Occurrence of race 3 of Phytophthora nicotianae in North Carolina, the causal agent of black shank of tobacco.
Black shank, caused by the oomycete Phytophthora nicotianae, causes significant annual yield losses in tobacco. Race 3 of P. nicotianae is reported here for the first time from North Carolina. It was identified from a North Carolina tobacco field with a history of tobacco varieties with Phl gene resistance and numerous field sites with no known deployment of varieties with the Phl gene. Race 3 was originally described from cigar-wrapper tobacco in Connecticut in the 1970s, but has not been reported in any other location since. Race 3 was defined as overcoming the Phl gene from Nicotiana longiflora but not the Php gene from N. plumbaginifolia. Stem and root inoculations were conducted on a set of host differentials to determine the virulence of North Carolina isolates. Stem inoculation was unable to distinguish between races 0 and 3 of P. nicotianae and is not a reliable method of identifying these virulence types. Race 1 gave a unique phenotype using stem inoculation. Root inoculation was the only reliable means of distinguishing between races 0 and 3. This is the first report of race 3 in North Carolina and the first report of damage to seedlings from root inoculations and to plants containing the Phl gene in naturally infested soil.