Diseases & rots

Powdery mildew

A white, flour-like coating on leaf surfaces that wipes off, thriving in still air with humid nights.

What you'd see

White or grey powdery patches on upper leaf surfaces that smear when rubbed
Patches spreading to stems and buds; leaves yellowing and dropping beneath
Distorted new growth in bad cases

Where to look

Crowded plants in still, humid air with big day–night swings. Begonias, African violets, kalanchoe and herbs indoors.

How it spreads

Wind-borne spores; each species of mildew tends to stick to its own plant family.

Is it really this? Look-alikes

Mealybugs — Mealybugs are tufted and in crevices; mildew is a flat film on the leaf surface.
Hard-water spots — Mineral spots are ring-shaped and do not spread.

What's going on

Powdery mildews are fungi that grow on the surface of the leaf and push feeding threads into the outer cells. Unlike most fungi they do not need wet leaves — humid air and stagnant conditions are enough.

The classic setup is warm days, cool humid nights and no air movement.

Why?

Because this fungus lives on the surface and hates flowing air, the remedies are about the surface and the air: wipe it, change the surface chemistry, and move the air. A fan does more than most sprays.

Grandmother's remedies

What people have always done — with an honest note on each.

Milk spray
Milk diluted 1:9 with water, sprayed weekly in the morning.
Verdict: Surprisingly well-supported by trials on cucurbits and roses; works as a preventive and on early cases.
Baking soda spray
Half a teaspoon of baking soda and a drop of soap per litre.
Verdict: Raises leaf-surface pH; modest effect, can mark sensitive leaves.
Remove worst leaves
Pick off heavily coated leaves and bin them.
Verdict: Sound — removes spore factories.

Modern remedies

What growers and extension services reach for today.

Potassium bicarbonate
Used per label as a spray.
Verdict: Better than baking soda; effective on contact.
Sulfur or horticultural oil
Applied per label, not in heat or on oil-sensitive plants.
Verdict: Effective; oil and sulfur must never be used within two weeks of each other.
A small fan
Gentle continuous air movement across the plants.
Verdict: One of the most effective changes you can make.

Honest verdict

Mildew rarely kills a houseplant but it will keep returning while the air is still. Space plants out, move the air, and treat early.

Prevention

Space plants so leaves do not touch
Air movement — a cracked window or a small fan
Water the soil, not the leaves, in the evening
Choose resistant varieties of herbs and begonias