Ag Alert article describes fumigation alternatives

Strawberry growers rid soil of diseases

Ag Alert is a publication of the California Farm Bureau Federation

Issue Date: September 3, 2014

By Bob Johnson

Jonathan Winslow, farm services manager for Farm Fuel Inc. of Watsonville, checks a field of strawberries that had undergone anaerobic soil disinfestation to rid the soil of disease pathogens.
Photo/Bob Johnson

With the clock running out on the use of methyl bromide to fumigate before planting strawberries, an increasing number of growers are showing interest in a method of ridding the soil of disease pathogens without chemicals.

In anaerobic soil disinfestation, tons of rice bran are incorporated into the soil, the ground is tarped, irrigated to capacity and kept saturated for three weeks, and then holes are punched in the tarp for planting.

The California Department of Pesticide Regulation is providing grant funding to expand the commercial use of anaerobic soil disinfestation, or ASD, and University of California, Santa Cruz, researchers continue to explore the finer points of managing the system.

“Last year, people moved from one-acre trials to entire blocks,” said Jonathan Winslow, farm service manager for Farm Fuel Inc. of Watsonville. “For 2014, we’re already past last year, and we’ll probably jump up when we get to the late-year planting in the Watsonville and Salinas area. We are working to implement ASD on a commercial scale.”

Farm Fuel contracts with farmers to supply rice bran, with or without mustard meal, and monitors changes in soil oxygen, moisture and temperature.

Growers used this method to prepare 130 acres of ground for berries in 2012, then expanded to more than 430 acres in 2013. And disinfestation is attracting interest from a broader group of growers, as use in conventional berries increased from just five acres to more than 120 acres in one year.

“We’ve gotten between 85 and 110 percent of fumigant yields with nine tons of rice bran,” said Carol Shennan, UC Santa Cruz professor of environmental sciences. “We’ve gotten an 80 to 100 percent decrease in verticillium, which means you may still get the disease if you start with very high levels.”

The system works to control diseases, observers said, but it does not work with the consistency of fumigation with methyl bromide plus chloropicrin.

Winslow cautioned that growers must carefully calibrate the application rate of the rice bran, incorporate it evenly, rapidly apply the plastic mulch and completely saturate the beds.

“The first irrigation is very important. Use one inch for sandy soil and two inches for clay soil,” said Joji Muramoto, UC Santa Cruz associate researcher in the department of environmental studies. “Start irrigating and tarping soon after incorporating the carbon. Maintain water at field capacity for three weeks. The total amount of water will be three to five inches, including the first irrigation.”

Because it is particularly important to tarp and saturate the ground as soon as possible after incorporation of the rice bran, growers are finding it works best to disinfest relatively small areas at a time.

“We just do five- to eight-acre blocks at a time. There’s no way I would go 20 acres at a time,” said Manuel Mercado, who grows both conventional and organic berries near Watsonville and Salinas at River Valley Farms.

He said that ASD gives him an alternative to fumigation on ground with disease problems, particularly with verticillium wilt problems.

“The first year, we tried just one acre with the rice bran, but last year we tried 20 acres of strawberries and 15 acres of raspberries,” Mercado said. “If you have disease pressure, it’s a good tool to use. If you don’t have pressure, you have to think about the cost.”

Mercado and other growers are hoping to reduce the cost of ASD by applying rice bran at less than nine tons an acre.

“I went to six tons an acre and I saw no difference. It’s about $1,000 less per acre,” Mercado said. “It costs about $3,000 an acre for strawberries, including the plastic and the labor.”

It only costs around $1,500 an acre to disinfest ground for raspberries using this lower rate of rice straw, according to Mercado.

Rice bran, molasses, wheat bran, crop residue and ethanol have all been used somewhere as the source of carbon, according to Shennan, and researchers are looking for more economical sources.

“Skin and seed grape pomace is 20 percent cheaper, and looks comparable to rice bran,” Muramoto said. “We are also looking at summer wheat and Sudan cover crops in combination with rice bran.”

Molasses has worked as a source of carbon in other areas, but not on the Central Coast. Researchers are working to learn if half rice bran and half molasses, with split applications, can be used to supply the organic matter.

Another complication is the early season flush of nitrogen from the carbon source, which should be considered in devising fertilizer plans.

“When you add nine tons of rice bran, you’re adding 300 pounds of nitrogen, and that’s a lot,” Shennan said. “With three trials that were randomized properly, we got no yield increase from pre-plant fertilizer when we used nine tons of rice bran, but I want to see more data.”

Mercado said he waits longer to apply fertilizer for berries planted in disinfested ground, as well as adjusting the chilling hours and planting dates for these unusually vigorous strawberries.

“Because of the nitrogen, the plants take off very fast, so you have to make sure they don’t collapse later. We don’t have to apply any nitrogen until May or June,” Mercado said. “You can see an increase in organic matter in your soil analysis right away. The nitrogen also went from around 20 pounds to around 100 pounds.”

Disinfestation does little to control weeds, according to Shennan, and management of the system must be tailored to fit different diseases.

“We need to know how we can make it work for specific pathogens by manipulating the carbon source, water and temperature,” Shennan said.

In general, warmer-weather disinfestation is more effective in killing disease pathogens.

“The higher the temperature, the stronger the disease control,” Muramoto said. “Verticillium can be controlled in the early fall; once you get to November, it’s too cold for verticillium in Watsonville. For fusarium, you need to do it in August or September with clear plastic.”

Verticillium can be controlled at soil temperatures of 65 degrees Fahrenheit or higher, but fusarium control takes 86 degrees.

He recommends using ASD in combination with other practices, like crop rotation and soil monitoring.

“If you have any disease, it’s very important to diagnose what disease you have, and what pressure,” Muramoto said. “Crop rotation is also very important. We had an outbreak of fusarium in the fields at Monterey Bay Academy because we didn’t rotate.”

With all the complications and caveats, disinfestation appears to bring benefits, even under marginal conditions, researchers said.

“It seems to be a somewhat forgiving process,” Winslow said. “Even in fields with high verticillium pressure and less than ideal anaerobic conditions, disease damage is still fairly low.”

He made his remarks while standing in a field of organic strawberries producing a decent crop growing in ground outside Watsonville that was heavily infested with verticillium, and was left untarped too long after incorporation of the rice bran.

The complications and limits of ASD seem more bearable in light of the limits of the alternatives.

“As people are moving away from methyl bromide, we’re seeing more diseases, and I expect to see nematodes in the next few years. Even if the field is prepped right and you apply chloropicrin under virtually impermeable film, we still see diseases creep in,” said Mark Bolda, UC Cooperative Extension farm advisor. “This is a critical juncture for the industry as we transition from methyl bromide to the alternatives. Methyl bromide has been used in the strawberry industry for 60 years and has done a good job of controlling the pathogens and weeds. Methyl bromide is going to end for caneberries this year, and for strawberries in 2016.”

(Bob Johnson is a reporter in Santa Cruz. He may be contacted at

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.


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