July and August often see summer turn into an overcast damp mush in the UK, and when that happens it’s a time to keep an eye open for blight. This serious disease, which affects both potatoes and tomatoes, is caused by the tiny, wind-blown spores of a fungus called ‘phytophthora infestans’. Blight was responsible for the Irish Potato Famine many years ago, and it’s still with us today. Not only that, it’s with us just about everywhere. Whether or not you actually get a blight infestation depends mostly on the weather, and partly on you.
While you’re unlikely to have potatoes growing in the polytunnel, tomatoes are a very common crop. Unfortunately, polytunnels tend to be havens for blight due to their high relative humidity levels – and, as is true with almost all disease and pest problems in the polytunnel, once you have it, it’s difficult to get rid of it.
Prevention is the best cure – as there is no ‘real’ cure. Blight can only take hold in damp or wet conditions, so make sure your plants have plenty of space around them. When they’re tall enough, remove all the leaves up to a height of 30cms. This improves ventilation, especially lower down where the air stays cooler, which in turn helps keep the leaves dry.
‘Smith Periods’ are weather patterns likely to result in the spread of blight spores. They are calculated using temperature and relative humidity data, and until recently predictions were restricted to a few Met stations dotted around the UK. The Blightwatch website is FREE to sign up, for Smith Period warnings that are now calculated based on data from the entire UK at postcode level. Once you register you can request warnings, either by email or text to your mobile phone of Smith Periods in up to 10 postcodes near your garden or allotment.
If blight arrives, all you can do is try to slow it down. Remove any affected leaves or fruit immediately, then spray everything – yes, the entire polytunnel – with a solution of bicarbonate (baking soda), 10g per litre. Add a couple of drops of a plant-based detergent to help the solution stick to the leaves. It‘s a good idea to begin spraying before you see any sign of blight, so make this a bi-weekly task from the beginning of the month.
Blight tends to affect older leaves first, but not always. Leaf edges begin to turn brown, watery, and quickly shrivel and collapse. Fruit will become discoloured at first, then decay quickly, preventing even short-term storage.
Advice about blight disagrees on whether or not it is truly capable of overwintering in the soil. Many, including the RHS, are convinced that it can. Either way, it is definitely capable of genetic mutation producing new strains capable of infecting varieties of potato previously considered ‘blight resistant’; all tomato varieties will eventually succumb to blight if the conditions are bad enough.
This, of course, is a very good reason to get an early start on tomato plants. If your area is prone to blight, choose very early varieties – then you have some hope of getting a crop before disaster strikes. I normally start a couple of ‘early’ plants indoors in February, and last year I had fruit from them in June.
Start sowing seeds for winter and the hungry gap this month! And yes, I know it’s the peak of summertime, but you have to give the seedlings a good start as they’ve got some pretty thin months ahead of them. Don’t forget cold-hardy lettuce varieties such as Rouge d’Hiver and Bronze Arrow which will survive even when there’s frost on their leaves. For detailed information on sowing, growing and harvesting times for all polytunnel vegetable crops, see ‘How To Grow Food In Your Polytunnel’.
Beetroot, cabbage, carrot, cauliflower, coriander, daikon, fennel, kohlrabi, lettuce, spring onion, pak choi, radish, rocket, strawberries, turnip.
Aubergine, broad beans, French and dwarf French beans, sprouting broccoli, cabbage, carrot, cauliflower, celery, chard, coriander, courgette, cucumber, elephant garlic, kohlrabi, lettuce, onion, spring onion, pak choi, peas, radish, rocket, spinach, strawberry, tomato.
And finally sweet corn and sweet peppers both make it on to the ‘harvesting’ list. Dinner is served...
Please see our Top Of The Crops for a list of over 80 Vegetables, Fruits, Herbs and Spices, Flowers and Exotics.