THREE CEDAR RUST FUNGI THEIR LIFE HISTORIES AND THE DISEASES THEY PRODUCE1 JAMES LEROY WEIMER The three fungi considered in these investigations are Gymnosporangium Juniperi-virginianae Schw., Gymnosporangium globosum Farlow, and Gymnosporangium clavipes C. & P. Except for a discussion of the hosts concerned, the fungi, together with the diseases that they produce, are treated separately. HOSTS Certain species of the genus Juniperus on the one hand and various species of the family Rosaceae on the other, serve as hosts for the alternate stages in the life cycles of the fungi named above. In the telial stage all three species occur on Juniperus virginiana L. Kern (1911)2 reports G. Juniperi-virginianae and G. globosum also on J. barbadensis L., and G. clavipes on J. communis L. Several horticultural varieties of J. virginiana are also known to be hosts. In this discussion, however, only J. virginiana will be considered as the telial host since it is the only species common in central New York State, in which locality the investigations were made. In their aecial stage these fungi occur on certain closely related members of the family Rosaceae. Among these are the cultivated and the wild varieties of apple (Pyrus malus L.) and crab apple (Pyrus coronaria L.), quince (Cydonia vulgaris Pers.), pear (Pyrus communis L.), June berry (Amelanchier spp.), mountain ash (Sorbus spp.), and numerous species of Crataegus.3 Also presented to the Faculty of the Graduate School of Cornell University, May, 1916, as a major thesis in partial fulfillment of the requirements for the degree of doctor of philosophy. AUTHOR'S ACKNOWLEDGMENTS. Grateful acknowledgment for assistance and suggestions is made to Professor H. H. Whetzel; to Dr. V. B. Stewart for help and criticism in preparation of the manuscript; and especially to Dr. Donald Reddick, under whose immediate direction the work was performed. ? Dates in parenthesis refer to Literature cited, page 548. For a more complete list of hosts see Kern (1911). THE DISEASE CAUSED BY GYMNOSPORANGIUM JUNIPERI VIRGINIANAE The disease caused by the fungus Gymnosporangium Juniperi-virginianae is generally known as cedar rust, although the galls are referred to as cedar apples or, more rarely, as cedar flowers. The term apple rust is most commonly applied to the aecial stage. Other names, such as leaf rust, stem rust, fruit rust, and orchard rust, are sometimes used to designate this disease. HISTORY AND GEOGRAPHICAL DISTRIBUTION The fungus is native to North America and has never been reported elsewhere so far as the writer has knowledge. Although it had already been known for a long time, it received but little attention prior to the work of Farlow in 1880. During the last decade, numerous investigations have been conducted bearing on the life history of the organism and on methods of control of the disease on the apple. The apple rust stage is widely distributed throughout the eastern half of the United States wherever cedar and apple occur together. ECONOMIC IMPORTANCE Owing to the fact that cedar trees occur in considerable numbers in but few States, apple rust has become of great economic importance only in certain localities. In some of the Southern States, where cedars grow in close proximity to the orchards, the disease causes an annual loss aggregating several thousand dollars, and there is considerable evidence that it is becoming more destructive each year. Pammel (1905) had never observed this rust on cultivated apples in Iowa prior to 1905. Emerson (1905), speaking of conditions in Nebraska, G. E. Stone (1911) in Massachusetts, and Giddings and Neal (1912) in West Virginia, state that the disease is becoming more serious each year. Stewart (1910) records several outbreaks in New York State, but says that the disease is rarely of much economic importance. R. E. Stone (1908) in Alabama, and Reed and Crabill (1915) in Virginia, list this as the most serious disease of apples in their respective States. In central New York the disease is very common on wild species of apple but is seldom found on cultivated varieties. The writer had two orchards under observation during the seasons of 1914 and 1915, one of which contained numerous cedar trees that were affected with G. Juniperi-virginianae, G. globosum, and G. clavipes, while the other was only about a half mile distant from a cedar grove that was severely For limits of geographical distribution see Kern (1911). infested with these three species of rust fungi. No affected apple leaves or fruit were found in either orchard. A few affected leaves and two affected apples were found in the Cornell University orchard in 1914. FIG. 136. GALLS OF GYMNOSPORANGIUM JUNIPERI-VIRGINIANAE The galls are in the winter condition and show the depressions from which the telial horns protrude in the following spring NATURE OF LOSSES Although the greatest loss from this rust occurs on the apple, cedar trees also may be materially injured. The injury to apple trees caused by the disease is largely due to premature defoliation and to a reduction in the vital activities of the less seriously affected leaves. Premature defoliation year after year greatly reduces the vigor of the trees and death may finally result. Reed, Cooley, and Crabill (1914) state that FIG. 137. where the disease is severe for several years in succession the trees make but little growth, become much weakened, and are more subject to attacks of insects and of other fungi. Another source of loss is due to the deformation of the affected fruit, such fruit in most cases being unsatisfactory for market. The young twigs may also become affected and die; in many such cases death of a tree may ensue before it reaches bearing age. SYMPTOMS On cedar On the cedar tree the first evidence of the disease caused by G. Juniperivirginianae is a minute. greenish swelling on the leaf, usually noticeable first on its upper, or inner, surface. The affected part of the leaf enlarges rapidly and becomes gradually darker in color, and by the last of September a nearly full-grown cedar apple is formed. At this time the gall is greenish brown in color, from globose to reniform in shape, and of a diameter varying from two millimeters to five centimeters. In New York State the slight pit-like depressions in the outer surface of the gall appear about October I (fig. 136). In the following spring the telial horns protrude from the depressions. These horns are golden brown in color and cylindric- FIG. 138. acuminate in shape (fig. 137). During warm The telial horns are shown as fully gelatinized spring rains they gelatinize and enlarge about two to three times (fig. 138). Later the galls die, but often they remain attached to the cedar tree for a year or more. Twig infections of a wild variety of apple were very common at Ithaca, New York, during the summer of 1914. |