[Illustration: FIG. 130. SPRAYING MACHINE] =EXERCISE= Watch the potatoes at the next harvest and estimate the number that is damaged by scab. You will remember that formalin is the substance used to prevent grain smuts. Write to your state experiment station for a bulletin telling how to use formalin, as well as for information regarding other potato diseases. Give the treatment a fair trial in a portion of your field this year and watch carefully for results. Make an estimate of the cost of treatment and of the profits. How does the scab injure the value of the potato? The late blight can often be recognized by its odor. Did you ever smell it as you passed an affected field? [Illustration: FIG. 131. CLUB ROOT] =Club Root.= Club root is a disease of the cabbage, turnip, cauliflower, etc. Its general effect is shown in the illustration (Fig. 131). Sometimes this disease does great damage. It can be prevented by using from eighty to ninety bushels of lime to an acre. =Black Knot.= Black knot is a serious disease of the plum and of the cherry tree. It attacks the branches of the tree; it is well illustrated in Fig. 132. Since it is a contagious disease, great care should be exercised to destroy all diseased branches of either wild or cultivated plums or cherries. In many states its destruction is enforced by law. All black knot should be cut out and burned some time before February of each year. This will cost little and save much. [Illustration: FIG. 132. BLACK KNOT] =Peach Leaf Curl.= Peach leaf curl does damage amounting to about $3,000,000 yearly in the United States. It can be almost entirely prevented by spraying the tree with Bordeaux mixture or lime-sulphur wash before the buds open in the spring. It is not safe to use strong Bordeaux mixture on peach trees when they are in leaf. [Illustration: FIG. 133. MOLDY PEACHES] =Cotton Wilt.= Cotton wilt when it once establishes itself in the soil completely destroys the crop. The fungus remains in the soil, and no amount of spraying will kill it. The only known remedy is to cultivate a resistant variety of cotton or to rotate the crop. [Illustration: FIG. 134. PEACH MUMMIES] =Fruit Mold.= Fruit mold, or brown rot, often attacks the unripe fruit on the tree, and turns it soft and brown and finally fuzzy with a coat of mildew. Fig. 133 shows some peaches thus attacked. Often the fruits do not fall from the trees but shrivel up and become "mummies" (Fig. 134). This rot is one of the most serious diseases of plums and peaches. It probably diminishes the value of the peach harvest from 50 to 75 per cent. Spraying according to the directions in the Appendix will kill the disease. [Illustration: FIG. 135. HALF OF TREE SPRAYED TO PREVENT PEACH CURL Note the difference in foliage and fruit on the sprayed and unsprayed halves of the tree, and the difference in yield shown below] CHAPTER VII ORCHARD, GARDEN, AND FIELD INSECTS