Evaluation of Tobacco Cultivars for Resistance to Races of Phytophthora nicotianae in South Africa

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Abstract
Black shank caused by Phytophthora nicotianae is a destructive root and stem disease of cultivated tobacco. P. nicotianae isolates associated with black shank in South Africa were selected in order to evaluate their aggressiveness, designate races of P. nicotianae and to determine which tobacco cultivars are resistant to them. Stem inoculations were conducted in the greenhouse using 32 P. nicotianae isolates from different tobacco growing regions in South Africa. The 32 P. nicotianae isolates differed significantly in levels of aggressiveness. Eight isolates were selected on the basis of geographical origin and virulence for race characterization using a set of differential tobacco cultivars. South African race 0 and race 1 isolates were used to evaluate black shank resistance of 11 commercially planted tobacco cultivars. Race 0 and 1 of P. nicotianae occurred in most of the tobacco growing regions. Commercially planted cultivars differed significantly in their resistance to race 0 and 1. Cultivars Vuma/3/46 and LK3/46 were highly resistant to both race 0 and 1 while cultivars LK33/60 and OD1 were highly resistant to race 0 but susceptible to race 1. Results of this study provide valuable information on cultivar resistance and selection of P.nicotianae isolates for future breeding programs in South Africa.