White Spots on Plant Leaves: Diagnostic Guide

White spots on plant leaves — how to tell powdery mildew from downy mildew, herbicide damage, and sun scald, with confirmation steps and treatment for each cause.

The diagnostic decision tree

Step 1: Is the white coating powdery and wiping off?

Rub the white material between your fingers. If it comes off easily and leaves a slightly grayish smear — that's the fungal mycelium of powdery mildew. If the surface feels rough or the spots don't rub off — proceed to step 2.

Step 2: Are the white spots discrete spots or a diffuse coating?

Powdery mildew starts as small circular white patches that expand and merge into a general coating. Downy mildew produces angular yellow spots on the upper leaf surface with gray or purple-gray fuzzy growth on the lower leaf surface — not typically white on top. Herbicide damage produces irregular pale patches or bleached areas that don't follow the typical spreading pattern of a fungal disease. Sun scald produces bleached, papery patches on the sun-facing side of leaves only.

Step 3: Which side of the leaf?

Powdery mildew: primarily upper surface, eventually both sides. Downy mildew: yellow or light patches on upper, fuzzy growth on lower. Herbicide damage: any distribution, often following a spray drift pattern (one side of a plant, plants near a fence or road). Sun scald: always the sun-exposed surface.