The end of October was pretty mild here in Cornwall, and we had damp weather too. I had to open up the zip doors of my little grow tunnel to let the air get in and circulate on several humid days which made me think that it is worth mentioning this here…. if you are growing winter crops under polythene/plastic grow covers like this one that I’ve been using, it can be a real breeding ground for fungal diseases, so it is necessary to open them up and let the air get in and around the plants.
One of the biggest problems can be grey mould – I’ve been lucky so far this year, but there have been other years when I’ve lost lettuces to it (and you quite often see it on strawberries for some reason.) Other fungal diseases will also quickly spread in the warmth and humidity of a grow tunnel like the one I’m using unless there is adequate ventilation, and larger polytunnels and greenhouses also need to have a window or door left open if the weather is as murky as it was last month.
Healthier plants will nearly always withstand fungal problems better, but even they can be affected if there’s no ventilation. It’s quite easy to know whether you need to open a door or not – if you put your hand in the grow tunnel, or poke your head into the greenhouse/polytunnel and it feels like that horrid kind of warm steamy air that you might associate with an overfull classroom of schoolchildren,, then it probably needs a bit of air moving around. If the air feels dry, the plants should be fine.