Spatial and Temporal Patterns of Aspergillus flavus Strain Composition and Propagule Density in Yuma County, Arizona, Soils

Aspergillus flavus Link:Fr. is a soilinhabiting fungus that saprophytically utilizes a wide range of organic substrates. From an agro-ecological perspective, it is an important degrader of crop debris contributing to nutrient recycling (11). A. flavus first attracted the attention of cotton researchers in the early 1950s, when it was associated with a fluorescent fiber condition of raw cotton (14). About a decade later, cottonseed meal was implicated in a widespread epidemic of trout liver cancer (18). The cause was subsequently found to be aflatoxins produced by A. flavus (12). Cottonseed is a preferred feed for dairy cows and, with the demonstration that aflatoxins were passed from feed to milk, aflatoxin content of cottonseed became regulated closely. An aflatoxin content greater than 20 μg/kg prevents dairy use and associated premiums. Thus, the aflatoxin content greatly influences the market value of cottonseed. In Arizona, A. flavus isolates can be divided into two strains, S and L, on the basis of sclerotial morphology (S for small, L for large) (4). Each strain is composed of many vegetative compatibility groups (VCGs) (2). Gene flow is limited among VCGs and, as such, sites are occupied by communities of A. flavus VCGs rather than a single A. flavus population (2). Strain composition is determined by the percentage of isolates that are S strain or L strain. S strain isolates consistently (> 98%) produce large amounts of aflatoxin in infected cotton bolls, whereas L strain isolates on average produce much less aflatoxin, with some producing no aflatoxin at all (4). The two strains can be differentiated on culture media, and strain incidence can provide a rapid estimate of the toxigenicity of an A. flavus community. The toxigenicity of the A. flavus community in a field may be an important determinant of the extent of aflatoxin production within crops. The average toxigenicity of A. flavus communities differs significantly across the southern United States, as well as within Arizona, and is significantly correlated with S strain incidence (9). Spatial and temporal analysis of plant pathogen communities may provide insights that are useful in plant disease management. Initial spatial analysis of a variable requires determination of the scale or range of distance over which most variability occurs. Subsequent research may then be directed at the scale of greatest variance and emphasize interpretation of spatial patterns at the appropriate scale. Factors varying at a scale similar to the pathogen may then be sought and the putative influence of these factors on the pathogen empirically evaluated. Patterns caused by landscape, crop sequence, climate, and tillage may each differ in scale, and the influence of each factor may cause variance at different scales in a pathogen population. Determination of spatial scale can be efficiently done using a nested analysis of variance (15,17). In the current study, our primary goal was to determine the spatial scales in which most variability occurs for both A. flavus strain composition and propagule density. In the process, we also assessed both the temporal stability of spatial scale and temporal variability in A. flavus propagule density and strain composition.