Most common pest Slugs and snails
Most destructive disease Late blight
First line of defence Prevention
Key principle Healthy plants resist better
95%

The proportion of carrot fly attacks blocked by a 60 cm fine-mesh barrier around a carrot bed, according to RHS research. Physical barriers are among the most reliable tools in organic pest management because they work consistently regardless of pest population levels or weather conditions.

The RHS position is that avoiding pests and diseases through good cultivation practice, careful variety selection, garden hygiene, and encouraging natural enemies should always be the first line of control. Intervention, whether physical, biological, or using approved organic products, comes second. This hierarchy matters because it is cheaper, less disruptive to the wider garden ecosystem, and more durable over time. A garden with good soil health, diverse planting, and a population of natural predators will experience fewer serious pest and disease problems than one managed reactively.

Prevention: the foundations

Most pest and disease problems can be reduced significantly by a small number of consistent practices:

Common pests

Slugs and snails

Slugs and snails are consistently the most frequently reported and most damaging pest in UK vegetable gardens, based on a survey of pests and diseases carried out by Garden Organic. They feed at night and in damp conditions, leaving characteristic ragged holes in leaves and slime trails on foliage, stems, and soil. Young seedlings are particularly vulnerable; an overnight slug attack can destroy an entire emerging row.

The most effective organic control is soil-applied nematodes (Phasmarhabditis hermaphrodita), microscopic roundworms that seek out slugs in the soil, enter through natural openings, and release bacteria that kill the slug within a few days. Applied as a watered-in drench, they are the only method that reaches soil-dwelling species including the keeled slug, which causes root damage and is never seen on the surface. Nematodes are harmless to all garden wildlife, beneficial insects, and pets. Apply in April and again in July to address the two peak slug breeding periods; soil temperature must be above 5°C for them to be effective.

Copper tape works reliably as a barrier on containers: run a band of self-adhesive copper tape around the outside of a pot to create a physical deterrent. However, an RHS study in realistic garden conditions found no significant reduction in slug damage from copper tape barriers around open beds, where any gap in the tape negates the effect. Crushed eggshells, grit, and similar abrasive materials have a poor evidence base and deteriorate quickly in wet UK conditions. Encouraging natural predators, hedgehogs, frogs, toads, ground beetles, and birds, provides background population control and is worth supporting regardless of other methods used.

Aphids

Aphids (greenfly and blackfly) are extremely common and can occur on almost any garden plant. They feed by piercing plant tissue and extracting sap, causing distorted growth, yellowing leaves, and sticky honeydew residue on which sooty mould develops. A colony can establish rapidly in warm weather; populations peak in June and naturally decline through July and August as predator populations build in response.

The most effective long-term management is encouraging a diverse community of natural predators. Ladybirds consume around 50 aphids per day; lacewing larvae kill up to 200 per week; hoverfly larvae, parasitoid wasps, and ground beetles all contribute. A garden with flowering plants throughout the season, structural features like log piles, and minimal pesticide use will support these populations naturally. The RHS notes that gardens where birds are encouraged tend to have lower aphid levels overall.

For severe infestations on individual plants, particularly in the early season before predator populations have built up, aphids can be dislodged with a firm jet of water or removed by hand. Biological controls (lacewing larvae, parasitoid wasps) can be purchased mail-order and introduced directly to a greenhouse or polytunnel where natural predators cannot reach.

Cabbage white caterpillars

Both the large and small cabbage white butterfly lay eggs on brassica leaves. The large white lays clusters of yellow eggs on leaf undersides; the small white lays single eggs. The caterpillars of both species, which are pale green with faint yellow markings, feed on the leaves and can strip a plant to the ribs if unmanaged. They can also work their way into the heart of a cabbage, where they are difficult to detect until the damage is already done.

The most reliable control is physical exclusion: cover brassicas with insect-proof mesh (0.8 mm holes or finer) from planting until harvest. The mesh must be secured at ground level with no gaps, as butterflies are persistent in finding entry points. Inspect covered plants weekly; caterpillars can still be present if the mesh was applied after eggs were already laid. Check the undersides of all leaves and remove eggs and caterpillars by hand as soon as they are found.

Carrot fly

Carrot fly (Psila rosae) is a small fly whose larvae tunnel into the roots of carrots and related crops including parsley, parsnips, and celery. The damage appears as rusty brown channels in the root flesh. There are two generations each year: the first lays eggs in May and June, the second in August and September.

The adult fly travels low to the ground and cannot fly higher than around 60–75  cm. A vertical barrier of fine insect-proof mesh (0.8 mm or finer) at least 60 cm tall, erected before sowing or transplanting and buried or weighted at the base, blocks around 95% of attacks according to RHS trials. Standard garden netting has holes too large to exclude the fly; the mesh must be fine enough to prevent the 8–9 mm adult from passing through. The barrier approach is the most reliable organic control available and removes the need for any other treatment.

Additional steps that reduce risk: avoid thinning in warm evenings when flies are most active (the scent of damaged foliage attracts them); water immediately after thinning to disperse the scent; and grow naturally deterrent companions such as rosemary and sage nearby.

Common diseases

Late blight

Late blight (Phytophthora infestans) is the most destructive disease of potatoes and tomatoes in the UK, causing an estimated £50 million in crop losses annually. It is caused by the same water mould responsible for the Irish Potato Famine of the 1840s. Symptoms appear as dark, water-soaked patches on leaves that spread rapidly, turning brown with a pale yellow border; white mould may be visible on leaf undersides in humid conditions. Infected tubers develop a dark, firm rot beneath the skin.

Blight spreads in warm, humid conditions: once temperatures are above 10°C and relative humidity stays above 90% for 48 hours (a “Smith Period”), conditions are ripe for an outbreak. Spores travel on wind, and outdoor tomatoes often develop blight one to two weeks after local potato crops show symptoms.

There is no organic cure once blight is established. The most effective organic strategies are resistant varieties and cultural controls. For potatoes, Sarpo Mira and Sarpo Axona have strong blight resistance; for tomatoes, outdoor varieties bred for resistance include Crimson Crush and Ferline. Never grow tomatoes adjacent to potatoes. Improve airflow around plants, avoid overhead watering, and earth up potato haulms to protect tubers. If symptoms appear, cut off and remove all affected material immediately; do not compost it.

Clubroot

Clubroot (Plasmodiophora brassicae) is a soil-borne pathogen that causes the roots of all brassica family plants to swell into distorted, club-shaped masses. Affected plants wilt in dry weather even when the soil is moist, and fail to develop properly. Above ground, the plant may appear stunted with yellowing leaves. The pathogen produces resting spores that can survive in the soil for 20 years or more, making clubroot effectively permanent once it becomes established.

Clubroot thrives in acid, poorly drained soil. The primary preventive measure is maintaining soil pH above 7.0 with lime applications; the pathogen is suppressed significantly above this threshold. Apply garden lime in autumn before planting brassicas and test pH annually. A rotation of at least four to five years away from all brassica family plants, including related weeds such as shepherd’s purse and charlock that also host the pathogen, is essential. On infected ground, raising transplants in large pots of fresh compost rather than direct sowing allows plants to establish a healthy root system before being exposed to the pathogen in the soil.

Clubroot-resistant varieties are available for some brassica crops and are worth using on any site with a history of the disease. No variety is fully immune, but resistance significantly reduces the severity of infection.

Powdery mildew

Powdery mildew is one of the most widespread fungal diseases in UK gardens, appearing as white or grey powdery patches on leaves, stems, and occasionally fruit. It affects a wide range of crops including courgettes, squash, peas, cucumbers, and gooseberries, typically becoming visible from midsummer onwards. Unlike most fungal diseases, powdery mildew can spread in warm, dry conditions as well as humid ones.

The key risk factor is plant stress, particularly drought. Plants growing in dry soil or compost are significantly more susceptible. Consistent watering, mulching to retain moisture, and correct spacing for airflow are the most effective preventions. Mildew rarely kills an established plant but reduces vigour and cropping if unchecked. Removing heavily affected leaves and improving conditions is usually sufficient. Some courgette and squash varieties have better mildew tolerance than others; this is worth checking when choosing what to grow.

Botrytis (grey mould)

Botrytis (Botrytis cinerea) is a grey-brown fuzzy mould that develops on dead, damaged, or weakened plant tissue and spreads rapidly to healthy material in cool, damp conditions. It is a common problem on strawberries, lettuce, tomatoes in enclosed spaces, and densely planted crops during wet summers. The mould releases masses of spores that are visible as puffs of grey dust when disturbed.

Management focuses on removing the conditions in which botrytis thrives: improve airflow by spacing plants adequately and removing dead or dying material promptly; water at the base not over the foliage; avoid leaving dead flowers on fruiting plants such as tomatoes and strawberries, as botrytis most often enters through dead floral tissue. In a greenhouse or polytunnel, ventilation is particularly important during cool, wet periods. There is no organic spray that cures established botrytis; removal of affected material and improved conditions are the only responses.

Physical barriers

Physical barriers are one of the most reliable and underused tools in organic pest management. They require no ongoing maintenance, have no side-effects on beneficial insects or soil organisms, and work consistently. The main options:

Encouraging natural predators

A diverse population of natural predators is the most sustainable long-term form of pest control in a garden. The key relationships:

Supporting these predators requires the same habitat features described in the habitat features guide: a log pile for beetles and hedgehogs, a pond or water feature for frogs and toads, flowering plants throughout the season for parasitoid wasps and hoverflies. A garden managed without synthetic pesticides will over time develop a self-regulating balance where pest populations are routinely kept in check by natural enemies. This takes longer to establish than a chemical approach but is far more durable and does not require ongoing expenditure or intervention.

What to avoid

Synthetic insecticides and fungicides, even those sold for garden use, have significant side-effects that work against the preventive approach described above. Broad-spectrum insecticides kill beneficial insects alongside pest species, removing the predator populations that provide natural pest control. A single application can set back a garden’s natural pest management for a full season or more as populations take time to recover.

Systemic insecticides, including some neonicotinoid-based products that are still available to gardeners despite restrictions on agricultural use, are taken up by plant tissue and appear in pollen and nectar, making them directly harmful to bees and other pollinators. The RHS advises against their use and recommends physical and biological controls as the default approach for all food-growing gardens.

If any product is used, check that it is listed as suitable for organic growing or approved for use in organic systems. Even then, treat it as a last resort after cultural and physical controls have been tried.

Further reading

Related guides