‘If it’s stinking, it’s working,’ says researcher studying decomposing carbon sources as alternatives to fumigation
NIAGARA FALLS, Ont. — An alternative to soil fumigation is catching on in one of the world’s largest strawberry production regions.
“Since the 1950s, fumigation has been the foundation of strawberry production in California,” according to Dr. Joji Muramoto, who spoke in Niagara Falls at the Ontario Fruit and Vegetable Convention in February.
“Anaerobic soil disinfection, ASD, is now being adopted very rapidly in the industry.”
The researcher at the Santa Cruz campus of the University of California said the practice was introduced in California in 2012. It’s now been applied to as much as 20 percent of all strawberry acres, including 80 percent of organic acres and about 20 percent of conventional acres.
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The shift to organic production, now more than 10 percent of production, and restrictions on the use of fumigants are driving the change.
The process begins with the incorporation of a readily decomposable carbon source. The soil is then moistened and covered with plastic mulch.
Moisture levels need to be maintained — California growers rely on irrigation — for the anaerobic decomposition process.
For California strawberries, the microbes require about three weeks to do the job.
In a trial in which rice bran was applied at nine tons per acre, Muramoto recorded yields 82 to 114 percent of those on soils treated with fumigants.
Verticillium dahliae was reduced by 80 to 100 percent and there was increased soil fertility, a reduction in pH and improved soil structure.
“If it’s stinking, it’s working,” Muramoto said.
There are limitations. Fusarium wilt disease in strawberries can be controlled only if soil temperatures are maintained at 30 C or higher for at least 300 continuous hours. Weed suppression may be limited, a lot of water is needed and the cost of rice bran can cost up to $3,000 per acre, although alternative carbon sources may also work and be less expensive.
Muramoto also suggested growers use a rotation and, for fusarium control, plant resistant varieties.
While rice bran has been the go-to carbon source in California, there are others.
These include molasses composted with broiler litter, molasses and wheat bran, and plant residue mixed with wheat bran. Efficacy can vary depending on the carbon source and the amount of the carbon source buried in the soil.
Researchers in Japan and the Netherlands, where the technology was developed, have determined it’s effective for a range of fungal and bacterial diseases, nematode pests and even weeds, although there’s a range in the efficacy.
Muramoto, who conducted research in Japan for 10 years before moving to the United States in 1996, suspects several modes of action are at play, including the production of organic acids, volatiles, reduction of iron and manganese in the soil, microbial community shifts, oxygen deprivation and reduced pH levels.
Tom Forge from Agriculture Canada’s Summerland Research and Development Centre in British Columbia, is also working with organic soil amendments. In this case the focus is on replant syndrome in orchards and strawberries.
“This can be devastating for orchardists. If you’re not getting up to full production quickly after planting new trees, it’s a big deal,” he said.
Forge said the syndrome is linked to pathogens and root lesion nematodes that have built up in the root zone. There may also be a connection related to restricted access to soil nutrients.
Fumigation has been the historic solution but regulatory changes, as in the U.S., have prompted a growing interest in alternatives.
A variety of options have potential, including the application of compost manure, seed meal and mulch.
“Compost is what we’ve concluded is the safest and most economic approach,” Forge said.
Forge said considerable research shows that a range of soil-borne pathogens can be directly suppressed with compost and by enhancing the health of the soil microbial community.
The concept is not new and has been in play since the 1930s, although not all treatments have been successful.
One interesting result arose from a cherry tree trial, part of thesis work by Tristan Watson.
In 2014, the best control of the root lesion nematode species, Pratylenchus penetrans, was achieved with a fumigant as compared to other treatments.
The following year, however, nematode numbers in the fumigated plots spiked, beyond those of even the control plot. Forge concluded this was likely because fumigants are non-selective, killing both pathogens and beneficial organisms.
Where compost or the combination of compost and mulch was applied, significant control of the soil-borne pest was achieved.
Sean Westerveld, ginseng and herb specialist with the Ontario Ministry of Agriculture, also spoke during the session.
Like the others, he said fumigation has been an industry mainstay for many agricultural crops but while it controls pests and pathogens, it has a negative impact on beneficial organisms.
There are three chemical fumigants registered in Canada, the registered biofumigant Mustgrow, which is a form of mustard meal, and mustard cover crops.
The challenge for growers is to seal the soil once the materials have been incorporated. Tarping is viewed as the best method but the soil can also be irrigated or rolled, using a power roller if possible, to help seal the soil surface.
When using registered products, Westerveld stressed that it’s important to follow the label. These usually involve a waiting period prior to crops being planted.
Fumigation is expensive, regardless of the type of material used.
With mustard crops, a mix of yellow and white types may provide control of a wider range of pathogens and may be less expensive, Westerveld said.
The plants need to be mown finely and incorporated into the soil as soon as possible and, in some instances, tarped.
During the discussion that followed Westerveld’s presentation, Forge said that other plant species, including sudan grass and pearl millet, have potential as biofumigants.
