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to grow smutted wheat because it has always done so, and because the extent of the loss and the ease of its prevention are so little realized. Formerly at least one-fifth of the cereal crops was annually destroyed by smut. Smutty wheat introduces a large element of speculation into the business of elevator men, for it produces a very low grade of flour. Such wheat must be washed, an expensive process which also endangers the quality of the flour. From smutted flour the baker gets a poor, darkened product that finds little market. As a result smutted wheat is justly thrown into a very low, or "rejected" grade.
Rust.—What is popularly termed wheat rust may be the result of one or more of a number of rust fungi, parasitic plants. This disease was mentioned by Virgil. It was known in Britain before 1592. Fontana (1767) is generally accredited with connecting rust of cereals with a specific fungus, which Persoon (1797) investigated more fully, and named. Three kinds of rusts are known to attack wheat. Puccinia coronata, the crown rust, is comparatively unimportant. The two distinctive rusts are Puccinia rubigo-vera, the early, orange leaf-rust, and P. graminis, the late, stem-rust. The former is popularly called "red" and the latter "black" rust. Both species, however, produce first reddish and then black spores, but in the orange leaf-rust the red spores are far more abundant than the black ones, while in the stem-rust the black spores are the more abundant.
Life History.—The wheat rusts belong to that type of fungi which have several stages of development represented by different types of spore formation and separated by two or more rest periods. The life history is the development of the fungus through all its stages, and it is said to be "known" when the experimenter can take one type of spore formation and from this produce artificially all of the other types in turn through the life cycle until a return is made to the type of spore formation with which he began. Usually the types
AECIDIA ON BARBERRY
of spores characteristic of the different stages in the life cycle of such a fungus are so different in form and character and so divergent in their modes of development and subsequent habits of growth as to mislead the investigator completely and excite no idea of relationship. Several entirely different types of hosts are frequently utilized in the life cycle. It is because of their complex life history that rusts have so long been shrouded in mystery and confusion, and inadequately understood.1
The wheat rusts produce in order, from spring to spring, four different forms of spores. (1) Aecidiospores (injuring spores) are the first spores found in the year. They occur on shrubs, or herbs other than grasses. (2) Uredospores (blight spores) appear in the early summer, and are often called summer spores.
TWO FORMS OF RUST SPORES COMMONLY FOUND ON WHEAT
At the left is shown the uredospores of the red rust, commonly found in early summer. At the right, the two-celled teleutospores or winter spores of the black rust.
These are the red spores that rust the leaf of the wheat. (3) Teleutospores (completion spores) are the last ones of the season. They are also known as resting or winter spores. Their dark color gave rise to the term "black rust." (4) Sporidia are very minute and delicate spore bodies formed in the spring on the germination tubes of the winter spores. The sporidia infect the plant that is host to the aecidium stage. The question of a breeding act in the rust life cycle is still an unsettled one.
When the secidiospore lodges upon the wheat leaf or stalk in the spring, it remains in a resting condition until a light
1 The works of Bolley, Carleton and Freeman will be found most useful In a study of rusts.
rain or dew furnishes sufficient moisture for germination. A small thread or filament is then sent out, which requires but an hour or two to pass through a breathing pore of the wheat plant, or, in the absence of a convenient breathing pore, to bore its way into the stem or leaf, within which a mycelium is formed. An ordinary dewdrop may contain hundreds of secidiospores that have been wafted to it upon the air. The time required for the rust to break out as a spot or pustule, after the germination of the infecting spore, varies from 8 to 14 days, for it is dependent upon atmospheric conditions. This
!,,, raBT% breaking out through the skin of the wheat ■ im/mplant is the result of the great numbers of ovoid spores that are formed, and these red summer spores, the uredospores, are thus enabled to drop off and float away upon the air to other wheat plants. If moisture is present they germinate at once, and the entire above process is repeated. Several generations of the red spores may be formed during one growing season. Countless myriads of spores are thus produced, a pustule 1-16 by 1-64 of an inch in size containing over 3,000 such spores. Under favorable conditions the rustiness of the grain increases with marvelous rapidity. In the meantime the spore beds which produced the first red spores are not inactive, but are producing teleutospores, that is, the black winter or resting spores. These are thick-coated, Indian-club shaped, and two-celled. They may now also appear in new spore beds in which no red spores Black And Eed have first formed. Over 2,000 of these spores Rust have been counted in a pustule 1-16 by 1-48 of an inch in size. No resting spores have been observed to germinate until late in the following winter. When they sprout a germ tube (pro-mycelium) proceeds from each of their two cells. These germ tubes soon divide into four cells which immediately produce severel minute, delicate cells known as sporidia. If sufficient moisture is present, the sporidia will germinate at once. If not, many of them may dry sufficiently to be carried by the wind, yet not enough to be injured. In the early spring months the damp straw and the ground and surface waters of an old wheat field may be swarming with countless millions of these sporidia, and clouds of them are wafted by the winds to distant points. No evidence has yet been secured, however, that the sporidia directly infect the wheat plant. Those of the stem rust infect the fruit and leaves of the barberry bush, and, as far as known, no other plants. Little cups and clusters of cups of yellow spor°s are formed. The floors of the cups start palisade-like chains of the spores, which, when mature, again take wings with the wind. These are the wheat infecting aecidiospores with which we began, and the life cycle is thus complete.
At the present writing the life history of all wheat rusts is not perfectly and certainly known in all its phases, but it is quite conclusively known that in some cases certain stages of the life cycle which is given above (and which describes the stem rust more correctly and fully than any other) are not essential. The cluster cup form of rust, found on barberry bushes in black stem rust, forms on common wild plants of the borage family in the case of orange leaf rust. In Europe it forms on hound's-tongue, but this stage of the rust seems to be absent in the United States. In crown rust of wheat the cluster cup stage commonly forms on the buckthorn.
It is now established that the uredospores of a number of the important rusts, including Puccinia graminis, can pass the northern winter in viable form. Dried and scattered by the August and July winds, a very large per cent of these rust spores germinated after a dry fall and a North Dakota winter.1 In the warmer climates the leaf rust not only survives the winter in the red spore stage, but forms new pustules every month of the year. Viable spores of both rusts successfully pass the winter frozen in snow and ice. The very early general infection by rust can hardly be explained by the wintering uredospores, however. Experiments by Bolley show quite conclusively that the infection comes by way of the air and not by way of the soil. It is thought possible that rust filaments passing the winter in green plants, and also broken particles of the mycelium from the crop of the previous year, may aid in 1 Bolley, Science, N. S. 22:50-1.
the infection. While it has been generally held that seed from rusted wheat will not transmit rust to succeeding crops,1 the observations of Bolley in 1904 and 1905 proved that seed from badly rusted wheat plants was quite uniformly infected internally, there being spore beds beneath the bran layer, containing both uredospores and teleutospores that subsequently germinated.2 This demands a new line of investigation, for it has not yet been demonstrated whether or not the internally infected seeds will transmit the infection to the plants grown from them. Variations in the spore forms and in the complicated life cycle of rusts give them great strength in selfperpetuation, the different methods of which present several chances of escaping threatening destructions. Many wild grasses also serve as hosts for the wheat rusts. They may be infected from wheat and wheat from them, which is another resource that aids rust in maintaining itself.
Distribution of Rusts.—Rusts being true parasites able to live only in the tissue of some host, their distribution is coextensive with that of their native hosts and that of the wheat crop, with one single restriction. Only in countries where no dews set and no rains fall are rusts absent, for moisture is essential to their first growth which causes the infection of the host plant. In irrigated regions where dew and rain are lacking, wheat grows without being rusted. The leaf rust is most regular in its occurrence and is also most widely and universally distributed. It is the most common rust of Australia and India, and in the United States it is most abundant in the Atlantic and southern states. The stem rust is irregular in its occurrence, usually missing one or two years in five or six, especially in some localities. In the United States the severe attacks occur most frequently in the central states, and in parts of Texas and California. This rust is very common in northern Europe, and in some seasons it is also quite abundant in Australia and Tasmania. It seems to be comparatively unimportant in India.3
Conditions Favorable to Rust Development.—There must first be such a wind from infected districts as will bring plenty of
1 U. S. Dept. Agr., Farm. Bui. 219 (1905), p. 8; Minn. Press Bui. 24.
2 N. D. Bui. 68 (1906), p. 646.
3 Carleton, Cereal rusts of U. S., pp. 21-22, 56-57.