Patchy leaves like the one pictured are a common sight in pumpkin, squash and courgette plants at this time of year. It’s nothing to be too concerned about, but as always it’s good to know what’s going on.
Powdery mildew is a fairly common issue for plants of the cucurbit family: courgettes, squashes and pumpkins. It often appears during late summer. The leaves start to develop a white/grey powdery coating before they eventually die off.
The mildew tends to affect older leaves first whilst new leaves continue to grow through perfectly healthily. Normally, the plant continues to produce edible fruits for several weeks afterwards and it is not until mid autumn that it eventually gives up the ghost. Our advice would be to remove badly affected leaves and allow healthy new growth to come through. You could try a milk/water spray (1 part milk to 3 parts water sprayed on the leaves) if you want, but remember that this is very much part of the plants’ natural cycle and will be difficult to prevent.
When the plant is severely affected, at the end of the season, just pull the plant up and call it a day!