Tell me what kind of plant it is and what you can see. You get the causes that fit — the ones that are not a disease first, because water, sun, cold and a stray spray cause more trouble than any fungus — each with the one check that tells it apart and what to do, in the order the extension services recommend.
Vegetable or herb · spots or blotches on leaves · started low and is moving up
Check first: early blight (tomatoes, potatoes)3 causes on our list fit what you described. They are in the order you should check them — the cheapest check first, and what is not a disease before what is. None of it is a diagnosis; the check is.
This lists what the extension services say fits what you describe, in the order to check it. It is not a diagnosis — when the checks do not settle it, a plant clinic will, for a small fee. No product here is named for a rate or a mix: the label on the product is the only place for that.
Causes and sources checked 2026-09-25.
Photo diary — is it spreading?
Photos are shrunk and kept in this browser, on this device. Nothing is uploaded. Take one today and another in a week: the two appear side by side.
Send a sample to your state’s plant clinic
When the checks do not settle it, the land-grant university clinics will — usually for a modest fee. Pick your state.
How to take it and pack it
Quick answer: when a plant looks sick, check the water first, then the light and the cold, then for insects, and only then for disease. Soggy soil and bone-dry soil both make leaves yellow and droop; a cold night blackens a houseplant; a lawn weed-killer drifting over the fence twists tomato leaves. Those are more common than any fungus, and they are the cheapest to rule out — which is why this tool puts them first.
How to use it
Say what kind of plant it is, what you see, the pattern, and how the soil feels two inches down. You get the causes that fit all of it, in the order to check them — what is not a disease first, then pests, then diseases — each with the one check that tells it apart from the look-alikes and what to do, in the order the extension services recommend: change how it is grown, then deal with it by hand, then let natural enemies help, and a product only last, by its label. Every cause is written in our words from the University of Maryland Extension page it names, and dated.
Underneath are two helpers. The photo diary keeps a photo on your device and puts it beside next week’s — “is it spreading?” is the question every guide asks next. The lab finder gives your state’s plant clinic and how to take a sample when the checks do not settle it.
The pattern tells you more than the spots
Which leaves? Older, lower leaves yellowing first points to nitrogen — the plant moves it from old leaves to new ones. New leaves yellow with green veins points the other way: iron, usually locked up by soil that is too alkaline, often beside a concrete path or foundation. A soil test settles both, and Maryland suggests one every three years anyway.
How many kinds of plant? If the tomatoes, the beans and the roses all went wrong at once, it is almost never a disease — diseases pick their hosts. Maryland’s three signs of weed-killer drift are exactly this: more than one kind of plant affected, damage on one side only, and nothing else explaining it.
How fast? Overnight damage is weather or chemistry. Weeks of slow spread from the bottom up is the classic pattern of early blight and Septoria on tomatoes. And a tomato that collapses in days in cool, wet weather, with white fuzz under the leaves, may be late blight — the one on this list you should report, because it spreads to neighbours and farms.
Water: the cause of most of it
Too much and too little look the same from across the garden: wilting, yellow lower leaves, brown edges. The difference is in the soil. Push a finger two inches down, or lift the pot. Waterlogged roots cannot take up water, so an overwatered plant wilts in wet soil; the fix is to let it dry and make sure water drains out of the bottom. A plant that droops at three in the afternoon and stands up again by evening, in moist soil, is simply hot — Maryland says short wilting does not harm it.
Pests you can see
Turn a leaf over. Aphids cluster on the undersides and new shoots and leave sticky honeydew below; a strong spray of water knocks them off, and lady beetles do the rest. Spider mites leave tiny pale dots — tap a leaf over white paper and watch the specks move. Hornworms strip tomato branches in late summer and are big enough to pick off by hand; leave any carrying white cocoons, which a parasitic wasp is already dealing with.
About products
The tool names the kind of product and never a rate or a mix: the plant and the problem must both be on the label, and the label is followed exactly. Maryland adds a caution worth repeating — plants badly damaged by spider mites can be hurt further by sprays, soaps and oils included. For fungus on palms specifically, see white fungus on palm trees.
What it does not do
It does not identify the plant, and it does not read photos. It does not cover fruit trees — they need a lab more often than a list — or lawns, which have their own Lawn Doctor. And it gives no likelihoods: the order is the order of the checks, cheapest first, never a guess at odds.