Selective medium was prepared by dissolving 1 g of yeast extract powder, 2 g of ammonium nitrate, 2 g of dipotassium phosphate, 0.5 g of magnesium sulfate, 0.5 g of potassium chloride as well as 17.5 g of agar in water to 1000 ml. After autoclaving the medium and then cooling to 45 C, 20 g of raw sweet potato starch (fumigated with formaldehyde for 24 h) was added and mixed thoroughly.