(1912). An experiment was set up in which the discharge of basidiospores began at half past six o'clock in the evening of one day and continued until after ten o'clock the next forenoon although at that time the rate of discharge was much slower. Evidently the dispersal of basidiospores can continue for some time after a period of rainfall if slow-drying conditions prevail. In observing the process under the microscope an abrupt sidewise movement of the basidiospore was always noticed several seconds previous to its discharge, and almost simultaneously a bubble appeared at its base.

The basidiospore farthest from the spore is the first to be formed, followed by the others in their respective order. The outermost spore is discharged first, followed by the next in order. Only about one minute elapses between the disappearance of the apical basidiospore and the one nearest it, but a much longer period elapses before the last two are discharged. Often the terminal basidiospore is mature before the sterigma of the basidiospore nearest the spore is even formed. This method of discharge. readily accounts for the wide dissemination of basidiospores by air currents. Germination of basidiospores.- Farlow (1886), Crabill (1913), and Reed and Crabill (1915), have contributed to the knowledge of secondary basidiospore formation. The basidiospore normally germinates by the development of one or more, rarely two, germ tubes from the side of the spore. Under certain conditions, instead of a germ tube a sterigma similar to those formed on the promycelium is put forth, and on the end of this a secondary basidiospore is produced. This secondary spore is identical in appearance with its parent except that it is somewhat smaller. Various stages of basidiospore germination are seen in figure 141, D (page 521). The chief factor influencing the production of the secondary spore is an excess of moisture.

Two cedar apples, one caused by G. Juniperi-virginianae and the other by G. globosum, with horns protruded, were subjected for twelve hours to a fine mist from a spray nozzle attached to a water tap. The temperature of the room was 23° C. and that of the water about 8° C. When the material was examined it was found that a large number of the spores had germinated abnormally, and that the basidiospores which were formed had already germinated by means of secondary spores. It is impossible to determine whether or not the excess moisture was the only cause of this abnormal germination, since the temperature factor may also have been of importance.

Aecial stage

Inoculation and infection of apple. The first basidiospores are usually disseminated in the spring about the time when the buds of the aecial hosts open, though some may be formed previous to this time. Infection

usually occurs on the dorsal surface of apple leaves. The germ tube penetrates the epidermis and the pathogene becomes established within the tissues of the host.

In these inoculation experiments a suspension of basidiospores in tap water was placed on various parts of both the upper and the lower surface of Wealthy apple leaves. After seven, fourteen, and twenty-one hours, respectively, parts of the leaves thus inoculated were removed, fixed, and embedded in paraffin. Several of these were later sectioned and examined. In one case, after a period of seven hours a germ tube of a basidiospore was found to have penetrated the lower epidermis directly and passed about two-thirds of the distance through the epidermal cell (fig. 141, E, page 521).

Several leaves from a small apple tree were inoculated by placing basidiospores in suspension on the foliage, with a camel's-hair brush. Some leaves were inoculated on the upper surface and others on the under surface. Infection was apparent after ten days on all the inoculated leaves. This demonstrates that infection can take place on either the upper or the lower surface of the leaf. In all cases, however, pycnia were produced only on the upper surface. Apparently, therefore, the production of pycnia on the upper surface of infected leaves is due, not to the fact that infection occurs there, but to some other factor. Pycnia have never been seen on the lower surface of leaves, although many aecia have been observed arising vertically from the upper surface. In 1914, and also in 1915, the first evidence of infection in nature was found about June 1. The mycelium is similar to that found in the telial hosts except that it is uninucleate and only a limited area of the host tissue is invaded.

Effect of environmental factors. It is evident that the amount of rust present in a given season will depend largely on weather conditions. Moisture is necessary for teliospore germination and for infection of the aecial host, and therefore the number of infection periods depends primarily on the number of rain periods.

An attempt was made in these experiments to determine the approximate amount of moisture necessary for infection of the accial host. Cedar apples were immersed in tap water for a few minutes and were then placed under a bell glass. After about four hours, when an abundance of basidiospores were being discharged, the gall was suspended over a small apple seedling. A lamp chimney inclosed both the seedling and the gall. The seedling was not moistened. The cedar apple retained its moisture for a long time in this position, and the basidiospores formed a yellow coating over the surface of the leaves of the seedling within a few hours. After eighteen hours the chimney was removed, and ten days after inoculation abundant infection was evident on nearly all the leaves. This

experiment was repeated several times and in each case the same results were obtained. Apparently sufficient moisture collected on the leaves from the water transpired and from that which evaporated from the telial horns to permit basidiospore germination and infection. A careful inspection failed to disclose any drops of water collected on the leaves inside the chimney.

Other experiments were attempted in which the lamp chimney containing the cedar apple was suspended over the apple tree so that the basidiospores fell on the tree but no opportunity was offered for the condensation of water on the leaves. No infection occurred under these conditions. This experiment was repeated on a large tree in the open. The basidiospores were allowed to fall on a few young leaves which were not inclosed within the chimney. On the night when the experiment was set up there was a heavy dew followed by forty-eight hours of precipitation. Abundant infection occurred and aecia were developed within the usual period of time.

From these experiments it is evident that but little moisture is necessary for infection. There must be sufficient moisture to cause the telial horns to gelatinize and to keep them in that condition for a period of from four to five hours, followed by conditions of high humidity to furnish the necessary moisture for infection. This is contrary to the opinion of Reed and Crabill (1915), who state that infection takes place only in the presence of abundant moisture. It is not clear whether they mean to include the whole process of basidiospore formation and infection or only the latter, since they also make the statement that infections followed short periods of rainfall.

Strains of the fungus. Since this disease is so destructive in West Virginia and Nebraska, specimens of cedar apples from each of these States were procured for the purpose of making comparative inoculation tests with the strain of the fungus found in the vicinity of Ithaca, New York. These specimens, obtained through the kindness of N. J. Giddings and E. M. Wilcox, were used to inoculate Wealthy apple trees in the open and apple seedlings in the greenhouse. Young leaves on different branches of each tree were inoculated with the three strains of fungi and their development was observed closely. Infection was apparent at exactly the same time in all cases and the development of the disease was identical in all particulars. In no case was there any evidence to show that one strain was more virulent than the others. The apples of West Virginia and of Nebraska may be more susceptible than those of central New York, which probably accounts for the fact that this disease is so destructive in the former States.

Varietal susceptibility of apple.- Numerous lists of susceptible and of resistant varieties of apples have been recorded by various writers. The

most important of these are by Emerson (1905) in Nebraska, Chester (1896) in Delaware, R. E. Stone (1908) in Alabama, Smith and Stevens (1910) in North Carolina, Reed, Cooley, and Crabill (1914) in Virginia, and Giddings and Berg (1915) in West Virginia. Stewart (1910) says that in New York State the varieties Wealthy, Boiken, and Rome are very susceptible, Hubbardston and Sutton are slightly susceptible, and McIntosh, Yellow Transparent, Gravenstein, Red Astrachan, Oldenburg, and Baldwin are resistant. The writer has had no opportunity to make observations on the susceptibility of different varieties of apples, but the following have been artificially infected several times: Wealthy, Wagener, Twenty Ounce, Tompkins King, Alexander, Baldwin, Rome Beauty, Bietigheimer, Baxter, Boiken, Banana, Black Gilliflower, Dartmouth.

The variety Wealthy is considered especially susceptible, although Stewart and Carver (1896) state that it proved to be resistant in Iowa. Seedling apples are very susceptible when artificially inoculated.

Several specimens of Salome apples were received in the autumn of 1913 and a large rust lesion was present on the blossom end of each. This variety should probably be included with those listed as susceptible in New York State.

Pycnia. The pycnia are the first fruiting bodies to appear in apple tissue attacked by the rust fungus. Masses of short-celled mycelium. collect at certain points under the epidermis and form the flask-shaped pycnia of the usual rust type. Hyphal branches extend into the pycnial cavity and from the ends of these the pycnospores are abstricted (fig. 143).

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Aecia. From two to four weeks after the pycnia become visible. depending largely on weather conditions, the aecia begin to break out on the lower surface of the leaves or from among the pycnia on stems or fruit. In New York State the aecia usually begin to break open about the first of August.

The tissues from which these fruiting bodies arise may be considerably hypertrophied, the spongy parenchyma especially being modified. Many septate strands of mycelium collect beneath the surface in the diseased area and from these the aecia are finally developed. The aecia are formed entirely within the host, but as they mature they break through the inclosing tissue, the peridium soon dehisces, and the spores are then scattered.

The aecia in all cases are composed of the inclosing pseudoparenchyma, the fertile spore-bearing stalks, and the aeciospores surrounded by the single layer of peridial cells (fig. 144). The aecia spores are binucleate and measure 16 to 24 μ by 21 to 31 μ. The spore wall varies in color from yellow to brown. When dehiscence occurs the peridium splits longitudinally between practically each row of cells. The ends of the cells remain attached, forming long strands which are one or more cells wide by several cells long. The individual cells are comparatively long and narrow, measuring 10 to 16 μ by 65 to 100 μ; they become much recurved when moist. The side walls are sparsely rugose with ridges extending the entire distance across. The aeciospores drop out of the aecia as they mature, and are carried by the wind

to cedar trees where they initiate the telial stage.