Types of Plants Affected. With the exception of gymnosperms, such as conifers, most of your plants are at risk of a powdery mildew infection if the conditions are right. It can grow on the top of the leaf or on the underside. Powdery mildew fungus favours high humidity and dry conditions. Mold or powdery mildew is a gray or white like substance that can be furry or powdery and grows on the leaves. It can spread rather rapidly so it is best to treat it as soon as you notice it. Powdery mildew is a common fungal disease of cucurbits that occurs in zucchini plants, as well as cucumbers, muskmelons, gourds and pumpkins. Water splash and air currents spread the spores of powdery mildew which grows on the surface of the leaf only. Powdery mildew on tomato plants often begins with pale yellow leaf spots that develop into the typical white, powdery growth. https://aussieorganicgardening.com/2012/01/powdery-mildew-on-zucchini Powdery mildew is a fungus which spreads a white or ash-grey film over the upper and lower surfaces of the leaves of plants-usually the older leaves. Powdery Mildew.