I spotted this on my tomato plant today! A sure sign that the plant is affected with blight. Tomato blight strikes in warm, wet conditions. These conditions are right for the spread of the disease. There must be two consecutive days with a minimum temperature of 10degrees C, AND each day has at least six hours with a relative humidity at 90%. This period of time is known as the 'Hutton Criteria' You can subscribe to a free warning service by the Met Office known as Blight Watch
where you will be notified when the Hutton Criteria is met,
The only thing you can do is good housekeeping at this stage. Remove all affected leaves and destroy them. Don't put them in your garden compost or you will be spreading the disease. If tomatoes are ripe then pick them and use them quickly.
Don't spread the disease further by watering the whole plant, just the soil and the roots if you must water. The tomatoes will be fine to eat as normal, but the fruit may develop brown marks as well if left to be affected.
Meanwhile on the plot, a couple of my Joe's Long Cayenne chilli have started to ripen.
I am doing a little bit of trial and error here. A few weeks ago I planted a second planting of courgettes and cucumbers. It may work or it may not. As long as the weather stays warm well into September (it may well do) then just as my first crop is getting old and tired, then I may just have a couple of new plants to give me a bit more of a crop into Autumn. If I don't.. then nothing lost... just a couple of seeds.
Here is one of my second crop cucumber plants ready to climb up a warm, South facing brick wall. Let's see what happens!