Monitoring trends in Aspergillus flavus
S strain incidence in Arizona
 

*New*  Sentinel Sites for Monitoring Aspergillus flavus Community Structure in Southwestern Desert Agricultural Soils  *New*

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Background.  Aflatoxin is a potent carcinogen produced in cottonseed infected by the fungus, Aspergillus flavus.   The aflatoxin content of cottonseed is strictly regulated.  Seed lots with aflatoxin levels above 20 ppb cannot be sold for dairy feed and, thus, the economic value of contaminated cottonseed is greatly reduced. A. flavus  is a soil-inhabiting fungus that lives off of decaying plant and animal debris.  Recent research has shown that not all strains of A.  flavus  produce aflatoxin and that some strains producing little or no aflatoxin can competitively exclude aflatoxin-producing strains from cottonseed.  The strain composition of the A.  flavus  community in the soil is very important in determining the potential for aflatoxin contamination of cottonseed. 

Aspergillus flavus  strain S (upper right) and strain L lower left Photo by P. J. Cotty (Click for an expanded view)

Distiguishing the S strain.  There are two major groups of A.  flavus:  strain S and strain L.  Strain S isolates generally produce much more aflatoxin than strain L isolates.  Strain S isolates can be easily distinguished from strain L isolates on V-8 juice medium (see picture).    The percentage of A.  flavus  isolates from the soil that are strain S is one indication of the toxigenicity of the A.  flavus  community.  We have found that the strain composition is relatively consistent over time at sample locations until recently.   Research by Cotty is underway to determine whether inoculating the soil with naturally-occurring atoxigenic strains of A.  flavus  can influence the toxigenicity of the fungal community and thus, reduce aflatoxin contamination in cottonseed.  Understanding regional spatial patterns of A.  flavus  strain composition will be a key component of managing aflatoxin in cottonseed. 

Geographic information systems and geostatistics are used extensively to study spatial patterns of A.  flavus  S strain incidence and propagule density. For more background refer to 

References:

Nelson, M. R., Orum, T. V., Jaime-Garcia, R., and Nadeem, A. 1998.  Applications of geographic information systems and geostatistics in plant disease epidemiology and management.  Plant Dis. 83:308-319.

Orum, T. V., Bigelow, D. M., Nelson, M. R., Howell, D. R., and Cotty, P. J. 1997. Spatial and temporal patterns of Aspergillus flavus strain composition and propagule density in Yuma County, Arizona, soils. Plant Dis. 81:911-916.

Orum, T. V., Bigelow, D. M., Cotty, P. J., and Nelson, M. R. 1999. Using predictions based on geostatistics to monitor trends in Aspergillus flavus strain composition. Phytopathology 89:761-769.
 

and for more details follow the links beginning with:

Using GIS and geostatistics to plan sampling and analyze results.

A Case Study in Methodology: Aspergillus flavus

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