Ridding soil of pathogens, weeds and nematodes remains an ongoing challenge for growers. To further research this problem, Dr. Francesco Di Gioia, assistant professor of vegetable crop science at Penn State, and Dr. Erin Rosskopf, USDA-ARS in Florida, worked on a multi-regional project funded by NIFA OREI (National Institute of Food and Agriculture/Organic Research and Extension Initiative).

The project was aimed of managing pathogens in vegetable cropping systems using anaerobic soil disinfestation (ASD). Di Gioia noted the important connection between soilborne pest management and soil health and explained various management options.

“One is the prevention or exclusion of the pathogens,” said Di Gioia, “and cultural practices including crop rotation, cover crops, organic amendments, conservative tillage practices and using resistant varieties and rootstock.”

Physical methods for managing soilborne pests and pathogens include solarization, soil flooding and soil steaming. The most often used strategy is chemicals, including chemical fumigation and chemigation. However, the goal is to eliminate the use of chemicals to the extent possible.

“A lot of specialty crops were using methyl bromide as a broad-spectrum fumigant,” said Di Gioia. “It was effective but dangerous to human health and the environment. The phasing out of methyl bromide led to research to discover alternatives to chemical fumigation.”

ASD was developed simultaneously and independently in Japan and the Netherlands. During World War II, growers in the Netherlands dealt with a major flood that lasted several years. Immediately after the flooding, bulb growers who had been dealing with nematodes and other soilborne pathogens realized that the flooding seemed to nearly eliminate pest issues.

Hoping they had discovered a solution to ongoing pest problems, growers attempted to replicate the flooding and found that eight to 12 weeks of flooding helped to control nematodes. At the same time in Japan, growers who flooded fields for rice production noticed similar pathogen control.

There was also research at the time to determine whether organic soil amendments helped manage diseases and soilborne pathogens. Researchers who were working on solutions throughout the world combined flooding with either soil solarization or organic amendments and saw positive results.

“Organic amendments have been used for centuries to improve soil health, and that has an impact on pests and pathogens,” said Di Gioia. “The combined principles of flooding with tarping and organic amendments led to the development of anaerobic soil disinfestation.”

The difference between solarization and ASD is temperature. “ASD is less dependent on nighttime temperature and can be applied in regions that don’t have high temperatures,” Di Gioia said.

ASD is applied in three steps. First, incorporate an organic amendment that is readily available to feed the microbes. “We’re providing sugar to activate microbes in the soil,” said Di Gioia, noting that the ideal C:N ratio is 30:1. “The other component is to tarp the soil with an impermeable film. That will allow the development of anaerobic conditions quickly in the soil.”

The third component is soil saturation. The soil isn’t flooded, just wet, which allows the distribution of byproducts which are the active ingredients that allow suppression of pests and pathogens.

The carbon source typically contains other nutrients such as nitrogen. “Incorporate the carbon source within the root zone, then mulch the soil with an impermeable film to limit gas exchange,” said Di Gioia, “then irrigate to saturation.”

These steps result in the rapid development of anaerobic conditions. Microbes feed on the carbon and develop a process similar to fermentation. Soluble sugar is transformed to organic acids, which are part of the active ingredients, and volatile organic compounds suppress pathogens. A combination of these factors results in suppression of pests and pathogens.

Rosskopf conducted extensive research and found ASD was comparable to traditional soil fumigation. She consulted with researchers from the Netherlands and Japan to further develop ASD. The target pests and diseases that had been previously controlled via soil fumigation included Phytophthora, Fusarium, bacterial wilt, southern blight, charcoal rot, root-knot nematodes and yellow and purple nutsedges.

“One of the most important things we learned from on-farm trials is that solarization as a requirement was unacceptable to growers,” said Rosskopf. “One of our goals was to conduct ASD under regular high-density polyethylene film.”

She said solarization was eliminated when total impermeable film (TIF) was used.

With information from Rosskopf’s trials, Di Gioia implemented research in Pennsylvania to determine whether high tunnel growers in the Mid-Atlantic and Northeast could successfully use ASD, especially in monoculture production that results in the proliferation of soilborne problems.

“We know that all three components – the carbon source (organic amendment), mulching and initial irrigation – are important for success,” said Di Gioia. “Also important are moisture, pH, temperature, CEC and nutrient content. All this is mediated by the soil microbial community.”

Because microbial activity is substantially reduced below 60º, the best ASD application window in the Mid-Atlantic is between April and October. Growers in the Midwest are also researching and adopting ASD.

Di Gioia said when applying ASD for the first time, the main consideration is the carbon source, which should be locally available and low cost, with physiochemical and microbiological properties suitable for ASD.

Several carbon sources are ASD options: cereal brans, vegetable waste, cover crops and molasses. Di Gioia compared two sources and found total carbon was much higher with molasses than with cover crops.

Research trials were conducted to determine whether ASD could be applied in spring for field-grown crops. Di Gioia used a variety of carbon sources including molasses, wheat middlings, soybean meal and poultry manure (which also provided nitrogen). Molasses showed the best anaerobicity level for a longer period.

Di Gioia concluded that it’s possible to apply ASD in spring, summer or autumn. The quality and quantity of the carbon source play key roles in treatment efficacy, nutrient availability and yield and quality of the following crop.

“It’s possible to integrate cover crops and organic amendments, which is one way to reduce the cost of the treatment,” said Di Gioia, adding that cover crop selection and amendments requires more research. “The C:N ratio of the cover crop and carbon source is key for ASD efficacy and impact on the crop following ASD. More research is need to evaluate the short- and long-term effects on soil microbes.”

by Sally Colby