Community Structure of Aspergillus flavus and A. parasiticus in Major Almond-Producing Areas of California, United States.

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.