Abstract: In order to develop a microbial agent that can prevent and control Fusarium wilt disease in melons，four pathogens（two for cucumber，one for watermelon and one for muskmelon）were tested，and vermicompost was used as the material to screen biocontrol bacterium strain. A new strain WQ-6 exhibiting antagonism towards all four pathogens tested，was screened out using the methods of plate-confrontation and inhibition mycelial growth and was identified as Bacillus siamensis. Based on single factor test and response surface analysis，an response surface analysis，an optimal fermentation condition for strain WQ-6（i.e. glucose 30 g ? L-1，peptone 30 g ? L-1，magnesium sulfate 1 g ? L-1，glycerol 1 g ? L-1，dipotassium hydrogen phosphate 1.5 g ? L-1，temperature 32.2 ℃，liquid-loading rate 19.2%，rotating speed 236.2 r ? min-1，pH 6.0，inoculation rate 4% and cultural time 24 hours）was obtained. Under the optinal condition，the inhibition zone diameter increased by 3.52–10.45 mm and the inhibition rate of pathogens increased by 2.28%–32.16%，as compared to the unoptimized control. The seed germination test（cucumber，watermelon and muskmelon）and pot experiment showed that the strain WQ-6 reduced the disease incidence of seeds by 32%–68%，increased germination rate by 100%–259.6%，and decreased the disease index of plants by 20%–34%. In addition，the WQ-6 fermentation broth（WFB）mixed with water extract of vermicompost significantly enhanced the root length，stem length and vigor index of seeds，and reduced the disease index of plants by 6.78%–36.44%，as compared with WFB alone. These results indicate that the strain WQ-6 has broad-spectrum antagonistic effect on Fusarium wilt disease in melons and presents a good option for promoting plant growth.

Key words: melon, vermicompost, Fusarium wilt disease, biocontrol bacterium, response surface analysis