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🌿 SOS Plants  /  Powdery Mildew and Fungal Diseases

SOS Plants · Diseases

Powdery Mildew and Fungal Diseases:
causes, treatment
and prevention

7 min read · Updated June 2025

SeverityModerate to urgent
⏱️Treatment2 to 4 weeks
🌱Salvageable?Yes if detected early

White powder on leaves, black spots, grey mold, or a musty-smelling substrate — these are all symptoms of fungal diseases. Fungi are among the most common enemies of indoor plants, and powdery mildew is their most visible representative. Unlike pests, fungal diseases don't come from outside — they develop from spores already present in the air or substrate, which find favorable conditions to proliferate.

This guide covers the 4 most common indoor fungal diseases, their causes, and effective treatments for each.

Understanding Fungal Diseases

Fungal diseases are caused by microscopic fungi that grow on or within plant tissues. Their spores are ubiquitous in the air — it's not their presence that's the problem, but when conditions allow them to multiply: excessive humidity, poor ventilation, plant injuries, waterlogged substrate.

The key is twofold: treat the existing infection AND modify the conditions to prevent it from returning.

"Fungi don't appear out of nowhere —
they wait for bad conditions to explode."

Powdery Mildew — White Powder on Leaves

⬜

Powdery Mildew (Erysiphales)

Moderate — treatable

Powdery mildew is the most recognizable fungal disease: a powdery white substance that appears on the top of leaves, initially as small spots then spreading. Unlike other fungal diseases, powdery mildew thrives in conditions of low air humidity combined with high temperatures — which makes it paradoxical and often surprising.

White powdery substance on the top of leaves Leaves yellowing under the spots Starts on young leaves Warm, low-humidity air Rapid progression if left untreated
✔ Treatment Remove and discard heavily affected leaves. Prepare a solution: 1 tablespoon baking soda + a few drops of liquid soap in 1 liter of water. Spray on all affected leaves. Alternate with a neem oil-based solution. Repeat weekly for 3 weeks. Improve ventilation around the plant.
✔ The baking soda test Baking soda alters the pH of the leaf surface, creating an environment hostile to fungi. It's the most effective natural treatment for powdery mildew — simple, economical, and safe for plants.

Grey Mold — Botrytis

🔘

Botrytis cinerea

Urgent — spreads quickly

Grey mold is a fuzzy grey-brown mold that develops on weakened or injured stems, leaves, and flowers. It thrives in cold, humid, and poorly ventilated environments. It's one of the most destructive diseases as it can invade an entire plant in a few days if conditions are right — especially dangerous in winter.

Grey-brown fuzz on stems or leaves Soft, waterlogged tissues under the fuzz Grey spores that disperse when touched Flowers rotting quickly Musty odor
✔ Emergency Treatment Isolate immediately. Cut and discard all affected parts in an airtight bag — do not compost them. Disinfect tools after each cut. Drastically reduce watering and improve ventilation. Treat with an approved fungicide if the infection is advanced. Botrytis is very difficult to eliminate without severe pruning.

Sooty Mold — Sticky Black Mold

⬛

Sooty Mold (Capnodiales)

Secondary — sign of infestation

Sooty mold is a black mold that develops on the honeydew left by sap-sucking insects — mealybugs, aphids, or whiteflies. It doesn't directly attack the plant but covers the leaves with a black layer that blocks photosynthesis. Treating it without eliminating the causal insects is useless — it always returns.

Black sooty deposit on leaves Sticky surface under the deposit Associated presence of mealybugs or aphids Leaves weakening and yellowing
✔ Two-step Treatment 1/ First, eliminate the responsible insects (mealybugs, aphids). Without this, the sooty mold will systematically return. 2/ Clean the leaves with a damp cloth soaked in soapy water to mechanically remove the black mold. On heavily affected leaves, a 50% diluted alcohol solution can be used.

Root Rot

🟤

Pythium / Phytophthora

Urgent — often fatal if late

Root rot is caused by aquatic fungi that proliferate in constantly waterlogged substrate. Roots blacken, soften, and rot, unable to conduct water and nutrients. On the surface, the plant shows paradoxical symptoms of dryness despite moist soil — soft leaves, yellowing, foliage drop.

Soft and withered plant despite moist soil Musty smell from the pot Soft or blackened stem at the base Brown, soft, foul-smelling roots when repotting Too frequent watering beforehand
✔ Emergency Treatment — immediate repotting Unpot and inspect the roots. Cut off all brown or soft roots with a disinfected tool until healthy white tissue. Let the roots air dry for 30 minutes. Repot in completely fresh, well-draining substrate. Water very lightly for the first 2 weeks. If more than 70% of the roots are affected, the chances of salvation are low but still try.

Conditions Favorable to Fungi

❌ What promotes fungal diseases
  • 💧 Constantly moist substrate
  • 🌬️ Stagnant air, poorly ventilated room
  • 💦 Water stagnant on leaves
  • 🌡️ Abrupt temperature changes
  • 🪴 Pots without drainage or full saucers
  • ✂️ Unhealed stem wounds
  • 🌿 Plants too close together
✅ What protects your plants
  • 💨 Good ventilation — air circulation
  • 🪴 Draining substrate that dries between waterings
  • ☀️ Sufficient light — fungi hate light
  • 💧 Bottom watering — never on foliage at night
  • ✂️ Disinfected tools for each pruning
  • 🌱 Well-spaced plants for aeration

Prevention — Air and Drainage First and Foremost

The vast majority of indoor fungal diseases can be avoided with two simple precautions: a properly draining substrate and regularly renewed air.

💨

Aerate regularly

Open windows for 10 to 15 minutes a day, even in winter. Regular air exchange prevents humidity stagnation that promotes all fungi. Avoid concentrating too many plants in a room without ventilation.

🪴

Adopt a draining substrate

A substrate that retains too much water is the ideal environment for root fungi. Add 20-30% perlite to your usual potting mix and ensure each pot has effective drainage. Empty saucers after each watering.

🌿

Never water in the evening

Water left on leaves or in the substrate overnight — without evaporation — is a major trigger for fungal diseases. Always water in the morning so excess moisture can evaporate during the day.

⚠ Never cut without disinfecting tools

Fungal spores are easily transmitted via undisinfected gardening tools. A blade used on a diseased plant then on a healthy plant can spread the disease in seconds. Use 70° alcohol on the blade before and after each use — systematically.

🌿 To treat and prevent

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🌿 Guide written by the Douceur Maison SOS Plants team.

We write practical guides to help enthusiasts care for their indoor plants. · sosplantes@douceurmaison.fr

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