Interactive lesson

A healthy polytunnel, step by step

Interactive lesson: a polytunnel without chemicals — climate against mould, a hand lens, mesh and traps, beneficial insects, tired soil, solarisation and hygiene.

Free sample Chapters 1 and 2 (8 steps) are free. The full lesson comes with the Polytunnel health module. Already bought? Unlock

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The whole lesson as text

1. Climate is the first protection

Most fungal diseases don't come because nobody sprayed, but because the leaf stayed wet too long.

Open in the morning — As soon as the sun is up, even for 15–30 minutes, in October and November too. Damp air leaves as the plants warm up.
Water at the roots — Drip, or a can at the base of the stem. Never over the leaves and never in the evening.
Remove the lower leaves — On tomatoes up to the ripening truss, so air moves along the ground.
Spacing and damp nights — Don't plant closer than recommended: a dense row is a damp row. In September and October leave the top vent in the end slightly open.

Why: The spores of grey mould and blight are almost always present, just waiting for moisture. Humidity of 85–90 % for more than a few hours and 6–8 hours of wet leaf are enough for mould and blight.

Anti-drip film gathers condensation into a thin film that runs down the sides. With new film it is worth paying extra.
Monitor humidity and temperature as described in module H11-04.

2. Spot it in time

A hand lens is the cheapest tool for tunnel health.

Lens and leaf undersides — Once a week, on the same day, turn over tomato, cucumber and pepper leaves and check the undersides with a 10 × lens.
Whitefly and spider mite — Whitefly rise when you touch the plant. Tiny pale speckles and fine webbing mean red spider mite.
Grey mould — Grey fluffy growth on fruit, stems and side-shoot scars. Remove infected parts into a bag and pinch out side shoots in the morning when dry.
Not every problem is a disease — A black, dry patch on the base of the fruit is blossom end rot: uneven watering. The fix is even watering and mulch, not spraying.

Why: You can't see mites, thrips and whitefly eggs well with the naked eye, but with a lens you see them in a second. A problem caught early is a small problem, so walk round for 15 minutes every week.

Tomatoes with mosaic, puckered leaves and yellow or brown wrinkled patches on the fruit: don't share plants or seed, wash hands and tools and contact the plant health authority (APHA in England and Wales, SASA in Scotland, DAERA in Northern Ireland, DAFM in Ireland, USDA APHIS or your state department of agriculture in the US). Suspected ToBRFV.
Many problems are not a disease but a mistake with water, heat or feeding. Spraying won't fix them.