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Title: Community structure of Aspergillus flavus and A. parasiticus in major almond producing areas of California, United States

item DONNER, MATTHIAS - University Of California
item FARIA LICHTEMBERG, PAULO SANTOS - Universidade Federal Do Parana
item DOSTER, MARK - Kearney Agricultural Center
item PICOT, ADELINE - University Of California Agriculture And Natural Resources (UCANR)
item Cotty, Peter
item PUCKETT, RYAN - University Of Arizona
item MICHAILIDES, THEMIS - University Of California Agriculture And Natural Resources (UCANR)

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2014
Publication Date: 12/3/2014
Citation: Donner, M., Faria Lichtemberg, P., Doster, M., Picot, A., Cotty, P.J., Puckett, R.D., Michailides, T. 2014. Community structure of Aspergillus flavus and A. parasiticus in major almond producing areas of California, United States. Plant Disease. doi: 10.1094/PDIS-05-14-0450-RE.

Interpretive Summary: Aflatoxins are toxic fungal metabolites that impair the immune system and cause cancer in humans. These poisons are produced by a diverse group of fungi. In order to develop management tools for limiting contamination, it is necessary to identify the fungi that cause the majority of aflatoxin contamination and to understand which competitive fungi might be useful as management tools. Aflatoxin contamination of almonds has been a problem for the export component of the almond industry in California over the past decade. As part of efforts to identify the causal agents of aflatoxin contamination on almonds, fungi associated with almond contamination in California were examined. Aflatoxin producers were found to vary over the three almond-producing regions with A. parasiticus and the L strain of A. flavus the most common aflatoxins producers. Isolates of the L strain of potential value as biocontrol agents were found throughout the sampled area. The results suggest that biological control strategies could be successful over the entire range of almond production but that efforts may be needed to select the most competitive atoxigenic fungi over the cropping area.

Technical Abstract: Several nut crops including almonds, pistachios, and walnuts 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 A. 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 five-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 five year period, higher frequencies of L strain were more often observed in the southern region (79.9% up to 95.1%, depending on year) compared to the northern region (21.4% up to 47.1%). In the north, A. parasiticus was the most common strain with frequencies that ranged from 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 in 2008 (75% of the isolates produced aflatoxins) from 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 five-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.