In 1925 a new disease of apple trees was described by Zeller and Childs and named Perennial Canker (Gloeosporium perennans). The results of the various investigators who have been working on the problem indicate a considerable difference of opinion as to the various factors responsible for the production and spread of the disease. This paper presents the results of a study of these factors and an interpretation of the part that each plays in the production of the disease. It is shown that its occurrence is entirely dependent on three factors; the annual inoculation of the host with the causal organism during the late summer and autumn; the presence of the woolly apple aphis which carries out this inoculation; and the exposure of the host after inoculation to periods of low-temperature. Other factors such as winter injury, time of pruning and precipitation, are contributory to its severity, but not essential for its general occurrence.