The Imported Cabbage Worm (Pontia Rapae) is generally recognized to be the most important insect enemy of cabbage and several closely related crops. The pest was first seen in North America in 1860 at Quebec, five years later in Maine. Now it is distributed throughout the United States. The butterfly is familiar to all garden makers. It is white, with a wing expanse of nearly 2 inches. There are two conspicuous black spots on each fore-wing of the female and only one on the fore-wings of the male. The full grown larva is over an inch long. It is nearly green and finely dotted with small black spots. A faint yellow stripe marks the middle of the back with a row of yellow spots on each side in line with the spiracles. The pale yellowish eggs are deposited singly, and usually on the underside of the leaves.
The butterflies appear early in spring, and in a few days begin laying eggs, which hatch in four to eight days, the larvae attaining full maturity in from ten days to two weeks. The chrysalis stage during the summer months lasts from one to two weeks. The insect also passes the winter in the chrysalis form. There are three broods in the North in one season.
Several natural enemies of this pest assist the grower in controlling it, but it is often necessary to use insecticides. All points considered, arsenate of lead is probably the most effective poison in destroying cabbage worms. Other remedies are hot water, kerosene emulsion, and pyrethrum.
The southern cabbage butterfly (Pontia protodice), the cross-striped cabbage worm (Evergestis rimosalis), the common cabbage looper (Autographa brassicae), the imported cabbage web-worm, the harlequin cabbage bug (Murgantia histrionica), cutworms, flea-beetles and leaf-beetles.
Club Root (Plasmodiophora Brassicae), also known as "club foot" and "clump foot," is unquestionably the most serious disease of crucifers. (Figure 72.) If uncontrolled it will soon spread through a community and render the profitable cultivation of cabbage and allied plants impossible for several years, a situation that has developed in a few sections of the United States. The malady has been known in Europe for more than a century and in this country for many years.
The disease is most destructive to cabbage and turnip, but also affects cauliflower, brussels sprouts, kale, radish, kohl-rabi, rutabaga, white mustard and many cruciferous weeds, shepherd's purse and hedge mustard being especially subject to infection.
The character of the disease and its relation to "clubbing" were determined by the Russian botanist, Woronin, who found that the micro-organism which causes the distorted enlargement of roots was a slime mold and not a bacterium nor a fungus. There is distinct "clubbing" in many cases, while in others irregular knots are formed. When the disease has advanced for several weeks, the deformed roots are incapable of supplying the plants with sufficient moisture and nourishment. The plants then become dwarfed and lighter in color. Affected plants wilt quickly in warm, sunny weather, especially if preceded by humid growing weather. The disease is less serious with the early than with the late crop, for with the best conditions for growth at this time many of the infected plants will mature, although the heads are usually small. Plants set in the field in June or July seldom mature marketable heads if attacked by club root.
Fig. 72. Club Root Of Cabbage.
The spores of this disease are inclosed by protective coverings that give them great vitality. It is not uncommon for the disease to recur in soils where crucifers have not been grown for 10 or 15 years. With annual tillage the spores soon become mixed with the soil and the malady may spread over the field or the entire farm or the community, and as no treatment of the plants has been found effective, the grower must resort to preventive measures. The following paragraphs relate to ways of infestation or dissemination and methods of prevention:
2. Rotation should be practiced, in which cruciferous plants should not be planted more frequently than every four years.
3. The disease thrives best in acid soils. In Belgium the calcareous soils were the last to become infested, and lime is the best-known means of soil treatment, although often unsatisfactory. The New Jersey Station reports that 75 bushels of stone lime an acre gave as good results as larger applications. The lime should be applied in the fall or at least several months in advance of planting.
4. Roots, stems and leaves from diseased fields should be burned. If fed to stock or used for composting such refuse will be a certain means of disseminating the malady.
5. In the purchase of all kinds of stable manures, the grower should make certain that the stock has not been fed plants which might cause infestation.
6. The disease is often found in small patches. The safest policy in such a case is to inclose the plat with a fence to prevent the spores being carried to other parts of the farm by means of implements, wagons, and the feet of horses and workmen.
7. Wild mustard, shepherd's purse and other cruciferous weeds should not be allowed to grow and serve as host plants in fields which will be used for cabbage or allied crops.
8. The grower should always guard against the purchasing of plants that have been produced in soils infested with club root, for this would be a certain means of disseminating the disease.