These practices include severe pruning, excessive nitrogen fertilization, and an unbalanced fertility program. Warm, moist conditions, particularly during flowering, greatly increase incidence of the disease. Blight development can occur at temperatures between 65 and 95 degrees F, but the 75 to 81 degrees F range is most favorable. Temperature extremes will slow development of the disease. In regions with mild winter temperatures, cankers may increase in size during the fall and winter on trees with a southerly exposure.
Rain is critical to the spread and development of the disease.
Fire blight outbreaks are encouraged by warm, cloudy weather following a rain. A humid environment promotes bacterial growth and spread. Other weather events like frost, high winds, and hail create wounds through which the bacteria can enter the plant. Typical 'Shepherds Crook' often seen at the end of fire blight-damaged shoots. The brown color of the blighted leaves indicates that this blighted shoot is on apple or crabapple.