Sources of the Early Infections of Apple

Notwithstanding the excellent work which has been done by previous investigators, the problem of determining the sources from which the early infections of the apple bitter-rot fungus (Glomerella cingulata) may arise has never been worked out with completeness sufficient to account for the heavy initial damage sometimes caused by this disease. Often bitter-rot will break out suddenly, and every apple {Malus spp.) in an orchard will be affected in an incredibly short time. In the Ozark region of Arkansas during the season of 1914 the crops of six of the orchards under the writer's observation were within two weeks almost entirely ruined by bitter-rot. Nearly all the apples on these trees were infected comparatively early in the season—i. e., about July 1—a large majority of them having from 50 to 100 points of infection. The suddenness of the appearance of the disease and the almost simultaneous infection of the fruit over the whole orchard strongly indicated that practically all the rotten spots were caused by spores which had washed down from primary sources of infection. In one orchard soon after the disease broke out nearly every apple was found to be affected with the small blister-like spots characteristic of the early stages of the disease (PI. VII, fig. 1). These spots had not yet developed far enough to produce acervuli and were evidently due to infection by spores from overwintering or primary sources. The later infections are, of course, easily accounted for because the fungus forms acervuli in the rot areas of the earlier infections and from these the spores may be washed by rain or carried by insects to sound apples, which, if conditions are favorable, may become diseased and in turn become sources of infection. Thus, we may have primary sources of infection, which may continue to act as such throughout the season, and secondary sources of infection, consisting of the diseased fruits of the current season. The primary sources of infection therefore become of great importance in the control of the disease, especially when they are present in great