« AnteriorContinuar »
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 æcidiospores 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
breaking out through the skin of the wheat
in new spore beds in which no red spores BLACK AND RED have first formed. Over 2,000 of these spores
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 sporos 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 æcidiospores 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.' 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
· 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,' 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. 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.
Conditions Favorable to Rust Development. There must first be such a wind from infected districts as will bring plenty of rust spores to the heading wheat. That the spores are thus brought to the wheat is shown by the fact that screened plants are not rusted, and that distilled water exposed to the air will gather great numbers of the spores in the short period of a half hour. The more soft and succulent the wheat straw is, the more open it is to rust infection. "The most effective rust infection weather may be described as muggy, showery, sultry, rather still hot days, with foggy, cool, dewy nights, at about the blossom period. Just following the infection, cool, moist, slow growing, showery weather may result in the most general rust infection, and in the greatest breaking out or rupturing of the straw."1
1 U. S. Dept. Agr., Farm. Bul. 219 (1905), p. 8; Minn. Press
How Damage Results.—Rust deprives the wheat grains of their nourishment. The grains may be only slightly shriveled, or the crop may be completely ruined. It has been claimed that orange leaf rust does little damage to wheat, and that in very wet seasons it may even be of benefit to the grain by preventing superabundant growth of the vegetative parts. This has been denied recently, however, and with good show of reason. It is pointed out that in 1904 and 1905 the leaf rust was so severe that the wheat grains wilted and shriveled before the stem rust was well developed on the straw. The leaf rust may also delay the ripening of the crop until injury from frost results.” Since the attack of the stem rust is the more direct, it is unquestionably the more virulent.
The Loss From Wheat Rust in the United States doubtless exceeds that caused by any other fungous or insect pest, and it may be greater than the loss from all other diseases combined. It is often not noticed because it is light. In one or another of the wheat growing sections, great areas are partially to nearly completely destroyed each year. While almost fabulous figures are required to record the estimated annual loss, the probabilities are that this is underestimated, for the slight, unnoticed attacks never enter the computation. If the loss is but 1 per cent of the wheat crop, it approximates $5,000,000 annually in the United States alone. Bolley examined the wheat fields of North Dakota for fourteen seasons, and, leaving out of consideration the years of great destruction, he estimated the average annual loss at 10 per cent. In 1903 the loss in southern Wisconsin was 50 per cent and in South Carolina 30 per cent. In 1904 the wheat crop of Minnesota and the Dakotas was most promising, but in a few days it was so damaged by rust that experts estimated the loss for the three states at 30.000.000 bushels, and the wheat that was produced in many instances weighed only from 36 to 48 pounds per measured bushel. Many fields were not harvested. Twenty million dollars seems a reasonable estimate for the average yearly loss from wheat rust in the United States. Immense annual losses are suffered by the Australian, Russian and Argentine wheat fields. In some years the loss in England is 50 per cent. Nor is this the only form of loss that must be attributed to rust, for it is one of the chief hindrances that entirely prevent the growing of wheat in parts of certain moist warm countries, such as China and Japan.
1 N. D. Bul. 68 (1906), p. 655. · N. D. Bul. 68 (1906), pp. 651-654.
Remedies.—Thus far rust has baffled every attempt at a remedy. Fungicides and spraying have been experimented with, but indirect methods are the only ones that have proved of any aid in the combat. All conditions are helpful that tend to mature the wheat crop before the rust becomes abundant. Fields should be properly drained. Good clean seed of a pure variety and of the best germinating powers should be sown in soil properly prepared. The seeding should be early, and the crop should be kept free frora smut and weeds. All of these things strengthen the wheat plant and hasten its growth. Rotation of crops is also advantageous. Wild grasses and weeds of the roadsides should be mown, and all barberry shrubs should be killed. The fields should be kept free from volunteer grains. The line of demarcation between the winter and spring wheat belts should be sharply drawn, for the winter wheat, ripening early, develops rust in such abundance that it will greatly injure the later spring wheat. The early maturity of winter varieties generally enables them to escape serious damage.
One of the most hopeful phases of the question is that some varieties of wheat are quite rust resistant. The different rusts are each more easily resisted by certain varieties of wheat. Thousands of varieties have been tested and bred to secure rust resistance. None are absolutely rust proof. “So far as the ordinary wheats are concerned, the resistant varieties are.