated for F. oxysporum f. sp. spinaciae CFU/g of soil from the real-time PCR assays, total F. oxysporum/g of soil enumerated on Komada's agar medium, incidence or severity of Fusarium wilt, and spinach biomass, as detailed in the original reports (14,24,25).…”

Lessons from 10 years of stakeholder adoption of a soil bioassay for assessing the risk of spinach Fusarium wilt

“…Reducing the number of replications enabled each grower to bring a single, 20 L bucket of soil instead of two 20 L buckets of soil, without compromising the ability to get robust quantitative assessment of the risk of Fusarium wilt. This determination was based on results for soils evaluated from 2010 to 2013 (Gatch & du Toit, 2015). Reducing the number of replications also enabled a larger number of soil samples to be screened in the greenhouse space available each year.…”