Gardening glossary
Mildew
Although gardeners often lump them together, powdery mildew and downy mildew are caused by completely different organisms and require different management. Distinguishing them is the first step to controlling them.
Powdery mildew is caused by fungi in the order Erysiphales (Podosphaera, Erysiphe, Sphaerotheca, and others). It produces a characteristic white to grey powdery coating on the upper surfaces of leaves, stems, buds, and sometimes fruit. The coating can be wiped off and is made of fungal hyphae and asexual spores. Unlike most fungal diseases, powdery mildew thrives in warm dry air with high humidity at the leaf surface but does not need free water to germinate. Crowded plants, shade, and poor airflow encourage outbreaks. It is host-specific — the powdery mildew on cucurbits will not infect roses.
Downy mildew is caused by oomycetes (water moulds) in the family Peronosporaceae, more closely related to algae than to true fungi. Symptoms appear first as pale yellow or angular patches on the upper leaf surface, bounded by leaf veins, with grey, purple, or white fuzzy sporulation on the leaf underside in humid conditions. Downy mildew needs free water — leaf wetness or high humidity above 85 percent — to germinate and spreads rapidly in cool wet weather. It can collapse a basil, cucumber, lettuce, or grape planting within a week.
Cultural prevention overlaps for both: space plants for airflow, water at the soil line in the morning, choose resistant cultivars, and remove infected debris. Powdery mildew responds well to potassium bicarbonate, sulfur, neem oil, and milk sprays applied at first sign. Downy mildew is harder to manage — copper-based fungicides applied preventively give partial protection but cannot rescue an established outbreak.