Management of Fusarium wilt in spinach seed crops in the maritime Pacific Northwest USA
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The maritime Pacific Northwest is the only region of the USA 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 10 to 15 years between spinach seed crops are necessary to reduce the risk of losses to this disease. Raising soil pH with limestone partially suppresses spinach Fusarium wilt, but the suppressive effect is transitory, and the disease still limits seed crop acreage in the region. Experiments were conducted to: 1) assess the potential for annual applications of limestone for three years prior to a spinach seed crop to improve Fusarium wilt suppression compared to the level of suppression from a single limestone amendment, 2) develop a soil-based greenhouse bioassay to characterize the spinach Fusarium wilt risk of soil samples submitted from stakeholders' fields, and 3) explore the mechanism(s) of limestone-mediated Fusarium wilt suppression. Annual applications of limestone for each of three years prior to a spinach seed crop were superior to a single limestone application for suppressing Fusarium wilt and increasing seed yield. A soil bioassay to assess Fusarium wilt risk was developed in which three spinach lines representing a range in Fusarium wilt susceptibility were used to test soil samples from growers' fields under consideration for spinach seed crops. In the four years that the soil bioassay has been offered as a risk assessment service, soil samples from 147 fields were submitted by stakeholders for evaluation. Follow-up visits to spinach seed crops planted in fields assessed with the soil bioassay validated the results. In vitro experiments demonstrated that deficiencies of iron, manganese, and zinc can reduce growth and sporulation of F. oxysporum f. sp. spinaciae. Furthermore, greenhouse experiments in naturally-infested field soil indicated that reduction in availability of these micronutrients in limestone-amended soils reduced Fusarium wilt inoculum potential. Together, these findings reveal relationships among soil properties and spinach Fusarium wilt development, increase the capacity for and profitability of USA spinach seed production, and will guide future research on soil-based management of this disease.