Courgette powdery mildew with large leaves: prevention and control

TL;DR - Courgette powdery mildew approach

Your courgette has white powdery spots on large leaves? That is powdery mildew (Podosphaera fusca). Prevention: good air circulation, water at roots (not leaves), proper spacing. Early intervention: remove affected leaves, spray with baking soda (1 tablespoon per liter water) or real fungicides. The sooner the better.

Why does courgette get powdery mildew?

Powdery mildew (Podosphaera fusca) is a fungus that attacks courgettes (and cucumbers, melons). The fungus grows on the OUTSIDE of the leaf - no penetration. That is why you see the characteristic white coating.

Why now? Courgettes grow fast and develop large leaves. Those leaves touch each other, humidity lingers, air does not flow. Perfect for fungus. By August it worsens because:

1. Leaves are large and crowded
2. Evening temperature drops, dew forms
3. Air is more humid (autumn)
4. The plant is tired from producing

Powdery mildew thrives at:

• High humidity (60-90%)
• Temperatures 18-25 degrees Celsius
• Still air
• Wet leaves

How do you recognize powdery mildew?

Symptoms:

• White, powdery coating especially on top of leaves
• Starts as small round spots
• Grows into larger patches
• Feels like powder (spores)
• Leaves turn yellow, then brown, drop off
• FIRST appears on old large leaves at the bottom

This is NOT the same as true fungal infections that penetrate (late blight, true mildew). Powdery mildew sits ON TOP.

How do you prevent powdery mildew?

Spacing and air

This is the main prevention. Courgettes need space.

• Plant spacing: Minimum 60 cm between plants. Better: 80-100 cm. Many gardeners plant too densely.
• Vertical growing: Grow courgettes upward on a trellis or string instead of sprawling. More air underneath, better against mildew.
• Pruning for airflow: Remove some bottom large leaves (at least 20 cm above ground) so air circulates.
• No plastic mulch: Black plastic traps moisture. Use straw mulch instead.

Watering

This is CRITICAL and many gardeners do it wrong.

• Water at roots, NOT leaves: This is key for anything that gets mildew (courgette, cucumber, melon). Water the soil, not the foliage.
• Early morning: If leaves accidentally get wet, do it in morning when sun arrives. It dries quickly.
• No evening watering: Wet leaves at night + night = mildew. Easy mistake.
• Drip line or emitter hose: Ideal. Water goes directly to soil.

Ventilation

• Open growing: Full sun, not shade. Shade means more humidity.
• Windbreaks away: Yes, windbreaks give protection, but still air helps mildew. Find balance.
• Air above soil: If growing in a greenhouse, ensure vents can open.

Nutrition

Healthy plants resist disease better.

• Potassium: Strengthens cells. Give tomato fertilizer once weekly (contains potassium).
• Not too much nitrogen: Excess promotes leaf growth, less resistance.
• Sulfur-containing products: Some use sulfur preventively. Very effective.

Treatment step by step

Step 1: Cut off what you can

As soon as you see mildew:

1. Remove all heavily affected leaves. Not halfway. Completely.
2. Throw them in the trash (NOT compost - spores survive).
3. Be careful: remove only leaves with HEAVY infection. The plant still needs foliage for photosynthesis.
4. Leave at least 30-40% of the green.

Step 2: Actions against regrowth

Now that you have cleared space, prevent return:

1. Baking soda (sodium bicarbonate): Mix 1 tablespoon per liter water. Spray all leaves (top and bottom). Repeat every 7-10 days. Works well if you act quickly.
2. Use sulfur fungicide: Available at garden centers (Nesulit, Asbex, etc.). Sulfur works better than baking soda. Follow instructions. Do NOT spray above 25 degrees Celsius (leaf burn).
3. Biological options: Bacillus subtilis (Serenade). Also effective, more environmentally friendly.

Step 3: Air and humidity management

Continue with what you have trimmed:

1. Ensure airflow (see prevention above)
2. Water only in morning at soil level
3. Mulch well with straw (no plastic)
4. Give potassium once weekly

Step 4: Repeat spraying

Spray every 7 days until mildew is gone. Sounds like a lot, but mildew spreads fast.

What does NOT work well

• Washing with water: Does nothing. Fungus is stuck on.
• Milk: Some swear by it. Research shows: mostly not effective.
• Natural oils: Neem, tea oil: against spider mites, not mildew.
• Cinnamon or baking soda without soap: Not effective.

Specific courgette varieties and sensitivity

Susceptible to mildew:

• Black Beauty
• Jade
• Classic dark green varieties

More resistant:

• Lungo
• Pattypan
• Round courgettes (mildew less visible)

Round varieties have the advantage that leaves get more air. If you fear mildew: choose a resistant variety.

Frequently asked questions

Can I eat courgettes from a plant if there is mildew on the leaf?

Yes. Mildew is on the leaves, not in the fruit. The courgette itself is perfectly edible.

Can I prevent mildew with biological predators?

Not really effective. Predators help against spider mites, not mildew. Mildew needs chemical or sulfur treatment.

How long does a courgette plant last after mildew?

Depends on severity. Mild mildew: plant stays productive. Severe: leaves drop, plant weakens, becomes worthless much sooner. Hence: early intervention matters.

If I have mildew do I need to clean tools?

Yes. Fungal spores stay in soil. For next season:

• Do NOT plant courgette in the same spot
• Remove all plant material from where mildew was
• Wash all garden tools clean

Why does courgette get mildew but tomato does not?

Courgette (and cucumber, melon) have large thick leaves and warm surroundings that mildew loves. Tomato is less susceptible (different leaf). Tomato struggles more with late blight (Phytophthora).

Step-by-step

Step 1: Recognize mildew early

Check bottom leaves every 2-3 days. White powdery spots? Mildew.

Step 2: Cut off affected leaves

Remove leaves with heavy mildew. Throw in trash.

Step 3: Spray baking soda or sulfur

Mix baking soda 1 tablespoon per liter water. Spray all leaves. Repeat weekly.

Step 4: Improve air circulation

Cut some large leaves for ventilation. Water only in morning at soil level.

Step 5: Maintain weekly spraying

As long as mildew is visible: spray every 7 days.

Discover your garden design

At [gardenworld.app](https://gardenworld.app) you can upload your vegetable plot and see how your courgettes, tomatoes, and other vegetables look in mature form. Plan spacing properly and prevent mildew through good design.