The Online Gardener's Handbook 2010
Chapter 4: Vegetables
Broccoli, Brussels Sprouts,
Cabbage, Chinese Cabbage,
Kale, Kohlrabi, Radish, Rutabaga

Table of Contents

  1. Introduction
  2. Alternaria Leaf Spot
  3. Bacterial Black Rot
  4. Caterpillars: Cabbageworms, Cabbage Loopers and Diamondback Moths
  5. Clubroot
  6. Downy Mildew
  7. Mosaic Virus
  8. Swede Midge
  9. Root Maggots
  10. Learn More


In this chapter, a description of various cole crop pests will be provided along with suggested management options. These management options will not include the use of pesticides. Some biopesticides and certain reduced risk pesticides are still available to the homeowner for controlling weeds and pests in lawns and gardens. For more information, refer to Chapter 2 of this handbook and the Ministry of the Environment's website. For suggestions on managing specific weeds and pests, consult local horticulturalists, Master Gardeners or your local garden supply centre.

Alternaria Leaf Spot

Alternaria leaf spot is a fungal disease that occurs on leaves. Spots are brown and target-like with concentric rings, and develop felt-like dark brown mould in wet weather. The fungus can be seed-borne, or survive in infected crop residue for up to two years.

Management Options

Follow proper watering and sanitation procedures. Water the soil rather than the leaves, and water in the morning to allow plants to dry by evening. Avoid overwatering. Remove and destroy crop residue at the end of the season. Purchase hot-water treated seed.

Bacterial Black Rot

Plants may become infected from seedling stage to harvest and in storage. A wilted, often-yellowish V-shape appears on cabbage leaf margins. Veins may be black or dark brown. They can be seen in stalks and stems of rutabaga and turnip and roots of kohlrabi, showing as a black ring or arc if cut crosswise. The bacteria overwinter on or in the seed and in plant refuse and spread on dirty tools and by splashing water.

Management Options

Buy transplants from a reliable retailer or buy hot-water treated seed. If black rot is found, collect plant remains and destroy them. Avoid overwatering. Disinfect tools and gloves after working with infected crop residue.

Caterpillars: Cabbageworms, Cabbage Loopers, Diamondback Moth

Cabbageworms are a common, annual problem throughout Ontario. They are pale green caterpillars, difficult to see, up to 30 mm in length, with a faint orange stripe. They often lie along the midrib of the leaf. White butterflies are a warning that eggs are being laid. Cabbage loopers, 4 cm long when mature, are pale green caterpillars with white stripes along back and sides. Their looping movement easily identifies them. They feed on leaves and can cause severe damage. Diamondback moth larvae are 1 cm long green caterpillars. When disturbed they often wriggle quickly backwards or drop off the plant on a silken thread. They eat small holes in the leaves with part of the leaf tissue still covering the hole. Their movement and eating patterns distinguish them from cabbageworms.

Management Options

Inspect plants frequently (daily if possible). Hand pick and destroy caterpillars or eggs when seen. Floating row covers may protect seedlings from egg laying adults of some species early in the season. These insects are susceptible to virus and bacterial disease, and have a number of parasites which may sometimes help to reduce populations.


Clubroot is a very serious disease caused by a soil-borne fungus, which will remain in soil indefinitely. Roots develop club-like swellings and when severe, the entire root system is deformed. Plants may be stunted and wilt on hot days. Garden soil can become infested by purchasing infected transplants or soil from infested fields. Infested soil clinging to tools and equipment can also spread disease. Acid soils favour development of the fungus. Cruciferous weeds, such as those in the mustard family, yellow cress, stinkweed, pepper grass and shepherd's purse, are also attacked, and maintain the fungus in the soil.

Management Options

Grow plants from seed or purchase only healthy transplants from reputable suppliers. Check transplants for clubs on their roots. Dig out all affected roots at harvest and discard them. Do this even if soil was infected before planting. Discard whole flat or box if any affected seedlings are found. If infected plants are noticed in the garden, remove and destroy as soon as wilting is seen. Do not compost diseased roots. As much as possible, eliminate cruciferous weeds in your garden. Incorporate agricultural lime into the soil to raise the pH to about 7.2. Hydrated lime (available at a building supply) will change the pH more quickly; use at the rate of 1 kg per 6 m2 and dig or rototill into soil about three weeks before planting. Have your soil tested to know the pH level.

Downy Mildew

Downy mildew is a fungus, which primarily occurs in cool, moist weather in spring and fall. Yellow patches develop on the upper surface leaves, later becoming brown and faintly netted with dark markings. On leaf undersides, patches may be covered with a downy, white fungus under humid conditions.

Management Options

Plant resistant varieties. Avoid close spacing to allow for adequate ventilation. Do not overwater and ensure plants have time to dry between waterings.

Mosaic Virus

Mosaic virus causes a long, crooked neck and a small, slender root on rutabagas and turnips. In late summer, cabbage, cauliflower and broccoli may also be affected, resulting in a continuous dropping of lower leaves, which leads to long bare stems and poorly developed heads.

The mosaic virus is spread by aphids.

Management Options

Plant early to minimize transmission by aphids. See the Aphids section in Insects and Diseases Attacking Many Vegetables for information on managing aphids.

Swede Midge

The swede midge is a gall fly native to Europe in Asia that was first found on Brassica plants in Ontario in 2000, and is now widely distributed in Ontario. It is a tiny, light-brown fly (1.5-2 mm), difficult to distinguish from other midge species in Ontario. The larvae are small (0.3-3 mm long), initially translucent maggots that become yellow-white when mature. They typically feed in clusters on young tissue, near growing points. Damaged seedlings often appear twisted and may have a noticeable brown scar or a gall at the growing point. Later feeding injury results in twisted and distorted heads. Damage can be confused with other problems such as nutrient deficiency, heat stress or frost damage. The first generation of swede midge adults appear from mid-May through early June. There are 4-5 overlapping generations in Ontario.

Management Options

The swede midge is very difficult to control. Swede midge is commonly spread to new areas by the movement of transplants from infested areas. Be confident about the source of your transplants and do not bring infested plant material into clean areas. Chop and bury infested residue. Most pupae are found within the top 5 cm of the soil. Deep tilling may reduce the number of pupae. Pupae may remain in the soil for 2 years, so avoid growing any crucifer crops in infested soil for 2-3 years. There is considerable variability among plant varieties in susceptibility to swede midge. Timing of planting can also help - fields planted prior to adult emergence in early June may have less severe damage than later plantings.

Root Maggots

Root maggots cause severe damage to young plants every year. There are three generations of the adult cabbage maggot which lay their eggs in soil near plants in May, early July and mid-August. The first generation causes the most damage. White maggots feed on roots, creating entry sites for rot. Plants become stunted and turn blue-grey; edges of leaves curl. Roots may break off when pulled up. Young plants are usually killed. Older plants survive, but growth is reduced.

Management Options

With rutabaga, avoid first-generation maggots by delaying sowing until early July (mid-June in Northern Ontario). Injury by the second and third generation maggots can be trimmed off before cooking. Sow seed in a frame tightly covered with mosquito screen or cheesecloth to prevent maggot flies from laying eggs near the seedlings. Make a collar from a 10 cm x 10 cm piece of tarpaper or stiff plastic with small a hole cut in the centre and slit down one side to slide over the stem of each plant when transplanting into the garden. The collar must lie flat on smooth soil; it is helpful to place a small stone on the cut.

Learn More


For more information:
Toll Free: 1-877-424-1300
Author: OMAFRA Staff
Creation Date: 21 July 2005
Last Reviewed: 25 June 2010