Effects of host resistance and soil fumigation on Thielaviopsis basicola and development of black root rot on burley tobacco.
Cultivars of burley tobacco with different levels of resistance to T. basicola were planted in fields naturally infested with the pathogen in North Carolina, USA. Severity of black root rot and populations of T. basicola were determined at transplant, midseason and after final harvest. Cultivars with monogenic resistance to T. basicola derived from Nicotiana debneyi developed little or no root rot in all tests and suppressed pathogen reproduction during the growing season. The severity of root rot and the reproduction of T. basicola were not related to the level of partial resistance. Cultivars with moderate levels of partial resistance frequently had higher root rot severity and similar or greater final populations of T. basicola than did cultivars with low levels of resistance. Moderately resistant cultivars gave good yields even with high root rot severity. The effects of soil fumigation with 1,3-dichloropropene, chloropicrin, methyl-isothiocyanate and combinations of these on disease and pathogen population dynamics were mixed. In some tests, fumigation significantly increased yield and decreased root rot severity and pathogen population; whereas in other tests, fumigation had little effect on the disease or the pathogen and failed to increase yields. Partial and complete resistance and, in some situations, soil fumigation were effective in the short- and long-term management of black root rot on burley tobacco.