Anaerobic soil disinfestation (ASD) use is growing, particularly among organic strawberry growers. While it seems like a newer practice, research in strawberry has been ongoing for well over a decade, and farmers in various crops are considering it as an alternative to fumigation.
What is it?
ASD is a method of disease and pest control that is being considered as an alternative to fumigation, particularly as fumigation tools become fewer and/or more regulated.
The treated area receives an application of substrate, drip tape is laid over it, and is then covered with plastic. The plastic helps to limit oxygen and increase soil temperatures, creating an anaerobic environment that encourages microbial activity that decomposes the carbon source and prevents weeds and disease.
The current practice is leaving the soil covered for 2-3 weeks, though the longer the plastic is down, the better the practice works. The substrate, in most cases for strawberry, is rice bran due to its carbon content and digestibility for soil microbes. It’s been used mostly in organic strawberry operations, as well as some organic cane berries, but the hunt is on for alternative substrates.
Exploring substrate options
For the last few years, researchers have been working with middlings (midds), a byproduct of flour production. There has also been some work in dried distiller’s grain (DDG) and enzyme-digested grains, both of which are byproducts of ethanol production. They’re also both used as dairy feed.
These substrates are very similar to rice bran in terms of composition of carbon, nitrogen, and micronutrients, but they also happen to be slightly more cost effective than rice bran.
“Which is what everybody likes to hear,” said Oleg Daugovish, University of California (UC) Cooperative Extension strawberry and vegetable crop advisor in Ventura County.
“We found out in the last couple of years that DDG worked pretty well as far as suppressing pathogens and enhancing strawberry growth,” he said.
ASD frequency is at grower discretion. Daugovish believes there is a beneficial effect in annual application because the practice gradually improves the soil. But budgeting for the annual purchase of a carbon source can strain some growers, which is why research is looking for more affordable options.
“If you skip it for two or three years, you might reverse a trend, but it seems like a stepwise improvement in soil health in terms of pathogen density in soil, soil structure, soil capacity, and so on,” said Daugovish. “And, obviously, the end result would be the performance of your crop.”
As everything is on the rise in terms of cost, it becomes a question of proportionality to each other to really determine where ASD falls in the pricing spectrum. With the availability of the carbon source, ASD would be fairly competitive with fumigation.
Can ASD be harmful?
Growing climates vary, and ASD isn’t suitable for every operation.
For example, fusarium is difficult to control in cool soils, and the pathogen can proliferate in an ASD environment that provides an ample food source.
On the Central Coast of California, “We don’t recommend ASD in fusarium-infested fields because you never know what kind of temperatures you’re going to get during those 2-3 weeks of ASD. You might have a nice warm period, but then you put your rice bran down onto infested soil, then you have June gloom, the temperatures drop into the sixties, and you might have the opposite effect,” Daugovish said.
It can work when the soils remain warm. But with the coastal climate fluctuations, there’s too much risk.
The timeline
Planting and harvest schedules are unforgiving. A two- to three-week window not only allows for the ASD process to work, but is a doable length of time for growers. However, research shows that maintaining ASD conditions for up to five weeks is ideal.
An Oxnard, California, grower for the specialty and direct sales market dedicated five weeks to ASD in summer for a fall planting and had phenomenal success in controlling both macrophomina phaseolina (charcoal rot) and yellow nutsedge. This demonstration field had several acres of both rice bran and wheat midds, with both having great results.
“This was like the Cadillac version of ASD,” Daugovish said.
The wheat midds, which researchers are hoping to be an economical alternative source, had a 95% suppression of macrophomina, and about a 95% control of nutsedge, “which is phenomenal for an organic treatment,” said Daugovish.
It does require a significant time investment and close attention to proper implementation, all on a very rapid schedule.
“Most folks don’t have time to do this. We’re turning things around, and people cannot wait. They have to keep moving,” Daugovish said. “But we learned that the longer you marinate [the soil], the more efficacy you get. So, we have to find a golden middle — how long to keep it in our conditions.”
Food waste
Food waste. More than a buzzword, it’s a very loud and significant issue. Strawberry researchers are exploring the potential use of dehydrated food waste from the state’s municipalities as a possible substrate.
Daugovish analyzed buckets of processed food waste samples he received from dehydrators, landing on a product from Harp Renewables for the trial plot that was planted in early October 2024, after favorable results from a small-scale trial that previous April.
Whether from school cafeterias, hospitals, or restaurants, once food waste is dehydrated and processed, it resembles the look of soil and has a consistent composition with a good carbon-to-nitrogen ratio and good pH. It also doesn’t smell and contains no pathogens.
Ideally, the food waste would ultimately become a resource for ASD, which would then be used to produce healthy strawberries.
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