In cultivating the mushroom, there are important several mushroom diseases including Mildew disease, Didymocladium ternatum. Studies on some factors affecting growth of D. ternatum and its chemical control were carried out. The optimum temperature for mycelial growth of D. ternatum was $15{sim}20^{circ}C,$ lower than that of Agaricus bisporus. Infection was very severe when it was infected between at casing and at the end of the first flush. Mycelial thermal death occurred at $50^{circ}C$ when it was treated for more than 20 minutes. For spore, $50^{circ}C$ for 60 minutes and $60^{circ}C$ for 20 minutes were required respectively as a thermal death point. Benlate, Homai and Dithane M-45 were selected as effective chemicals for controlling this disease.