This disease is variously known as cedar rust, cedar rust of apple, cedar-apple, apple rust and cedar-flowers. It affects not only the apple and wild crab, but also the red cedar. Both kinds of plants are necessary in the same immediate locality for the perpetuation of the rust-pathogene. If the cedar is absent, there can be no apple rust.
With respect to the apple, a great many lists of susceptible and resistant varieties have been published from various parts of the country. These lists agree in part, but it is of interest to note that certain varieties are classed as susceptible in one state and as resistant in another. The Ben Davis, for example, is said to be resistant in Delaware, Massachusetts, Rhode Island and Nebraska, but is reported as being susceptible in Iowa and Virginia. The Rome is susceptible in Alabama, Indiana, West Virginia and other states, but is resistant in Delaware. Similar examples of such variation are found in the Grimes and Red June. While there may be some variation due to the difference in location, it appears that the judgment of the individual as to what constitutes resistance or susceptibility is a more probable explanation. Different periods of rust - infection may give rise to confusing data, since the leaves of one variety may expand more quickly or may have a shorter or longer period of susceptibility than the leaves of another variety. It has been found that varieties are susceptible only while the leaves are young; after maturity, they are immune. It has been learned also that the morphological makeup of the leaf, as, for example, hairiness, has nothing to do with the question of its susceptibility.
In spite of these uncertainties with respect to varietal resistance, it is generally agreed that the York Imperial is very susceptible. Others in this class are the Rome, Wealthy, Jonathan, Smith and Ben Davis. Among the more resistant varieties are the Baldwin, Yellow Newtown, Grimes, Arkansas, Maiden Blush, Stayman Winesap, Yellow Transparent and Winesap.
Apple rust has been known to scientists for about a century. Important studies were made in New England about 1880. Since that date the rust has attracted more than usual attention in the states of Vermont, Iowa, Indiana, Alabama, Nebraska, West Virginia, Virginia, Kansas and North Carolina. The disease is now found to be widely distributed through the central and eastern portions of the United States.
The extent of the injury to apple-foliage is not always severe, but sometimes no less than 25 per cent of the leaves is taken from the tree. In many sections of the country the disease is serious on the fruits of the apple. The lesions not only mar the appearance of the fruit, but there is frequently a reduction in size and in quality. Rusted fruit is very likely to become infected with storage Rot pathogenes, thus entailing considerable loss. Rust itself does not induce trouble in storage. The red cedar is not only severely injured, but in extreme cases is killed. Where the tree is used for ornamental purposes this becomes an economic item. It is clear, then, that apple rust may become important in one or more of the following ways: (1) by defoliating the apple; (2) by dwarfing and reducing the quality of the fruit; and (3) by incurring damage to the red cedar.
With respect to the amount of the losses resulting from this disease it may be pointed out that in 1912 the crop of York Imperials was an entire failure in many orchards of West Virginia. Actual fruit - losses ranging from $2000 to $3000 an orchard, due to rust, were very common throughout the eastern part of the state that season. The growers of one county lost $75,000. The disease is equally destructive in Virginia, Alabama, Iowa and Nebraska.
The presence of apple rust in any locality may be detected by examining the apple or the cedar. On the apple leaves young spots may be found from April to June. At first these are seen on the upper surface as minute pale - yellow areas upper surface (three leaves on right).
Fig. 17. - Apple rust; on lower surface (on leaf at extreme left), and on (Fig. 17, right). These enlarge, become darker, and finally are orange-colored. On the upper surface of the spots the fruiting bodies of the pathogene develop, first as minute, yellow, flask-shaped pustules, which finally turn black (Fig. 17). The leaf tissues beneath the spots become swollen and on the lower surface another kind of fruiting body is produced (Fig. 17, left). This one is cup-shaped and considerably larger than the one on the upper surface. At maturity the walls of the cup-shaped structures split and recurve in a stellate manner. In cases of severe infection the leaves turn yellow and fall. Sometimes 200 to 300 spots may occur on a single leaf. The diseased fruits are recognized by the presence of the cup-shaped fruiting bodies which are usually aggregated at the calyx-end. Not infrequently the lesions develop on the side or on the stem-end. As previously noted the fruits are dwarfed as a result of their being affected when they are young. Rust is rarely found on the woody parts of the apple, and then only on very susceptible varieties. Later in the summer small, greenish, spherical enlargements of the leaf may be observed on the cedar. Very soon these take on their final shape, which in some cases isreniform (Fig. 18). The enlargements or galls continue their growth, becoming brown and attaining a diameter of two inches or less by the end of the season. During the late autumn and early spring, these galls, or cedar-apples as they are called, show numerous depressions over their surfaces (Fig. 18) and in early spring a brown horn projects for an inch or more from each depression (Fig. 19). In rainy weather these horns become gelatinous and orange-colored. When thus fully formed the cedar-apple resembles a flower, whence the popular name cedar - flowers.