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

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 dan-gerous 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.

Grapes ruined by black rot.

Fig. 1286. Grapes ruined by black-rot.

Smut of oats.

Fig. 1287. Smut of oats.

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 copp er-sulf ate 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.)

Loose smut of barley.

Fig. 1288. Loose smut of barley.

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-sulfide, 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 finely 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, ops, 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 c o w p e a, 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.

Strawberry leaf rolled up from the attack of the mildew.

Fig. 1289. Strawberry leaf rolled up from the attack of the mildew.