After the dry spell we've been having in England, summer now seems to be getting its revenge, maybe the weather knows we have family visiting all summer?
So, a warning - no, not about my family, they are lovely - but about wet weather. The two main things springing to mind are blight and slugs.
First late blight. Hopefully, if you garden organically, you already have the first step taken - by growing blight resistant varieties of tomatoes and potatoes. Also, keep greenhouses well ventilated, blight loves static, muggy air. Look out for brown patches appearing on tomato and potato leaves, but don't immediately assume it's blight (have a look at pictures online and compare, for example on the RHS site here).
If you have it on your tomatoes, no matter how upsetting you must dig them out and gather up all fallen material, leaves, rotten tomatoes and burn or bury. (It may be OK to take to the local recycling, but check with them first, I know in some cases their composting methods reach much higher temperatures than ours, and so can kill more pathogens).
Your potato tops may show signs, but if you have been earthing up well, then the tubers will be safe. Cut off the plants at soil level and leave the tubers in and don't dig until after a spell of warm and dry weather (hopefully we'll get some again one day).
Slugs. Well the toads help in my garden, but so also do the organic approved slug pellets I get from the Organic Gardening Catalogue, see here.