Study suggests combination of organic management techniques can minimize soilborne disease in strawberries

June 18, 2021


To identify the most effective methods to maintain strawberry yields and manage soilborne diseases using organic, economically viable techniques, UC Santa Cruz researchers studied the effects of crop rotation combined with biological alternatives to soil fumigants in strawberry fields at the UCSC Farm. The team of researchers included Carol Shennan, professor of Environmental Studies, and Joji Muramoto, UC Cooperative Extension Organic Production Specialist based at the Center for Agroecology & Sustainable Food Systems (CASFS). 

Soilborne diseases, particularly those caused by pathogens Fusarium oxysporum f. sp. fragariae (Fof), Macrophomina phaseolina (M. phaseolina), and Verticillium dahliae (V. dahliae), can devastate strawberry plants and have been a major driver of heavy chemical soil fumigant use in conventional strawberry production. These diseases present a major threat to organic strawberry growers. 

There are some organic techniques that have shown promise in mitigating the presence of soilborne pathogens in strawberry fields, including crop rotation and treating soil with a technique known as anaerobic soil disinfestation (ASD), a biological alternative to soil fumigants that involves incorporating rice bran into the topsoil, covering the soil with a plastic tarp to limit gas exchange, and adding water to initiate and maintain fermentation for at least three weeks. 

However, these methods come with some caveats: 1. Effective crop rotation can be challenging for growers because many crops, cover crops, and weeds host the same pathogens that affect strawberries; 2. While ASD is effective in managing M. phaseolina and V. dahliae in lower fall temperatures, it requires higher summer temperatures to be effective against Fof; because summer is the prime crop growing season, performing ASD during this time period means that growers lose out on income from crop sales. 

The researchers set out to find organic options for managing soilborne diseases that are not only effective, but also economically viable. They conducted an eight-year research trial at the UCSC organic farm to study the effects of crop rotation combined with either ASD or mustard seed meal, which has shown effective pest suppression when used as a soil treatment in some cropping systems but has not been extensively studied in organic strawberry systems.They assessed disease suppression and yield performance in both approaches

They compared a two-year crop rotation and a four-year crop rotation growing either broccoli, which suppresses V. dahliae, or lettuce, which hosts this pathogen, before planting strawberries. Within each of these rotation trials, four soil treatment methods were established: 1. a winter cover crop mixture of cereals and legumes with compost and fertilizer, plus late summer ASD preceding strawberry planting; 2. a cereal winter cover crop and incorporation of mustard seed meal; 3. a winter cover crop mixture of cereals and legumes with late summer ASD preceding strawberry planting; and, 4. bare, fallow soil followed by strawberry fertigation.

The researchers found that strawberry yields were highest when ASD was used in four-year rotations, while the two-year rotations showed higher strawberry wilt symptoms that negatively affected yield. ASD noticeably improved yields in the rotations that included lettuce, whereas mustard seed meal-treated fields resulted in higher strawberry yields when rotations included broccoli. However, the broccoli rotation-mustard seed meal combination produced equivalent strawberry yields to the broccoli rotation-ASD combination. ASD treatments in both two-year and four-year rotations resulted in strawberry yields significantly higher than mustard seed meal-treated and bare, fallow soil across all rotations.

An analysis of strawberry crown tissues showed that Fof, M. phaseolina, and V. dahliae were all present in the field, but while ASD did not eliminate the pathogens, it sufficiently suppressed disease to greatly enhance strawberry yields. Additionally, the presence of Fof was greater in the two-year rotations than the four-year rotations, demonstrating the effectiveness of a longer period between crop rotation for this host-specific, prevalent soilborne disease.

"Unlike chemically intensive conventional systems, soilborne disease management in organic systems relies upon complex ecological processes and requires integration of multiple approaches,” said Muramoto. “Depending on the production goals, each grower has to develop a site-specific strategy for soilborne disease management by integrating available tools such as crop rotation, use of resistant varieties, and ASD."