The matter of considering the growth periods of the crop becomes one of actual necessity when preparing for the work of spraying for prevention. It determines the time of spraying and the strength of solution that may be used with success.
Spraying for prevention.
Spraying for the prevention of plant diseases has now become a fixed practice in the better agri cultural regions throughout the world. It owes its existence to the simple fact that many of the special diseases which attack farm, garden and orchard crops are infectious by nature, and spread from plant to plant by means of small seed-like struc tures, called spores, which may be readily borne by the winds, water, insects or other agencies. When they fall on the growing plant, they begin to grow either by attacking the plant surface or by send ing filaments into the internal structures. It has been found that certain solutions, applied at the proper time, cut short the lives of these spores and their developing growths, preventing injury to the plant on which they fall, or on which they are spreading. The aim of spraying is to cover all surfaces that are likely to he attacked, or on which spores are likely to fall, with a film of some chem ical, either dry or in solution, that will prevent the germination of the spores and the development of the disease-producing organisms, and, at the same time, not injure the foliage and living tissues of the plant on which the spray falls. It is thus merely a matter of disinfection.
The time for spraying can be properly deter mined only by a close observation of the period at which the disease is spreading and by con sidering the stage of leaf, flower or fruit develop ment. Usually the earlier spraying is done on orchards and permanent plants, in order to destroy the first series of spores that may come from dis tant regions. Two or three, and in some cases four or five treatments are applied during the growing season for a like reason.
If spraying is done properly, one need not expect to see much indication of the diseases which are thus preventable in the sprayed crop. It is wholly a matter of prevention. Therefore, forethought must be exercised ; for when the disease is once started, spraying, in most cases, will not prevent the particular plant sustaining injury, as in the case of a potato plant which has become attacked by blight. Proper spraying, however, will prevent the disease spreading from this plant to other plants,—indeed, will keep it confined to the parts of the plant already attacked. Even the individual plants that are once attacked are benefited because their future growths may continue uninterrupted. Spraying has become so universal that one need only cite a few diseases that are thus preventable.
It must be remembered that, as each plant disease has a particular life-history and attacks its host plant in a particular way, there are special reasons for modifying spraying processes to fit each crop and each peculiar disease; therefore, one who wishes to take up the work should consult proper authorities, or bulletins dealing directly with this phase of the question.
The following list of diseases that may be pre vented by proper spraying is only an indication of the actual number : Apple ripe-rot, anthracnose, canker or bitter-rot, leaf-spot and scab ; aspara gus-rust; bean anthracnose; beet leaf-spot; celery blight; cucumber damping-off, mildew and blight; gooseberry mildew ; grape-rot and anthracnose ; lemon-scab; lettuce leaf-rot, leaf-mold and mildew; melon mildew and anthracnose; olive-scab; orange scab and mold ; peach leaf-spot and scab ; pear scab and leaf-spot ; plum-rot and shot-hole fungus ; potato early blight, late blight, rot and mildew ; raspberry anthrancnose ; squash fruit-blight, rot and mildew; tomato anthracnose, leaf-blight and damping off ; violet mildew, mold and blight.
Sanitary prevention.
Since all of the plant diseases that affect field crops and plants generally, excepting those that are due to improper agricultural technique or par ticular chemical nature of the soil, may be looked on as essentially infectious, either directly from plant to plant or from soil to soil, one may put the whole matter on sanitary bases similar to those which apply to the prevention of diseases among animals and man. An ounce of prevention is worth a pound of cure. In the case of farm crops and garden plants, it is clearly true that a slight amount of energy placed to the credit of proper methods of prevention adds greatly to the crop returns. The chief methods of prevention that are usually practiced have been cited when we mention seed treatment and spraying. These strictly belong to this heading of sanitary prevention, but, as they have become matters of common practice, the writer wishes to call attention to the fact that there are other sanitary methods of avoiding diseases in farm and garden crops aside from these two. Much may be done to put the environments of the crop in sanitary condition, as the cleaning up of the field after the previous crop, the elimina tion of diseased parts of permanent plants, trees and shrubs, the disinfection of bins, machinery, sacks, storehouses, elevators and all containers and contrivances that are to be handled in connection with the cultivation of the new crop. And, finally, the farmer should look to the breed, striving to procure breeds or strains that are resistant to the diseases that affect their race and variety.