Influence of Aspergillus flavus strains on aflatoxin and bright greenish yellow fluorescence of corn kernels

The objective of this study was to relate the diversity of a naturally occurring population of Aspergillus flavus to their ability to contaminate grain with aflatoxin and produce bright greenish yellow fluorescent (BGYF) kernels. A total of 19 strains of A. flavus isolated from a corn field near Kilbourne, Illinois were used as inoculum, including 16 genotypes (DNA fingerprinting), and representing both aflatoxin producers and non-producers. A commercial corn hybrid (Pioneer 3394) was grown in this field in 1996 and 1998. A total of 20 ears in the late-milk to early-dough stage of maturity were inoculated with each A. flavus strain using a toothpick-wound procedure. At harvest, 20 to 24 of the kernels nearest to each wounded site were separated into three categories: wound-inoculated kernels, intact BGYF kernels, and all other intact kernels. Sample weights of intact BGYF kernels in 1996 and 1998 grain samples averaged 5.0 and 9.5% of the total sample weight, respectively. Aflatoxin-producing strains were associated with a higher frequency (P < 0.05) of BGYF kernels for grain samples harvested in 1998. Removal of the individual wound-inoculated kernels and the intact BGYF kernels from corn ears inoculated with 13 aflatoxin-producing strains of A. flavus lowered mean aflatoxin values from 115 ng/g (range = <1 to 387 ng/g) to 2 ng/g for 1996 grain samples and from 744 ng/g (range = 20 to 1,416 ng/g) to 33 ng/g for 1998 grain samples. Results indicated substantial variation among A. flavus genotypes in their ability to produce aflatoxin in the germ and endosperm of infected BGYF kernels. The naturally occurring A. flavus population may include a majority of strains that produce no aflatoxin but exhibit BGYF and are thus aflatoxin "false positives" when corn grain is examined with an ultraviolet light at 365 nm. Intraspecific competition between aflatoxin-producing and non-producing strains would be expected to naturally suppress the severity of aflatoxin outbreaks within the Midwestern corn belt.