Detection of resistance in Sclerotinia sclerotiorum to carbendazim and dimethachlon in Jiangsu Province of China

Sclerotinia stem rot caused by Sclerotinia sclerotiorum, is the main fungal disease of oilseed rape (Brassica napus) in Jiangsu Province of China, causing oil quality declines and yield losses from 10 to 80 %. Chemical control, such as carbendazim (MBC) and dimethachlon, remains the major method to reduce the incidence of S. sclerotiorum. Isolates collected from Sclerotinia-infected stems of oilseed crops in Jiangsu province between 2010 and 2012, were tested for their sensitivity to carbendazim and dimethachlon in order to determine the level of resistance to these fungicides in the population of S. sclerotiorum in that province. Sensitivity tests showed that 375 isolates were resistant to carbendazim among the 3,701 isolates with a resistance frequency of 10.1 %; only one isolate in the Lian Yungang region of Jiangsu Province in 2010 showed normal growth at 5 mu g/ml dimethachlon. The proportion of carbendazim-resistant isolates ranged from 0 to 44 % in different regions of Jiangsu. The field MBC-resistant isolates showed comparable mycelial growth, sclerotial production, pathogenicity and osmotic sensitivity to the MBC-sensitive isolates, which suggested that the field MBC-resistant isolates might have sufficient parasitic fitness to compete with the field sensitive isolates in the field; whereas the dimethachlon-resistant isolates were less fit than their parental isolates in terms above. Moreover, S. sclerotiorum exhibited positive cross-resistance between dimethachlon and other dicarboximide fungicides such as iprodione and procymidone.