Fungicide sensitivity, race structure, and host pathogenicity of Phytophthora nicotianae in Tennessee and North Carolina

Phytophthora nicotianae, a hemibiotrophic oomycete, is the causal agent of disease of over 300 hosts, notably tobacco, tomato, citrus, strawberry, and ornamentals. Black shank of tobacco is a devastating disease that can result in up to 100% yield loss if left untreated, and buckeye rot of tomato compromises the marketability and yield of fruit impacted by P. nicotianae. Black shank and buckeye rot have been reported throughout the state of Tennessee, and the goal of this work was to analyze fungicide sensitivity of P. nicotianae isolated from tobacco and tomato, characterize the race structure of P. nicotianae, and evaluate host specificity of P. nicotianae on tomato and tobacco. The fungicide sensitivity study tested mefenoxam, oxathiapiprolin, mandipropamid, and fluopicolide on 155 P. nicotianae isolates collected in 2021 and 2022. Resistance was not found and baseline sensitivity levels were established. Subsets of the 2021 and 2022 isolates were screened in a race-typing host differential study utilizing the following tobacco varieties: Hybrid 404LC (fully susceptible to both race 0 and 1), ms KY 14XL8 (fully resistant to race 0, fully susceptible to race 1), and KT215LC (fully resistant to race 0, partially resistant rating 9/10 to race 1). The study revealed 92% of 2021 isolates as race 1, and 89% of the 2022 isolates were determined race 1, indicating race 1 to be the dominant race of P. nicotianae in tobacco fields. Greenhouse studies testing the pathogenicity of P. nicotianae isolated from tobacco on ten popular tomato varieties produced in Tennessee revealed no disease following root, fruit, and foliar inoculations. A field trial was conducted using the ten tomato varieties in a field with a history of repeated tobacco production and a known presence of P. nicotianae, and it did not yield disease on the tomato varieties. The concluding detached leaf and fruit assay revealed host specificity among P. nicotianae isolates aligning with their hosts of origin.