Popular Science Monthly/Volume 78/May 1911/Progress in Control of Plant Diseases

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PROGRESS IN CONTROL OF PLANT DISEASES
By Dr. F. L. STEVENS

PROFESSOR OF BOTANY AND VEGETABLE PATHOLOGY IN THE N. C. COLLEGE OF AGRICULTURE AND MECHANIC ARTS AND AGRICULTURAL EXPERIMENT STATION

PLANTS are subject to disease. As in the human being these diseases decrease vigor and productiveness of the organism or cause death. An attack upon valuable plant products such as ripe fruit, tubers and root crops, and mature timber may result in depreciation in value or even entire loss of the product. The rot of apples upon the tree or in the bin, the common blue mold seen upon lemons and oranges, the wide-spread blight of pear and apple twigs (Fig, 1) are familiar examples of plant disease.

In the early years of American history these afflictions were regarded as natural and inevitable, but during the last three or four decades scientific study has shown that rots, blights, molds, mildews, rusts, smuts, etc., are true diseases; that they do not constitute any part of the normal life stages of the plant affected. That they are caused by living parasites and, moreover, that they are often preventable.

Plant diseases have increased largely in number and destructiveness during recent years. This is due in part to the migration of disease from county to county and state to state; in part to the cultivation of weak or susceptible varieties and in part to long continued cropping in a given region, thus affording opportunity for the plant pests, which may at first have been weak and unimportant, to become thoroughly established and aggressive.

The asparagus rust, which has in some states nearly prohibited asparagus culture, offers an excellent example of migration in its progressive westward march across our continent. This invasion seems to have occupied the years between 1896 and 1902 since the rust was first noted at New Jersey in 1896, South Carolina 1897, Michigan 1898, Illinois 1899, Dakota, Nebraska and Texas 1900, California 1901 or 1902.

The destructive pear blight, our worst pear disease, made a similar journey, starting from the neighborhood of the Hudson valley, near the beginning of the last century, reached the Rockies sometime after 1886 and arrived in California about 1895.

Diseases have come to America from Europe. Grape anthracnose, cabbage club root, potato wart (Fig. 2), are noteworthy examples; similarly, PSM V78 D480 Healthy and diseased twigs by pear blight.pngFig. 1. Pear Blight, healthy and diseased twigs. serious American plant diseases such as potato blight, grape black rot, grape downy mildew and powdery mildew have made the European tour or even the world tour "personally conducted" under some unwitting guides.

The chestnut bark disease (Fig. 3) illustrates well the rapid and destructive invasion that is possible in case of new diseases. First noted in 1904 by Murrill in New York, and now well known in New York, New Jersey, Delaware, Connecticut, Rhode Island, Maryland and Virginia, it is rapidly spreading in every direction (Fig. 4). In Brooklyn 16,695 trees were killed on 350 acres, and the loss in and about New York city is placed at five to ten million dollars. The chinquepin and chestnut alone are susceptible. The attack is made upon the bark through wounds, but twigs and leaves are not directly affected. From the point of attack the disease spreads in all directions until the diseased parts meet on the opposite side of the branch, thus girdling the twig and killing it.

Sometimes it happens that a newly introduced disease causes much loss in its first years and later sinks to comparative insignificance. Such was the PSM V78 D480 Potato destroyed by the wart disease.pngFig. 2. A Potato half Destroyed by "Wart Disease"; the large "warty" outgrowth is soft and rapidly decaying. history of the carnation rust which about 1892 caused the loss of entire houses of plants, but which in a few years spent its force until it is now regarded as a disease of no unusual menace. In other instances imported diseases continually remain serious, as have the numerous grape diseases introduced into Europe from America and from Europe into America.

The diseases mentioned in Diagram I. are nearly all of great destructiveness. The potato blight is that which caused the famous potato famine in Ireland in 1845, in which year it swept Great Britain as well as much of Europe and America with a

PSM V78 D481 Chestnut trees destroyed by bark disease.png

Fig. 3. Large Chestnut Trees killed by the Bark Disease.

wave of disease. It to-day causes loss estimated at $36,000,000 annually in the United States. New York alone has incurred a loss of $10,000,000 in one year.

The asparagus rust prohibited successful asparagus culture in several states and threatened that industry in California, where the crop occupies yearly over 7,000 acres of land, until means of control were devised by scientific men. The rice smut was recognized in its

PSM V78 D481 Distribution of chestnut bark disease by county and state.png

Fig. 4. Map showing the Distribution of the Chestnut Bark Disease. Black shows area of severe infection; round dots, presence of disease prior to 1909; + indicates the spread of the disease during 1909. After Metcalf.

 
PSM V78 D482 Intercontinental migration of plant diseases.png
Diagram 1. Illustrating the Inter-continental Migration of Plant Diseases.

No. 1, Potato blight: Chili-Colorado-Europe. No. 2, Asparagus rust: Europe, 1805; New jersey, 1896, South Carolina, 1897; Michigan, 1898; Illinois, 1899; Dakota, Nebraska and Texas, 1900; California, 1901. No. 3, potato cercosporose: Europe, 1854; United States, 1903. No. 4, rice smut: Japan-South Carolina, 1898. No. 5, sorghum smut: Japan-United States 1884. No. 6, grape anthracnose: Europe-America, 1880, or earlier, now widespread. No. 7, cucumber downey mildew: Cuba 1868; United States, 1889. No. 8, grape black rot: North America, early France, 1885; Italy and the Caucausus, 1898. No. 9, potato wart: Hungary, 1896; England, 1900; Newfoundland, 1909; Boston and New York, 1910. No. 10, grape downey mildew: America early; France, 1873; The Rhineland, Savoy and Italy, 1879; The Tyrol and Algiers, 1880; Portugal and Greece, 1881; Alsace, 1882; the Caucausus 1887; Brazil, 1890. Now known in all countries except Australia. No. 11, grape powdery mildew: United States, early; England, 1845; Belgium and France, 1848; all Europe, 1849; Madeira, 1852. Known everywhere now. No. 12, chrysanthemum rust: Japan-England, 1895; America, 1896.

PSM V78 D483 Interstate migration of plant diseases 1886 to 1896.png

Diagram 2. Showing Interstate Migration of some Important Diseases.

1, asparagus rust: New Jersey, 1896; South Carolina, 1897; Michigan, 1898; Illinois, 1899; Dakota, Nebraska, Texas, 1900; California, 1901. 2, fire blight: the Hudson, 1792; California, 1895-7, 3, peach yellows: Philadelphia, 1806; Ontario, Michigan, Illinois, Maine, 1886.

first encroachment in South Carolina and an agressivecampaign against it with remedies which had recently proved efficient against smuts of other cereals checked it and completely subjugated it, so that it is no longer known in the Palmetto State. The cucumber mildew, which causes blanched spots on the leaves and often entire loss of crop, has rendered unprofitable the culture of cucumbers and cantaloupes in many sections. The two grape mildews are notorious in their destructiveness and have driven vineyardists to large expense in spraying or sulphuring.

Perhaps the most striking importation is that of the potato wart. This disease causes large unsightly knotty excrescences on the potato, rendering them worthless. It was first reported in England in 1902, found its way to Newfoundland, and it is known that two consignments of Newfoundland potatoes, probably infected, were shipped last year, one to New York, one to Boston. The spread of this disease in our states is an event to be predicted with confidence, especially as many sections of the country depend upon New England for their seed potatoes.

The large increase in aggressiveness of plant diseases has been met by a campaign of increased knowledge leading to new modes of subjugation. Though a few plant diseases are mentioned in early writings, (II. Chronicles 6-28, Shakespear's King Lear, III., Sc. 4; I. Kings 8-37; Moses 28, 22), the real significance of their presence, their nature and causes may be said to have been first recognized between the
 
PSM V78 D484 Advance knowledge of the spread of plant diseases 1.png
Table I
 

Table I (Continued)

PSM V78 D484 Advance knowledge of the spread of plant diseases 2.png

Table I. Showing Advance in Knowledge regarding some of the Chief Plant Diseases. The broken upper line indicates vague or indefinite knowledge; the solld line definite knowledge. The lower broken line indicates partial but unsatisfactory knowledge concerning treatment; the solid line the possession of facts sufficient for satisfactory treatment.

years 1846 and 1864, notwithstanding the fact that the first sulphur treatment for powdery mildew was recommended in 1821 and the first seed treatment for smuts was discussed in 1807.

PSM V78 D485 Advance knowledge of the spread of animal diseases 1.png

Table II. Showing Advance in Knowledge regarding some of the Chief Diseases of Domestic Animals. The broken upper line indicates vague or indefinite knowledge; the solid line definite knowledge. The lower broken line indicates partial but unsatisfactory knowledge concerning treatment; the solid line the possession of facts sufficient for satisfactory treatment.

PSM V78 D485 Advance knowledge of the spread of human diseases 1.png

{sc|Table III. Showing Advance in Knowledge regarding some of the Chief Human Diseases.}} The broken upper line indicates vague or indefinite knowledge; the solid line definite knowledge. The lower broken line indicates partial but unsatisfactory knowledge concerning treatment; the solid line the possession of facts sufficient for satisfactory treatment.

The most important advance in prophylactic procedure came with the introduction of the Bordeaux mixture, a compound of lime, bluestone and water, by Millardet in 3885. Since that date the battle has waged fiercely with successive conquests for plant pathology. Now many of the most important diseases are well understood and many successful modes of treatment have been devised.

Some of these with their most significant dates are represented in Table I.

During the seventies there were only two or three investigators in plant pathology in America. This number rapidly increased until between 1888 and 1900 over 4,000 papers upon this subject appeared. Only a few dozen economic plant diseases had been even cursorily described prior to 1880, while to-day a total of over 500, more than 250 of them serious, have been investigated. Countless diseases of wild plants also are now known more or less completely.

To enable comparison of the real advance made in plant pathology with the advances made in the sister fields of veterinary medicine and human medicine, two tables are given summarizing the history of these two fields as regards a few of the most important diseases (tables 11. and III.).

That plant pathology has made such a relatively good showing notwithstanding her tardy start is due to the late influence of the germ theory of disease in all of these fields.

With the continual increase of plant diseases in both number and aggressiveness every effort of the plant pathologist will be required to increase the efficiency of treatment. So also is there great need of a public opinion favoring rational plant sanitation. The chief diseases now prevalent as well as those which are liable to invade any section in the near future should be known to all plant producers. Especially needed are laws properly enforced to restrict the migration of diseases.