- Fruits: berries (strawberries)
- Crop Production: biological inoculants, municipal wastes, organic fertilizers
- Education and Training: extension, networking, on-farm/ranch research
- Pest Management: biological control, competition, cultural control, integrated pest management
- Production Systems: organic agriculture
- Soil Management: earthworms, organic matter, soil analysis, soil microbiology, soil quality/health
- Sustainable Communities: social networks
California produces more than 80% of fresh strawberries in the United States. Critical to this success has been the use of pre-plant soil fumigation with Methyl Bromide (MB) to manage soilborne diseases, especially Verticillium wilt caused by Verticillium dahliae and black root rot caused by a complex of soil dwelling organisms including Pythium ultimum. However, environmental regulations require a phase out of MB. Alternative fumigants have been developed but regulatory pressures may soon render them unavailable. Consequently, non-chemical alternatives for sustainable management of soilborne diseases are urgently needed.
Disease suppressive soils are those in which disease fails to develop despite the presence of a pathogen. Suppression is generally thought to be microbially-mediated and research has demonstrated that compost can provide the necessary factors to shift soil from disease conducive to suppressive. Four commercial composts, available to California’s Central Coast strawberry-growing region, were evaluated for suppression of disease. Composts were characterized by microbial and physiochemical properties to identify correlations with suppressiveness. Manure and mushroom compost significantly increased soil electrical conductivity, reaching field 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. All composts increased soil microbial activity when measured 2 weeks after compost incorporation and 7-11 months post incorporation.
The effect of four commercially available composts on V. dahliae suppression was assessed by quantifying the frequency of strawberry root infection, whereas an effect on P. ultimum was evaluated by quantifying the percentage emergence of cucumber seedlings. Compost amendments were associated with a reduction in root infection by V. dahliae in some cases, but results were not consistent across trials. Pythium ultimum was suppressed by all the composts in at least one trial, reducing incidence by 38-43% compared to non-amended soil.
The industry-wide shift in strawberry production generates a tremendous need for knowledge transfer and grower support. Accordingly, as a complement to the biological research, this study solicited industry perspective on the status of soilborne disease management through a questionnaire. Sixty questionnaires were completed at grower meetings, online and surface mail. When asked the most important tool in the absence of fumigation, the most respondents (46%) reported crop rotation. When given a choice of thirteen management tools, crop rotation also had the highest ranking by respondents as a practice always used/recommended. Avoidance strategies like cleaning equipment, avoiding rotation with known hosts and reducing shared equipment ranked highly as practices adopted/recommended. Organic growers had higher overall adoption of the thirteen practices compared to conventional growers. To incentivize adoption, respondents were most motivated by economic incentives like crop insurance, cash incentives and higher berry prices, but also highly incentivized by knowledge-based resources like research evidence, field trials and one-on-one advising. Lastly, respondents ranked the resource most used for gaining technical knowledge as fellow growers (27%) and in-house company meetings (28%). Online resources, books, and pamphlets were ranked low, only 38% of respondents reporting any use of one of those resources. Public meetings were highly ranked as a supplemental resource. The University of California had the highest number of respondents reporting use (81%) and was valued by 21% as their primary choice. In summary, respondents view crop rotation as critical to soilborne disease management and adoption of alternatives can greatly increase, with information dissemination focused on the University of California resources and all sources of social capital.
California strawberry growers yield 90-202 kg/ha, totaling approximately 1.28 billion kilograms of strawberry annually, roughly 91% of fresh strawberries grown in the United States (USDA, 2014). The high yields that are routinely obtained for California-grown strawberries reflect a sophisticated and intensive production system that has been refined over several decades. A key component of this system is pre-plant soil fumigation, which serves to reduce populations of soilborne pathogens. Where soil fumigation is not used, as in production of certified organic strawberries, growers must contend with soilborne pathogens, such as Verticillium dahliae and a complex of root-infecting organisms that cause black root rot (BRR).
Verticillium wilt, caused by Verticillium dahliae, is a major constraint on production of organic strawberries, and has become a problem for strawberry growers in California even where conventional practices are employed (Koike, 2011). Management of V. dahliae is particularly challenging due to its wide host range and survival structures (microsclerotia) that persist in soil for many years.
Black Root Rot (BRR) is seen one of the principal contributors to yield losses in non-fumigated soil, which can be as high as 25% (Yuen et al., 1991). The presence of each organism in BRR varies by location. The primary BRR-associated organisms isolated from strawberry roots growing in soil on the California central coast are Pythium spp. and Rhizoctonia spp. Pythium spp. have been found to represent 70% of the isolates identified in strawberry fields from Watsonville, CA, 88% from Santa Cruz and 50% from Salinas. Pythium ultimum was the most commonly recovered species (Martin, 1998). Damage caused by BRR is slow, but consistent, degradation of tertiary feeder roots, which may cause stunting and/or yellowing due to nitrogen deficiency.
Disease suppressive soils can provide effective long-term management of soilborne diseases (Weller et al., 2002). Virtually all soils possess some biological capacity to limit disease progression (Mazzola, 2004). Suppressiveness also can be induced, as illustrated by the build-up of antibiotic producing Pseudomonas fluorescences after years of increasingly severe Take-all disease caused by Gaeumannomyces graminis var tritici on serially monocropped wheat fields (Raaijmakers and Weller, 1998). The eventual reduction in disease incidence reflected the natural shift from disease conducive to suppressive soil. Development of soils suppressive to G. g. var tritici has been shown to be mediated by development of a soil microbiome suppressive to the pathogen.
The shift from conducive to suppressive soil can be achieved through the addition of active compost (Hoitink and Fahy, 1986; Hoitink and Boehm, 1999; Noble and Coventry, 2005; Mazzola, 2004; Stone et al., 2004; Termorshuizen et al., 2006). Research over the past 50 years has documented suppressive effects of compost on diseases caused by Pythium spp., Phytophthora spp., Fusarium oxysporum, Verticillium dahliae and others (Noble 2005). Spent mushroom compost reduced disease incidence of Verticillium wilt by 9% under field conditions (LaMondia et al., 1999). Composted horse manure + municipal compost, supplemented with a plant-growth-promoting rhizobacterium, Paenibacillus alvei, reduced severity of Verticillium wilt on Eggplant by 48% (Markakis et al., 2008). Municipal waste at 8-10% incorporation rate led to 63% and 43% reduction of P. ultimum in beetroot and pea respectively (Schuler et al., 1989). Sewage sludge at 25% amendment rate led to a 46% reduction of Pythium damping off in Impatiens (Ferrara et al., 1996). These results, among others, suggest that compost offers considerable potential to provide benefits in an integrated system for management of soilborne diseases affecting strawberry production.
Organic and conventional strawberry growers in California regularly apply compost, even though formal guidelines are lacking for type and application rate that will provide the best results. Compost type has been found to have an effect on suppression of some pathogens (Ringer et al., 1997 and Scheuerell et al., 2005). The objective of the study reported here was to determine whether suppression of Verticillium wilt and BRR can be achieved by application of commercial composts available to California strawberry production fields and if suppression differs by compost type.
The industry-wide shift away from principal reliance on soil fumigation and increasing disease pressure generates a tremendous need for knowledge transfer and grower support. Perceived grower needs, status of adoption of disease management practices, and channels of information dissemination are critical to maintaining appropriate research foci and outreach efficiency. In the larger context, especially in cases where voluntary adoption of an alternative practice is promoted, success is measured by how much individuals are prepared to change their practices, in turn influencing policy.
The California Strawberry Commission described two current industry objectives (2014): 1) to explore new approaches to outreach and 2) to get farmers to be a more “accepted and appreciated” part of the system as both food producers and environmental stewards. Future adoption of a new practice is more likely if growers are highly knowledgeable of the benefits of new practices and the estimated costs of implementation. Incentives can motivate growers to adopt practices identified as having costs or risks that outweigh the potential private benefits. By identifying the status of adoption, incentives for change and pathways of knowledge transfer, this research seeks to help the strawberry industry meet heightened environmental concerns and the trend toward sustainable agricultural practices while maintaining a thriving industry.
The study presented here used qualitative and quantitative survey data collected from growers, grower advisors and researchers in Monterey and Santa Cruz Counties to evaluate the current status of soilborne disease management. The aim was to gain insight into: 1) perceived needs in the absence of fumigation, 2) adoption of available disease management tools, 3) incentives to adopt new practices and 4) pathways for information acquisition. The results of this research provide an opportunity to integrate social determinants into disease management strategies in order to create a more comprehensive disease management package with higher success of adoption. Maintaining a thriving, multi-billion dollar strawberry industry in California is ideal because the optimal soil and climate conditions found on coastal California produce the highest strawberry yields in the world.
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- Evaluate locally available composts for suppression of Verticillium wilt caused by Verticillium dahliae and Black Root Rot caused by a complex of pathogen, one of which is Pythium ultimum.
- Test compost in certified organic and conventionally managed field soil greenhouse and field trials.
- Identify factors in compost, physical and/or microbial, that strongly correlate with disease suppression to enhance consistency and reproducibility of compost applications.
- Develop recommendations for compost production and application to induce disease suppressive soils.
- Solicit grower-identified needs for managing soilborne diseases so as to improve outreach and extension efficiency.