Disease

Disease in plants may be defined as any derangement or disorganization of the normal structure or physiological functions of the plant, as for example the formation of (jails, cankers or distortions, rotting of plant parts, or disturbances in the sap system resulting in wilting, or in the nutritive processes resulting in such symptoms as dwarfing, chlorosis, and the like. Forms of plant diseases are shown in Figs. 1279-1292.

It is often very difficult to distinguish clearly between diseased conditions and abnormalities of other types. Bud-sports, doubling of blossoms, fasciations and many other similar abnormalities, while often the result of reaction to some pathogen, are not apparently always so and they are often spoken of as teratological phenomena. While the reaction of plants to insect attacks in the formation of galls, cankers, and so on, is to be regarded as symptom of disease, the injuries produced by the mere eating away of parts of leaf, stem or fruit are not usually so to be regarded. Even here, however, it is often difficult to draw a sharp line of demarcation. While disease may usually be said to result in ultimate injury, there are apparently certain marked exceptions, as in the case of the root tubercles of legumes caused by the attacks of certain nitrogen-fixing parasitic bacteria. Here increased growth and crop-yield are generally held to result.

Diseases of plants are not something new or of recent development, as the grower is often inclined to think. The crops of the husbandman, from the earliest recorded history of his art, have been afflicted with diseases. In the historical writings of the Hebrews, the Bible, and in the writings of the Greeks and Romans, frequent mention is made of such diseases as rusts, smuts and mildews of grain and canker of trees. To be sure, the extensive and intensive crop-cultivation of modern times, together with the extraordinary worldwide transportation and exchange of crop-products, have greatly favored the distribution of plant pathogens (insects, fungi and bacteria), and afford them exceptional opportunities for destructive development. Nor are cultivated plants alone subject to disease. Disease epidemics among weeds and the wild flowers of the woods may be observed any season in localities in which weather conditions especially favor the causal organisms.

The study of the nature and control of plant diseases, however, is of recent development. The first man really to study plant diseases from the true modern economic point of view, that is, with the object of helping the grower to understand and combat or control diseases in his crops, was Julius Kuhn. This German, the son of a German land-owner and for many years himself the manager of a large agricultural estate, was the founder of an early German agricultural college. He interested himself, among other phases of agriculture, in plant diseases and their control and his book, "Die Krankheit der Kulturgewachse," published 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 Kunn's day there have been remarkable developments in the control of plant diseases. The discovery of bordeaux mixture by the Frenchman Millar- det in 1882; the discover}' of the formaldehyde treatment of seed for smut by the American plant pathologist, Arthur, in 1896; and the recent development of the use of lime-sulfur solutions and mixtures as a substitute 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. It has been conclusively demonstrated 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 mixture, from three to five times in the season. 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 ever}' grower. The value and efficiency of these means have been established beyond doubt. Their profitable application 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 practical 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 sinking symptoms characteristic of t hose diseases peculiar to the crops that he grows.

Disease may be exhibited in wrinkling or other distortions. There are such symptoms 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 blossoms, usually turning dark and drying up. Such symptoms characterize fire-blight of fruit trees, potato- blight (Fig. 1280, from Vt. Sta.), alternaria blight of ginseng and similar diseases, especially 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 ami the apple-scab are of this type. Another not uncommon 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 regarding 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 multiply 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 diseases 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, cucumber-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 leaf-curl, heart-rot and canker of trees, mildew of many plants, rusts and smuts of cereals (Figs. 1287, 1288, Kansas Experiment Station); in fact the mere enumeration of the more common fungous diseases of plants would fill many columns in this volume.

Algae, low forms of green plants, most of them living in water or very damp places. Few are known to produce disease in plants. The red rust of tea is one of the best known algal diseases.

Parasitic angiosperms,—flowering plants, of which there is no inconsiderable number, causing more or less injury to the plants upon which they live. These parasites are usually markedly degenerate in one or more respects, as a result of their parasitism, being often without true roots, or without leaves and frequently without chlorophyll green. As examples we may mention the mistletoes, dodders and broom rapes.

Insects (see page 1034) cause such diseases aa galls and similar malformations.

Nematode worms.—minute all but microscopic in size and multiplying rapidly, they constitute one of the greatest crop pests, especially in warm or tropical countries. They usually infest the roots, causing galls or swellings. Some species injure the plants oy destroying the fine feeding roots as in the case of the nematode parasites of oats so destructive in certain countries of northern Europe. Over 400 different plants are known to be subject to the nernatode root-gall disease. (See pp. 1041-2.)

Physiological disease is a term under which is included all those diseases the cause of which cannot be attributed to some parasitic organism. Their origin is variously attributed to abnormal enzymic activity, disturbed nutrition, and the like. The best-known of these are peach-yellows, chlorosis of the vine, tip-burn (Fig. 1291) ^mosaic disease of tobacco and leaf-roll disease of potatoes.

The various parasitic organisms cause disease in one of two ways, either by the secretion of toxines and enzymes which at once kill the plant tissues and change them into forms readily available as food for the invader; or the toxins and enzymes secreted merely stimulate or irritate the plant tissues in such a way as to result in abnormal tissue growth or diversion of the food substances of the host to the advantage of the parasite making its home between or in the cells of the host. Both types of disease-production have the same ultimate result, the serious injury or destruction of the infested plant, although the former is usually the more rapid and destructive. Of the first type, rots, blights and leaf-spots are the best examples, and are characterized by the rapid death and destruction of the affected tissues; of the second type, galls, leaf- curls, rusts and smuts are good examples and are characterized by a rather long period of association of the parasite with the living tissues of its host before marked injury or death of the plant results.

The causal agent is usually associated with the tissues of the host, either the dead or living, during its entire cycle of development. The apple-scab pathogen, Venturia imequalis, will serve admirably to illustrate. It passes the summer on the surface of leaf and fruit. In the autumn when the infested leaves fall to the ground, the fungus, which as a parasite has invaded only the cuticle of the leaf or fruit, now penetrates the dead tissues and develops there during the autumn the winter form of fruit bodies, the minute globose black perithecia, in which during the warm days of early spring the ascospores are rapidly developed. These ascospores (Fig. 1292), eight in a sac, ripen and are discharged by the spring rains that come during the blossoming period. The old leaves on the ground are filled with millions of these minute perithecia with many sacs of ascospores in each perithecium. The spores are shot into the air during the rain and being exceedingly light are carried to the opening leaves and forming fruits, where they germinate, sending out mycelial threads into the cuticle of leaf or fruit forming the characteristic dense dark green or black mats or crusts, the scab-spots. The leaves become crumpled and injured, the young fruits grow one-sided, or if the stem be attacked, soon drop from the tree, thus giving no set of fruit. On the scab-spots the conidia or summer spores cut off from the tips of upright branches in great numbers, are carried by the wind to other jeaves and fruits where, with the next rain, they germinate and give rise to new scab-spots and more conidia.

The life-cycle as given for the apple-scab fungus is typical of many of the fungous pathogens of our crops. It must be remembered, however, that each pathogen has habits peculiar to itself; hence the necessity for the most careful study of each that we may know its habits and peculiarities and thus be able successfully to combat it. The following illustrations will serve to explain and impress this point.

Plowrightia marbosa, the fungus causing black-knot of plums and cherries, requires two seasons to complete its life-cycle. The first season there appears on the knots only conidia, followed the second season by a crop of ascospores, produced in perithecia, which form a black crust on the surface where the conidia were earlier produced. Other pathogens like Exoascus cerasi, the "witches broom pathogen of the cherry, lives from year to year as mycelium in the branch and twigs of the broom-like growths it excites, producing each season a crop of spores on the under sides of the leaves. The blister-rust fungus of the white pine, Cronartium ribicolum, also lives from season to season in the tissues of the pine, producing each spring a new crop of spores. This pathogen exhibits another habit peculiar chiefly to certain of the rust fungi, namely that it has another stage or spore form on an entirely different host plant, in this case, the currant, especially the European black currant. The apple-rust pathogen, Gymiwsporanffium macropus, exhibits the same habit, passing the winter hi galls formed on the twigs of the red cedar. In the spring spores appear on these galls, which germinating in situ give rise to other minute spore bodies, the sporidia. These sporidia are carried by the wind to the young apple leaves and fruit, giving rise there to the rust disease so destructive to certain varieties like the Mclntosh and York Imperial. The spores formed on the rusted leaves and fruit of the apple are carried to the cedar, originating a new crop of galls and thus completing the life-cycle.

While some pathogens may develop in both living and dead tissues of their host, as we have seen in the case of the apple-scab fungus, other pathogens like the rust organism just described or the potato-blight pathogen, Phytophthara infestans, require to be constantly associated with the living tissues of their host The last-mentioned fungus passes the winter as mycelium in the tissues of diseased tubers, grows from thence up through the new shoots, slowly killing them and forming thereon the first crop of conidia, which, carried by the wind to nearby healthy plants, produce the primary infections of the season. The successive crops of conidia produced during the season on the blighted tops are washed into the soil by the rains, find their way to the newly formed tubers, and, infecting them, complete the seasonal cycle of the parasite.

Many fungous pathogens are now known to pass from one generation of the host plant to the next through the seed. The smut parasites of cereals afford remarkable examples of this habit. In the case of the oat- smut fungus, Ustilago catenae, the spores ripen as dusty black masses in the panicles of affected plants just as the healthy plants are in blossom. At this time the two hulls inclosing the grain are open. The wind-scattered spores lodge in the open flowers against the young kernel where they are soon safely housed by the closing hulls. They lie dormant along with the ripened seeds until they are planted. Then as the oat kernels germinate, the smut spores do likewise, sending forth their germ tubes which penetrate the young oat sprouts before they emerge from the hull. The mycelium grows along up through the growing oat straw, finally giving rise to the black spore masses in the unfolding panicle. In the case of stinking smut of wheat the seasonal life- cycle of the pathogen, TUlctia tritici, is much the same, except that the spores are disseminated at threshing time. Some very important differences in the habits of the loose smut pathogens of wheat, Ustilago tritici and of barley, Ustilago nuda, have recently been discovered (1902). The spores of these pathogens are also ripened and disseminated at blossoming time, but on falling within the open blossom they germinate at once, sending their germ-tubes into the tender young kernels. The affected kernels are apparently not injured but continue to develop and ripen. The mycelium of the pathogen within remains dormant until the seeds are planted and begin to grow, at which time the mycelium also becomes active. It grows out into the young shoots and up through the lengthening culms eventually to give rise to the black spore masses of the smutted heads. The bean anthracnose fungus, CoUetotrichum lindemuthianum, is also carried over in the seed. Here the fungus in the black spots or cankers on the pods penetrates into the tender cotyledons of the seed within, goes into a dormant condition as the seed ripenSj to become active again when the germinating seed lifts these cotyledons from the soil. A new crop of spores is produced, which, if the season be rainy, are splattered on to the stems and leaves of nearby healthy plants and the pathogen becomes established for another season.

While the wind is the most common disseminating agent of fungus spores, often carrying them for great distances, such agents as rain, flowing water, insects and even man himself, are frequently responsible. It is in the dissemination of bacterial pathogens, however, that insects most generally function. The dreaded fire-blight bacteria are disseminated only by insects or man. They pass the winter in a semi-active state in the half-living tissues along the margins of cankers on limbs or twigs, multiply rapidly with the rise of sap and the heat of spring. They ooze from the affected bark in sticky, milky drops. This ooze is visited by bees and flies, which with besmeared legs and moutnparts fly away to visit the opening apple or pear blossoms. Here they leave some of the bacteria in the nectar where they rapidly multiply, to be more widely distributed by each succeeding visitor. They soon penetrate into the tender tissues of the blossom, causing the blossom blight. From these blighted blossoms, sucking insects like the aphids carry the bacteria to the tips of the rapidly growing shoots when in sucking sap they introduce the organisms and twig blight follows. The striped cucumber beetle is probably the chief disseminator of Badllua trocheiphilus, which causes the cucumber-wilt.

Ecological conditions as affecting disease

By ecology is meant the influence of such environmental factors as climate, weather, soil and fertilizers, on the disease, its severity, epidemic occurrence, and the like. These factors may influence the severity of the disease by their effect on either the pathogen or the host, or both. For example, most fungous parasites require the presence of water on the host plant in which their spores may germinate, hence severe epidemics of such' diseases as potato-blight, apple-scab, brown-rot of

stone fruits and black-rot of grapes usually appear in wet seasons. Moreover, the attacking pathogen is especially favored by wet weather at certain seasons or periods in its development, especially the infection period. Continued spring rains about blossoming time favor apple-scab and peach leaf-curl. Late summer rains bring with them epidemics of late blight of potatoes, brown-rot of peaches or late infections of apple-scab. Frequent or continuous rains during June and July in grape regions are usually accompanied by severe attacks of the black-rot pathogen. The relation of rainfall to the pathogen explains why, when there has been a severe epidemic the previous season, the crop may escape if the following season be dry. There is ever a critical period in the development of the pathogen, usually when it is passing from its resting or winter stage to the active vegetative period of the growing season. Moisture and temperature conditions at such periods largely determine whether the disease will be epidemic or not. Of course the necessary abundance of spores to be disseminated is an evident necessity. Favorable weather alone cannot bring on disease as the grower too often believes.

The absence of rains at certain stages in their development is for other pathogens equally essential. The loose smuts of cereals afford good examples. Their spores are powdery and wind-borne and if rains fall when they are being disseminated, they are washed to the ground and perish instead of finding their way into the open blossoms of their host. Thus, clear sunny weather during the blossoming period i, of wheat and oats one season usually means a more or less severe epidemic of smuts the next, while rains at this time, even though there be an abundance of the disease, may mean a clean crop the following year.

On the other hand, weather conditions may determine the severity or absence of certain diseases by its effect on the host. Long-continued cold rainy weather in the spring, especially following a warm spell, results in a slow succulent growth of the developing peach leaves, rendering them especially susceptible to the attacks of the leaf-curl pathogen.

The application of certain fertilizers to the soils known to have a direct effect, either favorable or unfavorable, on different pathogens. The application of lime or of manure to the soil tends greatly to increase the scab of potatoes planted thereon; while, on the other hand, liming the soil prevents infection of cabbage and cauliflower by the club-root pathogen. Lime likewise favors the development of the root-rot of tobacco and ginseng caused by Thielama basicola, while applications of acid phosphate tend to prevent infection by this pathogen. The effect of fertilizers on the susceptibility of the host has also been shown to be marked in certain cases. Barley, when fertilized with nitrogenous manures, becomes very susceptible to attacks of the mildew Erysiphe graminis. Certain varieties of wheat have been observed in Denmark to suffer severely from attacks of the rust Puccinia glumarum only when nitrogenous manures are applied. Excessive applications of barnyard manure to greenhouse cucumbers often cause a physiological disease, the symptoms of which are a curling, and dying of the margins of the leaves, accompanied by marked chlorosis or yellowing. Fertilizers or late continued cultivation of pear trees, by lengthening the period of active twig-growth, favor fire-blight, the bacteria of which infect only tender actively growing tissues.

Control of diseases

By the term control is meant the profitable reduction of the losses ordinarily sustained from a given disease. The absolute prevention of many plant diseases is either impossible or unprofitable.

There are four fundamental principles upon which all methods of plant-disease control are based, viz.: (1) exclusion, (2) eradication, (3) protection and (4) immunization. Upon the first two are based those measures which are directed primarily against the pathogen, upon the last two those which are directed merely toward the protection of the host from pathogens commonly present in the environment. The order in which these principles are here presented represent the logical, though unfortunately not the historical or usual order of their development and application. We will consider briefly under each some of the more important methods now employed for the control of plant diseases.

1. Exclusion measures are directed toward keeping disease organisms, usually insects, fungi and bacteria, out of areas, regions or countries in which they do not occur. This is commonly attempted by the passing of laws forbidding the importation of plants affected with such parasites. As means of enforcing such regulations, some sort of inspection, either at port of entry or at point of destination, is provided. Inspection in the country from which they are exported is also often required. Absolute quarantine against all importation of certain plants from those countries in which dangerous diseases are known to occur is also being practised in some countries, as for example, prohibiting the importation of potatoes into the United States from those countries in which the black-scab is now known to occur. Exclusion measures, often undertaken when it is too late, are at best under present conditions of doubtful efficiency. Those interested in these methods of control should consult the various pest and disease acts of the different countries of the world. See Inspection, in Vol. III.

2. Eradication.—On the principle of eradication are based those measures which are directed to the elimination of pathogens already established. While absolute eradication is seldom to be effected, the pathogen may often be eliminated to such an extent as to reduce losses therefrom to a profitable minimum. In Denmark, the destruction of all barberry bushes, the alternate host of the grain-rust fungus, Puccinia graminis, has decidedly reduced the severity of this disease in recent years. The careful eradication'of all diseased plants is often quite effective even in a small area, like a raspberry or blackberry plantation suffering from the red rust. Here the mycelium of the pathogen lives from year to year in the roots of diseased plants, which each spring send up diseased shoots. On the under side of the leaves of these shoots, the orange-red spores are produced in great abundance, and serve to spread the pathogen to healthy plants. As diseased plants are readily detected in early spring by the pale clustered shoots, they may be removed before spores appear and the pathogen thus eradicated. The black-knot of plums and cherries is most readily and profitably controlled in a similar manner, the knot-affected limbs and twigs being cut out and burned early in the spring before spores appear. The fire-blight of pears is to be controlled only by systematic eradication, first of all cankers in autumn or early spring, then of all blossom blight as fast as it appears and later of the affected twigs when twig-blight comes on. To be effective, the trees must be inspected two or three times each week throughout the growing season and all diseased parts removed at once as soon as discovered.

Another method of eradication especially applicable to seeds, tubers or bulbs, on which spores of the pathogen pass the dormant period, is disinfection. This is accomplished by the application of chemical poisons, either in solution, as powder or as gas, at a strength and for a period of time sufficient to destroy the pathogen without injury to the host. When the pathogen lives over as mycelium in the seed or tuber, the application of heat is sometimes effective. Formaldehyde, as a gas or in solution in water, is now generally employed for the eradication of the smut of oats, the stinking smut of wheat and the potato-scab. (For details of method, see Formaldehyde, p. 1028). The spraying of peach trees with copper-sulfate solution, lime-sulfur solution or bordeaux, just before the buds start in the spring, disinfects the trees by destroying the spores of the leaf- curl fungus which pass the winter on the buds.

Pathogens which attack the underground parts of plants may sometimes be eradicated by disinfection of the soil before planting. Drenching the soil with a formaldehyde solution of a strength sufficient to distribute one gallon of the strong 40 per cent solution to each 100 square feet of surface, wetting the soil to a depth of 6 to 8 inches, has been found to be very effective against damping-off, root-rot and similar diseases in forest tree seed-beds, ginseng seed-beds and in the benches in greenhouses. It is also often effective in the eradication of nematodes in greenhouses. Steaming of the soil is also very effective, destroying insects and weed seeds as well as pathogenic fungi. It is not always conveniently applied.

3. Protection measures are to be employed in those regions in which the pathogen is very generally and very thoroughly established, or in which for one reason or another eradication is impossible or unprofitable. They aim to protect the crop against attacks of the parasite by means of some external barrier. Spraying is the most commonly employed protective measure. In spraying, the susceptible surfaces of the plant are coated with some slowly soluble poison, known as a fungicide. Fungicides are of various types. They are applied in suspension in water, in solution or dry, i.e., in the form of a fine impalpable powder. The fungicide most generally applied in liquid spraying is bordeaux, a colloidal compound formed by the union of lime^milk and copper-sulfate solution. Minute blue gelatinous membranes are formed which remain for a time suspended in the liquid. When sprayed upon the plants the water soon evaporates, leaving a coating of these dried membranes. The active fungicidal principal in these bordeaux membranes is the copper. When leaves or fruit are rewetted by rains enough of the copper in these membranes comes into solution to prevent the germination of the spores of the parasite that may have been deposited thereon. (See under Bordeaux, p. 1028.)

Bordeaux, however, is sometimes injurious to such plants as peaches, plums and apples, and has, within the last few years, been largely replaced as a summer spray, especially for apples. Lime-sulfur, unlike bordeaux, is a solution. It is made by boiling together in water, lime and sulfur. A concentrated solution of certain poly-sulfides of calcium, chiefly penta- and tetra- eulfide, is thus obtained which, when properly diluted is applied in the same way as the bordeaux. (For method of preparation, see Lime-sulfur, p. 1028). When this solution dries on the leaves and fruit, it is rapidly converted by the action of the atmosphere into other calcium compounds and free sulfur. The sulfur is in a very finely divided state and is the active principal of lime-sulfur. It becomes oxidized in the presence of moisture probably as sulfuric or sulfurous acid, which prevents the germination of the spores of the pathogen. Flowers of sulfur and sulfur-flour, when very finejy ground and applied dry by dusting or sprayed on in suspension in water, alone or with lime-milk (the so- called self-boiled lime-sulfur) are also quite effective against certain diseases. Dusting with sulfur is employed in combating powdery mildews of grapes, hops, roses and the rust of asparagus.

Lime-sulfur may not be used on potatoes and grapes, as it dwarfs the plants and reduces the yield, while bordeaux has just the opposite effect on these crops. Bordeaux, as already pointed out, is, however, injurious to leaves and fruit of the apple and to the foliage of peaches and certain varieties of plums. It will thus be seen that there is no universal fungicide and also that both the effect on the host and on the parasite must be considered. It is now known for example that while lime-sulfur is very effective against the apple-scab fungus, it has little fungicidal effect on the spores of the bitter-rot pathogen.

To be effective, fungicides must be applied before the disease appears. As the spores of most parasitic fungi germinate during the period of rainy cloudy weather, the fungicide, to be effective, must be applied before and not after such periods. They must not only be thoroughly applied to the susceptible parts but also at the proper stage of growth or development of the plant. To illustrate: the only effective periods for spraying apple trees for scab are: just before the blossoms open (not dormant); just after the petals fall; ten days or two weeks later; and again in late summer just before the late summer rains, to protect the rapidly developing fruit from late infection.

4. Immunization consists in establishing within the plant itself some condition which renders it immune or resistant to the attacks of the pathogens. Immune crops may be developed by selection and propagation of individuals naturally immune, whose immunity has been evidenced by their coming through an epidemic unscathed. Immune varieties may oe crossed with susceptible ones having other especially desirable qualities and then by segregation and propagation strains of the crop may be developed combining the resistance or immunity of the one parent with the desirable qualities of the other. Some striking results have been obtained in this line of disease control as witness the wilt- resistant cotton, cowpeas and watermelon, the nematode-free Iron cowpea, rust-resistant wheat, barley, and asparagus, and the anthrac- nose-resistant clover. Nevertheless, this method of control, while the most ideal, is beset with many difficulties and uncertainties. That pathogens, as well as crops, vary, giving strains capable of attacking host plants immune to other strains of the same pathogen, has generally been overlooked by breeders, and doubtless accounts for the frequent failure of supposedly resistant varieties when transferred to new localities. The production of artificial immunity by the injection of some substance into the plant or by the application of certain substances (fertilizers, etc.) to the soil is at most only in the preliminary stages of experimentation and as yet offers but little of practical value to the grower. H. H. Whetzel.

Fungicides

A fungicide is any material or substance that kills fungi or their spores. The word is used particularly for those substances employed in the warfare against parasitic fungi.

A satisfactory fungicide must be one that does not injure the plants and at the same time is effective against the parasite. For spraying, additional requirements are imposed: it should not dissolve readily in rain-water; it should adhere to foliage and fruit; in some cases it should be colorless in order not to make ornamentals more unsightly than when diseased. The fungicide which has been used most for general purposes is bordeaux mixture. Lately some other preparations, particularly lime-sulfur combinations, have come into use, and in many cases are supplanting bordeaux. There are in addition a large number of other substances which have fungicidal value and are in more limited use for specific cases. The following directions are taken, with modifications, from the author's part in Bailey's "Farm and Garden Rule-Book."

Practices

Destroying affected parts. —It is important that all affected parts should I* removed and burned, if pos- aible. In the fall all leaves and fruit that have been attacked by fungi should be raked up and burned. Diseased branches should be severed at some distance below the lowest visible point of attack. Fungous diseases often spread rapidly, and prompt action is usually necessary. Practice clean and tidy culture. Rotation of crops.—This is one of the most effective and practical means of head~ ing off fungous diseases. It is especially applicable to diseases of roots or root- crops, but also to many other <!i . ' - of annual plants.

Sterilizing by steam.— This is an effective fungicidal practice for several soil - inhabiting organisms which attack roots and stems. This includes nematode worms. It is especially applicable in the greenhouse, where it may be applied (a) through sub-irrigation tile or through specially laid perforated steam pipes in the bottom of the bed. Cover the beds with blankets, introduce steam under pressure of forty to eighty pounds for two hours. Insert thermometers at various places to see that the soil is being uniformly heated. (6) A large galvanized iron tight box may be constructed with finely perforated trays 4 to 0 inches in depth. Soil placed in these trays and steamed for two hours as above will be freed from parasitic organisms. In this case the frames should be sprayed with a solution of formaldehyde, one pint in twelve gallons of water. Steam sterilization of soil may be used on intensively cultivated areas or extensive seed-beds. A portable boiler ia necessary. The beds are sterilized after they have been prepared for seed, and just before the seed is sown. A galvanized pan of convenient dimensions and 6 to 8 inches deep is inverted, and the edges are pushed down into the soil 1 or 2 inches. The pan is connected with the steam boiler by means of a steam hose and live steam is run into the pan from twenty to forty minutes under a pressure of eighty pounds and up. The higher the pressure the deeper the soil will be sterilized. The pan must be weighted. Paths should be disinfected by spraying with copper aulfate one pound to fifty gallons of water or with formaldehyde solution one pint to twelve gallons of water. The cost of sterilizing is approximately three-fourths of a cent the square foot. It should be noted that soil-sterilization has an invigorating effect on many plants, and it will be necessary to run greenhouses at a lower temperature (5° to 10°) both night and day. Field-sterilization also kills weed seeds, and with the reduction of the cost of weeding mokes the process practicable.

Substances

Bordeaux mixture.—A bluish green copper compound that settles out when freshly slaked time and a solution of copper sulfate (blue-stone) are mixed. Many formulas have been recommended and used. The 5-5-50 formula may be regarded as standard. In such a formula the first figure refers to the number of pounds of copper sulfate, the second to the stone or hydrated time, and the third to the number of gallons of water. Bordeaux must often be used as weak as 2-2-50, on account of injury to some plants.

To make fifty gallons of bordeaux mixture, proceed as follows:

1. Pulverize five pounds of copper sulfate (blue vitriol), place in a glass, wooden, or brass vessel, and add two or three gallons of hot water. In another vessel slake five pounds of quicklime in a 0mall amount of water. When the copper sulfate is all dissolved, pour into a barrel and add water to make forty or forty-five gallons. Now strain the lime into this, using a sieve fifty meshes to the inch or a piece of cheese-cloth supported by ordinary screening. Stir thoroughly, and add water to the 6fty-gallon mark. The fiocculent substance which settles is the effective fungicide. Always stir vigorously before filling the sprayer. Never add the strong time to strong vitriol. Always add a large amount of water to one or the other first. Blue vitriol used alone would not only wash off quickly in a rain, but cause a severe burning of fruit and foliage. Lime is added to neutralize this burning effect of the copper. If the lime were absolutely pure, only slightly more than one pound would be required to neutralize this burning effect. For many purposes an excess of lime is not objectionable and may be desirable. For nearly ripe fruit and ornamentals an excess of lime augmenta spotting. In such cases the least amount of lime possible should be used. Determine this by applying the cyanide test (2).
2. Secure from the druggist 10 cents' worth of potassium ferroeyanide (yellow prussiate of potash) and dissolve it in water in an eight-ounce bottle. Cut a V-shaped slit in one side of the cork, Bo that a few drops of the liquid can be obtained. Now proceed aa before. Add lime with constant stirring until a drop of the ferrocyanide ceases to give a reddish-brown color.
3. When bordeaux mixture is desired in large quantities, stock solutions should be made. Place one hundred pounds of copper sulfate in a bag of coffee-sacking, and suspend in the top of a fifty-gallon barrel, and add water to the fifty-gallon mark. In twelve to fifteen hours the vitriol will be dissolved and each gallon of solution will contain two pounds of copper gulf ate. Slake a barrel of lime, and store in a tight barrel, keeping it covered with water. Lime so treated will keep all summer. It is really hydrated lime. This is often dried, pulverized, and offered on the market in paper bags of forty pounds each, under such names as ground time, prepared time, hydrated lime, and the like. If the paper u not broken, the time does not air-slake for a long time. One and one-third pounds of hydrated time equals in value one pound of quicklime. Air-slaked time cannot be used in preparing bordeaux mixture.

Arsenical poisons can be combined with bordeaux mixture.

Ammoniacal copper carbonate.—For use on nearly mature fruit and on ornamentals. Docs not discolor. Weigh out three ounces of copper carbonate, and make a thick paste with water in a wooden pail. Measure five pints of strong ammonia (26° Baume) and dilute with three or four parts of water. Add ammonia to the paste, and stir. This makes a deep blue solution. Add water to make fifty gallons.

Copper carbonate.—For use in the above formula, it may be secured as a green powder, or may be prepared as follows: Dissolve twelve pounds of copper sulfate in twelve gallons of water in a barrel. Dissolve fifteen pounds of sal-soda in fifteen gallons of water (preferably hot). Allow the solution to cool; then add the sal-soda solution to the copper-sulfate solution, pouring slowly in order to prevent the mixture from working up and running over. A fine precipitate is formed which will settle to the bottom if allowed to stand over night. Siphon off the clear liquid. Wash the precipitate by adding clear water, stirring, and allowing to settle. Siphon off the clear water, strain the precipitate through muslin, and allow it to dry. This is copper carbonate. The above amounts will make about six pounds.

Copper sulfate.—See Sulfate of copper.

Corrosive sublimate (mercuric chloride).—Used for disinfecting pruned stubs and cleaned-out cankers, at the rate of one part in 1,000 parts of water. Can be secured from the druggist in tablet form in vials of twenty-five each, and costing 25 cents. One tablet makes a pint of solution. Make and store solution in glass and label "poison".

Formaldehyde (40 per cent solution of formaldehyde gas in water).—A pungent, clear liquid, very irritating to eyes and nose. Obtained at any drugstore at about 40 cents a pint. Used for potato-scab, oat smut, bunt in wheat, soil disinfection, and so on.

Lime.—Offered for sale in the following forms: (a) Ground rock or ground limestone; air-slaked time is of the same composition, i.e. a carbonate of calcium. (6) Lump, barrel, stone, or quicklime; this is burned limestone, and should test at least 90 per cent oxid of calcium, (c) Prepared, ground, or hydrated time; this ia water- or steam-slaked quicklime, dried and pulverized. Used aa an applicant to the soil to correct acidity, for club-root of cabbage, and for preparing spray mixtures.

Lime-sulfur.—In the many possible combinations, lime-sulfur is coming to be equally as important as bordeaux mixture, in the control of many plant diseases.

1. Flowers of sulfur or very finely powdered sulfur is often dusted on plants for surface mildews.
2. A paste of equal parts of lime, sulfur, and water. This is painted on the heating-pipes in the greenhouse, and is valuable for keeping off surface mildews.
3. Home-boiled dilute lime - sulfur. This solution has been widely used in the past as a dormant spray, particularly for San .!"<" scale and peach leaf-curl. It is likely to be supplanted by (4) or (5). For preparation see page 1043.
4. Home-boiled concentrated lime-sulfur. —When a great deal of spraying is to be done, a concentrated lime-sulfur solution may be boiled at home and stored in barrels to be used as needed. For method of preparation see page 1043.
   Test with a Baume hydrometer, which has a scale reading from 25° to 35°. Dilutions are reckoned from a standard solution testing 32°. If the solution tests only 28°, it is not so strong as standard, and cannot be diluted so much as a solution testing 32°. The table shows the proper dilution for solutions testing 25° to 35° Baume: [table removed]
   Decimals are given in all cases, but for practical purposes the nearest even gallon or half gallon can be used, unless appliances for more accurate measurement are at hand. It is understood in making all dilutions that water is added to one gallon of the concentrate to make the stated amount. Do not measure out the stated amount of water and add the concentrated solution to it.
5. Commercial concentrated lime-sulfur.—As manufactured and placed on the market is a clear amber liquid, and should test 32° to 35° Baume. It costs about 20 cents a gallon retail, and comes ready to pour into the spray tank. For apple and pear diseases. Arsenate of lead can be used with this solution, and increases its fungicidal value.
6. Scott's self-boiled lime-sulfur.—This is a mechanical mixture of the two substances, and is really not boiled, the heat being supplied by the slaking lime. In a small barrel or keg place eight pounds of good quicklime. Add water from time to time in just sufficient amounts to prevent burning. As soon as the lime begins to slake well, add slowly (preferably through a sieve) eight pounds of sulfur flour. Stir constantly, ana add water as needed. As soon as all bubbling has ceased, check further action by adding a quantity of cold water, or pour into a barrel or tank and make up to fifty gallons. Keep well agitated. Very effective against peach scab and brown rot. Several other formulas have been used: in- 10 -.mi and 5-5-50. Arsenate of lead can be used with this mixture.
   By using boiling water and allowing the hot mixture to stand for half an hour, a stronger spray mixture than the above can be secured. It cannot be used safely on peaches, but has been used successfully on grapes for surface mildew. The addition of sulfate of iron or sulfate of copper, one or two pounds to fifty gallons, has been used for apple rust.

Potassium sulfid (liver of sulfur).—Simple solution, three ounces tn ten gallons of water. For mildew in greenhouses, on rose bushes and other ornamentals.

Resin-sal-soda sticker.—Resin, two pounds; sal-soda (crystals), one pound; water, one gallon. Boil until of a clear brown color, i.e. from one to one and a half hours. Cook in an iron kettle in the open. Add this amount to fifty gallons of bordeaux. Useful for onions, cabbage, and other plants to which spray does not adhere well.

Sulfate of copper (blue vitriol).—Dissolve one pound of pure sulfate of copper in twenty-five gallons of water. A specific for peach leaf-curl. Apply once before buds swell in the spring. Cover every bud. For use in preparing bordeaux mixture. Costs from 5 to 7 cents a pound, in quantity.

Sulfate of iron (copperas).—A greenish granular crystalline substance. Dissolve one hundred pounds in fifty gallons of water. For mustard in oats, wheat and the like, apply at the rate of fifty gallons an acre. Also for anthracnose of grapes as a dormant spray.

Sulfur (ground brimstone, sulfur flour, flowers of sulfur).— Should be 99 per cent pure. Valuable for surface mildews. Dust on dry or in the greenhouse used in fumes. Evaporate it over a steady heat, as an oil-stove, until the house is filled with vapor. Do not heat to the burning point, aa burning sulfur destroys moat plants. To prevent burning, place the sulfur and pan in a larger pan of sand and set the whole upon the oil-stove.

References

• Standard Cyclopedia of Horticulture, L.H. Bailey

See also

• List of plant diseases