Abstract: The effect of environment factors including temperature, relative humidity, dew period and light on the infection of Phoma herbarum strain SYAU-06 against dayflower (Commelina communi) were studied through analyzing three parameters of diseased leaf rate, disease index and reduction in fresh weight. The result showed 28-32℃ of temperature, over 80% relative humidity and at least 48 h dew duration were essential for perfect control effect. The illumination in dew duration is in inverse proportion to the disease severe and short-illumination benefited the disease development.