« AnteriorContinuar »
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
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
1 N. D. Bul. 68 (1906), p. 655.
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.
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.
as a rule, somewhat dwarfed, are close and compact, and stool but little. The leaves, comparatively few in number, are stiff, narrow and erect, with a more or less tough, dry cuticle, often with a glaucous or waxy surface; heads compact and narrow; and grains hard, red, small and heavy.
Varieties likely to prove considerably resistant to rust in the United States, if they are sown early, are, Kharkof, Turkey, Mennonite, Pringles No. 5, Rieti, Odessa and Pringle's Defiance for winter wheats, and Haynes Blue Stem and Saskatchewan Fife for spring wheats.
Durum wheats are much more resistant than other varieties. During the great rust attack of 1904 in the northwest, the maximum loss for durum wheat seems to have been about 10 per cent while that of ordinary wheats was frequently as great as 50 per cent. The different varieties of durum wheat also vary in their power to resist rust, two of the best being Iumillo and Velvet Don. "Rerraf' is one of the best rust resisters in Australia, but is quite non-resistant in the United States.
Other Diseases.—Leaf blight (Septosphaeria tritici Pass.) and Powdery mildew (Erysiphe graminis D. C.) occasionally cause slight losses in certain sections.
1 Carleton, Cereal Rusts of U. S., p. 21.
INSECT ENEMIES OF WHEAT
Species.—It has been estimated that there exist 1,000,000 species of insects of economic importance. About 100 species feed upon growing wheat, and about 50 more are found in granaries. Less than a dozen occasion enough loss to wheat to be of very great importance. Conditions in the United States åre most favorable for insects, because the continuous growing of the same grain crops over wide areas, and long, hot summers are very propitious for the multiplication of most species. On account of differences in climatic conditions and in the abundance of parasitic and other enemies, there is a periodicity in the recurrence of grain pests. Since a season favorable to one insect may be unfavorable to another, there is also a more or less marked rotation of different species.
Hessian Fly (Mayetiola destructor Say).-Wheat is the natural food plant of this insect, which is also supposed to be native to Asia. It was introduced into America from Europe. The Revolutionary patriots believed that it was contained in some straw brought over by the Hessian troops, hence its
Some of the ignorant Tory element claimed that General Washington was responsible for its introduction. It was first described technically in 1817.
DISTRIBUTION.—The natural spread of the Hessian fly has been estimated at 20 miles per year. It is now found in nearly all parts of the United States east of the 100th meridian, and on the Pacific coast (since 1884) probably from southern California to British Columbia. In Canada it has been found from Prince Edward Island to Indian Head, Saskatchewan. It also occurs in North Africa, western Asia, Europe and the British Islands. It has been an important grain pest in New Zealand since 1888.
DESCRIPTION AND LIFE HISTORY.—The adult Hessian fly is very fragile, dark-colored, and about 1/8 inch long. It is about half as large as the mosquito, which it resembles. Even when
comparatively abundant it will escape the notice of the ordinary observer. It can be caught with a sweeping-net, but is easily confused with other insects taken at the same time. The fly seems to be two brooded in all parts of the United States. In the north the broods follow each other in quick succession, while in the south they are widely separated. The egg of the insect is about 1-50 inch long. The newly hatched larva or maggot is slightly smaller than the egg. The fully developed larva is larger, and on account of its resemblance to a seed of flax it is known as the flaxseed. In fall wheat the fly passes the winter in the young plants, principally in the flaxseed stage, but also in the larval stage, not quite full grown. The flies
HESSIAN FLY :
a, FEMALE; b, MALE; C, EGGS.
emerge from the flaxseeds when the wheat is about 2 inches high. The time varies from March in Georgia to May in Michigan. Flies from wintering larvæ appear later. The eggs are deposited in the grooves on the upper surface of the wheat leaves, from 100 to 300 by each female fly. They are difficult of perception, even by one who has good eyesight. In a few days the eggs hatch into a pinkish larvæ that soon turn greenish, and descend to just above the roots, or, if the wheat has jointed, to the base of the particular leaves on which they were hatched. Sucking the juices from the growing wheat plant, these larvæ attain the flaxseed stage in about 4 weeks, the time being dependent on the weather. The prolonged southern summer during which there is little food for the