PlantPalYellow patches with fuzz underneath
Flip the leaf over. Fuzzy growth directly below a yellow patch can point to downy mildew.
What to check first
Look for pale or angular patches on top and grayish or purplish fuzz in the corresponding areas underneath, especially in damp weather. Different hosts can show different signs.
What to do now
Move a potted plant away from its neighbors if possible. Space plants out, water the soil and remove a few badly affected leaves while they are dry. Keep healthy foliage.
What to watch next
Check both sides of new leaves after a few days. If similar patches continue to appear, ask for a diagnosis for that host rather than assuming a powdery mildew treatment will work.