Characterization of soils properties associated with suppression of Fusarium wilt in spinach seed crops, and development of a quantitative molecular assay for Fusarium oxysporum f. sp. spinaciae.

Project Overview

GW08-005
Project Type: Graduate Student
Funds awarded in 2008: $19,960.00
Projected End Date: 12/31/2010
Region: Western
State: Washington
Graduate Student:
Major Professor:
Dr. Lindsey du Toit
Washington State University

Commodities

  • Vegetables: greens (leafy)

Practices

  • Crop Production: foliar feeding, tissue analysis
  • Education and Training: extension, on-farm/ranch research, participatory research
  • Farm Business Management: risk management
  • Pest Management: cultural control, economic threshold, integrated pest management, physical control
  • Soil Management: soil analysis

    Abstract:

    The maritime Pacific Northwest (PNW) is the only region of the U.S. suitable for spinach seed production. Washington and Oregon seed growers produce up to 50% of the U.S.and 25% of the world's spinach seed supply annually. However, acidic soils of this region are highly conducive to spinach Fusarium wilt caused by Fusarium oxysporum f. sp. spinaciae, which has become the most limiting factor for spinach seed production in the U.S. This project demonstrates the potential for limestone applications to reduce losses to Fusarium wilt and a soil bioassay for growers to identify low-risk fields for spinach seed production.

    Introduction

    Background:

    Spinach is a widely-consumed leafy vegetable with relatively high concentrations of vitamins, minerals, proteins and antioxidants that are important for prevention of human disorders such as age-related macular degeneration (Lucier, 2000; Pratt, 1998). Spinach is grown on approximately 56,800 acres in the U.S. annually for fresh and processed markets [USDA National Agricultural Statistics Service, (NASS)]. The primary states for spinach production are Arizona, Arkansas, California, Colorado, Maryland, New Jersey, Oklahoma, Texas and Virginia (USDA NASS). Spinach crops are planted at populations ranging from 30,000 to >2 million seed/acre, with the higher populations used for the expanding ‘baby leaf’ bagged spinach market (Holloway et al., 2003; LeStrange et al., 1996). The dense populations, particularly for ‘baby leaf’ crops harvested 30-45 days after planting, necessitate production of high quality spinach seed and have increased the demand for spinach seed.

    Spinach seed crops in the mild maritime region of western Washington and western Oregon produce up to 50% of the U.S. and up to 25% of the world's supply of spinach seed on 3,000-4,000 acres annually. Few areas of the world have the climatic conditions of the coastal Pacific Northwest (PNW) region of the U.S. that are required for production of high quality spinach seed, i.e., long summer day length (>16 hours) to trigger uniform flowering of this day length-sensitive species, dry summers to minimize pathogen infections on developing seed and mild summer temperatures for this heat-sensitive species (Metzger and Zeevaart, 1985). The few other countries with regions that meet these climatic requirements, and in which spinach seed is produced, include Denmark, France, Italy, Holland and New Zealand (Deleuran and Boelt, 2006; Ben Baerends, Seminis Vegetable Seeds, personal communication). Spinach seed growers in these countries compete directly with U.S. spinach seed growers.

    Many vascular wilt diseases caused by formae speciales of Fusarium oxysporum are economically significant, limiting the productivity and sustainability of susceptible crops in numerous areas of the world (Beckman, 1987). In the maritime PNW, depletion of virgin ground for spinach has resulted in Fusarium wilt, caused by the soilborne fungus F. oxysporum f. sp. spinaciae, becoming the limiting factor for high-yielding spinach seed crops in the U.S. (Foss and Jones, 2005). Seed treatments do not provide protection against infection of spinach plants by this pathogen after the seedling phase of growth, and soil fumigation is neither economically nor environmentally sound. Management tools are needed, particularly through the extended season for spinach seed production.

    Current efforts to manage spinach Fusarium wilt in the PNW are based on planting resistant cultivars and using very extensive crop rotations (Foss and Jones, 2005), but both these approaches have inherent limitations. Spinach cultivars with partial resistance are available, but many carry little resistance to this pathogen (Correll et al., 1994). In addition, seed crops are grown on a contract basis, so growers have little choice of the parent (inbred) lines they grow. Once introduced into soil, F. oxysporum f. sp. spinaciae can survive many years as a saprophyte and/or a pathogen on spinach and related asymptomatic host crops (e.g., beet and chard) (Armstrong and Armstrong, 1976; Reyes, 1979). When spinach Fusarium wilt was first identified in the PNW in the 1960s, the disease was managed by growing seed crops in fields not previously planted to spinach. Subsequent depletion of virgin ground for spinach led to Fusarium wilt becoming the main factor limiting production of high yielding spinach seed crops in this region (Foss and Jones, 2005). Losses to Fusarium wilt now necessitate rotation intervals of six to ten years for spinach lines with partial resistance and at least 12 to 15 years for susceptible lines. Confounding this is the need to isolate seed crops to avoid unwanted cross pollination of this wind-pollinated species. Therefore, selection of fields for spinach seed production is an increasingly complex and limiting factor for growers.

    Development of effective soil-based management approaches for F. oxysporum f. sp. spinaciae requires an intimate understanding of the effects of soil chemical and biological properties on growth and survival of these fungi. Soils differ widely in the capacity to suppress or promote these and other soilborne plant pathogens (Hornby, 1983). Some naturally-conducive soils can be rendered more suppressive to particular diseases, to various degrees and durations, using practices such as soil amendments, cover crops and selection of fertilizers (Beckman, 1987; Cook and Baker, 1983; Datnoff et al., 2007; Hornby, 1983; Stone et al., 2004; Ochiai et al., 2008). In general, Fusarium wilts tend to be promoted by acid soils, ammonium forms of nitrogen and high available concentrations of micronutrients such as iron, manganese and zinc (Datnoff et al., 2007). The relationship of these soil properties to growth and pathogenicity of the causal agent of Fusarium wilt of spinach has not been investigated and remains an important gap in our understanding of how to manage this soilborne disease and reduce the extensive rotation intervals required for production of high quality spinach seed.

    Goal 1: Management practices

    Over the past eight years, with >$52,000 in funding from the Puget Sound Seed Growers’ Association (PSSGA) and matching support from the state-funded Washington State Commission for Pesticide Registration (WSCPR), we have initiated a series of research trials to evaluate potential management tools for Fusarium wilt in spinach seed crops in the PNW. Based on evidence that alkaline and calcareous soils are naturally-suppressive to spinach Fusarium wilt in Denmark and Texas, we initiated field trials in western Washington in 2006 to 2008 to evaluate the potential efficacy of limestone amendments for suppression of Fusarium wilt (du Toit et al., 2007 and 2008). Agricultural limestone applied at >1.4 tons/acre suppressed Fusarium wilt significantly, and plant biomass was significantly greater in all plots treated with limestone compared to non-treated plots. For the Fusarium wilt-susceptible spinach female line evaluated, wilt was reduced 45% and seed yield increased 318% in plots amended at 3.5 tons/acre compared to control plots (du Toit et al., 2007). For the moderately susceptible spinach female line, seed yield was greatest on plots amended at 2.8 tons/acre, and decreased at higher and lower rates. This illustrates the need for research to optimize limestone amendments for suppression of spinach Fusarium wilt and to assess additional tools that may further enhance this suppression. For example, NO3- vs. NH4+ forms of N have been shown to suppress vs. promote, respectively, Fusarium wilt in other species (Datnoff et al., 2007; Huber and Watson, 1974). Woltz and Jones (1973) found that NH4+-N actually reversed the beneficial effects of limestone on Fusarium wilt management in tomato, demonstrating the need to determine whether the potential benefits of limestone applications in spinach seed production may be mitigated by the types of fertilizers used by spinach seed growers.

    Goal 2: Field site risk assessment

    Presently, the only factors used by growers to assess the risk of a field site for spinach Fusarium wilt is the duration of crop rotation between spinach seed crops and the level of partial resistance of the parent lines to Fusarium wilt. The latter is often not known for many of the parent lines used in spinach seed crops, and growers typically have no choice of the specific parent lines they are contracted to grow for seed companies. These two pieces of information are not foolproof, however, as some fields that had been out of spinach for as long as 12-16 years in northwestern Washington have developed severe Fusarium wilt (project director’s personal observations from 2000-2010). A better understanding of the relationships among soil properties and pathogen population, host-pathogen interactions and soil microbial communities could be used to develop a more effective tool for assessing the relative risk of Fusarium wilt in specific fields. A soil bioassay that correlates Fusarium wilt development on a set of standard parent lines to inoculum density, microbial diversity and fertility status of a representative soil sample from a field could be a valuable improvement in Fusarium wilt risk assessment and subsequent field site selection by spinach seed growers.

    Summary

    Development of effective risk assessment tools and sustainable management practices for Fusarium wilt are essential for the U.S. spinach seed industry to remain competitive with the few other countries that have suitable climates for spinach seed crops. U.S. growers currently do not know how to reduce rotation intervals to periods comparable to those in Denmark without incurring severe losses to spinach Fusarium wilt. Lack of such knowledge is important because, until it becomes available, U.S. farmers will continue to be restricted in their capacity to produce spinach seed on the limited acreage in the U.S. suitable for this crop. The research is expected to improve our understanding of the relationships of soil properties to suppression of spinach Fusarium wilt. This is expected to improve the ability of U.S. spinach seed growers to manipulate soil properties using cultural practices to render naturally-conducive soils of the PNW more suppressive to Fusarium wilt.

    We expect the outcome of the proposed research to contribute to reduced rotation intervals for spinach seed crops in the U.S. This contribution is significant because application of the new knowledge is expected to increase the capacity of each acre in the U.S. that can be used to produce spinach seed. This, in turn, will increase the sustainability and profitability of spinach seed as a U.S. crop, potentially equating to a larger slice of the world market for spinach seed, and decreased dependence on other countries for spinach seed by the many states that produce fresh and processed spinach. Furthermore, results generated by this research are expected to have a broader impact by providing information relevant to suppression and management of other Fusarium wilts, e.g., radish, onion, pea, and ornamental bulbs crops.

    Project objectives:

    Long-term survival of the Fusarium wilt pathogen in soils necessitates a rotation interval of 10 to 15 years, or longer, for spinach seed crops in the PNW. The overall objective is to reduce this rotation interval in order to increase the carrying capacity of PNW acreage for spinach seed production, based on an understanding of how soil chemical and microbial properties affect conduciveness of soils in this region to Fusarium wilt.

    Specific objectives are to:

    a. Develop a real-time PCR assay for specific detection of F. oxysporum f. sp. spinaciae to quantify populations of the spinach Fusarium wilt pathogen and determine soilborne inoculum threshold(s) for identifying fields suitable for planting spinach seed crops.

    b. Assess the potential of rapidly-dissolving hydrated lime applications to raise soil pH in western Washington and Oregon to suppress Fusarium wilt of spinach.

    c. Assess the effects of available calcium and micronutrients (specifically Zn, Mn, and Fe) on aggressiveness of F. oxysporum f. sp. spinaciae to spinach.

    As described in previous progress reports, I (Gatch) started on this project in 2008, first assisting du Toit’s program with a field trial evaluating rates of limestone amendments and parent lines with different levels of susceptibility to Fusarium wilt for suppressing this disease in spinach seed crops. Based on the results of this and previous experiments in du Toit’s program, the objectives listed above were modified from those of the original Western SARE Graduate Student Fellow Grant, as well as the Robert MacDonald Vegetable Seed Memorial Fund proposals. Additional federal funding was obtained (see Milestones section) to support progress toward these objectives.

    The modified objectives of this SARE project are as follows:

    a. Assess the potential for enhancing suppression of spinach Fusarium wilt using annual applications of limestone and nitrate-N fertilizers and monitor microbial and chemical changes in the soil associated with these practices over four years (modified objectives ‘b’ and ‘d’ of the original Western SARE proposal)

    b. Determine the effects of micronutrients, pH, calcium and form of nitrogen on F. oxysporum f. sp. spinaciae and on interactions of the pathogen with spinach (objective ‘c’ of the original Western SARE proposal)

    c. Develop a soil bioassay for spinach seed growers to determine the relative risk of field sites for Fusarium wilt (complementary to objective ‘a’ in the original Western SARE proposal on a real-time PCR assay).

    d. Monitor changes in populations of soilborne bacteria, particularly in the rhizosphere of spinach plants, that may be antagonistic to F. oxysporum f. sp. spinaciae, following limestone applications in the field trial in objective a, and correlate changes in soil microflora with other changes in soil properties such as pH.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.