Relationship of Isolate Origin to Pathogenicity of Race 0 and 1 of Phytophthora parasitica var. nicotianae on Tobacco Cultivars.

Flue-cured tobacco cultivars were evaluated for their reaction to race 0 and race 1 of Phytophthora parasitica var. nicotianae, the incitant of the disease tobacco black shank. Seventeen commercial tobacco cultivars having resistance derived from Fla 301 or a combination of Fla 301 and Fla 105 were subjected to root or stem inoculation by 22 different isolates of P. parasitic var. nicotianae collected from across the Georgia tobacco-growing belt. An adapted stem inoculation technique using field-grown tobacco indicator cvs. K-326, NC-71, Coker 371 Gold, and the breeding line NC-1071 was used to determine races of P. parasitica var. nicotianae. Typically, in greenhouse evaluations, cultivars stem inoculated with race 1 of P. parasitica var. nicotianae were killed. Cultivars that had resistance from both Fla 301 and Fla 105 (Ph gene) were not killed, with few exceptions, when inoculated with race 0. Fifty-seven tobacco specimens having black shank symptoms from the Georgia and Florida tobacco belt were evaluated for P. parasitica var. nicotianae race using the adapted stem inoculation technique. Of the samples evaluated from commercial tobacco fields, 83% yielded race 1 compared with a similar evaluation made in 1994, where only 16% of the samples yielded race 1. The increase in race 1 incidence may be related to the increase in use of cultivars which have the Ph gene for resistance. The use of rotations and metalaxyl or mefenoxam may be required where race 1 is found.