Powdery mildew starts out as small white random speckles on the leaves of pumpkins, squashes, zucchini, cucumbers and melons. These spots spread until whole leaves become covered in a whitish, dusty-looking mould.
Towards the end of summer large leaved plants like zucchini often develop powdery mildew on their leaves and stems due to humidity, poor air circulation or irregular moisture levels. You may notice, if you look closely, that there are small yellow and black ladybugs crawling around on your plants – these are well known for spreading the mildew spores from plant to plant. Powdery mildew is also spread, unwittingly, by gardeners as they touch and use tools on affected foliage and then move to other plants elsewhere in their gardens. It is often slow to take over entire plants and fruit is generally unaffected. If it does get hold though, powdery mildew will cause a plant to wilt and die due to the mouldy leaf covering interrupting the plant’s ability to photosynthesize. This can mean the end of fruitful fun if not promptly addressed.
Remove any leaves as soon as you see the first spotty signs. This should help to slow the spread of the disease and buy some time. Throw any affected foliage away in household rubbish – DO NOT compost or you’ll spread spores back through your garden. You can spray powdery mildew in the early stages with Baking soda spray but, given the size of a zucchini or pumpkin plant, this can be impractical.