We appreciate Verteramo and Gomez’s Life Cycle Analysis (LCA) work on conventional and organic strawberry production systems in California [...]

Nitrogen management in organic systems is challenging because complex organic forms of nitrogen originating from compost, manures, crop residue, and other organic materials must be converted by microbes into mineral forms of nitrogen to become plant-available. This overview and worksheet will help you estimate a nitrogen budget.

Anaerobic soil disinfestation (ASD) has been optimized and adopted as an organic alternative to chemical fumigation by strawberry growers in California. The ASD process relies on mixing labile carbon sources into the soil to generate chemical, physical, and microbiological changes aiding suppression of the soil-borne pathogens and enhancing fruit production. Continued ASD adoption is hindered by the increasing cost of rice bran, currently the most widely used carbon source. To address this need and to find suitable and economical alternative carbon sources, we conducted field evaluations of locally produced or sourced plant-based products. ASD with incorporated grass-sod clippings spent grain and coffee grounds from one supplier provided a 47% to 83% increase in fruit yields compared to untreated soil, but coffee grounds from a different supplier decreased strawberry yields. Carbon, nitrogen, and their ratios had important impacts on the efficacy of ASD with coffee grounds and grape pomace. ASD with wheat midds at 20 t/ha provided strawberry yields similar to chloropicrin-fumigated soil while substituting 30% of rice bran carbon rate with on-site grown cereal cover crop biomass resulted in yields similar to the full rate of rice bran but at a reduced cost. As we continue exploring cost-effective methods of soil disinfestation, we utilize ASD integrated with other pest management tools, such as the use of resistant cultivars and crop rotation for sustainable production.

Charcoal rot and Fusarium wilt, caused by Macrophomina phaseolina and Fusarium oxysporum f. sp. fragariae, respectively, are major soil-borne diseases of strawberry that have caused significant crop losses in California. Anaerobic soil disinfestation has been studied as an industry-level option to replace soil fumigants to manage these serious diseases. Studies were conducted to discern whether Gramineae carbon input type, incubation temperature, or incubation duration influences the efficacy of this disease control tactic. In experiments conducted using low rate amendment applications at moderate day/night temperatures (24/18 °C), and carbon inputs (orchard grass, wheat, and rice bran) induced an initial proliferation and subsequent decline in soil density of the Fusarium wilt pathogen. This trend coincided with the onset of anaerobic conditions and a corresponding generation of various anti-fungal compounds, including volatile organic acids, hydrocarbons, and sulfur compounds. Generation of these metabolites was associated with increases in populations of Clostridium spp. Overall, carbon input and incubation temperature, but not incubation duration, significantly influenced disease suppression. All Gramineae carbon inputs altered the soil microbiome and metabolome in a similar fashion, though the timing and maximum yield of specific metabolites varied with input type. Fusarium wilt and charcoal rot suppression were superior when anaerobic soil disinfestation was conducted using standard amendment rates of 20 t ha-1 at elevated temperatures combined with a 3-week incubation period. Findings indicate that anaerobic soil disinfestation can be further optimized by modulating carbon source and incubation temperature, allowing the maximum generation of antifungal toxic volatile compounds. Outcomes also indicate that carbon input and environmental variables may influence treatment efficacy in a target pathogen-dependent manner which will require pathogen-specific optimization of treatment protocols.

Soil-borne diseases and nitrogen availability are important limits on organic strawberry production. A trial using suppressive crop rotations to combat Verticillium wilt was conducted to see its effects on strawberry yields and net returns using a split-split-plot design. An ANOVA analysis was run to understand determinants of net returns. Results show that the suppression of wilt through the planting of non-host crops such as broccoli before the planting of strawberries can have significant effects on yield and net returns, and that suppressive crop rotations are potentially commercially viable.

Anaerobic soil disinfestation (ASD) has been adopted in over 900 ha in California strawberry production as an alternative to chemical fumigation. Rice bran, the predominant carbon source for ASD, has become increasingly expensive. In 2021–22 and the 2022–23 field studies, we evaluated 20–30% lower-priced wheat middlings (Midds) and dried distillers’ grain (DDG) at 21,800 kg ha−1 (in 2021) and 17,000 kg ha−1 (in 2022) as alternative carbon sources to rice bran. The study was placed at Santa Paula, California in September of each season in preparation for strawberry planting in October. Soil and air temperatures were 18–26 °C during that time. After the incorporation of carbon sources into the top 30 cm of bed soil, beds were reshaped, and irrigation drip lines were installed and covered with totally impermeable film (TIF) to prevent gas exchange. Beds were irrigated to saturate the bed soil within 48 h after TIF installation. Anaerobic conditions were measured with soil redox potential (Eh) sensors placed at 15 cm depth in all plots. Both DDG and Midds plots maintained Eh at −180 to 0 mV during the two ASD weeks, while untreated soil was aerobic at 200 to 400 mV. Permeable bags with inocula of Macrophomina phaseolina, a lethal soil-borne pathogen of strawberry, and tubers of a perennial weed Cyperus esculentus were placed 15 cm deep in the soil at ASD initiation and retrieved two weeks later for analyses. Two weeks after that, holes were cut to aerate beds and ‘Victor’ or ‘Fronteras’ bare-root strawberries were transplanted into them. ASD with DDG reduced viable microsclerotia of M. phaseolina by 49% in the first season and 75 to 85% with both carbon sources in the second season. Both ASD treatments reduced tuber germination of C. esculentus 86–90% compared to untreated soil in one of two years. Additionally, Midds and DDG provided greater sufficiency of plant-available nitrogen and phosphorus compared to untreated soil with synthetic pre-plant fertilizer and improved fruit yields by 11–29%. ASD with these carbon sources can suppress soil pathogens and weeds and help sustain organic strawberry production in California.

A team of researchers conducted a replicated on-farm experiment with the break period between strawberry crops (continuous strawberries with broccoli residue incorporation, one-year break, two-year break, three-year break, and seven-year break) as the main plot and cultivar as the split plot in Moss Landing, Central Coastal California. We hypothesized that the use of non-host rotation crops for Verticillium wilt plus bio-fumigation with broccoli, incorporation of mustard cover crop residues, use of relatively resistant strawberry cultivars, and compost application would suppress disease sufficiently to grow strawberries successfully in rotation every two or three years. Although a positive correlation between break period and marketable fruit yield existed, integrated use of biological and cultural practices allowed one to three-year breaks to have a statistically similar yield as seven-year break for this low Verticillium dahliae pressure field over a five-year period. This article is not subject to U.S. copyright law.