Downy Mildew

Plant disease

Description

Downy mildew is a group of plant diseases caused by several species of oomycetes, or water molds, rather than one fungus with a single scientific name. Different pathogens infect different hosts, including cucurbits, grapes, basil, lettuce and ornamentals. The disease usually develops when leaves stay damp and humidity is high; its preferred temperature varies by pathogen. Spores spread by wind or splashing water, and new infections can develop quickly in favorable weather. The most conspicuous signs are discolored patches on leaves and pale, gray or purplish downy growth, often on their undersides. Severely affected leaves may brown, collapse and fall, reducing plant vigor or harvest. Some downy mildews also affect stems, flowers or fruit, depending on the host. Because these diseases are host-specific and can resemble other leaf problems, identify the affected plant before choosing a control method.

Symptoms

• Yellowing leaves
• Spots on leaves
• Fuzzy gray mold
• Wilting
• Leaves dropping
• Stunted growth

How to Identify

Check both sides of affected leaves, especially early in the morning or after humid weather. Look for yellow, pale green or brown patches above, with fine white, gray or purplish downy growth directly beneath them. On cucurbits, patches are often angular because leaf veins limit their spread; patterns differ on other hosts. Powdery mildew usually makes a more conspicuous, powdery coating on leaf surfaces, while gray mold tends to produce thicker fuzz on dying tissue. Nutrient deficiencies can yellow leaves but do not produce downy growth. If sporulation is absent or symptoms are ambiguous, a local extension service or plant diagnostic laboratory can help confirm the cause.

Prevention

Choose resistant varieties when available, and start with healthy plants. Give plants enough space for air circulation, control weeds and avoid crowding under covers or in greenhouses. Water at soil level when practical; if overhead watering is necessary, do it early enough for foliage to dry. Avoid handling plants while leaves are wet. Inspect susceptible crops regularly, particularly during humid weather, so infections are noticed early. Remove volunteer host plants and manage nearby susceptible plants that may harbor the disease. Clear infected crop debris after harvest and follow host-specific guidance on rotation; rotation alone may not prevent spores arriving on the wind. In protected growing areas, improve ventilation and reduce persistent condensation. Where outbreaks recur, consult local forecasts or extension alerts for the affected crop.

Treatment

Begin by removing badly affected leaves or plants if the outbreak is limited. Bag infected material rather than leaving it beside the crop, and avoid working among wet plants. Improve airflow and keep foliage as dry as practical; these steps slow spread but cannot heal damaged tissue. Inspect remaining plants frequently. If the disease continues to spread or the crop is valuable, consider a product specifically labeled for downy mildew on that plant. Some copper-based products are options for home gardeners, but they work mainly as protectants, can damage foliage and may affect soil organisms; follow the label closely. Other registered fungicides may be available for particular crops and regions. Apply only as directed, observing harvest intervals and restrictions, and rotate modes of action where labels recommend it to reduce resistance. Severely infected annual plants are often best removed. If the diagnosis is uncertain, confirm it before spraying.

Plants Affected

Arugula, Basil, Bitter Melon, Bottle Gourd, Broccoflower, Broccoli, Chayote, Chives, Collard Greens, Cucumber, Endive, Fava Bean, Garlic, Grapes, Hebe, Impatiens, Ivory Gourd, Lettuce, Millet, Onion, Pansy, Pea, Poppy, Pumpkin, Quinoa, Radicchio, Ridge Gourd, Shallot, Snake Gourd, Snow Pea, Sorghum, Spinach, Sunflower, Turnip, Watercress, Winter Melon

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