Response to black root rot (Thielaviopsis tabacina Ferr.) of several flu-cured tobacco (Nicotiana tabacum L.) genotypes in different testing environments.

Several breeding lines and tobacco varieties were tested for degree of resistance to black root root caused by the fungus Thielaviopsis basicola. The entries in the study represented an array of forms from fully resistant to very susceptible. Artificial black root rot testing included „chlamydospore test” performed at cotyledonary stage and testing at post-transplant stage by either planting 5-leaf plants in black-root rot inoculated soil or immersing their roots in inoculum suspension prior to transplanting. The results were compared with those from a naturally infested field with two levels of the disease. The most inconsistent results were obtained from the chlamydospore test which failed to differentiate between different levels of tolerance of black root rot. It could only be used to make distinction between fully resistant and the remaining entries. Testing at the post-transplant stage showed, with one exception a rather good fit with the results from the field study, especially when transplants were transferred to pre-inoculated soil. Although the general order of response to black root under field conditions was similar regardless of disease severity, some varieties were more sensitive than others to an increased level of black root rot in the soil.