Aspergillus flavus causes aflatoxin contamination in both cottonseed and corn. Corn-cotton rotations are common in South Texas, where reduced tillage frequently results in long-term residence of corncobs on soil surfaces. Corncobs are colonized by A. flavus either prior to harvest or while in the soil. This study sought to determine the potential of corncobs as sources of inoculum for cotton and corn in South Texas. A. flavus communities in corncob and soil samples were collected during the planting seasons of 2001 to 2003 from 29 fields extending from Calhoun and Victoria Counties in the north to the Rio Grande Valley. In order to assess persistence of A. flavus in corncobs, A. flavus communities in corncobs and soil were contrasted every 2 to 3 months in four fields throughout the 3-year study. To assess seasonal variation, similar contrasts were performed in two fields on a biweekly basis. The results indicate that corncobs are major sources of A. flavus inoculum. Corncobs from the previous season contained, on average, over 190 times more A. flavus propagules than soil from the same field, and 2-year-old corncobs still retained 45 times more propagules than soil. There was no significant difference in the incidence of A. flavus strain S on corncobs and soil. The quantity of A. flavus in corncobs decreased with corncob age (r
2 = 0.54; P = 0.002).