Association of morphology and mycotoxin production with vegetative compatibility groups in Aspergillus flavus, A. parasiticus , and A. tamarii

Abstract

Vegetative compatibility groups within populations of Aspergillus flavus, A. parasiticus, and A. tamarii from soil and peanut seeds in a peanut field were examined for differences in morphology (colony color and Sclerotium characters) and mycotoxin production (aflatoxins, cyclopiazonic acid, and kojic acid). Aspergillus tamarii was divided into types A and B based on morphological differences and the lack of vegetative compatibility between the two types. Using digital color image processing, the four taxa were easily distinguished by colony color through analyses of peak color intensities for red, green, and blue. Color comparisons of A. flavus vegetative compatibility groups were not possible because of poor sporulation by many of the isolates. Vegetative compatibility group 1 of A. parasiticus differed significantly from groups 2–9 in colony color, and groups 1–3 of A. tamarii type A and groups 1–3 of A. tamarii type B were also significantly different within each type. Color image processing of filtered conidia indicated that the color difference of A. parasiticus group 1 was due primarily to the flocculose texture of the colony whereas group differences in A. tamarii types A and B were the result of conidium pigmentation. Aspergillus flavus and A. parasiticus showed significant differences among groups in number of sclerotia, Sclerotium volume, and Sclerotium shape (length/width ratio). Isolates of A. tamarii type B often produced irregularly shaped sclerotia; type A isolates were nonsclerotial. Among the 11 groups of A. flavus, significant differences were detected in total aflatoxin (aflatoxins B1 + B2), cyclopiazonic acid, and kojic acid. Aspergillus parasiticus groups also showed significant differences in total aflatoxin (aflatoxins B1 + B2 + G1 + G2), ratio of G1 + G2/B1 + B2, and kojic acid; cyclopiazonic acid was not produced by A. parasiticus. Nonaflatoxigenic isolates of A. flavus and A. parasiticus were restricted to certain groups and in A. parasiticus, all nonaflatoxigenic isolates accumulated O-methylsterigmatocystin, an immediate precursor of aflatoxin B1. Isolates of A. tamarii type A produced cyclopiazonic acid and kojic acid whereas those of type B produced only kojic acid at concentrations six-fold higher than type A; few differences in mycotoxin production were detected among groups in the two types of A. tamarii. The high proportion of variation among isolates accounted for by vegetative compatibility groups suggests that isolates within groups are closely related.