How do I keep humidity down enough to avoid botrytis in a closed winter greenhouse?

Botrytis needs three things: high humidity, still air and dead or damaged tissue to colonise. Removing any one of them substantially reduces the problem, and in winter the practical lever is air movement.

Ventilate even when it is cold. Opening a vent for an hour around midday, when the outside air is warmest and driest, exchanges the moisture-laden air for drier air and costs less heat than you would expect. A small circulating fan running continuously is even more effective — it does not remove moisture but it prevents the still, saturated boundary layer at leaf surfaces where spores germinate.

Water in the morning rather than the evening, so foliage and surfaces dry before the temperature drops overnight and relative humidity rises. Water the compost rather than the plant. And water less overall in winter; plants use a fraction of what they do in summer, and evaporation from wet surfaces is a major humidity source.

Remove dead material relentlessly. Fallen leaves, spent flowers, damaged stems and old compost are the entry points — botrytis is a weak pathogen that rarely attacks healthy intact tissue directly. A weekly ten-minute clean-up is the most effective single habit.

Space plants so air moves between them, and raise pots off solid staging onto slatted benching or gravel so the undersides are not sitting on a permanently damp surface.

A maximum-minimum hygrometer is worth having. If relative humidity is sitting above about eighty-five per cent overnight you have a problem developing regardless of whether you can see it yet; keeping it below that most of the time makes botrytis a rare event rather than an annual one.