Notwithstanding all general preventive measures, spraying is often necessary to avoid heavy damages. The fundamental principle involved is that of protection. If the operator uses an insecticide it serves as a poison or a repellent and should be applied before the enemy has made a serious attack; and if injury is expected from fungous diseases, by the application of a fungicide the parts in danger of infection are armored with material which will prevent the entrance of the parasites.
Five things are important in successful spraying: (1) Know your enemy; (2) select the most effective poison for its control; (3) spray thoroughly; (4) spray as often as may be necessary under existing conditions; (5) spray at the proper time.
Inexpensive pumps are seldom satisfactory. The best materials should be used in the construction of spray pumps; all metal parts which come in contact with the solutions should be made of brass or copper to prevent serious corrosion. A large air chamber is an advantage in securing an even and continuous discharge. Over 100 pounds pressure is essential to the best work in orchard spraying, but less than this is effective in garden operations.
Various types of pumps are available. The bucket pumps, which may be bought for a few dollars, answer the purpose in small home gardens. Knapsack sprayers are very convenient on small areas where the crops are planted close together, and in large plantations when growth is so far advanced as to prevent the use of barrel or power sprayers. It is not an easy task to carry and operate a knapsack sprayer, but it is unquestionably the best pump under certain conditions. For example, aphides sometimes infest cabbage after the plants are too large to permit the use of barrel or power sprayers, and in addition, they colonize on the under sides of the leaves as well as on the upper. These conditions make the knapsack sprayer the most desirable pump for this work, especially if used with a crooked extension rod.
Barrel pumps are more satisfactory and less laborious to operate than knapsack sprayers, and are most popular with commercial growers. Chain and sprocket power machines are frequently employed. By their use large areas may be covered in a day. With a good pump and efficient nozzles properly placed and adjusted, the work is satisfactory. Gas, compressed air and gasoline engine pumps are not used extensively in commercial gardening. Various powder guns on. the market are effective in the application of powders.
For its value in spraying vegetables a nozzle depends upon its ability to break up solutions and mixtures into the finest particles. In orchard spraying and sometimes in garden treatment, an additional factor is important; namely, the ability of a nozzle to project the spray with the greatest possible force. The best known and most popular nozzles used by commercial vegetable growers are the "Vermorel" and the "Friend".