Pests and diseases of vegetables
Traditional organic growing techniques produce strong, healthy crops, resistant to pests and diseases. But if your crops do get infected, use natural remedies where possible.
BLACKFLY AND GREENFLY
These aphids collect in colonies on stems and flower heads of artichokes and most other leafy vegetables. Companion planting can help combat these pests: chervil, marigolds, nasturtiums and savory are all reckoned to be effective deterrents.
- Symptoms: The leaves of infested plants are covered with varying-sized holes made by yellowish brown or green caterpillars. On cabbages, these caterpillars bore into the heart and ruin the edible parts with their excrement.
- Plants affected: Brassicas, swedes, turnips.
- Treatment: Check brassicas regularly and remove the eggs, which are laid in groups of twenty to a hundred on the undersides of the leaves, and caterpillars. If the larvae are numerous, beforethey burrow into the heart leaves, spray with Bacillus thuringiensis, a bacterium which will kill the caterpillars within a few days, or an approved insecticide.
CABBAGE ROOT FLY
- Symptoms The maggots of the cabbage root fly (Phorbia brassicae) tunnel into the roots, which become riddled with holes and discoloured. The plant is weakened and its growth retarded, while the leaves wilt in hot weather.
Serious attacks can cause plants to die. It is mainly the first generation of maggots that tunnel into the roots, while subsequent generations attack the aerial parts of the plant.
- Plants affected Brassicas.
- Treatment Destroy the overwintering larvae by practising crop rotation, and use companion crops such as onions to prevent infestation.
As the fly lays its eggs at soil level, placing collars around the stems where they meet the soil can help to prevent the larvae getting to the roots. Feed the plants with liquid nettle manure to strengthen their natural resistance to all types of pests and diseases.
CABBAGE WHITE BUTTERFLY
- Symptoms: The leaves are ragged and full of holes, and the heart is riddled with tunnels. Inside are the blue-green or greenish yellow caterpillars (Pieris brassicae), responsible for the damage. If you do not act quickly, only the veins of the leaves remain. Cabbage white caterpillars often live in colonies and are easily recognised by their colour, longitudinal stripes and black markings.
- Plants affected: All members of the cabbage family, including broccoli, sprouts.
- Treatment: Take action at the first sign of attack, before the caterpillars burrow into the heads of the plants. Use an insecticide containing rotenone (derris), natural pyrethrins or Bacillus thuringiensis. Remove and crush any eggs that have been laid.
- Symptoms: White maggots of the celery fly or leaf miner (Eulia heraclei) tunnel within
the leaves, causing brown spots to appear. These maggots later become brown pupae. The leaves shrivel and dry up and, if harvested, the stalks have a bitter, burnt taste.
- Plant affected: Celery.
- Treatment: Pick off affected leaves and destroy. Spray the foliage with an approved insecticide such as derris or pyrethrins. Liquid feed the crop with nettle manure to strengthen growth.
- Symptoms: Small white maggots tunnel within the leaves, and sometimes the leaf stalks, which eventually shrivel and dry up. Damage is caused by the maggots of the chicory fly (Ophiomyia pinguis}.
- Plant affected: Chicory.
- Treatment: Pick off affected leaves and destroy them. Spray the foliage with an approved insecticide.
- Symptoms: Swollen and deformed roots are caused by a soil-borne fungus (Plasmodiophora brassicae),which penetrates the plant and forms swellings. The leaves turn yellow and wilt in sunny weather. Symptoms are similar to those of the gall weevil. The spores that are released by the swellings can remain in the soil for up to ten years before becoming active.
- Plants affected: All members of the cabbage family.
- Treatment: Crop rotation is essential to prevent the disease recurring year after year. Treat
young plants with an approved club root treatment before planting out. Lift and burn any affected plants. Make sure that the land is limed and well drained as this fungus flourishes on very acid soils.
Before planting, check that the roots of bought brassicas and related plants are healthy.
Improve drainage by deep digging and incorporating plenty of humus into the soil.