Sexual Reproduction in Aspergillus flavus Sclerotia Naturally Produced in Corn

Aspergillus flavus is the major producer of carcinogenic aflatoxins worldwide in crops. Populations of A. flavus are characterized by high genetic variation and the source of this variation is likely sexual reproduction. The fungus is heterothallic and laboratory crosses produce ascospore-bearing ascocarps embedded within sclerotia. However, the capacity for sexual reproduction in sclerotia naturally formed in crops has not been examined. Corn was grown for 3 years under different levels of drought stress at Shellman, GA, and sclerotia were recovered from 146 ears (0.6% of ears). Sclerotia of A. flavus L strain were dominant in 2010 and 2011 and sclerotia of A. flavus S strain were dominant in 2012. The incidence of S strain sclerotia in corn ears increased with decreasing water availability. Ascocarps were not detected in sclerotia at harvest but incubation of sclerotia on the surface of nonsterile soil in the laboratory resulted in the formation of viable ascospores in A. flavus L and S strains and in homothallic A. alliaceus. Ascospores were produced by section Flavi species in 6.1% of the 6,022 sclerotia (18 of 84 ears) in 2010, 0.1% of the 2,846 sclerotia (3 of 36 ears) in 2011, and 0.5% of the 3,106 sclerotia (5 of 26 ears) in 2012. For sexual reproduction to occur under field conditions, sclerotia may require an additional incubation period on soil following dispersal at crop harvest.