fruit is often deformed and bears a circular, flattened, black lesion devoid of aecia near its base. Aecia are produced rarely. On Crataegus The lesion produced by G. globosum on Crataegus leaves is almost identical with the rust lesions on apple foliage. The red border about the margin of the spot is not so common, however, and the aecia are rarely arranged in the form of a circle (fig. 148). The twigs of Crataegus are not commonly affected by this rust, but an occasional twig infection has been observed. The lesion is yellow, similar to that on the leaf, but practically no swelling of the twig is apparent. Pycnia are produced in this discolored area, followed later by aecia (fig. 149). Infected fruits have been found, but these are not common. again a yellow spot is formed but little or no hypertrophy results. pycnia and the aecia follow in the same lesion. Here The FIG. 149. AECIA OF GYMNOSPORANGIUM GLOBOSUM The aecia are developing from the under surface of Crataegus leaves. The stem and the leaf petiole are also affected ETIOLOGY Nomenclature The fungus now known as G. globosum was first named by Farlow in 1880. He gave it the name G. fuscum var. globosum, but later (Farlow, 1880) changed the name to G. globosum. Life history The details of the life cycle of this species are almost identical with those of G. Juniperi-virginianae. The aeciospores of the two species mature at approximately the same time. The time of infection of the cedar has not been determined, but it is presumably during the period when the aeciospores are being dispersed. Rust-infected Crataegus leaves were collected on September 26, 1914, and exposed to the weather in a wire screen. At this time aeciospores taken from these leaves failed to germinate. Subsequent tests were made and germination was obtained until December 15, but all attempts to germinate these spores after this date failed. Since aeciospores will germinate during and even later than the time of their dispersal, the writer sees no reason for assuming that infection does not take place until the following spring, as Reed and Crabill assume for G. Juniperi-virginianae. Although the penetration of the germ tube has never been observed, there is but little doubt that it enters the stomata. The mycelium develops within the cedar leaf for a period of from ten to twelve months before any sign of infection becomes apparent. Telial stage Development of telial horns.— The mycelium of this species is practically identical with that of G. Juniperi-virginianae and there is almost a complete absence of haustoria in the young galls. The telial horns are developed from a stromatic layer in the same manner as are those of G. Juniperi-virginianae. They begin to develop in the autumn but it is not until early the next spring that they become far enough advanced to penetrate the surface of the gall. In the spring of 1915 the epidermis over the papillae had begun to break open on March 29, while at that time no evidence of this breaking could be found on the galls of G. Juniperi-virginianae. The telial horns were apparent on April 10. No growth in plant life was evident at that time and there was still considerable ice and snow on the ground. Spores capable of germination were present in these tentacles on April 15. The telial horns continue to increase in size so that when gelatinization first takes place they may be from 1.5 to 3 millimeters thick by from 2 to 5 millimeters broad at the base and from 6 to 12 millimeters high. The number of horns on a gall varies from one to one hundred or more. They are distributed on the gall unevenly and are chestnut brown in color. Instead of standing singly they may coalesce and form a continuous band around the gall. The horns of G. Juniperi-virginianae have never been seen to fuse in this way. The first gelatinization period usually coincides with the first warm rain period after the horns are protruded, and the number of times this process may occur during a season varies greatly. In 1914 the horns gelatinized four times and fell off on May 20, while in 1915 twelve such periods were recorded before the horns became dry on June 2. The telial horns of this species may be more than double in size when swollen, and are then thinner in consistency than the jelly-like horns of G. Juniperi-virginianae under similar conditions. After each protrusion the horns of the latter species dry down to their normal form with the exception of the tips. In the case of G. globosum drying occurs until the last gelatinization takes place, at which time the horns form a solid mass of thin, jelly-like substance over nearly the entire surface of the gall, and this substance intermingles with the adjoining leaves and twigs. When drying occurs this material clings to the leaves or twigs and is pulled loose from its attachments. The galls do not die as do those of the other species, but live and fruit year after year. The teliospores of G. globosum closely resemble those of G. Juniperivirginianae. They are practically of the same width, from 15 to 21 μ, but are often somewhat shorter, ranging in length from 37 to 54 μ. There are also the same number of pores and these are similarly located. The spore stalks are cylindrical in form. Teliospore germination is similar to that of G. Juniperi-virginianae (fig. 150). Accial stage The pycnia and the aecia of G. globosum are similar to those of G. Juniperi-virginianae, the greatest difference being in their size. The size and shape of the peridial cells is somewhat different for the two species. The peridial cells of G. globosum are broadly lanceolate in face view and measure 15 to 23 μ by 60 to 90 μ, and are linear-rhomboid FIG. 150. VARIOUS TYPES OF TELIOSPORES in side view, measuring from 13 to 19μ thick. The outer wall is smooth and about 1.5 μ thick, while the inner and side walls are slightly thicker and are rugose with ridge-like papillae of varying lengths. Acciospore germination. Most attempts to germinate the aeciospores of G. globosum have yielded negative results. On two occasions slight germination was obtained, as is shown in table 3. Inoculation of cedar trees. Following the methods described under G. Juniperi-virginianae, many attempts have been made during a period of three years to obtain infection of red cedar with G. globosum, but thus far no positive results have been obtained. |