Common Pests and Diseases:
Cabbages and other brassicas can be extensively holed by caterpillar feeding by the end of summer. Holes are eaten in the outer leaves of all brassicas and damage may also be seen on the inner leaves of cabbages when the heart is cut through. Caterpillars and their excrement are often found on the plants.
Caterpillars are the larval stage of various butterflies and moths. There are several species that feed on cabbages and allied plants.
There are three common culprits:
- Caterpillars of the large cabbage white butterfly (Pieris brassicae).
- Small cabbage white butterfly (Pieris rapae).
- and the cabbage moth (Mamestra brassicae).
All three may be present at the same time. Caterpillars of cabbage moth and small white butterfly are more damaging as they bore into the hearts of cabbages, whereas the yellow and black caterpillars of the large cabbage white stay mostly on the outer leaves.
How to control them:
- Inspect plants regularly and pick off the pale yellow butterfly eggs and caterpillars when seen.
- A pathogenic nematode, Steinernema carpocapsae, is available by mail order from some suppliers of biological controls as well as some garden centres. The microscopic worm-like nematodes are sprayed or watered on to the foliage, preferably in the evening when the leaves are likely to stay wet for longer. The nematodes enter the caterpillars’ bodies and infect them with a bacterial disease.
- Spray with pyrethrum (Py Spray Garden Insect Killer, Scotts Bug Clear Gun for Fruit & Veg, Doff All In One Bug Spray) or bifenthrin (Bayer Sprayday Greenfly Killer Plus, Scotts Bug Clear Gun or Doff All-In-One Garden Pest Killer).
- Both insecticides have a one-day harvest interval but use of bifenthrin is limited to one treatment during the growing season.
- The adult butterflies and moths lay eggs on the underside of leaves.
- The butterflies have two generations during the summer; cabbage moth has one generation.
- Large white caterpillars are likely to be seen in June-July and late August-September; small white caterpillars feed May-June and July-early September; cabbage moth caterpillars are active in July-September.
- When fully fed, the caterpillars leave the plants to pupate.
- Cabbage moth pupates and overwinters in the soil.
- Cabbage white butterfly larvae pupate on suitable vertical surfaces above ground level.
Carrot fly is the most problematic pest of carrots and allied vegetables. It can make a large proportion of the crop inedible. Carrot fly larvae tunnel into carrots, causing them to rot.
Carrot fly is a small black-bodied fly whose larvae feed on the roots of carrots and related plants. Rusty brown scars ring the tap roots of carrot and other susceptible vegetables, making them inedible, and susceptible to secondary rots. When the roots are cut through, tunnels are revealed, often inhabited by slender creamy-yellow maggots up to 9mm (3/8in) long.
How to control them:
- Sow sparsely to avoid thinning the seedlings. Female carrot flies searching for egg-laying opportunities are attracted by the smell released when surplus plants are removed.
- Late sown carrots (after mid-May) avoid the first generation of this pest; similarly carrots harvested before late August avoid the second generation.
- Protect vulnerable crops by surrounding them with 60cm (2ft) high barriers made of clear polythene to exclude the low-flying female flies, or cover the plants with horticultural fleece. It is essential to practise crop rotation with these methods, otherwise adult carrot flies may emerge within the protected crop from overwintered pupae in the soil.
- Choose carrot cultivars that are less susceptible to carrot fly.
None of the insecticides currently available to amateur gardeners is approved for use against this pest.
- The maggots hatch from eggs laid in late May–June and in August-September.
- Newly-hatched larvae feed on the fine roots but later bore into the tap roots. The brown scars are where tunnels near the root surface have collapsed.
- Two or three generations of carrot fly can develop between late spring and autumn, with the pest overwintering as larvae or pupae.
Slugs are familiar slimy pests that cause havoc in the allotment, eating and making holes in leaves, stems, flowers, tubers and bulbs.
There are about seven species of slugs that are garden pests. They can cause damage throughout the year on a wide range of plants, but seedlings and new growth on herbaceous plants in spring are most at risk and may need protection.
Most slugs feed at night, and the tell-tale slime trails, if present, will alert you to the level of activity. Damage is most severe during warm humid periods.
You may see the following symptoms:
- Slugs sometimes leave behind slime trails, which can be seen as a silvery deposit on leaves and stems.
- Slugs cause irregular holes in plant tissue made by their rasping mouth parts. They can kill young seedling by completely eating them.
- Black keeled slugs (Milax spp.) live underground and tunnel into potato tubers and bulbs.
How to control them:
Slugs are so abundant in gardens that some damage has to be tolerated. They cannot be eradicated so target control measures on protecting more vulnerable plants, such young vegetable plants.
A biological control ('Nemaslug') specific to slugs, with no adverse effect on other types of animal, is available in the form of a microscopic nematode or eelworm that is watered into the soil. The nematodes (Phasmarhabditis hermaphrodita) enter slugs' bodies and infect them with bacteria that cause a fatal disease.
A moist soil and soil temperatures of 5-20ºC (41–68ºF) are required, therefore control is most effective during spring to early autumn. Best results are achieved by applying in the evening to moist but well-drained soils; control may be less successful in heavy soils, such as clay. The nematode is available by mail order from suppliers of biological controls.
Other non chemical controls
Preventive measures you can take include:
- Transplant sturdy plantlets grown on in pots, rather than young vulnerable seedlings. Protect transplants with plastic bottle cloches.
- Place traps, such as scooped out half orange, grapefruit or melon skins, laid cut side down, or jam jars part-filled with beer and sunk into the soil near vulnerable plants. Check and empty these regularly, preferably every morning.
- Place barriers, such as copper tapes around pots or stand containers on matting impregnated with copper salts. Gel repellents can also be used to create barriers around plants.
- Go out with a torch on mild evenings, especially when the weather is damp, and hand-pick slugs into a container. Take them to a field, hedgerow or patch of waste ground well away from gardens or allotments, or destroy them in hot water or a strong salt solution.
- Some birds, frogs, toads, hedgehogs, slow-worms and ground beetles eat slugs and these predators should be encouraged in gardens.
- Rake over the soil and remove fallen leaves during winter so birds can eat slug eggs that have been exposed.
Scatter metaldehyde slug pellets thinly around vulnerable plants, such as seedlings and young shoots on herbaceous plants.
Pellets may harm other wildlife, pets and young children if eaten in quantity, although slug powders based on aluminium sulphate are less toxic.
Most plants, once established, will generally tolerate slug damage and control measures can be discontinued.
Slugs vary in size from the grey field slug (Deroceras reticulatum), which is no more than 5cm (2in) long, to the large black slug (Arion ater), which can be 12cm (5in) when fully extended. Some slugs vary in colour; Arion ater can be black, orange-brown or buff coloured.
Most slugs live in or on the soil surface, but keeled slugs (Milax species) live and feed mostly in the root zone.
Slugs remain active throughout the year, unlike snails, which are dormant during autumn and winter. Warmer weather, combined with damp conditions greatly increases their activity. Slugs are most active after dark or in wet weather.
Reproduction occurs mainly in autumn and spring, when clusters of spherical, yellowish-white eggs can be found under logs, stones and pots.
Potato and Tomato Blight
Potato and tomato blight is a disease of the foliage and fruit or tubers of tomatoes and potatoes, causing rotting. It is most common in wet weather. It is a serious disease for potato and outdoor tomatoes, but not as common on tomatoes grown in greenhouses.
You may see the following symptoms:
- The initial symptom of blight on potatoes is a rapidly spreading, watery rot of leaves which soon collapse, shrivel and turn brown. During humid conditions, a fine white fungal growth may be seen around the edge of the lesions on the underside of the leaves.
- Brown lesions may develop on the stems.
- If allowed to spread unchecked, the disease will reach the tubers. Affected tubers have a reddish-brown decay below the skin, firm at first but soon developing into a soft rot as the tissues are invaded by bacteria. Early attacks of blight may not be visible on tubers, but any infected tubers will rot in store.
- The symptoms on tomato leaves and stems are similar to those on potatoes.
- Brown patches may appear on green fruit, while more mature fruits will decay rapidly.
How to control them:
Infected material should be buried or burned rather than composted. Earthing up potatoes provides some protection to tubers. Early-harvested potatoes are more likely to escape infection. Operate a rotation to reduce the risk of infection, ideally of at least four years.
Tomatoes are generally very susceptible. It is probably best not to rely on host resistance for blight control in tomatoes.
Because infection is so dependent on specific combinations of temperature and rainfall that periods of high risk can be predicted accurately. Advisory services issue warnings for commercial potato growers on which they can base their spray programmes. Met. Eireann usually give blight warning in their weather reports.
Gardeners must rely on a more restricted range of protectant fungicides containing copper or mancozeb (Dithane), since the more effective systemic products are not approved for amateur use.
When wet weather is forecast from June onwards, protectant sprays are advisable, especially for outdoor tomatoes.
The blight pathogen is a microscopic, fungus-like organism whose sporangia (spore-bearing structures) easily break away from infected foliage and may be wind-blown for long distances. The actual infective spores are released from the sporangia into water and need to swim in a water film before settling on the plant wet summers. The pathogen then spreads rapidly, killing the cells. Under humid conditions, stalks bearing sporangia grow from freshly killed tissues and the disease can spread rapidly through the crop.
The pathogen overwinters in rotten potatoes left in the ground or by the sides of fields. However, the great majority of infections in gardens arise from wind-blown sporangia originating in other gardens, allotments and commercial crops. Outbreaks may occur from June onwards.
The fungus can also produce resting spores (oospores) in the plant tissues that can contaminate the soil. Little is known about their survival and their potential as a source of the disease. The investigations into oospores are continuing and more information may be available in a few years.
Late blight attacks defoliate potato crops, but if the disease arrives after the tubers are set and they are harvested before they become infected, little is lost. However early attacks can also be devastating and blight is the most important commercial disease of potatoes. Outdoor tomatoes are at high risk of infection if the weather is suitable. The disease is less of a problem under glass as the spores have to find their way into the glasshouse through doors and vents. If, however, blight establishes in a glasshouse the high humidity inside usually leads to very rapid development of symptoms.