The United States is the world’s largest producer of strawberries, producing over 36 billion pounds valued at $2.2 billion in 2012, and accounting for 29 percent of the total world’s strawberry production. California growers produce about 90% of the strawberries grown in the US. Consumption of strawberries in the U.S. has rapidly increased over the past 20 years, and strawberries are now the 5th most popular fruit with consumers. This increase in demand has in part been created by improvements in yield and year-round availability from domestic growers. However, the U.S. strawberry industry was built around the use of soil fumigation with Methyl Bromide (MeBr) to effectively manage a number of soilborne diseases, which are the primary impediments to economic viability. With the phaseout of MeBr, producers have shifted to the use of other fumigants, but this has led to increased crop losses to emerging diseases. Furthermore, increasing restrictions on alternative fumigants due to health and environmental concerns, especially in California, make the development of effective reduced/non-fumigant based disease control options critical for the economic survival of this specialty crop production system.
Biologically-based techniques, notably anaerobic soil disinfestation (ASD) have emerged as a promising alternative to soil fumigation in strawberries for control of some major soil borne pathogens.
What is ASD?
ASD works by creating anaerobic (no oxygen) soil conditions. This is done by incorporating readily available carbon-sources (like rice bran, grape pomace, grass hay, molasses etc.) into topsoil that is irrigated to saturation and covered with a plastic tarp to prevent oxygen moving into the soil. The tarp is then left in place and soil soil moisture maintained above field capacity for about 3 weeks. During this time soil microbes that thrive in the absence of oxygen are able to respire using the added carbon, which results in the build-up of anaerobic by-products that are toxic to pathogens, but that are degraded rapidly once the tarp is removed or after holes are made through the tarp prior to transplanting strawberries. Interestingly, unlike fumigation ASD leads to increased numbers of bacteria and fungi in the soil. We have also found that major shifts in the soil microbial community composition occur with ASD, in a manner that is dependent on the rate and type of carbon source added. It is thought that some combination of the by-products of anaerobic respiration (organic acids and other volatiles, reduced iron) and the changes in microbial community composition and numbers are responsible for controlling the pathogens. Microbial changes may also lead to longer term soil suppressiveness that reduces the re-colonization of the soil by pathogens. For more information see the posted webinar and the publications page. The following article summarizes the development and research on ASD and its use worldwide:
Shennan, C. Muramoto, J, Mazzola, M, Butler, D, Rosskoph, E., Kokalis-Burelle, N. Momma, K, Kobara, Y, Lamers, J. 2014. Anaerobic Soil Disinfestation for Soil Borne Disease Control in Strawberry and Vegetable Systems: Current Knowledge and Future Directions. Acta Horticulturae 1044:165-175.
Current status and research directions
In the 2014/15 season around 1000ac of strawberries and raspberries were grown using ASD, with grower interest increasing each year. Major challenges at this point are to improve the reliability of ASD for controlling a wider range of pathogens in different environments (soil type, climate), and to find ways to reduce the cost of the technique. We know that soil temperature, the level of anaerobic conditions obtained, the specific carbon source used, and soil type all can affect how well ASD controls disease and improves yield. So at this point we are focusing on understanding the mechanisms of how ASD works, and how to optimize it in terms of timing, soil temperature, and carbon source used for specific target pathogens. While use of rice bran as the carbon source and fall application has worked well for Verticillium control, for example, this combination is not effective against Fusarium.