SPATIAL AND TEMPORAL PATTERNS OF ASPERGILLUS-FLAVUS STRAIN COMPOSITION AND PROPAGULE DENSITY IN YUMA COUNTY, ARIZONA, SOILS

Aspergillus flavus isolates from Arizona can be divided into S and L s trains on the basis of sclerotial morphology. These genetically distin ct strains differ in aflatoxin production. To help understand factors influencing the aflatoxin producing potential of A. flavus communities , spatial and temporal patterns of strain incidence were compared with patterns of A. flavus propagule density in Yuma County soils. Strain S isolates were found in all sampled fields, but the percentage of str ain S isolates ranged from 4 to 93%. A nested analysis of variance was used to determine the spatial scale at which most variability in stra in composition and propagule density occurred. For both variables, the largest component of variance occurred among fields within areas at a spatial scale of 1 to 5 km. There was also spatial structure (12 to 2 1% of the variance) at the subregional level (> 20 km) in strain compo sition, but not in propagule density. Temporal patterns for both varia bles were similar. The sampling periods with the highest incidence of strain S isolates, August 1994 (60%) and July 1995 (62%), occurred dur ing cotton boil formation. The regional average for A. flavus propagul e density was near 1000 propagules/g in the summer, but less than 100 propagules/g in the spring. The results suggest that insights into fac tors influencing the toxigenicity and propagule density of A. flavus c ommunities might be achieved most readily by contrasting fields in clo se spatial proximity.