Community Structure ofAspergillus flavusandA. parasiticusin Major Almond-Producing Areas of California, United States

Abstract

Several nut crops, including almond, pistachio, and walnut, can become contaminated with mycotoxins. Of greatest economic significance are aflatoxins, which are mainly produced by members of Aspergillus section Flavi. The distribution of the two sclerotial-size morphotypes of Aspergillus flavus (i.e., S and L strains) and A. parasiticus, the main species responsible for aflatoxin production among section Flavi, was monitored in the soil of almond orchards in California over a 5-year period from 2007 to 2011, excluding 2009. In total, 4,349 Aspergillus isolates were collected from 28 almond orchards located in the northern, central, and southern Central Valley in California. Overall, A. flavus L strain was the most frequent, followed by A. parasiticus and A. flavus S strain. However, variations in the spatial distribution of these three taxa were found between the three regions. Over the 5-year period, higher frequencies of L strain were more often observed in the southern region (79.9 to 95.1%, depending on year) compared with the northern region (21.4 to 47.1%). In the north, A. parasiticus was the most common strain, with frequencies of 28.5 to 61% for the various years. In addition, the frequency of aflatoxin-producing isolates among L strains fluctuated from year to year. A significant increase (P = 0.0001) was observed from 2008 (75% of the isolates produced aflatoxins) to 2007 (59%), and a decrease was observed from 2010 (61%) to 2011 (53%). Aflatoxin-producing L strain isolates were significantly more prevalent than atoxigenic isolates in each region during the 5-year survey, except in 2011 in the north, where more isolates were atoxigenic (56%) than aflatoxin-producing (44%). Our results indicate that the structure of A. flavus and A. parasiticus communities in the soil and the proportion of toxigenic isolates vary across regions and years. Such knowledge may help devise appropriate aflatoxin control strategies, including the use of atoxigenic isolates as biological control agents adapted to the soil environments in each region.

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

“…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].…”

The vegetative compatibility group to which theUSbiocontrol agentAspergillus flavusAF36 belongs is also endemic to Mexico

“…The enzyme coded by this gene is essential for aflatoxin biosynthesis, and thus the aflC SNP confers atoxigenicity to YV36 and several other atoxigenic VCGs (Ehrlich and Cotty 2004). However, SNPs, insertions, and deletions in the aflatoxin biosynthesis gene cluster all may cause atoxigenicity within A. flavus (Chang et al 2005;Donner et al 2014).…”

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

“…Influence of atoxigenic VCGs on aflatoxin contamination of both almond and pistachio kernels co-inoculated in y All A. flavus associated with almond were recovered from almond orchard soils and were previously classified as atoxigenic based in earlier studies (Donner et al 2015;Picot et al 2018). Isolates from fig and pistachio agroecosystems belong to the fungal collection of our laboratory and were not previously screened for aflatoxin-producing potential; − indicates that isolates from those counties were not available (the crops are either not grown on those counties or the fungal collection does not contain isolates originating from those counties).…”

“…For example, in almond, isolates of CAG, CAM, CAP, and CA012 were more efficient against A. parasiticus. A strain of one of those VCGs may increase the efficacy of a biocontrol mixture, especially in Northern California, where A. parasiticus thrives (Donner et al 2015).…”

“…In California, over the last decade, there has been an increase in unacceptable aflatoxin levels in pistachio, almond, and fig (Donner et al 2015;Doster et al 1996Doster et al , 2014Picot et al 2017). This has resulted in rejection of tree crop lots, preventing entry into European markets (Bui-Klimke et al 2014;RASFF 2017).…”