Fungal Diseases in Plants: Identifying and Treating Mildew, Root Rot, and More

Besides pests like aphids and spider mites, there’s a second major threat to houseplants: fungal diseases. Powdery mildew, root rot, and leaf spot disease can seriously damage your plants in a very short time. In this article, you’ll learn how to recognize the most common fungal diseases and what you can do about them.

Powdery Mildew: When Leaves Look Dusted in White

Powdery mildew is one of the most common fungal diseases and easy to recognize: a white, powdery coating on the leaves that can be wiped off. Despite its name, powdery mildew actually prefers warm, dry conditions — it’s sometimes called the “fair-weather fungus.”

Susceptible plants: Roses, cucumbers, and zucchini, but also houseplants like begonias and African violets.

Fighting it with home remedies:

• Milk and water mixture: Mix 1 part fresh whole milk with 8 parts water and spray the plant every 2–3 days. The lactic acid bacteria in the milk fight the fungus.
• Baking soda solution: Dissolve 1 teaspoon of baking soda in 1 liter of water, plus a dash of dish soap. The alkaline solution destroys the fungal cells.
• Garlic infusion: Steep 2 crushed garlic cloves in 1 liter of water for 24 hours, strain, and spray onto the plant.

Downy Mildew: The Sneaky Sibling

Downy mildew appears as a gray-purple fungal growth on the underside of the leaf, while the top of the leaf shows yellow spots. Unlike powdery mildew, it loves moisture and cool temperatures.

Control: Remove affected leaves immediately (dispose of them in the household trash!). Improve air circulation, and avoid watering the plant from above. With a severe infestation, only fungicides from a garden center will help.

Root Rot: The Silent Killer

Root rot is caused by various soil-borne fungi and is especially insidious because the symptoms only become visible when it’s almost too late. The cause is almost always waterlogging — watering too often, poor drainage, or pots without drainage holes.

Symptoms:

• Wilted, limp leaves — even though the soil is moist (this confuses a lot of people!)
• Yellow leaves that drop off
• Soft, mushy stem base
• Musty, rotten smell coming from the soil
• Brown, mushy roots when you remove the plant from its pot

Emergency measures:

1. Remove the plant from its pot immediately and cut off all rotten, brown roots with clean scissors.
2. Carefully rinse the root ball under lukewarm water.
3. Repot into fresh, dry soil and a clean pot with good drainage.
4. Do NOT water for the first week — the roots need to dry out and heal first.
5. As prevention, add a layer of expanded clay pebbles at the bottom of the pot for drainage.

Leaf Spot Disease: Brown Spots with a Yellow Halo

Small brown or black spots on the leaves, often surrounded by a yellow halo — this is typical leaf spot disease. It’s caused by various fungi and bacteria and occurs especially in high humidity.

Control:

• Remove and dispose of affected leaves immediately.
• Don’t water from above — wet leaves encourage the disease to spread.
• Don’t crowd your plants too closely together — good air circulation helps prevent it.
• Fungicides from a garden center can help with a stubborn infestation.

Gray Mold (Botrytis): A Fuzzy Coating

Gray mold appears as a gray, fuzzy coating on leaves, flowers, and stems. It occurs especially in high humidity and poor ventilation — typical for plants that are too crowded, or potted plants overwintering in a basement.

Control: Remove affected plant parts immediately, improve air circulation, and lower humidity.

General Prevention Against Fungal Diseases

• Don’t water from above — water on the leaves encourages fungi.
• Ensure good drainage and avoid waterlogging.
• Leave enough space between plants for good air circulation.
• Regularly remove dead leaves.
• Only use clean, disinfected tools.
• Always dispose of diseased plant parts in the household trash, never on the compost pile!