Downy mildew is caused by Bremia lactucae, a fungus-like organism called an oomycete — the same general group of organisms that includes the pathogen responsible for late blight.

This bastard likes things cool and wet. Moisture on the leaves is required for infection, and prolonged damp conditions, high humidity, dew and poor airflow create ideal conditions for an outbreak. Spores develop primarily in the white downy growth on the undersides of infected leaves and are carried to other plants by air currents during moist conditions.

The disease generally starts on older outside leaves and can spread farther into the plant. Seedlings can be severely damaged or killed. The damaged tissue can also provide an opening for secondary rot organisms, so an infected lettuce can go from spotty and disgusting to genuinely rotten.

How to Fix It

Start with cultural control. Reduce the amount of time the leaves remain wet. Water at soil level where possible, water early in the day, give plants adequate spacing, control weeds and improve airflow so foliage dries quickly.

Remove infected leaves as soon as you find them. If a plant is heavily infected, remove the whole plant. Clean up infected crop debris rather than leaving it lying around the garden.

If downy mildew is a recurring problem, look for resistant varieties. Resistance is one of the best controls available.

For organic treatment, copper-based products labelled for downy mildew may provide some protection when applied preventively or very early. But copper isn't magic here; UC IPM notes that copper treatment can be only marginally effective. Moisture management and resistant varieties are the heavy hitters.