Abstract: Muskmelon fruit rot caused by Rhizoctonia sp. is a new disease first found in Liaoning Province in recent years. The disease appears more serious on greenhouse muskmelon. The casual agent was identified as AG-4 HGI of Rhizoctonia solani by the studies of the morphology，pathogenicity，anastomosis groups and ribosomal DNA-ITS. The biological characteristics of the results showed that the best mycelium growth was obtained at 28 ℃ and pH 6.0–8.0；The melon juice culture medium was most conducive to mycelium growth in the 10 tested media，The optimum one was soluble starch in the 9 tested carbon sources，the optimum nitrogen source was peptone. The effect of light on the mycelial growth was not evident. The ideal sclerotium generation was at 25 ℃，pH 7.0，in darkness and with Czapek culture medium. The beneficial carbon source for the formation of sclerotia was Sorbitol；The beneficial nitrogen source for the formation of sclerotia was KNO₃. Sclerotium germination was well with carrot juice culture medium；In the 9 tested carbon sources and 10 tested nitrogen sources，sclerotium germination was in favor of malt sugar and peptone；The optimum temperature for sclerotium germination was 25 ℃；The optimum pH is 5.0–8.0；Dark conditions was benefit for sclerotium germination. The mycelial lethal temperature was at 50 ℃ for 10 min. The lethal temperature for sclerotium was at 53 ℃ for 10 min.

Key words: muskmelon, muskmelon fruit rot, Rhizoctonia solani , rDNA-ITS, biological characteristics