The Online Gardener's Handbook
Chapter 4: Vegetables
Broccoli, Brussels Sprouts,
Cabbage, Chinese Cabbage,
Kale, Kohlrabi, Radish, Rutabaga
Table of Contents
- Alternaria Leaf Spot
- Bacterial Black Rot
- Caterpillars: Cabbageworms, Cabbage Loopers
and Diamondback Moths
- Downy Mildew
- Mosaic Virus
- Swede Midge
- Root Maggots
- 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.
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.
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,
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.
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.
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 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.
Plant resistant varieties. Avoid close spacing to allow for adequate
ventilation. Do not overwater and ensure plants have time to dry
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
The mosaic virus is spread by aphids.
Plant early to minimize transmission by aphids. See the Aphids
section in Insects
and Diseases Attacking Many Vegetables for information on managing
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.
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 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.
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.