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Virulence and Cultural Characteristics of Two Aspergillus flavus Strains Pathogenic on Cotton. P. J. Cotty, Research Plant Pathologist, Southern Regional Research Center, U.S. Department of Agriculture, Agricultural Research Service, New Orleans, LA 70179; Phytopathology 79:808-814. Accepted for publication 22 March 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/Phyto-79-808.

Seventy Aspergillus flavus isolates from Arizona desert valleys were sorted into two distinct strains on the basis of sclerotial size, cultural characteristics, and virulence to cotton. Strain L isolates produced large sclerotia (over 400 m in diameter), and strain S isolates produced small sclerotia (less than 400 m in diameter). Strain S isolates produced greater quantities of sclerotia in a wider assortment of media and a broader temperature range than strain L isolates. Isolates of both strains exhibited pH homeostasis in culture. However, the strains maintained different pH values. Strain S isolates produced more aflatoxin than strain L isolates in culture; however, the strains produced similar aflatoxin levels in developing cottonseed. Strain L isolates were more aggressive than strain S isolates at deteriorating cotton boll locks (locules) and spreading within bolls, and this tendency partly explains the difference between in vitro and in vivo production of aflatoxin. The aflatoxin level in infected seeds was not correlated with either the aflatoxin level in vitro, intraboll fungal spread, or lock deterioration, but it was correlated with the product of the aflatoxin level in vitro and the intraboll spread parameter (r = 0.750.91). The correlation increased slightly (r = 0.780.93) when lock deterioration was factored into the equation. These results indicate that pathogenic aggressiveness contributes to the ability of an isolate to contaminate cottonseed with aflatoxins. The lack of correlation between pathogenic aggression and aflatoxin production in vivo suggests aflatoxin does not enhance virulence in the cotton-A. flavus interaction.