Why Are My Chilli Leaves Curling?

Broad mite (Polyphagotarsonemus latus)

Broad mite is one of the most damaging and hardest-to-diagnose pests of chilli under glass. The adult mites are 0.1 to 0.2 millimetres long, completely invisible to the naked eye, and live and feed on the youngest tissue at the growing tips, inside the tightly folded young leaves. Their feeding causes the young leaves to curl upward and inward very tightly, become thickened and leathery, and develop a bronzed, greasy, or lacquered sheen on the upper surface. New leaves emerging from the growing point are often so distorted that they crumple and fail to unfurl normally. There is no webbing (distinguishing it from spider mite). The stem tissue just below the growing point may also become scarred and corky. The overall appearance is of a plant with severely stunted, deformed growing tips, often mistaken for a virus, calcium deficiency, or herbicide contamination.

What to do

• Examine distorted leaf tips with a hand lens (at least 20x magnification): broad mites are visible as tiny, slowly-moving, pale oval specks on the youngest leaves and in the growing point. Compare with a healthy plant.
• Introduce predatory mites (Amblyseius cucumeris or Amblyseius californicus) as soon as broad mite is suspected. These predators are highly effective against broad mite and are commercially available on slow-release sachets for hanging in the plant canopy.
• Isolate affected plants immediately to prevent spread to healthy plants: broad mite moves between plants by walking and is also dispersed by insects and human contact. Clean tools and wash hands after handling affected plants.
• Remove and destroy the worst-affected growing tips to reduce the mite population on the plant before introducing predators.
• Hot water treatment: immerse badly affected plant cuttings or small plants in water maintained at 43 to 44°C for 30 minutes. This temperature kills the mites without damaging the plant tissue if held accurately.

Aphids

Peach-potato aphid (Myzus persicae) colonises chilli shoot tips and the underside of young leaves, causing the leaves to curl downward and distort around the feeding colonies. Aphid-infested growing tips often look different to broad mite damage: the curling is softer and less dramatic, the leaf surface texture is normal, and the aphids are easily visible as small, pale green insects. Aphids also excrete honeydew, which promotes sooty mould on the leaves below the colony.

What to do

• Inspect chilli growing tips weekly from the time plants are brought under glass in spring. Remove colonies by wiping with a damp cloth. Treat with insecticidal soap. Introduce the parasitoid wasp Aphidius colemani for persistent infestations: highly effective against peach-potato aphid on chilli under glass.

Two-spotted spider mite

Two-spotted spider mite (Tetranychus urticae) affects chilli in the same way it affects aubergine and tomato under glass. The first sign is pale, mottled stippling on the upper surface of older leaves, caused by the mites piercing the leaf cells. Fine webbing appears on the leaf underside as the population grows, and the leaves progressively yellow, curl, and drop. Spider mite thrives in hot, dry conditions.

What to do

• Inspect the underside of chilli leaves regularly. Introduce the predatory mite Phytoseiulus persimilis at the first sign of spider mite activity. Maintain humidity by misting pathways in the greenhouse. Insecticidal soap or neem oil provides contact control but requires thorough coverage of the leaf undersides.

Overwatering

Chilli roots need well-drained, aerated compost: waterlogged conditions prevent oxygen from reaching the roots and cause the plant to lose its ability to take up water, leading to paradoxical wilting and downward leaf curling (epinasty) in an apparently wet pot. The lowest leaves yellow and drop first, followed by progressive upward decline.

What to do

• Water chilli only when the top few centimetres of compost are dry. Ensure pots have drainage holes and are not standing in water. Use a free-draining compost mix with added perlite if waterlogging is a persistent problem. Allow plants to recover in a warm position before resuming watering.

Frequently asked questions

Why are my chilli leaves curling?

Chilli leaves curl for several reasons, the most common being broad mite (Polyphagotarsonemus latus), aphids, spider mite, and overwatering. Broad mite causes a very distinctive tight upward curling and distortion of the young leaves at the growing tips, often accompanied by a bronzed or greasy appearance of the leaf surface; the mites are far too small to see without magnification. Aphids cluster on the growing tips and cause the young leaves to curl downward and inward. Two-spotted spider mite causes pale stippling, fine webbing, and progressive yellowing and curling of older leaves as the population grows. Overwatering causes the leaves to curl downward (epinasty) and may also cause yellowing from the base of the plant.

What is broad mite and how do I identify it on chilli?

Broad mite (Polyphagotarsonemus latus), also known as cyclamen mite, is a microscopic mite that is one of the most damaging pests of chilli under glass in the UK. The mites are 0.1 to 0.2 millimetres long, far too small to see without a hand lens of at least 20x magnification, and are most numerous on the youngest leaves at the shoot tip. Their feeding causes very characteristic damage: the young leaves at the growing tip curl upward and inward very tightly, become thickened, dark, and have a bronzed, oily, or glazed appearance on the upper surface. New leaves emerging from the growing point may be so distorted that they appear crumpled and fail to expand normally. In contrast to spider mite, there is no visible webbing. The growing tips of severely infested plants become stunted and produce only deformed leaves. Broad mite spreads by being carried on clothing, tools, and by insects visiting the plants. Diagnosis can be confirmed by examining a distorted leaf tip under a hand lens or by sending a sample to a plant diagnostic service.

How do I treat broad mite on chilli?

Broad mite is difficult to treat because it lives and feeds inside the developing leaf tissue at the growing tip, protected from contact sprays by the tightly curled leaf. Options include: predatory mite introduction using Amblyseius cucumeris or Amblyseius californicus, which are effective biological controls for broad mite and can be introduced on sachet slow-release systems; sulphur-based acaricides (in UK amateur use, sulphur-based products are the main option); and hot water treatment for affected plants (immersing cuttings or small plants in water at 43 to 44°C for 30 minutes will kill broad mites without harming the plant). The most effective approach for protecting a greenhouse population of chilli is prophylactic introduction of predatory Amblyseius mites early in the season before broad mite populations establish. Isolate any newly acquired plants for two to three weeks before introducing them to the greenhouse, as broad mite is often introduced on bought plants.

Why are my chilli leaves curling downward?

Chilli leaves curling downward (cupping downward, with the leaf edges turning under) is typically caused by overwatering, heat stress, or aphid infestation. Overwatering is the most common cause: chilli roots need oxygen and waterlogged compost deprives them of it, causing the plant to lose its ability to regulate water in the leaves. The leaves droop, cup downward, and eventually yellow from the base. Check the compost moisture before watering: chilli should be watered when the top few centimetres of compost are dry, not on a fixed schedule. Heat stress on very hot days causes temporary downward curling as the plant conserves water by reducing the exposed surface area; this reverses as temperatures drop in the evening and is not harmful. Aphids on the growing tips cause downward curling of the young leaves immediately around the feeding colonies.