A text-book of mycology and plant pathology . f internal. Theexternal factors of disease are the chemical conditions of the soil, as adetermining cause, also the physical character of the soil. The influ-ence of a superabundance of water, or its absence, is important. Cli- 1 Cook, Mel T. and Taubenhaus, J. J.: The Relation of Parasitic Fungi to theContents of the Cells of the Host Plants, i. The Toxicity of Tannin, Bull. 91,Delaware College Agric. Exper. Stat., Feb. i, 1911. GENERAL CONSIDERATION OF PLANT DISEASES 75 matic and meteorologic conditions may be influential, when these dis-turb the
A text-book of mycology and plant pathology . f internal. Theexternal factors of disease are the chemical conditions of the soil, as adetermining cause, also the physical character of the soil. The influ-ence of a superabundance of water, or its absence, is important. Cli- 1 Cook, Mel T. and Taubenhaus, J. J.: The Relation of Parasitic Fungi to theContents of the Cells of the Host Plants, i. The Toxicity of Tannin, Bull. 91,Delaware College Agric. Exper. Stat., Feb. i, 1911. GENERAL CONSIDERATION OF PLANT DISEASES 75 matic and meteorologic conditions may be influential, when these dis-turb the normal life of the plant. Light, heat, cold, rain, dew, hail,frost, wind and lightning play an important role. The gaseous emana-tions from gas pipes, smelter works, smokestacks, including soot, dustfrom cement works, acids, poisons, and dye stuffs, which pollute streams,all are determining causes of disease. Traumatism or mechanic injurymay be of various sorts and the effects are dependent upon the form andseverity of the injury, or Fig. III.—Rose-chafer {Macrodaclylis subspinosus). a. Adult or beetle; b,larva; c, d, mouth parts of same; e, pupa, /, injury to leaves and blossoms ofgrape with heetles at work. (From Marlalt in Quaintayice, A. L. and Shear, C. L.,U. S. Farmers Bull. 284, 1907.) Living organisms, whether animal or vegetal, may be the cause ofdisease. All groups of animals may be considered, but the mammals,worms and insects (Fig. iii) are of most importance and depredations of plants are notorious and insects occupy firstplace in their destructive effects on plants (Fig. 112). Various para-sitic flowering plants are known, as well, as the bacteria and fungi,for their disease-producing powers. 276 GENERAL PLANT PATHOLOGY As an internal determining cause, the formation of enzymes underabnormal cx)nditions must be reckoned as causal, as well, as nutritivedisturbances which produce monstrosities and the like. Having classified the chief caus
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