Downy Mildew
Disease · Commonly affects: Cucumbers and other cucurbits, Basil, Lettuce, Spinach, Grapes, Onions
Downy mildew is a group of foliar diseases caused by fungus-like water molds (oomycetes) that thrive in cool, damp, humid weather. It matters to home gardeners because it spreads fast during wet spells and can strip the leaves off a healthy plant within days, cutting yields and sometimes killing young seedlings outright.
How to identify it
Look for pale green to yellow blotches on the upper leaf surface, often boxed in by the leaf veins so they take on an angular, blocky shape. Flip the leaf over and you will usually see a fuzzy gray, purplish, or downy growth on the underside directly beneath those spots, which is the telltale sign. Unlike powdery mildew, which sits as a white dusting on top of the leaf, downy mildew's fuzz is on the underside and shows up in damp, cool conditions.
Damage
The infection destroys leaf tissue, so spots dry to brown, leaves curl and drop, and the plant loses the foliage it needs to feed itself. Heavy outbreaks can defoliate a plant quickly, stunt growth, reduce fruit size and flavor, and leave exposed fruit prone to sunscald, while seedlings may collapse entirely.
Prevention
Choose resistant or tolerant varieties when they are offered, since seed catalogs list them for cucumbers, spinach, lettuce, and basil. Space and stake plants generously and rotate susceptible crops to a new bed each year so air moves freely and spores do not build up in one spot. Keep the garden clean by removing fallen leaves and old plant debris at season's end, and build healthy, well-drained soil so plants grow vigorously and dry out quickly after rain.
How to manage it
Water at the base of the plant with a soaker hose or drip line early in the day so leaves dry fast, and avoid overhead sprinkling that keeps foliage wet. As soon as you spot infected leaves, prune them out and pull heavily affected plants, then bag and discard that material rather than composting it. Open up crowded plants by thinning and pruning to boost airflow, and stay out of the beds while foliage is wet so you do not carry spores from plant to plant on your hands and tools.
General guidance for US home gardens, focused on prevention and non-chemical control. For a confirmed diagnosis and options specific to your area, contact your local Cooperative Extension office. Page updated Aug 4, 2026.