Plant Disease Checker

Why plant diseases are hard to diagnose

The same symptom — yellow leaves, brown spots, wilting — can be caused by a fungus, a bacterium, a virus, a pest, a nutrient shortage, overwatering or a cold night. That is why this checker ranks possibilities instead of declaring an answer, and why the best advice it can give is to confirm the diagnosis before treating. A few details help narrow it down a lot: where the problem started (lowest leaves first usually means a soil-borne disease splashed up by rain; new growth first often means a nutrient or pest issue), how fast it is spreading (late blight can take a tomato plant in days; powdery mildew takes weeks), and the weather (most fungal diseases need wet leaves, while powdery mildew prefers warm, dry days with poor airflow). A photo of the whole plant and a close-up of the symptoms is what an extension service will want to see.

First aid for a sick plant

Whatever the disease turns out to be, the first steps are the same. Remove the affected leaves and stems and put them in the bin — not the compost heap, where many diseases survive. Do not handle healthy plants with the same hands or tools until you have washed them. Improve airflow around the plant by spacing, pruning or moving it, and water the soil rather than the leaves, in the morning so any splashes dry during the day. Stop fertilising a sick plant: soft new growth is more vulnerable, and a stressed plant cannot use the feed. For houseplants, isolate the plant from its neighbours so the problem does not spread across the shelf. These steps alone often stop a mild fungal problem without any spray at all, and they are the right first move even if you later decide a treatment is needed.

Prevention: the disease triangle

Plant pathologists talk about a “disease triangle”: a disease needs a susceptible plant, a pathogen, and the right conditions — usually wet leaves and the right temperature. Remove any one side of the triangle and the disease cannot take hold. Choose resistant varieties where they exist (many tomato and rose varieties are bred for resistance), rotate vegetable crops so soil-borne diseases do not build up in one bed, and space plants so air moves between them. Mulch stops soil splashing onto lower leaves, which is how early blight and many leaf spots start — the mulch calculator works out how much you need. Water in the morning at the base of the plant, not overhead in the evening. Clean up dead leaves and spent plants at the end of the season instead of leaving them to harbour disease over winter. And buy healthy plants: inspect new purchases for spots and mould before you bring them home.

When the problem is not a disease

Before you treat for disease, rule out the simpler causes. Yellow leaves with wet soil are usually overwatering or root rot; yellow leaves with dry soil are usually underwatering. Brown leaf tips are often dry air, salts in tap water or over-fertilising. Spots that appear after a cold night may be cold damage, and pale, stretched growth is usually a light problem — check with the sunlight calculator and the humidity and temperature checker. If insects are present, use the pest identifier helper instead. If the plant is simply weak and pale, the watering calculator and fertiliser calculator help check the basics. And when in doubt, ask your local extension service — a confirmed diagnosis is worth more than any guess.