Powdery Mildew is a common disease of all cucurbits, which means it affects Squashes, Cucumbers and Courgettes.
The images top left and right, and the image left here are all showing a significant powdery mildew infestation, and potentially, if left unchecked, it can damage the plant and reduce yields.
Powdery mildew looks different to courgette mosaic virus, illustrated in the centre.
Powdery Mildew is more common in warm environments with high humidity. Plants grown in a greenhouse are more likely to suffer from powdery mildew.
When powdery mildew strikes, one of the first steps is to improve air flow which can include moving plants outside or spacing out their containers. I did this recently to combat mildew and moved 80% of squashes and courgettes outside, some in containers and some I planted in the veg plot.
Here you need to recall that some cucumbers, plants and seed are sold as "outside" and some "indoor" varieties, the latter will have to remain in the greenhouse, irrespective of the powdery mildew. If you plant or place the courgettes and squash outside, ensure lots of space for air to circulate.
Next, remove the worst affected leaves and dispose of them. If you use pruning shears or any tools, disinfect them to prevent cross contamination.