Mark Bolda: I think in many ways the soilborne diseases are probably the most constraining of the diseases and pests that we face in strawberries. Watsonville is a community that is supported by the strawberry industry. If as an industry we start to lose farms because we can't handle these soilborne diseases, that would be a tragedy.
Joji Muramoto: In this area, thanks to the climate, strawberry harvest usually starts in late March and continues until October or even November. But if plants have soilborne diseases, harvest can finish in June or July—so that is very big damage for growers. There are three major soilborne diseases of strawberries in California: Verticillium dahliae, the pathogen that causes Verticillium wilt; Fusarium wilt caused by Fusarium oxysporum; and charcoal root rot caused by Macrophomina phaseolina.
Steve Pederson: Verticillium is probably the number one most problematic soilborne disease. The problem with being a diversified organic grower is that we grow lots of vegetables that are potential hosts.
Chapter 2: Anaerobic Soil Disinfestation (ASD): Principles and Mechanics
Joji Muramoto: Anaerobic Soil Disinfestation, known as ASD, is a biological process that can control a range of soilborne pathogens using the principle of acid fermentation. There are three steps to doing ASD. The first step is to apply a readily decomposable carbon source to the soil, which increases the microbial activity in a very short period of time. Then we cover the soil with plastic. Then we use drip tape to saturate the pore space with water, which starts the anaerobic digestion of the carbon source we incorporated. We usually leave it for three weeks, during which anaerobic decomposition, like a fermentation process, takes place.
Carol Shennan: These fermentation processes are the key to a lot of the disease suppression that we get with ASD. When there is no oxygen in the soil, bacteria have to use other pathways than the normal respiration pathways to break down the carbon. And there are various byproducts produced—organic acids, volatiles—that are toxic to certain pathogens and pests. Different microbes flourish under that new environment. Not only is it different, but there are actually more bacteria and more fungi than we started with. So it’s not sterilizing the soil in any way—in fact we’re creating more biological activity in it—it’s just a different kind of community. One of the interesting things about that, is that it seems like that may confer some ability of the soil to resist future disease. It’s great to be able to control something immediately, but it’s even better if you can make a soil that’s more resistant to reinfection down the road.
Chapter 3: ASD: A Biological Process
Carol Shennan: With ASD, we are relying on the soil microbial community to do the work for us and they require particular conditions. We have to be careful about the soil temperatures when we do ASD. For certain pathogens like Verticillium, soil temperatures of around 70-75 degrees F are fine, but for other pathogens like Fusarium wilt, you need to have much higher soil temperatures for ASD to effectively control it. We have even found that the carbon source may be important—some carbon sources are better able to control a particular pathogen than another. How to manage the water to get good anaerobic conditions is going to be different if you have a heavy soil than if you have a more sandy soil.
That’s where we are with the ASD work at this point—we know that it can work for some things in some places, and now we are trying to work out how to optimize it for particular locations and particular pathogens.
Chapter 4: ASD: Growth & Challenges
Carol Shennan: Four or five years ago we had maybe 1 or 2 acres being tested. In the fall of 2014 we had 1,000 acres—which is a huge growth rate—and that wouldn’t have been possible without the partnership that we built from the beginning with a local company called Farm Fuel who imports all the carbon material, and they also provide technical assistance to the growers on how to do ASD. That has been really important—having that capacity to scale up.
Tim Campion: The potential of ASD that we have seen is favorable results with increase in yields in the plants, and overall health of the plants. It is pretty obvious just looking out in the field—comparing the ASD plants with the rows right next to it—the vigor of the plants and the health, and the stronger plants, better pest-resistance and disease-resistance. One concern is the cost with the increased labor and materials.
Jaime Lopez: Our first year doing ASD was only a 5-acre test plot and each year it has doubled. Right now we are at about 120 acres and we are about to add more acres in our other districts. The hurdles that we have when applying ASD is that we have a very scarce labor force. So trying to have a turnaround time of one week incorporating the ASD into the soil—putting the mulch, putting the drip tape, irrigating within a week’s time—I think is one of the biggest issues that we have.
Carol Shennan: The most successful growers with ASD start off doing it on a small area, working out the kinks and then scaling it up. Because it is a lot, you need to be able to have a way to get the carbon into the soil, get the beds made, and get the plastic on and apply the water as quickly as possible. Otherwise, that carbon is broken down aerobically, which won’t have the benefits.
There are a lot of mechanics to work out. We really recommend that growers talk to other farmers who are doing it, about how they have been able to get it to work, and then try it in a small area first.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.