Metalaxyl Resistance of Phytophthora palmivora Causing Durian Diseases in Thailand

Thailand is the leading producer and exporter of durians worldwide. Serious diseases in include root rot, stem fruit which are caused by Phytophthora palmivora, P. nicotianae, Pythium cucurbitacearum, respectively. Thai farmers have applied fungicides for more than 20 years to control but it remains difficult control. Thus, monitoring fungicide-resistance development pathogens important disease management. Pathogens were isolated from naturally infected between 2016 2017 southern Thailand. The sequences internal transcribed spacer (ITS) 5.8S regions rDNA used identification their species. Seventeen out twenty isolates confirmed be palmivora. All tested mycelium-growth sensitivity metalaxyl, azoxystrobin, dimethomorph. results showed that nine resistant metalaxyl with 50% effective concentration (EC50) higher 100 mg L−1. By contrast, all sensitive both azoxystrobin dimethomorph, EC50 < 1 Metalaxyl-resistant not controlled (−25.6% 22.2%) treatment detached leaves ‘Monthong’ durian L−1 prior inoculation, metalaxyl-sensitive moderately metalaxyl-resistant better (33.0% 62.6%). These clearly indicate strains present populations palmivora