First Report of Dimethachlon Resistance in Field Isolates of Sclerotinia sclerotiorum on Oilseed Rape in Shaanxi Province of Northwestern China.

Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic fungal pathogen causing diseases in a wide range of plants, including oilseed rape (3). Substantial economic losses caused by S. sclerotiorum have been reported in the United States, Canada, Brazil, South Africa, Hungary, India, Nepal, and Japan (1). Application of fungicides is the principal tool for controlling S. sclerotiorum because of lack of high level of host resistance. Dicarboximide fungicides such as dimethachlon have been widely used to control S. sclerotiorum in recent years in China and field isolates with reduced sensitivity to dimethachlon have been reported in Jiangsu Province of eastern China (2). In order to understand the current status of dimethachlon resistance in S. sclerotiorum isolates of northwestern China, 196 and 344 isolates of S. sclerotiorum collected from oilseed rape fields in 10 counties throughout Shaanxi Province in 2011 and 2012, respectively, were assayed for sensitivity to dimethachlon using 5 μg ml dimethachlon as a discriminatory dose. Mycelial plugs (6 mm in diameter) cut from the margin of a 48-h-old colony were placed in the center of petri dishes containing potato dextrose agar (PDA) amended with 5 μg ml dimethachlon; PDA without fungicide served as the control. Cultures were incubated at 26°C and colony growth was measured after 72 h of incubation. Isolates that showed growth on PDA amended with fungicide were tentatively considered resistant to dimethachlon, whereas the completely inhibited isolates were considered sensitive. Results showed that 1.02% or 2 isolates of the 196 isolates collected in 2011 and 3.78% or 13 isolates of the 344 isolates collected in 2012 were resistant to dimethachlon. For all the isolates considered resistant and 42 randomly selected sensitive isolates, 50% effective concentrations (EC) were determined on PDA amended with a series of dimethachlon concentrations. The average EC value of dimethachlon for sensitive isolates was 0.29 ± 0.02 μg ml Resistance ratios (EC of resistant isolate / average EC of sensitive isolates) for the two resistant isolates detected in 2011 were 10.28 and 23.83, respectively, whereas resistance ratios for the 13 resistant isolates detected in 2012 ranged from 24.90 to 101.97. The average EC value of dimethachlon for the 13 resistant isolates detected in 2012 was 19.05 μg ml, and EC values for the two resistant isolates detected in 2011 were 2.98 and 6.91 μg ml, respectively. These results indicated that both resistance frequency and resistance level increased from 2011 to 2012. Bioassay results of three resistant isolates indicated that there was positive cross-resistance between dimethachlon and other dicarboximide fungicides such as iprodione and procymidone. To our knowledge, this is the first report of dimethachlon resistance in S. sclerotiorum in Shaanxi Province of northwestern China. The molecular mechanism of dimethachlon resistance in field isolates of S. sclerotiorum remains to be studied. Although resistance frequency is low at present, dimethachlon resistance should be kept in mind and fungicide resistance management tactics such as use of biological control agents, fungicide tank-mixing, or alternating dimethachlon with other fungicides having different modes of action is recommended in controlling S. sclerotiorum. References: (1) M. D. Bolton et al. Mol Plant Pathol. 7:1, 2006. (2) H. X. Ma et al. Plant Dis. 93:36, 2009. (3) L. H. Prudy. Phytopathology 69:875, 1979.