Plant Health Problems
Crucifers comprise a large variety of vegetable crops ranging from mustard and cabbage grown for their edible foliage to cauliflower and broccoli grown for their edible flower stalks. All share the same plant pathogens.
Diseases caused by Fungi:
Black leg, Phoma lingam.
This disease, caused by a fungus which lives in soil, rots away the stem of young plants. The leaves turn yellow and the plants fail to grow. The disease is most important in cool, moist soil.
Good soil drainage, crop rotation, and removal of plant refuse help control this disease.
Downy mildew, Peronospora parasitica.
This fungus disease is not common on cabbage and related crops in Connecticut, but is usually found on seedlings in cold frames when it does appear. The disease appears as faint, yellow spots on the leaves, and may show a white, fuzzy growth on the underside of the spots. These spots rapidly grow together and may kill the whole seedling. These symptoms usually appear only on the outer leaves of plants in the field. The fungus overwinters in the soil.
To avoid an outbreak, it is best to keep the cold frame as dry and warm as possible, without inhibiting plant growth.
Yellows, Fusarium oxysporum f. sp. conglutinans.
Infected plants appear stunted, sickly and yellow. The leaves turn purple to brown, and the older leaves drop off. Scraping the lower stem will show streaks of discoloration. A different form of this fungus, Fusarium oxysporum f. sp. raphini, causes Fusarium wilt of radish and does not affect the other brassicae. The fungus which causes cabbage yellows survives in the soil for many years and attacks the cabbage plants through the roots. The disease is favored by high soil temperature and high soil moisture. Fortunately there are many cabbage varieties very resistant to the cabbage yellows fungus. Now the disease is rarely seen on cabbage in Connecticut. It shows up occasionally during a hot summer, on cabbage planted in a low, wet spot in the field.
Planting resistant varieties avoids the disease.
Black leaf spot, Alternaria brassicae.
This fungus disease appears as large, black target-spots. This is a leaf spot usually attacking the older, outside leaves of plants of the cabbage family.
The disease does not usually require special control measures.
Clubroot, Plasmodiophora brassicae.
Plants with clubroot have stunted, yellowed leaves which wilt on hot days. The roots are enlarged and misshapen, cracked, and often rotted. The organism which causes this disease lives in the soil and enters the plant through the roots. Clubroot attacks all members of the cabbage family, including weeds such as mustard.
To avoid carrying the disease into the field, only healthy seedlings should be taken from the plant bed for transplanting. Avoid planting infested fields with any crop belonging to the cabbage or mustard family. Since acid soils (pH <7.2) favor the development of clubroot, proper application of lime according to a soil test may help control the disease.
Diseases caused by Bacteria:
Bacterial leaf spot, Pseudomonas maculicola.
This disease shows as small, dark spots on the leaf. The spots appear translucent or water-soaked when the leaf is held up to the light. The spots may grow together, and cause the death of part or all of the infected leaf. This bacterial disease is not common in Connecticut. When it does appear, it is more likely to attack cauliflower than any other cabbage-family crop. The bacteria are most likely to infect during prolonged periods of warm, driving rains.
No special control measures are required.
Black rot, Xanthomonas campestris.
This bacterial disease appears on the leaves as yellow, wedge-shaped areas with darkened veins. These leaves are usually wilted. There are black streaks visible in the stem when it is split open. The head finally rots. Black rot is a wet-weather disease spread in the field by splashing rain or insect feeding. The bacteria live over in the soil, in plant refuse, or in seed from diseased plants.
The only control at present is to use clean hot-water treated seed, crop rotation, and removal of plant refuse the previous season.
Cabbage aphid, Brevicoryne brassicae.
This whitish, powdery aphid occurs in dense clusters on the leaves where it feeds on plant sap. The leaves of infested plants crinkle and curl, and growth is retarded. There may be as many as 16 generations in a season. The aphid overwinters as glossy, black eggs on the leaves and stems of old plants remaining in the field. See Aphid fact sheet.
Cabbage looper, Trichoplusia ni.
This caterpillar, which humps its back or loops when it crawls, usually feeds on leaves but also burrows into cabbage heads, especially in cool weather. It is light green and striped lengthwise with white and darker green. Its body is nearly smooth and is narrowest at the head. The adult is a grayish-brown moth with a small silvery spot resembling a figure 8, near the middle of each forewing. It flies at night when it deposits small round greenish-white eggs, singly on the leaf surface. This species does not overwinter in Connecticut, but migrates up from the south in some years. Among the compounds registered for control of this pest in Connecticut are Bacillus thuringiensis var. kurstaki (Bt), Bt var. aizawai, spinosad, and carbaryl. A higher rate of Bt may be needed than is used against the imported cabbageworm, and Bt will be more effective when both the days and nights are warm. For either insecticide, consult the label for dosage rates, safety precautions, and preharvest intervals.
Cabbage maggot, Delia radicum.
This insect infests the stems of early set crucifers. As a rule, later planted cabbage are not injured, but late radish and turnip may be damaged. The white maggots infest the stems just below the surface of the soil and infested cabbage and cauliflower plants may wilt and die without forming heads. The gray flies, which look like small house flies, appear about the middle of May and the females lay eggs on the surface of the soil near the stem of the plant. There are three generations each year and the insect overwinters as pupae in the soil.
As mentioned above, planting after the end of May is often enough to reduce damage to cabbage and cole crops where the edible product is above the ground. Covering plants with spun-bonded row cover or other fine-mesh material can keep the adult flies from being able to lay eggs on the plants. Be careful to rotate the planting away from ground where plants in the cabbage family have been grown before, and to bury the edges of the cover thoroughly. Because the eggs are laid within a few inches of the plant, barriers of tarpaper or old carpet padding used to cover the soil around the base of each plant can prevent infestation. The barrier (6-10" in diameter, with a hole for the stem in the center and a slit so that it can be slid around the plant) must be fit carefully to prevent the fly from getting underneath, but not restrict the growth of the stem. The beneficial nematode, Steinernema feltiae, that attacks the maggots in the soil is also available. Diazinon is registered for control of cabbage maggots in Connecticut. To be effective, the chemical must be applied to the soil at the time of planting. Consult the label for dosage rates, safety precautions, and preharvest intervals. When wilting or other damage is visible, it is already too late.
Cross-striped cabbageworm, Evergestis rimosalis.
This caterpillar is about 4/5" long when fully grown and bluish gray with tiny black stripes across the top of the body. This insect is not common, but can be damaging on a single plant or in a small garden. The eggs are laid in masses of 20-30, so there may be large numbers of caterpillars on one plant. They prefer to feed on young leaves and terminal buds. Bacillus thuringiensis var. kurstaki and spinosad are registered for control of this pest in Connecticut. Consult the label for dosage rates, safety precautions, and preharvest intervals.
Diamondback moth, Plutella xylostella.
Although usually a minor pest, at times this insect can damage cabbage and related plants by mining in and eating small holes through the outer leaves. The larvae are pale green and only about a quarter inch long when fully grown. They are very active, and when disturbed, wriggle and drop to the ground. The pupa is enclosed in an openwork net attached to the leaves and overwinters in this stage. The adult is a small gray moth with light yellow rear margins on the forewings. It has a wingspread of 5/8".
The diamondback moth is usually kept under control by its many effective natural enemies, except when insecticides have eliminated the enemies or in unusually hot weather. Among the compounds registered for control of this pest in Connecticut are spinosad and formulations of Bacillus thuringiensis var. kurstaki (Bt), although populations of caterpillars resistant to Bt are occasionally brought up from the southern U.S. on infested seedlings for transplant. These larvae may be sensitive to Bt var. aizawai. Consult the label for dosage rates, safety precautions, and preharvest intervals.
Flea beetles, Phyllotreta sp. and Psylliodes punctulata.
Flea beetles are tiny (1/12 to 1/10" long) jumping beetles that eat small, round "shotholes" in the leaves. There are several species of flea beetles that feed on plants in the cabbage family, including Phyllotreta cruciferae, a black, shining beetle which is the most common species, the striped flea beetle, Phyllotreta striolata, which is black with two curving stripes on the back, and the hop flea beetle, Psylliodes punctulata. See the Flea Beetle fact sheet.
Imported cabbageworm, Pieris rapae.
The cabbageworm is velvety and bright green in color and is about an inch long when mature. It eats large holes in the outer leaves, and as it grows it often moves toward the center of the plant and may feed directly on the cabbage heads. This caterpillar is the larva of the white butterfly that is common around cabbage fields. The butterfly has a wingspread of about one and 3/4" and is white with a tinge of sulfur yellow. The tips of the forewing are gray, and the male bears one and the female two black spots on each forewing. The eggs are yellow and are laid singly on the under surface of the leaves.
See Imported Cabbageworm fact sheet.
Purplebacked cabbageworm, Evergestis pallidata.
This caterpillar is about 3/4" long when mature, bristly and with the body tapering at both ends. The back is purplish-brown, the underside green and there is a yellow stripe running along each side. It sometimes feeds on leaves, webbing them together, or bores into the stem and roots. There are 2-3 generations per year and the late generation is most damaging. This insect is rare, however, and control is not usually necessary.
See fact sheet.
These shiny, hard-skinned, wire-like larvae are the immatures of click beetles (Elateridae). Wireworms feed on seeds, roots, and stems, producing weak plants. They are mainly problems on root crops such as turnips or rutabaga. Several species of wireworms are involved and, like the white grubs, are more injurious to crops planted on soil that is the first or second year out of sod. The corn wireworm, Melanotus communis, or the wheat wireworm, Agriotes mancus, usually occur in heavier organic soils and tend to disappear with continued cultivation. Light sandy soils favor the eastern field wireworm, Limonius agonus. Continued cultivation encourages this species. The presence of wireworms in soil may be detected with baits, such as a cut potato, a mixture of untreated wheat and corn seeds, or rolled oats, buried a few inches deep in the soil, marked with a flag, then dug up 2-3 weeks later. If present in large numbers, they may also be visible when the soil is turned over.
Wireworms are difficult to control with available insecticides. Some tactics which may reduce damage are: 1) delay planting for several weeks after plowing so that little food is available, 2) use plastic mulch between rows to heat the soil above the wireworms' preferred temperature.
Zebra caterpillar, Melanchra picta.
This black caterpillar with bright yellow markings is a general feeder, being found on nearly all kinds of vegetables as well as many flowering plants. It is about two inches long when fully grown. The moth is reddish brown and without prominent markings. The rear wings are nearly white with brown margins. There are two generations each year. Like the cross-striped caterpillar, this insect is not common, but large numbers of caterpillars may be found on a single plant.