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.
[Illustration: Fig. 206. Wheat leaf with red-rust, natural size.]
[Illustration: Fig. 207. Portion of leaf enlarged to show sori.]
[Illustration: Fig. 208. Natural size.]
[Illustration: Fig. 209. Enlarged.]
[Illustration: Fig. 210. Single sorus.
Figs. 206, 207.—Puccinia graminis, red-rust stage (uredo stage).
Figs. 208-210.—Black rust of wheat, showing sori of teleutospores.]
=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.
[Illustration: Fig. 211. Head of wheat showing black rust spots on the
chaff and awns.]
[Illustration: Fig. 212. Teleutospores of wheat rust, showing two cells
and the pedicel.]
[Illustration: Fig. 213. Uredospores of wheat rust, one showing
remnants of the pedicel.]
=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).
[Illustration: Fig. 214. Barberry leaf with two diseased spots, natural
size.]
[Illustration: Fig. 215. Single spot showing cluster-cups enlarged.]
[Illustration: Fig. 216. Two cluster-cups more enlarged, showing split
margin.
Figs. 214-216.—Cluster-cup stage of wheat rust.]
=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.
[Illustration: Fig. 217. Section of an æcidium (cluster-cup) from
barberry leaf. (After Marshall-Ward.)]
=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.
[Illustration: Fig. 218.
Section through leaf of barberry at point affected with the cluster-cup
stage of the wheat rust; spermagonia above, æcidia below. (After
Marshall-Ward.)]
[Illustration: Fig. 219.
_A_, section through sorus of black rust of wheat, showing
teleutospores. _B_, mycelium bearing both teleutospores and
uredospores. (After de Bary.)]
[Illustration: Fig. 220. Germinating uredospore of wheat rust. (After
Marshall-Ward.)]
[Illustration: Fig. 221. Germ tube entering the leaf through a stoma.]
=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_.
[Illustration: Fig. 222. Teleutospore germinating, forming promycelium.]
[Illustration: Fig. 223. Promycelium of germinating teleutospore,
forming sporidia.]
[Illustration: Fig. 224. Germinating sporidia entering leaf of barberry
by mycelium.
Figs. 222-224.—Puccinia graminis (wheat rust). (After Marshall-Ward.)]
=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.