Race distribution and distribution of sensitivities to mefenoxam among isolates of Phytophthora parasitica var. nicotianae in Guizhou province of China

Black shank, caused by the hemibiotrophic oomycete Phytophthora parasitica var. nicotianae, is a major disease of tobacco (Nicotiana tabacum) in southwest China. Mefenoxam is one of the most commonly used fungicides for its management. A survey was conducted to determine the level of mefenoxam sensitivity in the pathogen population in Guizhou province and to determine the race distribution of selected isolates. In 2012, 179 isolates of P. parasitica were screened for sensitivity to mefenoxam on lima bean agar. Zero percent of the isolates was classified as resistant, 41.9% as intermediately resistant, and 58.1% as sensitive. Race was determined for 32 isolates from 12 tobacco districts using five discriminative cultivars: NC-1071, Fla 301, L8, Xiaohuangjing 1025 and K-326. Twenty seven isolates were race 1 and five were race 0. Race 1 was present in all regions sampled and presents a major concern for the use of tobacco cultivars in Guizhou province in the future. Based on the results of our survey, we conclude that a combination of practices that includes partial resistance, cultural controls, and fungicides will be needed for long term management of tobacco black shank in Guizhou province of China.