Curse of the cucurbits
I hope you are all coping with the heat, getting your gardening done early in the morning and staying out of the sun. Unfortunately, all we can do for our gardens is water first thing in the morning, and arrange a bit of shade for the most vulnerable plants.
Vegies, and particularly large-leafed plants like cucurbits (zucchini, squash, cucumber, pumpkins, etc), feel the heat badly. The combination of dry soil and humid air is an invitation to powdery mildew, which is one of the worst cucurbit diseases. Plants that are affected will be stunted and will produce far less and smaller fruit.
Prevention is better than cure. When you are planting out seedlings, make sure that you leave plenty of room around each plant. Keep the soil evenly moist with drip irrigation and a layer of mulch. It can be helpful to spray the leaves regularly with a mixture of nine parts water and one part full-cream milk. This is a preventative spray only, and won’t cure established mildew infections.
If you find a patch of silvery grey mildew on the foliage you know to take stern measures. Cut off any leaf on which you see the mildew appear and put it in the bin. Cover the remaining foliage with dusting sulphur and check that your watering system is keeping the soil moist.
If plants are still very stressed by the heat, arrange some shade over them. This can be a simple as sprinkling a light layer of straw over the foliage in the morning.
Dusting sulphur should not be applied during the heat of the day or if you expect a really hot day to follow. It is not a systemic fungicide, but forms a protective layer over the leaf surface, so you will need to reapply it after heavy rain.