Strawberry diseases and pests

Why strawberries rot: grey mould up close

Grey mould, caused by *Botrytis cinerea*, is the classic strawberry rot. The RHS calls it a very common disease that causes a soft decay of plant tissue.

Why it shows late. The RHS grey mould guide explains a trick of the fungus. It gets into the flowers and colonises the developing fruit, but does not rot it until ripening raises the sugar content. The RHS says this is very noticeable with strawberries. So the infection often starts weeks before you see it.

What favours it. High humidity, overcrowded plants and poor ventilation. Plants that are stressed or wounded are more vulnerable, though healthy plants can be infected too. Airborne spores are always about, and the fungus survives on dead plant material and as small black resting bodies.

What helps. The RHS strawberry guide keeps water off the crown and fruit and waters in the morning, so any splashes dry in the day. Dry straw under the forming fruit keeps berries off damp ground. Mulch should not touch the crown, which can rot, so leave a small gap. The RHS grey mould guide removes dead and dying leaves and flowers promptly and bags them as it goes. The RHS says no fungicides are approved for use against grey mould by gardeners.

Powdery mildew: what the commercial advice says

The RHS strawberry guide does not cover powdery mildew. The detailed UK information comes from AHDB, which advises commercial growers, mostly with crops under tunnels. Read its advice with that in mind.

AHDB names the fungus *Podosphaera aphanis*. Its factsheet says the first warning sign is leaves that cup or curl upwards and stay curled for 24 hours or more, still visible early in the morning. Dark red blotches may follow, depending on the variety. White growth appears first on the underside of the leaves, then on top, and can spread to flower buds and to ripe and unripe fruit.

The fungus overwinters on crop debris and old leaves, says AHDB, and up to 70% of young plants can carry small colonies. In new crops, it says, the infection often starts during propagation. Hot, humid air under closed tunnels speeds it up.

AHDB's cultural measures for growers are resistant varieties, avoiding excessive feeding and watering, removing old debris from second and third year crops in spring and, under cover, venting to keep humidity below 75%.

Keep the bed young and on the move

Several strawberry problems live in the soil or build up in old plants. The RHS answer is to renew the bed.

Spacing. Plant 35–40 cm apart in rows about 75 cm apart. In containers the RHS spaces plants about 20 cm apart.

Renewing. A bed usually stays healthy for up to four years in well-drained, fertile soil, the RHS says. Replant every three or four years to avoid disease build-up, and make the new bed in a different spot to avoid replant disorder. The RHS strawberry guide also names verticillium wilt, a soil-borne fungal disease that enters through the roots, and says several viruses infect strawberries, causing poor vigour and low yield. It gives no cure for either.

After cropping. Cut the old leaves of summer-fruiting plants about 10 cm above the crown, so new leaves can grow. On perpetual strawberries, remove only dead, damaged or diseased leaves at the end of the season.

How to plant and look after a new bed is on our pages when to plant strawberries and how to grow strawberries.

Strawberries in pots: watch for vine weevil

Growing in containers reduces slug and snail damage to the fruit, but the RHS says vine weevil grubs are particularly fond of strawberries and a bigger threat in pots.

The adults are dull black beetles about 9 mm long that cannot fly and feed at night, notching leaf edges. The grubs live among the roots. Most plant losses happen from September to March.

What to do. On mild spring or summer evenings, check plants by torchlight and pick off the adults. Repot container plants to remove grubs. For biological control, the RHS lists nematodes applied in August or early September. *Steinernema* species need a soil temperature of 5–20 °C and *Heterorhabditis* species 12–20 °C. They work best in light soils and open composts.

Slugs, birds and fruit flies

Slugs and snails are drawn to damp straw, the RHS says, so remove the straw in autumn. Our page how to get rid of slugs covers what works.

Birds and squirrels. Before the berries ripen, cover the plants with taut, secure netting. Chicken wire deters squirrels. Remove the protection after harvest, or grow strawberries in a fruit cage.

Spotted wing drosophila is a fruit fly that can damage unblemished soft fruit, including strawberries. Look for collapsing fruit or tiny puncture holes. Pick fruit as it ripens, clear overripe and fallen fruit, and never compost infested fruit. Insect-proof mesh finer than 0.98 mm keeps it out once fruit has set.

The RHS also lists frost damage, known as black eye, fruit aphids and phyllody among strawberry problems.

What about sprays?

The RHS says it doesn't support the use of pesticides and encourages non-chemical methods. If chemical controls are used, it says they should be a last resort, used in a minimal, highly targeted way.

Only pesticides approved for home gardeners are legal to buy, and following the label is a legal requirement. The RHS advises against household products such as vinegar or washing-up liquid as pesticides.

For grey mould, the most common strawberry rot, the RHS says no fungicides are approved for use by gardeners.