Dried bacterial discharge often forms strands which are carried by wind. Pear flowers are invaded very rapidly through nectaries and pistils, which are ideal sites for growth of the bacteria. Apple flowers are usually invaded through stigmas and anthers. Soft, succulent shoots are very vulnerable to the disease. Mature apple and pear tissues are much more resistant to infection than young tissues.
Cultural practices favoring rapid shoot growth increase the plants' susceptibility to attack.
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.