Plant disease identification, symptom by symptom

Tell a disease from a watering, light or pest problem

Four questions sort most cases before you reach for a disease name.

1. Is the whole plant affected at once? Care problems hit the plant evenly or by position: every lower leaf, the side facing the window, the leaves that touched the cold glass. Infectious diseases usually start on a few leaves and spread from there. The Maryland Extension page puts it this way: noticing the pattern and progression of symptoms is what leads to the cause.
2. What does the soil say? Push a finger into the soil below the surface. Clemson lists yellowing, wilting and leaf drop as overwatering symptoms, because waterlogged soil cuts the oxygen roots need; soft, dark roots mean roots are dying. Bone-dry soil with crisp, curled leaves is the opposite problem. Our yellow leaves guide walks through the watering and light causes in order.
3. Can you see an animal or its traces? Look under leaves and along stems with a phone flashlight. Insects, eggs, webbing, honeydew, slime trails and droppings are signs of pests, not disease. The RHS notes that slugs and snails feed at night, so ragged holes and slime trails are often all you will find in the morning.
4. Did something change recently? A move, a repot, a cold spell, a new fertilizer or a window opened in winter often comes a week or two before the symptoms. Clemson lists chilling and cold water among the causes of leaf drop and pale patches.

If the answers point to care, fix the care and watch the new growth. Leaves already damaged by spots or scorch will not turn green again, so judge progress on the leaves that appear next.

First steps the sources agree on

Whatever the cause, these non-chemical steps come up again and again in the extension sources cited here:

• Remove affected leaves. Clemson advises removing diseased material for both bacterial and fungal spots; the RHS recommends pruning out mildew-infected shoots promptly and clearing fallen infected leaves. Bag them or put them in the trash rather than leaving them on the soil.
• Keep water off the leaves. Water the soil, not the foliage, especially indoors; wet leaves spread bacterial and fungal spots.
• Improve airflow and light. Space plants so leaves do not touch, and move shade-weakened plants into brighter light. The RHS and UC IPM both link powdery mildew to still air and shade.
• Fix drainage. Use pots with holes, empty saucers after watering, and let the top of the soil dry before watering again. Clemson suggests cutting out a few rotten roots and repotting in fresh, sterile soil; if most roots are dead, starting again with a new plant is the better choice.
• Deal with the insects behind honeydew. UC IPM suggests pruning heavily infested parts and using a strong stream of water; once the insects are under control, sooty mold fades and can be washed off. For spider mites it recommends a daily water spray on the leaf undersides and keeping plants watered so they are not drought-stressed.
• Isolate a sick houseplant from your other plants until you know what you are dealing with.

If you consider a pesticide or fungicide, follow the label and local rules, and identify the cause first: a spray does nothing for a watering or light problem.

Take a sample to a plant clinic

When a valuable plant, a whole bed or a crop is at stake, a lab answer beats any guess. In the US, start with your county Cooperative Extension office; many states run a university plant disease clinic that accepts samples. The Cornell University Plant Disease Diagnostic Clinic, for example, takes samples from homeowners to commercial growers, tests plant material for bacteria, fungi and viruses, and lists its fees on its site. Its sample collection tips are a good checklist for any clinic:

• Send several stages of damage and every affected part of the plant.
• Include the margin between dead and healthy tissue, where early symptoms are easiest to read; skip fully dead material, which carries secondary organisms that hide the real cause.
• Collect the sample before applying any pesticide.
• Wrap it loosely in an unsealed plastic bag inside a sturdy container; wrap roots separately from foliage, and do not water the plant or dampen the packing.
• Ship early in the week so it does not sit in a post office over the weekend; refrigerate it or keep it out of the sun if you cannot ship right away.
• Add photos of the damage pattern and of the whole site.

Call the clinic or your extension office before you send anything: rules, forms and fees differ by state and country.

What a photo can and can't tell

A clear photo shows a lot: the shape and color of spots, a halo, rings, a powdery film, webbing, insects large enough to see. It cannot show roots, cannot tell a wet pot from a dry one, cannot smell rot, and cannot separate diseases that look alike on the surface but need a lab to name. Take several photos: the whole plant, one affected leaf top and bottom in daylight, and the stem at soil level. Our guide to photographing a plant for identification covers light and framing.

Plant Identifier's Diagnose tool reads one photo and writes an AI suggestion: a likely cause, the affected areas, treatment steps and prevention, with a severity badge that the AI writes for that photo. Use it as a first pointer to what to check in the table above, not as a diagnosis. Ask the follow-up chat under the result what else could cause the same symptom, then confirm with the soil, the roots and the undersides of the leaves. If you are not sure which plant you have, the houseplant identification key helps you name it first, because many problems are specific to one kind of plant.