Gossypium barbadense (Pima cotton) was planted in a naturally infested field containing 600 cfu/g of Thielaviopsis basicola on two planting dates in 1986 and in 1988 near Coolidge, AZ. Soil temperatures ranged from 18-20 C and 24-26 C at a depth of 15 cm on the first and second planting dates, respectively. During both seasons, disease incidence approached 100% with 75-100% cortical root decay 1 mo after the first planting. Two wk after the second planting, there was an 89% disease incidence in 1986 and 92% in 1988 with 50-75% cortical root decay in both instances. Plant stands were reduced by 28% in 1986 and 32% in 1988 in the first planting and 11% in 1986 and 8% in 1988 in the second planting. In October 1986, 32% and 5% of the plants in the first and second plantings, respectively, had darkened stelar root tissues near the crown that contained hyphae and aleuriospores of T. basicola. G. hirsutum was grown at 20 and 28 C in a growth chamber in soils containing 0, 90, and 600 cfu/g of T. basicola. Seedling stunting and cortical decay increased with inoculum density at both temperatures but were more severe at 20 C. SEM demonstrated that phialoconidia and aleuriospores germinated, produced appressoria, and penetrated within 12 and 48 hr, respectively. Five days after inoculation infected cells were filled with hyphae and aleuriospores. Populations of T. basicola in the test field, in 1986 after 4 yr of planting cotton, were 596, 481, and 142 cfu/g 1, 2, and 7 mo after planting. During 1987 the field was split into two plots. One was planted with wheat and the other was fallowed. In 1987 populations of T. basicola remained stable. In 1988 disease incidence and populations of T. basicola were monitored in the wheat and fallow plots currently planted with Pima cotton. In February 1988 soil populations of T. basicola dropped to 87 cfu/g and 13 cfu/g in the wheat and fallow treatments, respectively. However, in April 1988, 1 mo after planting, Pima cotton growing in the wheat rotation plot had a disease incidence of 71% with an average cortical decay of 47%. In contrast, cotton planted in the fallow treatment had a disease incidence of 43% with an average cortical decay of 23%. At this time populations of T. basicola were 50 and 10 cfu/g in the wheat and fallow treatments, respectively.