1022 DISEASES AND INSECTS 



DISEASES AND INSECTS 



book, "Die Krankheit der Kulturgewachse," pub- 

 lished in 1858, is to be regarded as the first book of real 

 economic importance on the subject of diseases in 

 plants. In this remarkable volume is given a concise 

 statement of the thoroughly digested and personally 

 tested knowledge of his time, on the nature and control 

 of plant diseases. He also describes a number of new 

 methods, especially for seed treatment of cereals 

 against smuts, which have become the foundation for 

 many of our present-day practices. 



Since Kiihn's day there have been remarkable 

 developments in the control of plant diseases. The dis- 

 covery of bordeaux mixture by the Frenchman Millar- 

 det in 1882; the discovery of the formaldehyde treat- 

 ment of seed for smut by the American plant patho- 

 logist, Arthur, in 1896; and the recent development of 

 the use of lime-sulfur solutions and mixtures as a sub- 

 stitute for bordeaux in the spraying of apples and 

 peaches, are but the most noteworthy of the many 

 discoveries and developments in the remarkable growth 

 of this economic science within the last half century. 



The economic importance of plant diseases can 

 scarcely be overestimated, as they constitute one of the 

 chief losses in our agricultural resources. The loss 

 from 5 to 25 per cent of many crops from diseases 

 alone each year is so common as to be the general rule. 

 The loss from potato diseases each season in the United 

 States has been carefully estimated at not less than 

 $36,000,000. Yet, it has been conclusively demon- 

 strated by extensive experiments among potato-growers 

 during a continuous period of ten years, that an annual 

 average increase of over forty bushels per acre may be 

 expected from spraying the crop with bordeaux mix- 

 ture, from three to five times in the season at a total 

 average cost of about $5 per acre. The loss from oat- 

 smut commonly averages from 5 to 25 per cent of the 

 crop, yet it may be absolutely prevented by seed 

 treatment at almost insignificant cost. The loss from 

 scab in the apple crop of New York State often totals 

 not less than $3,000,000 and for the United States a 

 corresponding loss of over $40,000,000. In 1900, the 

 peach-growers of Georgia lost $5,000,000 by brown rot, 

 while the average annual loss from the same disease in 

 the entire United States is never less. Yet in each case 

 here mentioned, as well as in most of the other of our 

 common and destructive diseases, cheap and effective 

 means of control are within the reach of every grower. 



The value and 

 efficiency of 

 these means 

 have been estab- 

 lished beyond 

 doubt. Their 

 profitable appli- 

 cation requires 

 only intelligence 

 and practice on 

 the part of the 

 grower. 



Symptoms of 

 disease in plants 

 are so varied in character as to 

 make an attempt at wholly 

 satisfactory grouping for practi- 

 cal purposes of doubtful value. 

 Mention of some of the more 

 common types, however, may be 

 useful. The grower must learn 

 by study and experience the 

 more striking symptoms charac- 

 teristic of those diseases peculiar 

 to the crops that he grows. 



Disease may be exhibited in 



1279. Effects of the malformations of the leaf, stem, 

 leaf -curl fungus on peach ro t or fruit, as for example, 

 foliage. (XM) knots, galls, tubercles, curling, 



1280. Early blight of potato. 



wrinkling or other distortions. There are such symp- 

 toms in crown-gall of trees, black-knot of plums and 

 cherries and leaf-curl of the peach (Fig. 1279). Another 

 type are cankers, dead sunken or roughened areas 

 in the bark of trees or the outer rind of herbaceous 

 stems, as for example 

 in the New York apple- 

 tree canker, the brown- 

 rot canker of peaches, 

 frost cankers of many 

 trees, and anthracnose 

 of beans, melons, and 

 others. The blight 

 type of lesion is also 

 very common. Here 

 are the more or less 

 sudden death of leaves, 

 stems, shoots or blos- 

 soms, usually turning 

 dark and drying up. 

 Such symptoms char- 

 acterize fire-blight of 

 fruit trees, potato- 

 blight (Fig. 1280, from 

 Vt. Sta.), alternaria 

 blight of ginseng and 

 similar diseases, espe- 

 cially in their last stages. The leaf- or fruit-spot type is 

 also very common. Brown or black spots appear in 

 foliage or fruit. They may be brown dead or rotted 

 areas, or spots due to the growth of the parasite 

 on or under the surface. Bordeaux-injury spots on 

 apple foliage, shot-hole leaf injury of stone fruits, leaf- 

 spot of the currant (Fig. 1281), celery or alfalfa, the 

 tar-spot of the maple, the black-spot of the rose and 

 the apple-scab are of this type. Another not uncom- 

 mon type is that exhibited in certain bacterial and 

 fungous diseases, where the pathogen infests the sap- 

 tube regions of the stems or petioles, resulting in a 

 sudden wilting of leaves and shoots. The wilt diseases 

 of cotton, cucumber, ginseng, watermelon and cowpeas 

 are characterized by this symptom. The yellowing of 

 the foliage, either suffused or localized as spots, rings, 

 and blotches and often accompanied by dwarfing and 

 wrinkling of the affected organs is a common symptom 

 of certain so-called physiological diseases like the peach 

 yellows (Figs. 1282, 1283), little-peach, mosaic disease 

 of tobacco, infectious chlorosis and nitrogen-poisoning 

 of greenhouse cucumbers (Fig. 1284) and other plants. 



The causes of disease in plants. 



Etiology, or the cause of disease, has been more 

 generally and carefully investigated than any other 

 phase of the subject, so that we now know much regard- 

 nig the agents primarily responsible for most plant 

 diseases. These agents may be grouped as follows: 



Slime molds, lowly organisms having characters of 

 both plants and animals (see article Fungi). The club- 

 root of cabbage, cauliflower and other crucifers, is 

 the best known slime-mold disease. 



Bacteria, microscopic unicellular plants which mul- 

 tiply very rapidly by simple fusion (see article Fungi). 

 While most species are harmless scavengers of dead 

 organic matter, and a few are known to cause dis- 

 eases of men and animals, not less than 150 different 

 diseases of plants are now known to be due to the 

 attacks of parasitic bacteria. Some of the commonest 

 bacterial diseases of plants are, fire-blight, crown-gall, 

 olive-knot, soft-rot of vegetables, potato-scab, cucum- 

 ber-wilt and black-leg of potatoes. 



Fungi (see Vol. Ill) are perhaps responsible for far 

 the greater number of the diseases of plants. They are 

 the causal agents in such well-known diseases as apple- 

 scab, brown-rot of plums and peaches (Fig. 1285), 

 black-rot of grapes, (Fig. 1286) bitter-rot of apples, 

 brown-rot of lemons, late blight of potatoes, peach- 



