Recent damp weather (that we’ve had in Cornwall, at least) provides the ideal conditions for fungal infections to spread. It is worth getting on top of any problems as quickly as possible…
Most fungal infections will affect the leaves first, so check leaves regularly. Look for brown spots, white patches, mould, black patches, downy coatings. There are a few images below that show some common problems.
As a general rule, if you see any problems, you should remove the affected leaves as soon as possible and get rid of them to stop the disease from spreading. Ensure plants have adequate spacing so that air can circulate well, and be careful when watering to water in the mornings so that plants can dry again and try to avoid getting the leaves wet as much as you can.
You may find a mulch or liquid feed helps in some cases, by providing the plants with a nutrient boost to help them fight off infection. If the plant is too badly damaged, you may have to pull it up, but if you can see healthy new leaves forming at the growing tips, then all is not lost, so hang in there!
Blight on potato
Leaf spot on spinach
Powdery Mildew on squashes
Downy Mildew on pumpkin