A new selective medium (TB-CEN) was developed for isolating Thielaviopsis basicola, cause of black root rot of tobacco, from soil. TB-CEN medium contains etridiazol and nystatin to inhibit the growth of undesired fungi, and unautoclaved extract from carrot to selectively enhance for the growth of T. basicola. Inoculum and/or population densities of T. basicola in five burley tobacco fields were 74-166 propagules per g of soil, and 0-12 propagules per g of soil in three other burley fields. Inoculum and/or population densities of T. basicola in 12 flue-cured and 2 sun-cured tobacco fields were 0-26 propagules per g of soil, and 101 and 402 propagules per g of soil in two other flue-cured fields. Environmental factors apparently had a strong effect on black root rot development, since root rot and plant stunting were severe in two burley fields that had 148 and 158 propagules per g of soil, but were not severe in the two flue-cured fields that had 101 and 402 propagules per g of soil. All of the cultivars planted in the four fields were susceptible. Black root rot was the major disease associated with the stunting of tobacco plants in the burley region of Virginia, but not in the flue- and sun-cured regions. No evidence was found to indicate that endomycorrhizae were involved in tobacco stunting in Virginia. T basicola inoculum density-disease severity studies were conducted both in soil-temperature tanks and in the field. Tobacco seedlings were grown in temperature tanks (20-23 C) for 30-31 days in naturally infested field soil (pH 6.5). For all cultivars tested (Burley 21, NC95, and Va Gold), the mean percent of roots that were rotted increased significantly (P=0.001) as inoculum density increased (R² range for regressions=0.93-0.97). Severe levels of root rot occurred at inoculum densities of 50-200 propagules per g of soil. Significant (P=0.01) reductions in plant growth occurred at inoculum densities as low as 5-10 propagules per g of soil. In a study conducted on a commercial burley tobacco (cv. B21-Ky10) field, inoculum densities of 150 and 683 propagules per g of soil were associated with moderate and severe levels of black root rot, respectively. Differences between soil-temperature tank and field studies appeared to be due to variations in environmental- and host-related factors. In another burley field study, the fungicide imazalil, which completely inhibited the growth of T. basicola when amended into agar media at a concentration of 1.0 μg a.i./ml, failed to control black root rot when it was added to transplanting water (50 ml/plant) at concentrations as high as 1,500 μg a.i./ml.