Pick the symptom that matches your plant, not the plant first — the same symptom often has the same handful of causes across completely different species. Each one links straight to the relevant section of the full species guide.
Almost always a watering issue — usually too much, occasionally natural aging of an older leaf.
Usually underwatering, low humidity, or mineral buildup from tap water and fertilizer.
Could be fungal leaf spot, cold-water damage, or physical/pest damage — the shape and edges of the mark are the tell.
The one to act on quickly — almost always overwatering or water sitting where it shouldn't.
Usually a water-stress signal in one direction or the other — check the soil before changing anything.
A light problem almost every time — the plant is reaching for something brighter, not asking for food.
New growth coming in plainer or greener than the rest of the plant — a light issue, not a disease.
Almost always about light — either not enough of it, or a recent change the plant hasn't adjusted to yet.
Things that look alarming but are just how the plant behaves. Worth ruling out before you do anything.
Don't see your plant yet? The field guide currently covers nine species — browse the full list. More are added regularly.