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valley should yield good crops without commercial fertilizers for over a thousand years. "From 7 to 135 pounds of nitrogen, from 3 to 55 pounds of phosphoric acid, and from 3 to 36 pounds of potash are sold with every ton of produce leaving the farm.'' * Eighty billion pounds of nitrogenous material entered into the creation of one harvest in the United States in the early nineties. The cereals annually took from the earth nearly 3 billion pounds of phosphoric acid, and the loss of potash was not less than 4 billion pounds.
1 Yearbook U. S. Dept. Agr., 1897, p. 301.
Introductory.—Studies in plant pathology of any great practical bearing or importance are decidedly and characteristically modern and recent. In 1885 there were three institutions in the United States besides the department of agriculture which were making systematic efforts in experimental work with plant diseases, and in disseminating such knowledge as then existed in these lines. Ten years later over one hundred special investigators were devoting their time to this work, and 50 colleges and stations were endeavoring to solve its practical problems. The science of plant pathology has had its highest appreciation during the last decade, for some of the outlying problems have been solved, a working foundation has been laid from the facts that were acquired, and a body of institutions with specialists and resources has been developed for scientifically prosecuting the work.
The Classification here followed is practical and inclusive, rather than scientific and exclusive. There is nothing pathological in the sudden destruction of a field of wheat by floods or locusts, but excessive moisture or the presence of a parasite may each bring about diseased conditions, and every gradation of phenomena between these two types must be considered.
Sources of Injury and Weakness are of three kinds: (1) Unfavorable inanimate environment; (2) unfavorable animate environment; and (3) poor seed wheat. When these three sets of factors occur in combination, as they frequently do, their relations and inter-relations are so intimate and intricate as to be inseparable. Only in a general way can they be individually studied.
WEATHER AND SOIL INFLUENCES
Unfavorable Inanimate Environment.—Drought; Hail, Wind And Rain Storms; Floods; Fire. The operation of these destructive influences needs no further elucidation than a mere 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 radiation 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.
Unfavorable Animate Environment.—Weeds—Plants out of place are called weeds. They deprive wheat of its 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.