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Downy MildewIdentification & Treatment Guide 2026

Peronospora spp. / Plasmopara spp. · Family: Peronosporaceae

Downy mildew produces angular yellow patches on upper leaf surfaces with grayish-purple fuzzy growth underneath. Caused by Peronospora or Plasmopara oomycetes, it thrives in cool wet weather. Manage with resistant varieties, preventive mancozeb or copper sprays, good air circulation, and avoiding overhead irrigation. It requires free water to infect — keep leaves dry.

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Quick Diagnosis

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Downy Mildew

Affects: leavesstemsflowersfruit
Peak Season: Cool wet weather (10-20°C), high humidity, extended leaf wetness periods
Spread Risk: Fast
Spread By: Airborne sporangia, rain splash, contaminated seed (some species)

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Symptoms: What does Downy Mildew look like?

Visual identification guide — tap each stage for details

Pale yellow angular spots on upper leaf surface from early downy mildew — Angular yellow-green spots on upper leaf surface, bounded by veins

Gray-purple fuzzy sporulation on leaf underside from downy mildew — Diagnostic gray-purple fuzzy growth on leaf underside

Severe downy mildew causing rapid leaf browning and death — Rapid leaf browning and defoliation from advanced infection

초기 단계

Small pale green to yellow spots on upper leaf surface
Light green to yellowish spots appear on the upper leaf surface. Spots are often angular, bounded by leaf veins in some hosts (cucurbits, grape).

중간 단계

Gray-purple fuzzy growth on leaf underside
Characteristic grayish to purplish-brown fuzzy sporulation develops on the underside of leaves directly below the upper surface spots. This is the diagnostic feature distinguishing downy from powdery mildew.

후기 단계

Rapid leaf death and crop loss
Lesions expand rapidly under favorable conditions. Leaves brown, curl, and die. Defoliation can be complete within days. Fruit on affected cucurbits may show surface lesions and be unharvestable.

Causes: Why is my plant getting Downy Mildew?

Environmental triggers and disease cycle

Free water on leaves for 6+ hours (rain, heavy dew, overhead irrigation)
Cool temperatures (10-20°C depending on species)
High humidity (>80%)
Poor air circulation from dense planting
Overcast, cloudy weather preventing leaf drying

Disease Lifecycle

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Host Plant

Susceptible crop

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Pathogen

Peronospora / Plasmopara

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Conditions

Cool wet weather (10-20°C), high humidity, extended leaf wetness periods

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Infection

Penetrates tissue

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Symptoms

Visible signs

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Spread

Airborne sporangia

Often confused with: Powdery Mildew, Potato Late Blight, Tomato Late Blight. See our comparison guide.

How to Treat Downy Mildew

Tick each step as you complete it — works offline too

🌿 OrganicSafe for all farms

Organic Treatment Steps

4 steps

Apply copper-based fungicide (Bordeaux mixture 0.5% or copper hydroxide 2 g/L) preventively every 7-10 days during risk periods.
Spray potassium phosphonate (phosphorous acid) at 3 ml/L for systemic protection.
Remove and destroy infected leaves and plant debris to reduce spore source.
Use drip irrigation exclusively — never overhead irrigation. Water in the morning if overhead is unavoidable.

🧪 ChemicalUse protective equipment

Chemical Treatment Steps

4 steps

Apply mancozeb (Dithane M-45) at 2.5 g/L as protective spray every 7-10 days during cool wet weather.
Use metalaxyl + mancozeb (Ridomil Gold MZ) at 2.5 g/L for systemic + protective action.
Alternate with dimethomorph (Acrobat MZ) or mandipropamid (Revus) to manage resistance.
Begin sprays BEFORE disease appears when weather forecasts indicate cool, wet conditions.

Prevention & Seasonal Calendar

Year-round protection plan for your crops

Pre-Planting

Plant resistant varieties when available
Use drip irrigation — avoid wetting foliage
Space plants for good air circulation

Mid-Season

Ensure morning sun exposure to dry dew quickly
Remove crop debris after harvest — oomycetes survive in residue
Avoid working in fields when foliage is wet to prevent mechanical spread

Post-Harvest

Begin protective fungicide sprays before onset of cool wet weather

🛡️ Resistant Varieties

Varies by crop — check with local extension

View full seasonal calendar for your region →

Frequently Asked Questions

Common questions about Downy Mildew

What is the difference between downy mildew and powdery mildew?

Downy mildew produces fuzzy gray-purple growth on the UNDERSIDE of leaves and requires wet, cool conditions. Powdery mildew produces white powder on the UPPER leaf surface and thrives in warm, dry conditions. They are caused by completely different organisms — downy mildew by oomycetes (water molds) and powdery mildew by true fungi (Ascomycota).

Can downy mildew survive between seasons?

Many downy mildew species produce thick-walled oospores that survive in soil and plant debris for years. Some species are also seed-borne. This survival ability means the pathogen can re-emerge from residues each season when conditions become favorable.

Is the fuzzy growth under leaves infectious?

Yes, the fuzzy growth consists of sporangiophores bearing sporangia (spore-containing structures). These sporangia are released into the air and spread by wind and rain to new leaves, where they infect if free water is present for sufficient time (usually 6-12 hours).

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Related Disease

Powdery Mildew

Erysiphe spp. / Podosphaera spp.

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Related Disease

Potato Late Blight

Phytophthora infestans

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Affected Plant

Maize (Corn)

Zea mays

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Treatment Guide

Full Treatment Protocol

Detailed spray schedule & dosage guide

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