A continuing concern about high aflatoxin levels in pistachios has led to some new recommendations for growers who use atoxic AF36 to out-compete the toxic strain of fungi found in orchards.
Aflatoxins are toxic chemicals produced by strains of the common fungi Aspergillus flavus and Aspergillus parasiticus. These fungi produce several types of aflatoxins, with B1 being the most potent. The regulatory limit in the U.S. for aflatoxins on food products is 15 parts per billion (ppb). The limit for the B1 strain is 10 ppb. The European Union limit is 10 ppb and 8 ppb for B1. Pistachio samples over those limits are unmarketable.
AF36 is a strain of Aspergillus flavus that occurs naturally in an orchard, but it does not produce aflatoxin. It can be found in orchards at 3 percent 13 percent of the fungi present. When the fungus is grown on sterilized seed and spread in an orchard, it has been shown to displace the toxic strains. The product is manufactured by the Arizona Cotton Research and Protection Council.
Themis Michailides, University of California plant pathologist and researcher at the Kearney Center, told growers attending the annual Pistachio Day event at Visalia that while initial AF36 trials were successful in reducing aflatoxin levels, efficacy has been reduced.
Starting in 2008, AF36 was used in commercial pistachio orchards, and initially achieved a 90 percent to 95 percent displacement of the toxic strains. In pistachio nuts, there was a 40 percent reduction on samples, and a 55 percent reduction on re-shake samples. Soil samples in 2012-2016 showed displacement at only 50 percent to 70 percent.
In 2012, AF 36 was labeled for use in pistachios as AF36 Prevail. The initial carrier was killed wheat seed, but that was replaced with sorghum after research showed higher sporulation rates. Jeff Chedester of Western Milling, a distributor of AF36 Prevail, says the killed sorghum was also found to be better carrier for AF36, and was easier to distribute in orchards. The product was labeled for almonds and figs in 2017.
Field testing found it was effective in displacing aflatoxin-producing strains, and that with annual applications it tends to build up in orchards.
Very high levels of navel orangeworm infestations in pistachios in recent years have contributed to increased levels of aflatoxins, Michailides says, but there are some other reasons why AF36 has been less effective in reducing aflatoxin levels. One was placement of the product. The wheat or sorghum seeds are killed so they won’t germinate, but the AF36 will not sporulate without some moisture. The product will also fail if it falls on very wet ground. The optimum soil moisture level is in the 13-18 percent range.
The current recommendation for rate is 10 pounds per acre, but besides needing the right amount of moisture, timing of application is important. Studies have shown the best time for placement in the orchard is mid-July.
Birds and insects can eat or damage the seeds. Another fungi, Fusarium, can also colonize on the seeds, reducing the amount of AF36 that can sporulate.
Suggestions to increase the efficacy of AF36 include better application methods and a higher rate. It is typically applied by ground, using ant bait spreaders. The 10 pounds per acre rate works well in field crops where AF36 is used, but Michailides suggests that higher rates might be necessary in orchards.
Applying AF 36 at the right time, and with optimal soil moisture conditions, will increase effectiveness, he says. Seeds should not be covered by raking, and no herbicides should be applied for at least two weeks. Controlling insects, especially ants, can also improve efficacy.
Michailides also offers suggestions to make AF 36 use more practical. Pre-wetting the seed carrier could allow spreading on dry ground, or a spore suspension could be applied to male flowers. An insect repellent seed coat could reduce insect damage.