CHAPTER XX.

FUNGI CONTINUED.

“Rusts” (Uredineæ).

=400.= The fungi known as “rusts” are very important ones to study, since all the species are parasitic, and many produce serious injuries to crops.

=401. Wheat rust (Puccinia graminis).=—The wheat rust is one of the best known of these fungi, since a great deal of study has been given to it. One form of the plant occurs in long reddish-brown or reddish pustules, and is known as the “red-rust” (figs. 206, 207). Another form occurs in elongated black pustules, and this form is the one known as the “black rust” (figs. 208-211). These two forms occur on the stems, blades, etc., of the wheat, also on oats, rye, and some of the grasses.

=402. Teleutospores of the black rust form.=—If we scrape off some portion of one of the black pustules (sori), tease it out in water on a slide, and examine with a microscope, we see numerous gonidia, composed of two cells, and having thick, brownish walls as shown in fig. 212. Usually there is a slender brownish stalk on one end. These gonidia are called _teleutospores_. They are somewhat oblong or elliptical, a little constricted where the septum separates the two cells, and the end cell varies from ovate to rounded. The mycelium of the fungus courses between the cells, just as is found in the case of the carnation rust, which belongs to the same family (see Parag. 186).

=403. Uredospores of the red-rust form.=—If we make a similar preparation from the pustules of the red-rust form we see that instead of two-celled gonidia they are one-celled. The walls are thinner and not so dark in color, and they are covered with minute spines. They have also short stalks, but these fall away very easily. These one-celled gonidia of the red-rust form are called “uredospores.” The uredospores and teleutospores are sometimes found in the same pustule.

It was once supposed that these two kinds of gonidia belonged to different plants, but now it is known that the one-celled form, the uredospores, is a form developed earlier in the season than the teleutospores.

=404. Cluster-cup form on the barberry.=—On the barberry is found still another form of the wheat rust, the “_cluster-cup_” stage. The pustules on the under side of the barberry leaf are cup-shaped, the cups being partly sunk in the tissue of the leaf, while the rim is more or less curved backward against the leaf, and split at several places. These cups occur in clusters on the affected spots of the barberry leaf as shown in fig. 215. Within the cups numbers of one-celled gonidia (orange in color, called æcidiospores) are borne in chains from short branches of the mycelium, which fill the base of the cup. In fact the wall of the cup (peridium) is formed of similar rows of cells, which, instead of separating into gonidia, remain united to form a wall. These cups are usually borne on the under side of the leaf.

=405. Spermagonia.=—Upon the upper side of the leaves in the same spot occur small, orange-colored pustules which are flask-shaped. They bear inside, minute, rod-like bodies on the ends of slender threads, which ooze out on the surface of the leaf. These flask-shaped pustules are called _spermagonia_, and the minute bodies within them _spermatia_, since they were once supposed to be the male element of the fungus. Their function is not known. They appear in the spots at an earlier time than the cluster-cups.

=406. How the cluster-cup stage was found to be a part of the wheat rust.=—The cluster-cup stage of the wheat rust was once supposed also to be a different plant, and the genus was called _æcidium_. The occurrence of wheat rust in great abundance on the leeward side of affected barberry bushes in England suggested to the farmers that wheat rust was caused by barberry rust. It was later found that the æcidiospores of the barberry, when sown on wheat, germinate and the thread of mycelium enters the tissues of the wheat, forming mycelium between the cells. This mycelium then bears the uredospores, and later the teleutospores.

=407. Uredospores can produce successive crops of uredospores.=—The uredospores are carried by the wind to other wheat or grass plants, germinate, form mycelium in the tissues, and later the pustules with a second crop of uredospores. Several successive crops of uredospores may be developed in one season, so this is the form in which the fungus is greatly multiplied and widely distributed.

=407a. Teleutospores the last stage of the fungus in the season.=—The teleutospores are developed late in the season, or late in the development of the host plant (in this case the wheat is the host). They then rest during the winter. In the spring under favorable conditions each cell of the teleutospore germinates, producing a short mycelium called a _promycelium_, as shown in figs. 222, 223. This promycelium is usually divided into four cells. From each cell a short, pointed process is formed called a “_sterigma_.” Through this the protoplasm moves and forms a small gonidium on the end, sometimes called a _sporidium_.

=408. How the fungus gets from the wheat back to the barberry.=—If these sporidia from the teleutospores are carried by the wind so that they lodge on the leaves of the barberry, they germinate and produce the cluster-cup again. The plant has thus a very complex life history. Because of the presence of several different forms in the life cyle, it is called a _polymorphic_ fungus.

The presence of the barberry does not seem necessary in all cases for the development of the fungus from one year to another.

=409. Synopsis of life history of wheat rust.=

_Cluster-cup stage on leaf of barberry._

Mycelium between cells of leaf in affected spots. Spermagonia (sing. spermagonium), small flask-shaped bodies sunk in upper side of leaf; contain “spermatia.” Æcidia (sing. æcidium), cup-shaped bodies in under side of leaf. Wall or peridium, made up of outer layer of fungus threads which are divided into short cells but remain united. At maturity bursts through epidermis of leaf; margin of cup curves outward and downward toward surface of leaf. Central threads of the bundle are closely packed, but free. Threads divide into short angular cells which separate and become æcidiospores, with orange-colored content. Æcidiospores carried by the wind to wheat, oats, grasses, etc. Here they germinate, mycelium enters at stomate, and forms mycelium between cells of the host.

_Uredo stage (red-rust) on wheat, oats, grasses, etc._

Mycelium between cells of host. Bears uredospores (1-celled) in masses under epidermis, which is later ruptured and uredospores set free. Uredospores carried by wind to other individual hosts, and new crops of uredospores formed.

_Teleutospore stage (black rust), also on wheat, etc._

Mycelium between cells of host. Bears teleutospores (2-celled) in masses (sori) under epidermis, which is later ruptured. Teleutospores rest during winter. In spring each cell germinates and produces a promycelium, a short thread, divided into four cells. Promycelium bears four sterigmata and four gonidia (or sporidia), which in favorable conditions pass back to the barberry, germinate, the tube enters between cells into the intercellular spaces of the host to produce the cluster-cup again, and thus the life cycle is completed.

=410. Other examples of the rusts.=—Some of the rusts do great injury to fruit trees and also to forest trees. The “cedar apples” are abnormal growths on the leaves and twigs of the cedar stimulated by the presence of the mycelium of a rust known as Gymnosporangium macropus. The teleutospores are two-celled and are formed in the tissue of the “cedar apple” or gall. The teleutosori are situated at quite regular intervals over the surface of the gall at small circular depressions, and can be easily seen in late autumn and during the winter. A quantity of gelatine is developed along with the teleutospores. In early spring with the warm spring rains the gelatinous substance accompanying the teleutospores swells greatly, and causes the teleutospores to ooze out in long, dull, orange-colored strings, which taper gradually to a slender point and bristle all over the “cedar apple.” Here the teleutospores germinate and produce the sporidia. The sporidia are carried to apple trees where they infect leaves and even the fruit, producing here the cluster-cups. There are no uredospores.

G. globosum is another species forming cedar apples, but the gelatinous strings of teleutospores are short and clavate, and the cluster-cups are formed on hawthorns. G. nidusavis forms “witches brooms” or “birds nests” in the branches of the cedar. The mycelium in the branches stimulates them to profuse branching so that numerous small branches are developed close together. The teleutosori form small pustules scattered over the branches. G. clavipes affects the branches of cedar only slightly deforming them or not at all, and the cluster-cups are formed on fruits, twigs, and leaves of the hawthorns or quinces, the cluster-cups being long, tubular, and orange in color.