A lack of effective soil-borne disease management practices besides crop rotation and the high cost of weed management are two great challenges in organic strawberry production in coastal California.
Verticillium wilt, caused by Verticillium dahliae, is a lethal soil-borne disease for strawberries and poses a great threat for organic strawberry production in the state. To avoid this disease, organic strawberry growers have to implement long-term crop rotations with non-host crops of V. dahliae. Even with a longer crop rotation, however, the disease may not be avoided. This has been the case for the organic farm of the Center for Agroecology and Sustainable Food Systems (CASFS) at the University of California Santa Cruz (UCSC). This organic farm has been facing multiple outbreaks of Verticillium wilt on strawberries in spite of its 7-year crop rotation system. As a small-scale, diversified organic farm, it is difficult to rotate only with non-Verticillium hosts, and when host crops are planted this can result in increased populations of V. dahliae in the soil. In addition, weed management costs in organic strawberries can be very expensive; it is not unusual to cost as much as $2,000/acre or more.
To combat these challenges, the research team at UCSC, led by Dr. Carol Shennan, looked for alternative solutions for the wilt and weed control problems. They examined the effects of three approaches:
- Anaerobic (without oxygen) soil disinfestation (ASD), a biological alternative to methyl bromide fumigation developed in the Netherlands and Japan
- Mustard cake (MC) (a byproduct of biodiesel production) application into the soil
- Broccoli residue incorporation
These three potential solutions were applied individually and in combinations with the other treatments. Field trials were conducted in Watsonville, CA, the organic farm at UCSC, and a demonstration trial at the Agriculture and Land-Based Training Association, (ALBA), in Salinas, CA.
Results from these experiments showed disease suppression using ASD in combination with MC and alone. V. Dahliae microsclerotia number in the soil continued to be lower even after 2 years from treatment application. The broccoli residue application did not show any effect. Weed suppression using ASD or MC had limited effect. An economic analysis showed that the strawberry’s marketable fruit yield using ASD and ASD plus MC was highest in the organic systems. ASD plus MC was also comparable to the conventional fumigation systems for early to mid-season yields.
Overall the ASD application worked well in V. wilt suppression and yield increase. However, there was no additive or synergistic effect using MC alone or with the broccoli rotation. Yield increases using ASD appears to be caused by a combination of providing nitrogen early in the season and disease suppression late in the season. The net return above land costs increased approximately 30 percent using ASD and ASD plus MC, suggesting the economic benefits of ASD not only in strawberry production but also in overall crop rotations compared in the trials.