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Common Pests and Diseases:

Cabbage Caterpillars

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:

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:

Non-chemical control

Chemical control

Biology

Carrot Fly

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:

Non-chemical control

Chemical control

None of the insecticides currently available to amateur gardeners is approved for use against this pest.

Biology

Slug

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:

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.

Non-chemical control

Biological control

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:

Chemical control

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.

Biology

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:

Potatoes

Tomatoes

How to control them:

Non-chemical control

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.

Chemical control

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.

Biology

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.

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