Identify Plant Disease: Symptoms, Photos and Free Tools

Start by ruling out care problems, then match what you see to a disease. Most sick houseplants are not infected: the University of Maryland Extension calls overwatering the number one reason indoor plants fail, and Clemson's Home & Garden Information Center (HGIC) says yellowing of a whole plant is most often caused by overwatering. A true disease needs three things at once, which plant pathologists call the disease triangle: a susceptible host, a pathogen and a favourable environment (Wikipedia).

Work through these steps:

• Name the plant. Diseases are host-specific, so the same spot means different things on a tomato and on a rose. A photo in an identification app gives you the species in seconds.
• Look at the pattern. A University of Florida plant pathologist explains that pathogens take time to spread, so an infection usually starts on a single leaf or stem, while wilting or mushiness spread across the whole plant points to a physical cause such as water or cold.
• Check the roots. Slide the plant out of its pot. Healthy roots are white and firm; overwatered roots are soft and brown or black, and the soil may smell sour (University of Florida).
• Look for the organism itself. Powder, coloured pustules, fuzzy grey mould, sticky ooze or tiny black dots in dead tissue are signs of a pathogen, not just symptoms (Clemson HGIC; University of Minnesota Extension).
• Photograph it and get a second opinion from an app, an extension office or a diagnostic lab.

• By eye: symptoms and signs. Symptoms are the plant's reaction (yellowing, spots, wilting); signs are the pathogen you can see (mildew, rust pustules, ooze). The spot key below is based on this.
• By pattern and history. Which leaves are affected, how fast it spread, and what changed recently: watering, a move, cold, repotting. Plant Doctor, one of the photo tools ranking for this search, asks for exactly this care history alongside the photos.
• By laboratory test. Diagnostic clinics examine samples under the microscope, culture them, or run antibody and DNA tests. Wikipedia names ELISA, an antibody test, as the most widely used detection method worldwide, alongside PCR and related DNA methods. Cornell's Plant Disease Diagnostic Clinic works for homeowners as well as growers under the motto 'Test, Don't Guess'.

The University of Florida gives a quick key for leaf spots:

• Fungal: dry, papery spots, often with concentric rings like a target. Clemson adds that tiny black dots, the fungus's fruiting bodies, may be visible in the dead tissue.
• Bacterial: dark, black, wet-looking areas with angular edges. Clemson describes water-soaked spots of even size, sometimes with a yellow halo and sticky ooze.
• Viral: green spots or mottling; viral infections cannot be cured.

Not every bump is a disease. Clemson describes edema, rough corky swellings on leaf stalks and the undersides of leaves, caused by wet soil, poor light and low temperature rather than a pathogen, and cold water spots, white or straw-coloured patches from watering with cold water.

• Root and stem rot. Wilting, soft brown or black roots, and sometimes a brown or black ring girdling the stem at soil level; Clemson HGIC lists several soil fungi as causes, among them Rhizoctonia, Pythium and Phytophthora, and names overwatering and poorly draining soil as the trigger.
• Powdery mildew. White to grey powdery spots or felt-like mats on leaves, stems and buds; favoured by cool, humid nights and warm, dry days of 70 to 80 F (University of Minnesota).
• Fungal leaf spot. Small brown spots with yellowish margins, sometimes ringed, with black dots in the dead tissue (Clemson).
• Bacterial leaf spot. Water-soaked spots of even size, sometimes with a yellow halo and ooze (Clemson).
• Anthracnose. Leaf tips turn yellow, then tan, then dark brown, and the browning may spread right round the leaf (Clemson).
• Gray mould (Botrytis). Dark brown to black blight of flowers, buds, leaves and stems, with large round leaf blotches that show target-like rings; worse in wet weather and on wounded or ageing tissue (University of Minnesota).
• Rust. Yellow, orange or red spots on top of the leaf and raised, powdery pustules on the underside (University of Minnesota).
• Early blight of tomato and potato. Round brown spots up to half an inch across, with target-like rings, starting on the oldest leaves near the ground (University of Minnesota).
• Fire blight. A bacterial disease of apple, crabapple, mountain ash and other rose-family plants such as hawthorn: young shoots wilt, turn grey-green and bend into a shepherd's crook, dead brown or black leaves cling to the branches, and a badly affected tree looks scorched (University of Minnesota).
• Viral mosaic and mottling. Green spots or mottled patterns; viral infections cannot be cured (University of Florida).

Yes, several tools let you upload a photo at no charge, but treat the answer as a list of possibilities. Plant Doctor's web tool analyses one to five photos for free and states plainly that its results are possible causes, not a laboratory confirmation; it recommends asking a horticulturist or extension service when symptoms spread fast, on food crops, on rare plants, or when pets or children may be affected. PlantIn offers a disease identifier with free and paid tiers.

The sources we opened contain no independent accuracy test of disease-diagnosis features, so there is no figure to quote for how often these tools are right. The step apps do well is naming the plant. PlantLens, our app, identifies the plant from a photo; knowing the species tells you which diseases are likely and which care guide applies. See our comparison of plant identification apps.

Plant Doctor's photo checklist is a good standard for any app or for an email to an extension office:

• the whole plant in bright natural light;
• the worst symptom, close up;
• both sides of an affected leaf;
• the stems and the soil surface;
• the pot's drainage;
• any insects or residue.

Add a note of recent watering, light changes, repotting, fertiliser and temperature, and when the symptoms started. If you can, unpot the plant and photograph the roots too; the University of Florida's root check (white and firm, or soft and brown) settles many cases before any disease is involved.

Send a sample when the plant is valuable, when a food crop or a tree is involved, or when the problem keeps spreading after you have fixed watering and light. University clinics test for fungi, bacteria, viruses and nematodes; the University of Minnesota's Plant Disease Clinic serves the public on a fee-for-service basis, and Cornell's clinic publishes sample submission instructions. Many extension services also answer questions through Master Gardener programmes; the University of Minnesota invites you to submit one online.

Do not reach for a fungicide on a guess. The University of Minnesota states that fungicides will not cure existing powdery mildew or rust infections and can only protect healthy tissue, and that gray mould is usually controlled by better air flow and watering at the soil rather than sprays. Removing infected leaves, spacing plants and watering the soil instead of the foliage come first in all of their guides.

Research and drafting for this page were done with the help of AI, and every figure was then checked line by line against the sources listed at the end of the page. Sources were opened on 2026-09-25. Where sources disagree, we give each figure with its source rather than picking one. If you find a mistake, tell us and we will correct it.

A plant diagnosis app takes one or more photos, often plus a few questions about care, and returns a ranked list of likely problems. The better ones are open about that. Plant Doctor's site says its results are ranked possibilities with confidence scores, that it looks at diseases, pests, nutrient deficiencies and care history (watering, drainage, light, repotting, fertiliser, recent changes), and that it is not a substitute for professional advice on rare, valuable, edible or rapidly declining plants. It also claims 95 percent accuracy in internal validation when users supply clear photos and care context; that is the company's own test, not an independent one.

Independent evidence is thin. A 2022 study by Siddiqua and colleagues found 606 plant disease apps across three app stores, of which 17 met their criteria for evaluation. Most scored acceptably on looks, usability and performance but poorly on AI-based functions, and the authors concluded that the apps cannot be used as a complete solution to plant disease detection, diagnosis and treatment. Only one, Plantix, which is aimed at crops, managed all of the following: identifying the plant, detecting the disease, keeping a plant database and suggesting a treatment.

The limit every app shares is that it sees only what you photograph. Plant Doctor itself lists normal ageing, overwatering, nutrient stress, pests, root damage, light changes and temperature shock as possible causes of yellow leaves, and the root check described above needs the plant out of its pot.

Before you subscribe to anything, check these points:

• Does it ask for more than one photo and for care history? Plant Doctor asks for a whole-plant photo, close-ups of affected leaves, stems, soil and pests, and details of watering and light. A single leaf photo leaves out most of the evidence.
• Does it show several possibilities with a confidence level, or one confident answer? Ranked results are more honest.
• Does it name the plant first? Diseases are host-specific, so the species narrows the list.
• What is actually free? Many apps are free to download with limits on scans; read the limits on the store page before starting a trial.
• Does it tell you when to see a person? Good tools point you to an extension office or a plant clinic for food crops, trees and valuable plants.

PlantLens, our app, starts with the plant's identity: it names the species from a photo with a confidence indicator, helps you recognise common signs of stress and disease, and points you to what to check next. It is not a laboratory diagnosis; for anything serious, the university plant clinics listed above test samples.