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Chinese cabbage - Diseases, Pests and Problems Basic Information
http://www.essentialgardenguide.com/garden-vegetables-problems/22/Chinese-cabbage/

Problem: Cabbage worms, Cabbage loopers
Affected Area: Leaf
Description: Loopers are caterpillars that are generally green or greyish, taking on the color of the host plant. They may blend well and be difficult to see. They eventually develop into white- or yellow-winged butterflies often seen fluttering about the plants. Caterpillars feed on the underside of leaves leaving ragged holes sometimes to the extent that plants starve and die.
Control: Spray with Bacillus thuringiensis (Bt) Comercially available as Dipel or Thuricide.) Cover with remay or similiar product in spring.

Problem: Cutworms
Affected Area: Stem
Description: Plants chewed off just above ground level. Cutworms are caterpillars 1 1/2" long and mottled or striped green, brown or gray. When they are disturbed, they roll up in a coil. They usually position themselves at the moisture line in the soil moving up and down according to the water content. If the soil surface is dry, they will be found a couple of inches below the surface where the moisture begins. When newly watered, they will be at the surface.
Control: Put cardboard collar around new transplants to extend 1" to 2" above and below soil level.

Problem: Root Maggots
Affected Area: Root
Description: Leaves wilt and growth is stunted. Insect is common, white, root feeding maggot. 1/4" long adult flies emerge from the soil about the time cherries bloom and lay eggs at base of plants in surrounding soil. Legless larvae feeds on host plant for three weeks, riddling the roots with brown tunnels before they pupate Two or three generations can occur each growing season.
Control: Use Diazinon before planting as a soil treatment. Rotate from year to year.

Problem: Flea Beetle
Affected Area: Leaf and Root
Description: Tiny holes ?pinholes? chewed in leaves by adult insect. Adult insects are 1/16" long, hard shelled, shiny, dark-colored beetles that jump when disturbed. Slender, whitish, cylindrical larvae feed in or on roots but root damage is generally minimal.
Control: Dust with Rotenone. Keep debris removed. Rotate location of planting from year to year.

(Homemade soap pesticide- few tblsp of a mild liquid soap, canola oil, and at least a gallon of water? Add a little garlic and hot pepper powder if you like for more insecticdal power. here)
ATTRA
Golden Harvest Organics - yellow sticky tape

Problem: Aphids
Affected Area: Leaf
Description: Green, red black or white insects that cause curled yellow leaves and exude a honeydew substance
Control: Insecticidal soaps or a strong stream of water or most labeled insecticides like Diazinon or Sevin. A layer of aluminum foil under plants reflects light to underside of leaves making them an undesirable habitat for aphids.

Problem: Slugs and Snails
Affected Area: Entire Plant
Description: Large portions of young plants missing.
Control: Slugs and snails are very susceptible to desiccation (drying) and require a moist, shady place to live. Cultural practices which promote a sunny, dry environment will discourage them. Avoid too-frequent waterings allowing soil surface to dry out between irrigations. Keep garden free of debris, boards, bricks, and stones where they hide. Hand picking these pests is very effective. Create ?traps? for hand picking by laying boards in the garden. Slugs and snails will congregate under them. Lift the boards each morning and collect the slugs and snails. Kill and dispose of them in garbage as they will crawl back if tossed out of the garden and eggs inside dead pests can still hatch to produce more of these pests. Slug and snail bait containing metaldehyde can be placed near food plants as long as they do not contact edible portions of the crop. Most effective when moistened, but not water logged. Snail bait attracts slugs and snails from several feet away so bait stations are effective. Stations help protect birds, pets and other non-target animals which are also attracted to the bait. Place small piles of bit under a slightly propped up board or use container such as a cottage cheese or yogurt carton. Bury carton to the mouth of the container. Place small amount of commercial bait inside and moisten with apple juice, orange juice or water. Cut hole in lid to allow access and place lid on container. Containers may also hold beer or yeast water to attract slugs and snails in where they drown. Place bait stations wherever slugs and snails are active or around perimeter of garden.

Problem: Damping Off
Affected Area: Seedling
Description: Young seedlings wilt and die
Control: Use treated seed and let soil dry out between waterings

Problem: Bacterial soft rot
Affected Area: Leaf and Stem
Description: Leaves turn yellow (chlorotic) beginning at margins and spreading inwards. Veins within area turn black. Infection enters main stem turning the inside black.. Plants either die or are dwarfed when young, become defoliated if more mature.
Control: Plant resistant varieties and rotate crops from year to year.