Compost-Induced Disease Suppressive Soils for Control of Verticillium Wilt of Strawberry

Final Report for GW13-011

Project Type: Graduate Student
Funds awarded in 2013: $24,992.00
Projected End Date: 12/31/2016
Grant Recipient: UC Davis
Region: Western
State: California
Graduate Student:
Major Professor:
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Project Information

Summary:

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.  

Introduction

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.

References

Ferrara AM, Avataneo M, Nappi P. 1996. First experiments of compost suppressiveness to some phytopathogens. In: De Bertoldi M, Sequi P, Lemmes B, Papi T, editors. The Science of Composting. London: Blackie Academic & Professional. p 1157-1160.

Hoitink, H.A.J., Fahy, P.C. 1986. Basis for the control of soilborne plant pathogens with composts. Annual Review of Phytopathology 24, 93–114.

Hoitink, H.A.J., Boehm, M.J., 1999. Biocontrol within the context of soil microbial communities: a soil-dependent phenomenon. Annual Review of Phytopathology 37, 427–446.

Koike, Steven. 2011 “Operating a State-wide Strawberry Disease Diagnostic Services Center.” California Strawberry Commission Annual Production Research Report 2010-2011, p25-29.

La Mondia JA, Gent PN, Ferrandrio FJ. 1999. Effect of compost amendment or straw mulch on potato early dying disease. Plant Disease 83: 361-366.

Markakis Ea, Tjamos Se, Chatzipavlidis I, Antoniou Pp, Paplomatas Ej, 2008b.Evaluation of compost amendments for control of vascular wilt diseases. Journal of Phytopathology 156, 622-7.

Martin, Frank. 1998. Strawberry root rot and the recovery of Pythium and Rhizoctonia spp.

Mazzola, Mark. 2004. Assessment and Management of Soil Microbial Community Structure for Disease Suppression. Annual Review of Phytopathology 42: 35-59.

Noble, R and E. Coventry. 2005. Suppression of soil-borne diseases with composts: A review. Biocontrol Science and Technology 15 (1): 3-20.

Raaijmakers JM, Weller DM. 1998. Natural plant protection by 2,4-diacetylphloroglucinol-producing Pseudomonas spp. in take-all decline soils. Molecular Plant Microbe Interactions 11: 144–52.

Ringer, C. E., Millner, P.D., Teerlinck, L. M., and Lyman, B. W. 1997. Suppression of seedling damping-off disease in potting mix containing animal manure compost. Compost Science and Utilization 5: 6-14.

Schuler, C., Biala, J., Bruns, C., Gottschall, R., Ahlers, S., and Vogtmann, H., 1989. Suppression of root rot on peas, beans and beetroots caused by Pythium ultimum and Rhizoctonia solani through the amendments of growing media with composted organic household waste. Journal of Phytopathology 127: 227-238.

S.J. Scheuerell, D.M. Sullivan, W.F. Mahaffee. 2005. Suppression of seedling damping-off caused by Pythium ultimum, P irregulare, and Rhizoctonia solani in container media amended with a diverse range of Pacific northwest compost sources. Phytopathology 95: 306–315.

Stone, A. G., S. J. Scheuerell, and H. M. Darby. 2004. Suppression of soilborne diseases in field agricultural systems: organic matter management, cover cropping, and other cultural practices. In Soil Organic Matter in Sustainable Agriculture, eds. F. Magdoff, and R. R. Weil, 131–177. Boca Raton, FL: CRC Press.

Termorshuizen, A.J., van Rijn, E., van der Gaag, D.J., Alabouvette, C., Chen, Y., Lagerlof, J., Malandrakis, A.A., Paplomatas, E.J., Ramert, B., Ryckeboer, J., Steinberg, C., Zmora-Nahum, S., 2007. Suppressiveness of 18 composts against 7 pathosystems: variability in pathogen response. Soil Biology and Biochemistry 38: 2461-2477.

Weller, D.M., J.M. Raaijmakers, B.B.M. Gardener, and L.S. Thomashow. 2002. Microbial populations responsible for specific soil suppressiveness to plant pathogens. Annual Review of Phytopathology, 40:309–348.

Yuen, G. Y., Schroth, M. N. Weinhold, A.R., and Hancock, J.G. 1991. Effects of soil fumigation with methyl bromide and chloropicrin on root health and yield of strawberry. Plant Disease 75: 416-420.

Project Objectives:
  1. 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.
  1. Test compost in certified organic and conventionally managed field soil greenhouse and field trials.
  1. Identify factors in compost, physical and/or microbial, that strongly correlate with disease suppression to enhance consistency and reproducibility of compost applications.
  1. Develop recommendations for compost production and application to induce disease suppressive soils.
  1. Solicit grower-identified needs for managing soilborne diseases so as to improve outreach and extension efficiency.

Cooperators

Click linked name(s) to expand
  • Dr. Tom Gordon

Research

Materials and methods:

Suppression of Verticillium dahliae: Frequency of root infection assay

 Verticillium dahliae-inoculated sand was used to infest potting soil into which strawberry crowns of the cultivar Albion were planted. There were 5 treatments: non-amended control (potting mix only) and four compost-amended mixtures (manure, mushroom, yard trimmings, and vermicompost). Three to four replicate pots (3.79L) were established per treatment in a randomized complete block design. Plants were maintained in a controlled environment chamber with a 12 hour photoperiod and day/night temperatures of 25/18°C. After 21 days, roots were gently removed from the soil, cut at the mesh line, and stored in a plastic bag at 4°C for up to one week before being assayed. Composite soil samples were removed from each pot and assayed using dilution plating to determine the inoculum density of V. dahliae for each replicate pot (Gordon et al., 2002). This experiment was repeated three times.

 New roots were cut and washed, first in 1% sodium hexametaphosphate followed by two washes in sterile deionized water to release microsclerotia adhering to the outside of the root. Using a guide, 50cm of roots were carefully placed end-to-end on each of several plates of NP-10 (Kabir et al., 2004). When possible, a total of 200cm (4 plates) was assayed per replicate pot and placed in the dark at room temperature (24±2°C). After seven days, plates were placed under a dissecting scope and infection sites were identified by the emergence of V. dahliae colonies, which were identified by their distinctive pattern of microsclerotia formation. Data were expressed as the number of infections per unit length of root.  

Suppression of Pythium ultimum: Cucumber Surrogate Assay

 Cucumber is highly susceptible to Pythium damping-off caused by P. ultimum and was used as a surrogate in lieu of strawberry plants. Because infection occurs during seed germination, symptoms of damping-off develop quickly and provide a sensitive assay for disease suppression. Cucumber seeds (Cucumis sativas ‘Marketmore’) were surface sterilized in 1% sodium hypochlorite for two minutes, rinsed with sterile deionized water and air dried before use. Pythium ultimum was cultured on 1.5% water agar at room temperature. Treatments were prepared by amending autoclaved Sunshineâ potting mix (Sun Gro Horticulture) with 20% compost (v/v) and adding one 10 cm diameter Petri plate per 171ml substrate of 1.5% water agar colonized by P. ultimum. In the non-amended control, the equivalent number of non-P. ultimum-colonized 1.5% water agar plates were added. The ten treatments included: Sunshine potting soil only and Sunshine soil amended with the four previously described composts, with and without P. ultimum. For each treatment, four replicate pots (9.5cm L x 9.5cm W x 8.3 H, 573ml) were filled with treated potting soil, moistened to field capacity and maintained at 24°C. After three days, twenty surface sterilized cucumber seeds were sown per pot, placed in a randomized complete block design in a growth chamber (25°/18°C, 12h day/night, respectively), and irrigated with nutrient water (“Grow More 4-18-38” (no boron), combined with de-ionized water to achieve a macronutrient ratio of 2:1:2 fertilizer water). After eight days, emerging seedlings were enumerated and seedling stem height was measured. This experiment was repeated five times.

Microbial activity was measured by fluorescein diacetate hydrolysis

 Fluorescein diacetate (FDA) hydrolysis was used to provide a quick and sensitive measure of total microbial activity (Schnurer and Rosswall, 1982). Two gram soil samples of each compost were combined with 60mM potassium phosphate and 200μl of FDA. Spectrophotometer absorbance at 490 nm was used to measure the amount of FDA hydrolyzed. For all samples, soil moisture was determined and used to express microbial activity per gram of dry potting soil. For measurements of microbial activity in compost-amended field soil, a standard curve was generated for each compost.

 Total fungal and bacterial counts

 These were conducted by serial dilutions using an automated spiral plater of 50ul per plate on two media types, 1/10 strength tryptic soy agar amended with natacid to control for fungal growth and 1/10 diluted potato dextrose agar amended with antibiotics (chlortetracycline (100mg/L), streptomycin sulfate (100mg/L), and Neomycin sulfate (100mg/L) to control for bacterial growth. Plates were incubated in the dark at 20°C for 72 hours. Colonies were enumerated and converted to CFU/ml in the original suspension using an automatic colony counter (Flash & Go, IUL Instruments). Colony forming units per gram dry media were then calculated.

Data Analysis

 The Shapiro-Wilk test was used to confirm data were normally distributed. Analysis of variance (ANOVA) was used to test for significant treatment effects. Percentage data were transformed as the arcsin square root of mean recovery rates prior to analysis. Where significant F values were obtained, the significance of mean differences was assessed using Tukey post hoc multiple comparison test. All procedures were implemented using R statistical computing software (R Core Team, 2013).

 Grower needs assessment

We conducted a survey of California central coast (Monterey and Santa Cruz Counties) strawberry growers and grower advisors, including pest control advisors, private consultants, researchers and extension agents. Survey development included contributions from eight researchers and professionals in the strawberry industry. The survey collected 60 responses through dissemination at three grower meetings, online and through 168 mailings using addresses obtained from 2014 Pesticide Use Permits for strawberry production in Monterey County (Monterey Agricultural Commissioner Office). This represents approximately 30% of the central coast growers.

The question asking which alternative approaches to fumigation are being adopted /recommended was studied in more detail using a subset of 37 respondents. This data set consisted of yes/no responses for 37 people who provided answers to 13 questions about alternative practices for disease management. Positive responses were coded as 1s and negative responses as 0s. The binary data were used to calculate a distance matrix among respondents using the “binary” method in the R “dist” function. This procedure calculates the distance (as the complement of similarity) by assuming the joint presence of a binary character implies a similarity between any two respondents while a joint absence does not. The distance matrix was used to perform a principal coordinates analysis (PCoA) and a hierarchical cluster analysis (HCA). The HCA used a group-average clustering algorithm to construct a dendrogram linking all 37 respondents together.

Research results and discussion:

Suppression of Verticillium dahliae

Manure and mushroom composts are both high in salts and symptoms of salt damage were evident. Salt toxicity, which reduces root elongation (Munns, 2002), may lead to a higher density of root infections because roots are growing more slowly. Furthermore, saline conditions have been shown to increase susceptibility to V. dahliae (Levin et al. 2003, 2007). In field trials, plots amended with manure compost showed symptoms of salt damage and even death during early plant establishment. Whereas root development and total yield were comparatively good compared to other non-salt damaged plots, this twenty-day study evaluating frequency of root infection by V. dahliae was insufficient to leach the soil and allow plants to recover in the case with field trials.

Vermicompost had the lowest frequency of infection in two of the three trials, and second to lowest in the third trial. Although it was not significantly lower than the non-amended soil, it shows promise as a potential candidate for disease suppression, especially compared to other composts. One source of variability that may be obscuring potential differences is inoculum density. Accurately describing inoculum density in soil is a challenging component of this research.

Verticillium dahliae coexists with other members of the microbial community, which may influence the frequency with which V. dahliae infects roots. Because V. dahliae is not an aggressive saprophyte, when abundant and/or antagonistic soil microbes are present, it can be outcompeted for nutrients or root niches (Schreiber and Green, 1963). Microbial populations and activity increase following the addition of compost and remain higher than in non-amended soil for several months (Chapter 2). In the present study, increased microbial activity provided by compost, did not reduce the frequency of root infection. In fact, the non-amended control commonly had a lower frequency of infection than some compost amended treatments.

Suppression of Pythium ultimum

One of the aims of our research was to determine if composts derived from different source materials differ in their effects on disease suppression. Overall, when manure is excluded, the non-amended control treatment had the lowest percent emergence in four of the five trials in the P. ultimum-infested treatments, two of which were significantly lower than a compost-amended soil. Cucumber emergence in vermicompost without P. ultimum was the highest, above 90% in all trials. In the two trials where compost treatments had a significant treatment effect, yard trimmings compost provided suppression in both trials, and vermicompost and mushroom compost provided suppression in one trial. Previous research found that 20% compost (v/v) was commonly required to provide disease suppression, although suppression at lower incorporation rates was possible (Noble, 2005). Manure compost, which was phytotoxic at 20%, is not a good candidate for use at high incorporation rates. Lower rates were not tested, but should be considered for inclusion in future evaluations. These results are limited to describing the effect of compost on P. ultimum over 8 days and may differ based on duration of the experiment.

Percent emergence was the most useful indicator of the effect of P. ultimum infestation, whereas height was not despite usefulness in other studies (Chen and Nelson 2008). Used in combination with percent emergence, height may be useful as an indicator of late infection, if it in fact reflected stunting due to Pythium damping off. We did not assess seedlings past 8 days so this approach was not tested.

Significant experiment*treatment interactions reflect variation that is not associated with main effects. One potential source of this variation is the compost itself. Three different batches of compost were used during this study. Although the compost retailers are commercial facilities with compost recipes and controlled processes, natural variation in ambient temperature, source materials and storage durations may affect the microbial community. Differences in sources of organic matter can influence disease suppression, perhaps due to corresponding differences in the associated bacterial communities (Davis, 2011). Changes in organic matter composition with storage could also lead to different outcomes.

Pythium ultimum sporangia are stimulated to germinate by seed exudates (Windstam and Nelson, 2008). Within 6 hours, seeds can be fully colonized by P. ultimum and within 24 to 48 hours, may sustain high levels of embryo infection. The opportunity for disease suppression is before seeds are exposed to resident P. ultimum spores and/or between spore emergence and seed infection. It has also been documented that suppression of P. ultimum can be provided by a consortia of seed colonizing bacteria that are metabolically active in the spermosphere at the time of seed sowing (Chen and Nelson, 2008). Specific bacterial taxa have not yet been identified but nascent research suggests species of Bacillus and actinobacteria may play important roles. In our study, bacterial abundance and microbial activity associated with each compost were not predictive of the effectiveness of composts in disease suppression. A relationship has been established between disease suppression of Oomycete-induced damping-off diseases and high microbial activity in compost as measured by hydrolysis of fluorescein diacetate (Keener et al. 2000), as well as an association between seed colonizing bacterial communities and suppression. Yard trimmings provided the most consistent suppression, but had lower bacterial abundance and microbial activity than vermicompost and mushroom compost. Total bacteria in yard trimmings was 4.6 x 106 CFU per ml and microbial activity measured 0.11 ugFDA/gDw*min. This may suggest a threshold above which higher abundance or microbial activity does not enhance suppression.

Cucumber seeds were used as a surrogate for strawberries because they are highly susceptible to Pythium damping off and are easy to cultivate. Numerous studies have demonstrated that suppression is effective across plant species, like cucumber, pea, wheat, beetroot and bean (Chen et al., 1988, Chen and Nelson, 2008, Schuler et al., 1989). All of these plant species are started by seed and seed-colonizing bacteria may be the source of suppression. Commercial strawberry crops are established by planting dormant crowns. Bacteria stimulated in the rhizosphere of a newly emerging root may be different from those stimulated in the spermosphere. Whether these bacteria can provide the same type of suppression as the seed-colonizing bacteria remains to be determined. A model proposed by Weller et al. (2002) would suggest that bacterial suppression of root infections by P. ultimum is a general phenomenon of the rhizosphere, and so findings based on cucumber seedlings may be applicable to strawberry crowns.  

The results of this study demonstrate that suppression of Pythium-induced damping off can be achieved by incorporation of yard trimmings, vermicompost, and mushroom compost over an 8 day period. The significance of these effects was variable across experiments, perhaps due, in part, to differences between batches of compost from the same supplier. As the strawberry industry moves into the post-fumigation era, additional studies to confirm compost effects on suppression of strawberry root infections would be appropriate.

Grower needs assessment

Extension of research findings remains critical to grower adoption of new management strategies. Whether as written materials, oral presentations, on-farm field trials and demonstrations, or farm advising, the majority of growers rely on these sources of information for making decisions about adopting new practices. Cash incentives, crop insurance and higher prices paid for strawberries can incentivize a certain population of growers; however, they are also seeking knowledge-based resources to consider adopting new practices. Early adopters are important to advance research, but are not likely to be the primary or sole driving source of widespread adoption.

Information directly from people, such as fellow growers, public meetings, in-house meetings and pest control advisors, known as social capital, remains the most important mechanism of information dissemination. Information online, in books and from pamphlets were used by less than half of all respondents. The most widely used resource among respondents was the University of California, with 81% of respondents reporting use. The University of California was not clearly defined, but could include UC farm advisors, UC publications, and UC websites. The highest primary source of information are fellow growers and in-house meetings, both reported by 27% of respondents, followed PCAs (25%) and the University of California (20%). In-house meetings refer to meetings organized internally by grower cooperatives, of which most growers are members. “Fellow growers” was selected by 60% of respondents as one of their top three information resources. However, the University of California and public meetings had a strong response rate across all six rankings, suggesting that they are valued as both primary, secondary, and tertiary information sources as well as a supplement to other preferred information channels. For about half of respondents, public meetings are one of the top three resources, and for the other half, they are supplementary to the top three. Webinars, books, pamphlets, and web pages are not a replacement for in-person public meetings.

The disease management practices that growers were asked to respond to are a combination of innovative and cooperative processes. For example, alternative chemicals, ASD, and switching to non-fumigant reliant crops rely on innovation to succeed. Whereas selective leases, growing a broccoli rotation and avoiding land previously in lettuce require cooperation between land owners, vegetable growers and/or previous growers. Results show that growers are adopting practices that require both innovative and cooperative processes. Whether a practice is innovative or cooperative could influence the type of outreach efforts, the approach taken in a presentation, and the target stakeholders for information dissemination, that would maximize adoption. For example, innovation processes often require new technical skills sets and techniques that may be better communicated to growers through field demonstrations, early grower adopters, PCAs and technical meetings through the University of California or a private company. Whereas cooperative processes may benefit from meetings that include more stakeholders like vegetable growers or land owners, and materials that explicitly describe the disease management strategy as a cooperative process.

The most notable shared natural resource in strawberry production is uninfested, pathogen-free soil. Lettuce and strawberries are the two dominant high value crops grown in the central coast of California because of suitable climate and soil. Lettuce has benefited from pathogen-free soil following methyl bromide-fumigated strawberry rotation. However, in 1995, a lettuce field was decimated by disease in the Pajaro Valley in Watsonville, a predominantly strawberry growing region, and soon confirmed to be the first record of Verticillium dahliae causing disease on lettuce. Subsequent research showed that two to three successive lettuce crops will restore V. dahliae pathogen populations to greater numbers than pre-fumigation levels (Atallah, 2011). Consequently, a rotation that includes lettuce is not a good option for strawberry growers in the absence of fumigation.

The inclusion of broccoli as a rotation crop may be on the rise because there is interest among 91% of respondents. A cost-return study by Subbarao et al (2007) shows a broccoli-strawberry rotation to be profitable, despite the loss of yearly strawberry production. Whereas some strawberry growers see the positive environmental and profitable outcomes of including vegetables such as broccoli in a rotation, it can be difficult to adopt due to specialty equipment and/or skill set required for successful, commercial production. Outsourcing vegetable production is one option for addressing this problem.

With less control over seasonal land management and limited knowledge of pathogens that may be present, growers are becoming more selective of land leases. The only practice that conventional growers adopted/recommended more than organic growers is more selective land leases. On the California central coast, 10-12% of strawberry production is organic, so it is likely that organic growers have fewer options, due to organic certification requirements, compared to conventional growers. Growers and landowners are deciding the fate of available pathogen-free soil. Is pathogen-free soil a resource requiring greater governing and protection?

Managing a shared resource such as pathogen-free soil may require collective action and alternative processes to promote adoption. The traditional ‘diffusion of innovation’ model proposed by Rogers (2003) describes the process of information spread as including innovation, communication channels, time and a social system, and the adopters range from early to laggards. However, this widely accepted and applicable model assumes private benefits. Private benefits, those that directly benefit the decision maker, are easy to adopt when benefits out-weigh the costs. In contrast, practices that increase social benefits are more challenging for adoption because benefits to the adopter are not as direct or may not be as easily measured, such as the value of pathogen-free soil, or may depend on simultaneous adoption by a large number of growers.

Under conventional practices, soil fumigation with methyl bromide permits fields to be used as private economic goods by individual growers because fumigation mitigates disease carryover effects, as demonstrated by benefits to lettuce growers following strawberry production. Withdrawal of methyl bromide increases the mutual dependence of growers to maintain the economic value of fields as production units. Consequently, we can expect a shift toward management of a field as a social good requiring mutually beneficial management practices, as demonstrated in growers becoming more selective of their land leases. Management practices requiring collective action or cooperation are more challenging to adopt and the absence of a strategy for a shared resource could lead to a demise in saliency of the industry should land become less productive. Governance of natural resources like pathogen-free soil may become a good management strategy for the sustainability of strawberry production post-fumigation. There are benefits in agreeing on common rules and practices, coordinating usage, engaging in conflict resolution, sharing information, negotiating tradeoffs and building common knowledge (Folke, 2005). A strong social network and research extension system lay a good foundation for collective resource management for sustainability of the industry into the future. At a pivotal time when land is still productive but pathogens are becoming more widespread, a regional plan for maintaining uninfested, pathogen-free soil has an opportunity to emerge as the foundation for a sustainable industry in the absence of fumigation.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

PUBLICATIONS

Lloyd, M, and Tom Gordon. 2016. Growing for the future: Collective action, land stewardship and soilborne pathogens in California strawberry production. California Agriculture 70(3):101-103.

Lloyd, M., and T.R. Gordon. 2016. Evaluation of Four Commercial Compost on Strawberry Plant Productivity and Soil Health. International Journal of Fruit Science, submitted.

Lloyd, M., and T.R. Gordon. 2015. Strawberry: Plant Productivity and Management of Soilborne Diseases in the Post-Fumigation Era. University of California Press. PhD Dissertation.

Lloyd, Margaret and T.R. Gordon. 2014. Verticillium Wilt and Compost Amendments. University of California at Davis, 10 October, www.gordonlab.net/composting/?p=641. Accessed 25 August 2016.

ONLINE RESOURCES

Lloyd, Margaret and T. R. Gordon. “Compost for Soil Borne Disease Control”. http://www.gordonlab.net/composting/. WordPress.

Lloyd, Margaret. “Pivotal Times”. Online video. YouTube. September 17, 2014.

Contribution to the UC Sustainable Agriculture Research and Education Program’s Solutions Center for Nutrient Management ucanr.edu/sites/Nutrient_Management_Solutions/stateofscience/Compost/

PRESENTATIONS and POSTERS

Lloyd, M. 2015. The ABC’s of Soilborne Wilt Disease Management in Organic Production. Woodland California Association of Pest Control Advisors and Organic Fertilizer Association of California, Sustainable/Organic Production Seminar in the Sacramento Valley, oral presentation. December 3, 2015, Woodland, CA.

Lloyd, M. 2015. How cultural practices can influence productivity and plant health in strawberry UCCE Annual Disease Seminar, oral presentation. November 4, 2015, Salinas, CA.

Lloyd, M. 2015. The Effect of Four Commercial Composts on Disease Suppression and Strawberry Health. UCCE Strawberry Meeting, oral presentation. March 9, Sacramento, CA.

Lloyd, M. 2015. The Effect of Four Commercial Composts on Disease Suppression and Strawberry Health. UCCE Strawberry Meeting, oral presentation. February 20. Watsonville, CA.

Lloyd, M. 2015 The Effect of Four Commercial Composts on Disease Suppression and Strawberry Plant Health. North American Strawberry Symposium, plenary speaker. February 5, 2015, Ventura, CA.

Lloyd, M., and T.R. Gordon. 2014. Effect of four types of compost on strawberry plant health and productivity.  American Phytopathological Society Annual Meeting, poster. August 9-13, 2014, Minneapolis, MN.

Lloyd, M. Composting in California: Microbial Suppression of Soilborne Diseases. Composting in California California Environmental Protection Agency , California Deparment of Pesticide Regulation and CalRecycle brown bag lunch, oral presentation. 18 November 2014, Sacramento, CA.

Lloyd, M., and T.R. Gordon. 2013. Microbial Suppression of Soilborne Diseases in Strawberries. August 10-14, Austin, TX.

Lloyd, M., and T.R. Gordon. 2012. Effect of four types of compost on strawberry plant health and productivity. American Phytopathological Society Annual Meeting, poster. August 4-8, 2012, Providence, RI.

Lloyd, M., and T.R. Gordon. 2011. Evaluation of Compost for Plant Growth and Suppression of Soiborne Diseases. American Phytopathological Society Annual Meeting, oral presentation. August 6-10, 2011, Honolulu, HI.

Project Outcomes

Project outcomes:

Through on-farm research and significant outreach, there is/are

  • increased understanding of microbial suppression of soilborne diseases.
  • increased knowledge of the local compost materials and the impact it may have on the soil or soilborne pathogens.
  • additional tools for growers to select compost best suited for their farm and goals.
  • identification of soilborne disease management practices adopted by growers or interesting to growers.
  • awareness of the resources used by growers to learn about new practices.

Economic Analysis

No economic assessment was performed for this research. Compost suppliers were selected based on those used by commercial strawberry growers located on the central coast.

Farmer Adoption

This research developed significant baseline information that will help growers understand the differences between compost types. This baseline information included macro and micro nutrients, salinity, microbial activity, and microbial abundance.

Pythium ultimum was consistently suppressed by compost and provides compelling information for growers who are concerned about or currently need to manage P. ultimum caused diseases such as black root rot.

Verticillium dahliae was not consistently suppressed by compost, therefore, we cannot provide a recommendation to growers who are looking to manage this pathogen.

Several of the growers included in the field studies have either adopted or switched their compost source based on our collaboration.

In my new position as a UCCE farm advisor, I continue to work with the farmers in the Central Valley. Recently awarded grant funds are supporting ongoing work with these farmers, specifically to provide compost to their farms along with extension and outreach efforts. With continued outreach and education, I hope to increase farmer adoption over time.

Recommendations:

Areas needing additional study

In our research evaluation of V. dahliae suppression using compost, differences between treatments were observed but consistency was not been achieved. Further studies are needed to better understand how composts may affect the ability of V. dahliae to infect roots useful to elucidating differences in infection useful in developing management strategies.

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.