Aspergillus flavus in Soils and Corncobs in South Texas: Implications for Management of Aflatoxins in Corn-Cotton Rotations

Aspergillus flavus is the main causal agent of aflatoxin contamination in several agricultural products, including cottonseed and corn (2,11,15,22). Aflatoxins are toxic fungal metabolites that are limited in food and feed by government regulations throughout most of the world (26,32). A. flavus can be divided into S and L strains based on morphologic, genetic, and physiologic criteria (3,10,16,17). The S strain produces numerous small sclerotia (average diameter < 400 μm) and high levels of aflatoxins, whereas the L strain produces fewer, larger sclerotia (average diameter > 400 μm) and, on average, less aflatoxin (10,13,18). Aflatoxin producers from North America with the S strain morphotype produce only B aflatoxins (8). A. flavus S strain isolates have been implicated as important aflatoxin producers in several areas worldwide, including Arizona (10,13,25), Texas (14,19,20), Louisiana, Mississippi, and Alabama (13) in the United States, Thailand in Southeast Asia (28), and Argentina in South America (24). A. flavus typically lives as a saprophyte in the soil depending on organic matter to propagate and survive (1,6). Crop residues, including corncobs, are the main source of organic matter in soils where corn crops are produced and these support large populations of microorganisms (9,23,29,30). Unlike most crop residues that readily decay in agricultural soils (23), corncobs reside in soil for several years before being totally decomposed (5,7). Harvest and cultivation practices influence how long crop residues reside in soil (7). Cultivation practices vary widely in South Texas, with reduced tillage practices being very common in some areas. A high percentage of corncobs are colonized by A. flavus before their incorporation into soil (33), making them a potentially important source of inoculum for susceptible crops planted in fields previously cropped to corn. These infested corncobs may play a major role as reservoirs of A. flavus in corn–cotton rotations. The objective of this study was to examine the potential importance of corncobs both as a source of inoculum of A. flavus in corn–cotton crop rotations and as a refuge for the long-term maintenance of A. flavus communities in agricultural environments in South Texas.