AtoxigenicAspergillus flavusIsolates Endemic to Almond, Fig, and Pistachio Orchards in California with Potential to Reduce Aflatoxin Contamination in these Crops

Abstract

In California, aflatoxin contamination of almond, fig, and pistachio has become a serious problem in recent years due to long periods of drought and probably other climatic changes. The atoxigenic biocontrol product Aspergillus flavus AF36 has been registered for use to limit aflatoxin contamination of pistachio since 2012 and for use in almond and fig since 2017. New biocontrol technologies employ multiple atoxigenic genotypes because those provide greater benefits than using a single genotype. Almond, fig, and pistachio industries would benefit from a multi-strain biocontrol technology for use in these three crops. Several A. flavus vegetative compatibility groups (VCGs) associated with almond, fig, and pistachio composed exclusively of atoxigenic isolates, including the VCG to which AF36 belongs to, YV36, were previously characterized in California. Here, we report additional VCGs associated with either two or all three crops. Representative isolates of 12 atoxigenic VCGs significantly (P < 0.001) reduced (>80%) aflatoxin accumulation in almond and pistachio when challenged with highly toxigenic isolates of A. flavus and A. parasiticus under laboratory conditions. Isolates of the evaluated VCGs, including AF36, constitute valuable endemic, well-adapted, and efficient germplasm to design a multi-crop, multi-strain biocontrol strategy for use in tree crops in California. Availability of such a strategy would favor long-term atoxigenic A. flavus communities across the affected areas of California, and this would result in securing domestic and export markets for the nut crop and fig farmer industries and, most importantly, health benefits to consumers.

Present Status and Perspective on the Future Use of Aflatoxin Biocontrol Products

“…The four types of aflatoxins are associated with a myriad of health threats including stunting, impaired food conversion, immunosuppression, liver The magnitude of the aflatoxin contamination problem differs between developed and developing nations. Regulations and strict limits (tolerance levels) are enforced in developed nations to protect consumers from contaminated foods and feeds [12][13][14][15]. In contrast, regulations in emerging and developing nations, if they exist, are poorly enforced with few exceptions.…”

“…During a 10-year research effort, it was found both that VCG YV36, to which AF36 belongs, was the atoxigenic VCG most commonly encountered in California and that AF36 was successful in limiting aflatoxin content in commercially produced pistachio. Therefore, USEPA granted registration of AF36 for use in pistachio grown in California and Arizona (2012), and eventually (2017) in almond and fig in California [15,51]. The biocontrol product AF36 is now used in several states across the US, which makes it the most widely used aflatoxin biocontrol product in the world.…”

“…The US Texas Corn Producers Board is seeking the registration of FourSure [64]. In California, several atoxigenic VCGs have been identified by the University of California-Davis (UC Davis) in collaboration with USDA-ARS and different tree-nut growing organizations, and isolates in those VCGs are valuable resources as constituents of multi-isolate products to treat tree nut crops [15,65,66]. In North Carolina, combinations of atoxigenic strains have been tested in a field of a research station [67].…”

“…This can result in selection of isolates that may not be the most appropriate and widely adapted to limit aflatoxin contamination. Area-wide adaptation of atoxigenic VCGs can be revealed by examining several thousand A. flavus isolates associated with multiple crops collected in several hundred locations across a target country/region [15,26,47]. Such studies require intensive microbiological, chemical, and molecular tests.…”

“…Subsequently, production of aflatoxin is quantified, and reduction of aflatoxin contamination is calculated with respect to grains inoculated only with an aflatoxin producing isolate [61,77]. Competition experiments have also been conducted using almond and pistachio kernels because those were the target crops [15].…”