Fig. 18. - Cedar - apples in winter condition.
The apple rust pathogene is a fungus, Gymnosporangium Juniperi-virginiance, with a complex life-history as well as a long name. (The apple rust fungus common in New England is Gymnosporangium globosum Farlow. This species occurs also on pear; see page 341.) Fortunately, however, the cycle of the parasite is well known to botanists and to many apple-growers. The fungus was first found and described on the red cedar, but no suspicion of its connection with the apple disease was then held. Several years elapsed before it was recognized that another stage of G. Juniperi - virginiance occurred on the apple. Prior to this discovery the stage on the apple was referred to as Rcestelia Pyrata. After the hetercecious habit of the fungus was established, the last name was discarded and the former name was, and is now, used to designate it in all of its stages on the cedar and the apple.
Fig. 19. - Cedar - apples in spring conditions. Note the horns.
In the spring basidiospores, or sporidia, are blown from the gelatinous horns of the cedar-apple to the young leaves and fruits of the apple. In some cases they may be carried several miles, but most of them do damage only at a mile or less. Under favorable conditions the sporidia germinate. Low temperatures, such as prevail in April and May, are most favorable to the germination of the sporidia. The germtubes penetrate the apple leaf through the cuticle on the upper surface. The mycelium develops locally within the leaf, occupying the spaces between the cells and sending haustoria into the cells. As a result of the stimulative action of the mycelium, the leaf is hypertrophied, due to an excessive enlargement and multiplication of the cells in the lower part of the leaf (spongy parenchyma). These cells become elongated and the intercellular spaces of the spongy parenchyma are obliterated. The apple-fruit tissues are at first enlarged, but finally growth of the fruit at the point of attack is stopped, which fact explains the marked dwarfing and reduction in size. About one month after infection, the mycelium forms the first fruiting bodies of the season. These are on the upper surface of the leaf or on the fruit and are called pycnia (spermagonia). Spores are developed within the pycnia and are called pycnospores (sper-matia); their function is unknown. They do not reproduce the fungus. Within a short time after the spermagonia appear, the cup-shaped bodies develop on the lower surface of the leaf opposite the spermagonia; these are known as aecia. About the middle of July these open and their spores, called aeciospores, are discharged. The wind carries them to the red cedar where they cause the formation of the cedar-apple. It is not definitely known whether the seciospores germinate in the fall or in the spring, but at any rate mycelium develops from the germtube and the cedar leaf-tissues are stimulated as are those of the apple. As a result of this over-growth a gall is formed. This is the cedar-apple. It will be observed that one year elapses between the inoculation of the apple and the appearance of the gall on the cedar. This gall continues to grow throughout the summer and by fall has attained its full size. The next spring brown horns push out over the surface of the gall; these horns are composed of another kind of spores - telio-spores. With the advent of spring rains the horns become gelatinous and the teliospores germinate. This occurs approximately two years after the first inoculation of the apple. A peculiar germtube is produced by the teliospores - one which is four-celled and which is called a promycelium. Four spo-ridia, or basidiospores, are developed on each promycelium; each sporidium is discharged at maturity and is blown to the apple. Thus the complete life - cycle involves a period of about two years.
The teliospores do not all germinate at once. As already noted, during a wet period they germinate and produce sporidia. If the air then becomes dry, the sporidia are snapped away and caught by the wind. With the return of rainy weather the process of gelatinization of the horns and germination of the teliospores is repeated. As evaporation occurs the second crop of sporidia is disseminated. In certain years there may be but one crop, in others there may be as many as six crops. The number and frequency of these yields of sporidia are determined by the weather conditions. The air must remain highly humid for at least six hours in order to allow gelatinization, germination, and sporidial production; but within this short period a great number of sporidia may be produced and with the return of dry air they are promptly carried to the apple. It is clear, then, that following a short rain it is but a question of a few hours before the fungus is actually inside the apple leaf.
The relation of the development of G. Juniperi - virginiancp to the weather is very striking, and is a question of great importance as well as of great interest. From what has been said it is obvious that the weather factors concerned are moisture, temperature and wind. Moisture is necessary for sporidial production. Winds and warmth aid in evaporation and in sporidial liberation. Moisture again is necessary for germination of sporidia, seciospores and teliospores. Temperature is likewise a limiting factor in this connection. Winds appear to be the only agents of inoculation. It is not necessary that the winds be strong, since the spores are very light and float easily in air currents.
The simplest and most effective method of handling apple rust consists in the eradication of the red cedars. This is practiced by larger growers where the disease has become important. This work must be done thoroughly. All red cedars within a radius of at least one mile about the apple orchard must be destroyed. The cost of this work has been found to be less than forty-eight cents an acre in West Virginia. It has been found that galls may produce teliospores and sporidia two months after the trees are cut. It is therefore advisable to burn any red cedars which are cut later than March first. The removal of galls from the red cedars is a commendable practice only when the cedars are more desired than the apples. Such may be the case in public parks and on private estates. The galls should be removed in advance of sporidial production, that is, before March first, to make sure of effectiveness. The work must be done every year. Spraying the cedars likewise is to be done only where the cedars are few and highly valued on account of their ornamental qualities. In any case the value of the operation is as yet uncertain. Spraying apples to protect them against sporidial infection is unreliable. This is because effectiveness is so dependent upon the time of the application. A delay of one day after telial gelat-inization makes spraying the apple of no value. The time in which effective spraying may be done is not long enough to allow for covering a large orchard efficaciously without undue outlay for machinery and labor. Where orchards are far enough from the cedars to escape severe infection and where only moderately susceptible varieties exist, the application of a fungicide may prove effective. Under such conditions spray the leaves as soon as they unfold and keep them protected until the first week in June. Subsequent applications should be made often enough to protect the new leaves as they appear. Lime sulfur 1 - 40 may be used. Wherever cedars occur and it is not feasible to destroy them, the young orchards should be set to resistant varieties, avoiding particularly the York Imperial, Rome and Wealthy.
Reed, H. S., and Crabill, C. H. The cedar rust disease of apples caused by Gymnosporangium Juniperi-virginianse Schw. Virginia Agr. Exp. Sta. Tech. bul. 9:3-106. 1915. Heald, F. D. The life history of the cedar rust fungus Gymnosporangium Juniperi - virginiansB Schw. Nebraska Agr. Exp. Sta. Rept.
22: 105 - 120. 1909. Giddings, N. J., and Berg, A. Apple rust. West Virginia Agr. Exp.
Sta. Bul. 154: 5-73. 1915. Coons, G. H. Some investigations of the cedar rust fungus. Nebraska Agr. Exp. Sta. Rept. 25: 217-246. 1912. Jones, L. R., and Bartholomew, E. T. Apple rust and its control in Wisconsin. Wisconsin Agr. Exp. Sta. Bul. 257: 3-30. 1915. Farlow, W. G. The Gymnosporangia or cedar apples of the United States. Boston Nat. Hist. Soc. Ann. mem. 1880:1-38. 1880. Pammel, L. H. The cedar apple fungi and apple rust in Iowa. Iowa Agr. Exp. Sta. Bul. 84: 1 - 36. 1905. Stewart, Alban. An anatomical study of Gymnosporangium galls.
American Journ. Bot. 2: 402 - 417. 1915.