Imágenes de páginas
PDF

mention. Very little specific information can be found as to the extent of damage caused. Such scattering data as have been collected can be most advantageously presented under the subject of insurance.

Frosts.—The most usual injury by frost is the winterkilling of fall wheat. This may occur whenever the ground freezes to any appreciable depth, and in two ways. The plants either freeze to death, or are lifted out of the soil by alternate freezing and thawing. A good covering of snow is very protective. Seeding with a press drill lessens the danger. Frost may also injure wheat when it is filling, or it may cause the stems to burst after they have jointed.

Hot Waves Or Winds are most liable to occur during a period of drought. It is thought that these waves can be forecasted for a period of about four days. At such times the eastward circulation of the atmosphere is practically suspended, and radi.ation is at a minimum. A hot wave is defined as a period of three or more consecutive days with a maximum temperature reaching or passing 90° F. In years when hot waves are unusually severe, the harvest returns are decreased by one-fourth in quantity, and the quality is quite inferior. The heat seems to mellow the ground, however, and to put it in such ideal condition as to increase the crop of the following season. Hot winds have a velocity of 20 to 30 miles per hour, a temperature often ranging from 100 to 106° F., and 20 to 30 per cent of relative humidity. The roots cannot supply moisture, even if it is present in abundance, as fast as it is evaporated from the leaves by this great blast of hot, dry air. The cells are completely desiccated, and the whole structure of the plant collapses. A hot wind is most destructive immediately after a rain, which temporarily checks and lessens the transpiration of which the plant is capable. In the United States these winds are most apt to occur in the central prairie regions. In Argentina, a similar dry, hot wind known as the pampero comes up suddenly, destroys all vegetation "and even cracks furniture and timber in buildings." A wind-break of trees, or anything else that tends to lessen the movement of the air, has a remedial effect.

Excessive Moisture.—This may be injurious in a number of ways. If too much water is present, the wheat may be "drowned." It also tends to develop the straw indefinitely, and at the expense of the grain. Rains during heading are apt to prevent filling, and are by far the most common cause of blight. In a very wet harvest wheat is apt to germinate before it can be threshed. One-third of the wheat crop of lower Canada was lost in 1855 by the grain germinating in the straw.

Unfavorable Soil.—The soil texture may be such as to deprive the roots of the proper air supply. Certain elements, as in the case of alkali, for example, may be present in such abundance that their chemical action upon the wheat plants causes disease. Some of the essential plant foods may be absent, or present in improper proportion.

All types of disease mentioned thus far arise from physiological variations due to abnormal variations in the growth factors, are not transmissible, and consequently never spread from plant to plant or field to field, as do the infectious wheat diseases. » f. * *

PLANT INFLUENCES

Unfavorable Animate Environment.—Weeds—Plants out of place are called weeds. They deprive wheat "of i^ nutriment and ordinarily give very little in return, except in the case of certain legumes. Weeds once introduced into a region spread rapidly. With runners, rootstocks, running roots and apparatus for throwing seeds, they effect a dispersion of their kind independently of any external agencies. Wind, water and animals are the natural agencies that aid in the dispersion, but rarely carry seeds long distances. Man aids weed migration more than all natural means combined, and consequently its general direction is in the wake of the progress of cultivation. Commerce in wheat makes some weeds cosmopolites. These plants have a wide range of adaptability, which grows wider under conditions of cultivation. Some seeds, especially those of cockle, have a tendency to approximate the wheat grain on account of selective influences arising from cleaning seed wheat. Those which differ most from wheat are the ones removed. Sowing the remaining ones develops a strain more closely resembling the wheat grain.

Weeds injurious to wheat may be divided into three classes, based on the point of incidence of the damage caused: (1) Those which choke the crop, preventing its growth; (2) those which interfere with harvesting and curing; and (3) those whose seeds injure the commercial value of the grain by mingling with it. Deterioration in the quality of the grain by the third class is perhaps the greatest damage resulting from weeds.

Kinds of Weeds: Chess Or Cheat (Bromus secalinus L.)—This is an annual grass that will not produce seed unless sown in the fall, and consequently it is not found in spring wheat. It is less vigorous than wheat, but more prolific, and also more resistant against cold and insects. One pound of seed has been known to multiply 99-fold in one generation, and one seed 3,000-fold. The ordinary observer cannot distinguish the young chess plant from wheat. Chess injures flour and must be cleaned from the wheat before grinding. Seed wheat properly cleaned by a fanning mill is quite free from chess. If wheat is treated for smut by stirring it in a solution, the chess seeds will rise to the surface and can be skimmed off. A pound of chess and a bushel of wheat have about the same number of seeds.

Russian Thistle or Cactus (Salsola kali tragus L.).—This weed is neither a thistle nor a cactus, but a saltwort, closely related to the tumbleweed, lamb's quarters, and pigweed. In parts of Russia, where it has been known over 150 years, extending now to northern Russia and central Siberia, it is known as Tartar or Hector weed. It was first introduced into the United States in 1873 or 1874, being sown in South Dakota with Russian flax seed. As the weed prefers a dry climate, it could not have found a more congenial habitat. When the plants were uprooted in the fall they rolled across the prairie with the speed of the wind, scattering seeds at every bound, and stopping only when they were worn to pieces, or when the wind ceased, for in the early Dakotas there were few fences, forests, or streams to stop their course.

Thus they covered an advance of 5 or 10 miles in a season, though stray plants went much farther. As a rapid traveler thoroughly covering territory it surpassed any other weed known in America, and very few cultivated plants intentionally distributed have such a record for rapidity. Within 20 years it infested a continuous area of about 35,000 square miles, and caused at least $1,600,000 damage to wheat every year. By 1894 it had crossed to the west side of the Missouri river, and was spreading in Minnesota, Iowa and Nebraska. In 1895 the injuries which it caused extended from Michigan to Colorado, Idaho and California, but the greatest damage resulted in the Dakotas and Nebraska.

The Russian thistle is an annual with a dense, yet light growth of circular or hemispherical form. The average plant is 2 to 3 feet in diameter, weighs 2 to 3 pounds when matured and dry, and is estimated to bear 20,000 to 30,000 seeds. Single plants have been found 6 feet in diameter, weighing about 20 pounds when thoroughly dry, and estimated to bear 200,000 seeds. It is ideally fitted to be carried by the fall winds, which easily break off or pull out its slender roots. A severe frost kills the plant at any time, but it produces seeds abundantly as far north as the Canadian boundary.

In Russia no effectual method of exterminating the weed is known. It is continually growing worse, and as a consequence the cultivation of crops has been abandoned over large areas in some of the provinces near the Caspian sea. Laws for its eradication were passed in South Dakota in 1890, in Iowa in 1893, and in Kansas, North Dakota, Minnesota and Wisconsin in 1895. In 1892 and again in 1893, the department of agriculture sent an assistant botanist to the Dakotas to work out the whole life history of the plant, and the method of dealing with it was established chiefly on the basis of the knowledge thus obtained. It is claimed that if this had been done before 1885, a saving would have been effected for the wheat growers of the northwest "sufficient to pay the cost of maintaining the whole department of agriculture for many years to come."1 Wetter seasons, more intensive farming, the building of fences, and the planting of trees reduced the Russian thistle to the ranks of comparatively unimportant weeds in the Dakotas, but in Nebraska stringent measures were necessary. Farmers cooperated in the work, and the weed was "hunted almost as strenuously as game would be," so that for some years it has not been an important factor in wheat growing in Nebraska.

Darnel (Lolium temulentum L.) has its widest distribution in Europe. It also occurs in the wheat fields of California, where 1 Yearbook U. S. Dept. Agr., 1897, pp. 96-96.

it is mistaken for chess. It is an annual grass, and can be cleaned from seed wheat by the chess method.

Cockle (Agrostemma githago L.) is a member of the pink family and a widely distributed weed of the wheat fields since ancient times. In size and weight its seeds resemble wheat grains so closely as to be removed with difficulty. They are easily seen in the grain, however, and are injurious to flour, consequently they render wheat less marketable. As cockle is usually not abundant, grows over a foot high, and is conspicuous because of its large pink blossoms, it can easily be pulled from the growing wheat. The seeds have great vitality and will germinate even if they have lain in the ground several years.

Wild Garlic (Allium vineale L.) is most troublesome to wheat in eastern United States. It grows about two feet in height. The flour is spoiled when the bulblets of the plant are ground with the wheat. These can be removed by careful screening. Badly infested land should be put into cultivated crops for a few years.

Wheat-Thief (Lilhospermum arvense L.).—Other names by which wheat-thief is known are bastard alkanet, corn gromwell, redroot and pigeonweed. Its greatest damage is to meadows following wheat in rotation. Cultivated crops are the best remedy.

Wild Mustard or Charlock (Brassica sinapistrum L.) is so uniformly found in spring wheat that flouring mills make a byproduct of its seed. When not very abundant it is easily pulled in the field, for it grows nearly as high as the grain and has conspicuous yellow flowers. In small wheat fields where it is very abundant it can be killed by spraying the field with a 3 per cent solution of copper sulphate, using about 50 gallons of solution to the acre. It is claimed that the wheat is not injured.1 If wild mustard seed is covered with at least five inches of soil it will not grow, but thus buried it will' retain good germinating power for over 56 months. It comes up most abundantly through one inch of soil.

Thistles are of two varieties: Canada thistle (Cnicus arvensis), and common or sow thistle (C. lanceolatus). The latter is also known as spear, bur, and bull-thistle. It grows 2 to 4 feet high and has the better hold on the land where both 1 Cornell Bui. 216 (1904), p. 107.

« AnteriorContinuar »