“Sometimes you get a situation that’s a win-win,” says Bob Klein, manager of the Administrative Committee for Pistachios. “This is one of those.”
He’s describing a voluntary program California’s pistachio growers began adopting last year for treating their orchards with a natural fungicide to help protect their crop from aflatoxin contamination -- the industry’s top food safety concern.
It’s a win for consumers by significantly reducing the threat to their health posed by aflatoxins, one of the most carcinogenic of naturally-produced compounds. And, it’s a win for the pistachio industry by protecting the market for pistachios and by reducing the costs to growers and processors of controlling aflatoxins.
This programs involves the use of AF36, a non-toxic variant of the common soil fungus, Aspergillus flavus. When spread on the orchard floor, dew and soil moisture activate growth of the AF36 spores that colonize pistachios, preventing growth of other strains of this same fungus which produce aflatoxins.
By displacing these carcinogenic strains in the soil, AF36 offers growers a way to control aflatoxins directly. Up to now, they have relied on indirect measures, since no conventional fungicides have been developed to control the toxin-producing fungal strains. Roasting the nuts doesn’t destroy aflatoxins, either.
So, growers have directed their efforts at controlling the navel orangeworm, which, by feeding on the nuts, exposes them to aflatoxin infection. These measures include orchard sanitation to remove mummies which can harbor over-wintering worms, insecticide sprays targeted at the third generation NOW eggs and early harvest.
Meanwhile, processors spend millions of dollars a year sorting through the nuts to identify and remove those damaged by NOW and testing shipments for aflatoxin contamination. In the United States, processors are prohibited from shipping loads containing more than 15 parts per billion of aflatoxins. In Europe, the limit of allowable contamination is 10 parts per billion. Hundreds of thousands of pounds of pistachios are rejected each year because of excessive aflatoxin levels and must be resorted and retested.
Farmers have been using AF36 for the past several years to reduce aflatoxin levels in cottonseed, corn, and peanuts. Last year AF36 was registered for use in California pistachio orchards.
This followed 11 years of testing by scientists at the UC Kearney Agricultural Research and Extension Center in Parlier. In these trials, the amount of aflatoxin-contaminated nuts was reduced by as much 45 percent. Researchers expect even lower levels of contamination as the use of AF36 continues over time.
In 2012, California pistachio growers treated about 75,000 acres with this biofungicide, which is also approved for organic production. This year processors offered growers a premium of up to 2 cents per pound to treat their orchards with the material.
“We thought we’d get about 75 percent of California’s 200,000 acres of pistachios treated this year,” Klein says. “But, the incentive program worked so well that we ended up with AF36 being applied to, essentially, 100 percent of the acreage this year.
The treatment uses sterile wheat, inoculated with AF36, which is applied between mid-May and mid-July. That’s when soil moisture and air temperatures are most favorable for growth of the fungus and before the toxic strains of Aspergillus flavus release their spores.
“The AF36 spores get to the ecological niche first, preventing the toxic strains from developing,” Klein says.
The material, which costs $1 per pound, is broadcast over the berms at the rate of 5 pounds per acre using a modified ant bait spreader mounted on an ATV. Application cost is about $2 an acre.
“The AF36 treatment doesn’t eliminate the need to control NOW because consumers don’t like to eat insect-damaged nuts,” Klein says “But, for a relatively modest cost, the industry is getting back considerably more in product safety and economic benefits.”
In native soils, AF36 and the other non-toxic strains of Aspergillus flavus represent about 95 percent of all the strains of the fungus, Klein notes. However, after the soil is tilled and is used for crop production, the balance reverses almost completely in favor of the toxic strains
“By increasing the level of non-toxic strains with the AF36 treatment, growers are returning their soils to a native state, at least with respect to Aspergillus,” he says.
In the first year after treatment, the level of toxic strains in research plot soils was cut in half. After the second year it was reduced to just 10 percent of all the strains. A year later, the toxic strains represented just 5 percent of all the fungus strains.
Currently, the idea is to treat orchards with AF36 annually. However, in the future it may be possible to reduce application frequency to every other year or every third year, Klein notes.
Next year, the industry may replace sterile wheat with sterile sorghum as the carrier for AF36. The rate of spore formation with sorghum is higher and the round the seed makes it easier to apply than oblong-shaped wheat kernels, Klein says. Plus, the price of sorghum is lower.
“We have great confidence that AF36 will dramatically reduce aflatoxin levels in pistachios,” Klein says. “Buyers overseas, particularly those in Europe, are quite excited about it.”
Research continues on the use of AF36 to control aflatoxins in California’s almond and fig orchards.