In Vitro Sensitivity to Metalaxyl of Phytophthora parasitica var. nicotianae Isolates from Burley Tobacco in Korea

In disease surveys from 1986 ti 1998, disease incidence of tobacco black shank was gradually increased in burley tobacco from 1996. To study the causes of the disease occurrence, one hundred and fourteen isolates of Phytophthora parasitica var. nicotianae (Ppn) were collected from burley tobacco-growing areas in the southern part of Korea during 1996-1997, and tested in vitro for meatlaxyl sensitivity which was determined by measuring the mycelial growth on corn meal agar (CMA) amended with metalaxyl. Of the tested isolates, 78.1% showed sensitive to metalaxyl, having $\textrm{ED}_{50}$ values less than 1.0 $\mu\textrm{g}/$\textrm{ml}, while 1.7% was resistant weth $\textrm{ED}_{50}$ greater than 100 $\mu\textrm{g}/$\textrm{ml}. Ppn isolates from three provinces, Chungnam, Chonbuk and Chonnam showed similar distributions of metalaxyl sensitivity. Metalaxyl-resistant isolates were not significantly different from metalaxyl-sensitive ones in mycelial growth rate, chlamydospore formation capacity and size of the spore, and pathogenicity on tobacco plant (cv. Burley 21). These results suggest that the metalaxyl-resistant Ppn in burley tobacco may be one of the major factors to cause the higher occurrence of the tobacco black shank in the burley tobacco-growing area.