Downy Mildew

• Angular yellow patches on the upper surface of leaves — bounded by leaf veins
• Gray or purple fuzzy sporulation on the underside of the same patches
• Rapid progression in cool, humid weather with heavy dew
• Infected tissue browns and collapses quickly
• Often confused with powdery mildew — but downy mildew grows on the underside, powdery on the surface

Downy mildew is caused by oomycetes (water molds), not true fungi. It requires leaf wetness and cool temperatures (50-70°F) to infect. Spores travel by wind and can spread from farm fields to home gardens. Cucumber downy mildew (Pseudoperonospora cubensis) is tracked regionally — spore forecasting maps are available online in the eastern US. Each downy mildew species is largely host-specific.

1. 01Avoid overhead watering; water in the morning so foliage dries by midday
2. 02Improve airflow through plant spacing and trellis training
3. 03Remove infected leaves as soon as they appear; dispose of, do not compost
4. 04Apply copper-based fungicide preventively when regional spore maps show active movement into your area
5. 05Plant resistant varieties where available — cucumber downy mildew resistance has improved substantially in recent years
6. 06Rotate out of susceptible crops for a season in beds with recurrent issues

If this keeps happening in your garden, the single most effective change is often the seed packet. These varieties carry documented resistance.

If downy mildew hits your cucumbers or lettuce hard two or more seasons in a row, an extension vegetable specialist can advise on currently resistant varieties in your region — resistance in downy mildew can shift as new pathogen races emerge.

• Cucumber Downy Mildew Forecasting— Cucurbit Downy Mildew ipmPIPE
• Downy Mildew on Vegetables— University of Maryland Extension