Page:The New International Encyclopædia 1st ed. v. 06.djvu/342

* DISEASES OF ANIMALS. 292 DISEASES OF PLANTS. Of the chief infectious diseases of animals, ac- tinomycosis, anthrax, gUmders. fooluntlaiouUi disease, rabies, tetanus, and tuberculosis may be transmitted to man. DISEASES OF PLANTS. In addition to biini: of i.Tcat sciciitilic inliTcst, diseases of phints arc of iiiiniciisc ecoiioniic ini]inrtance. Losses runnin;,' into millions of dollars have been caused in ditferent countries by certain plant diseases. In Australia the vhcat-rii>t often greatly depreciates the wheat crop; phylloxera has ravajjed the vineyards of France: potato-rot is held partly responsible for some of the famines in Ireland; mildew and black rot are serious enemies to firape culture in America, and in parts of California grape-growing has been aban- doned on account of disease. While the oc- currence, though not the causes, of many plant diseases has long been kno«-n, their dctinitc study (phytopathology) has been devel- oped since about 1S85. Since that date our knowledge of the causes of diseases and of the means of preventing them has been growing with remarkable rapidity, and no country has added more to this information than the United States, through the plant pathologists of the Depart- ment of .griculture and of the several agri- cultural exjieriment stations. For ordinary purposes, plant diseases may be classified, according to their causes, into four categories — namely: (1) Those caused by fungi ; (2) those caused by bacteria: (3) those due to nematodes and insects; and (4) those due to physiological causes. Taking these in the order of enumeration, the diseases due to fungi are given first rank, and they are perhaps the most widely spread and destructive of all. There is hardly a crop or a species of plant that is not liable to attacks of fungi through either its roots, stem, leaves, or fruit. The spores of fungi are exceedingly mi- nute, and through them disease is spread. They are carried everywhere by the wind, by insects, and other agencies. Finding lodgment upon a plant, if the conditions arc favorable, it is at- tacked. Plants not in perfect physical and physiological condition olfcr ai^ easy entrance to the germ tube sprouted from the spore, and the threadlike mycelium spreads the disease throngli the host plant, as it is called, the fungus being referred to as a parasite. All plants are not subject to the attack of the same fungi, since certain conditions render a given plant imnuine to some and susceptible to others ; nor are all fungi parasitic, since some live use- fully as saprophytes upon dead and decaying vegetation. Some fungi gain entrance to plants b_v sending their germ tuljes through the minute breathing pores of the leaves ; others dissolve their way through the cuticle by n progres.ses with great rapid- ity, from individual to individual, and becomes a veritable epidemic. The host plant is injured in a number of ways, such as being robl)ed of its nourishment, having its water-supply shut o(T, etc. Some parasites stimulate the host to un- usual growth, as may be seen in the galls. knots, and swellings, .-ome of which are of pro- tescjue shapes; still others produce stunted growth or destroy the host plant entirely. Many fungi which cause plant diseases are interest- ing on account of the fact that they exist in two forms, each of which lives u]ion a dilfei-ent plant. The phases which belong to one cannot be made to grow upon the other, but the phages of each host readily inix?u- late the other. This i, true of the fungus causing apple-rust, the alternate form of which is foiuul in the so-called apples of the red cedar or juni|>er. Different stagi's of one of the wheat- rusts ( Puccinia graminis) are found upon wheat and upon the barberry-leaves. A disease of pine occurs in another form upon currant-leaves, and many other similar e.amples arc well known. Familiar examples of diseases caused hy fungi are the rusts and smuts of grains, potato-rot, grape-rot, clubroot. crown-gall, plum-knot, an- thraenose of many plants, leaf-spots, wilt dis- eases, mildews, scab, etc., all more fully described under the nanus of their respective hosts. The second class of plant diseases to be con- sidered are those due to bacteria, over some of which there has been considerable controversy. Some writers deny their bacterial origin. But. the invest igiit ions of Dr. E. F. Smith and others .seem to prove that in a number of well- known instances bacteria are the immediate cause of disease. These investigators, by carefully conducted experiments, have succeeded in pro- ducing the diseases in question by inoculation experiments with the bacteria. Diseases of fun- gous (jrigin for the ino»t part attack certain defi- nite portions of plants, as the leaves, fruits, etc., destroying them and indirectly affecting the rest of the plant ; but the bacterial diseases, es|x>cially upon soft, herbaceous plants, attack any part of the plant and cause its sudden collapse without nnu-h delay. Prominent jimong the diseases at- tributed to bacteria may l)e mentioned the fire- blight of apples and pears, black rot of cabbage, a wilt disease of cucumbers and melons, a brown rot of potatoes, egg-plant, and tomatoes, a disease of sweet corn, a hyacinth disease, and a disease of celery. Most of these are fully characterized under their special names. Among the <liseases of the third class may be mentioned the phylloxera, which attacks the roots of the grape, devastating the vineyards of France. Italy. (lermany, and Europe generally, wherever varieties of grapes which belong to Vitis vinifera are grown : nematodes oreelworms, which attack the roots of many plants in parts of -the I'nitcd States, Europe, and elsewhere, causing galls to be formed to the great detriment of the plant: and thrips and aphides, whose punctures cause a disease of carnations and doubtless many other plants. The diseases of the last class arc due to some interference with the proper exercise of the func- tions of the plant — by improper nutrition, too nnich or too little water, light, heat. etc. These functional diseases manifest tliemselves in a manner similar to the b.acterial diseases, the whole plant being affected. For these diseasea the corrective means suggest themselves, the only dilTiculty lieing the discovery of the causes. The investigations of the past few years have led to the discovery not only of the cau'^es of many plant diseases, but also of the means where-