Diagnose your plant by symptom
Start with what you see — yellow leaves, holes, spots, curling, decline, or fruit problems. Land on the likely cause in two clicks. A working decision tree, not a disease encyclopedia.
This page is a working decision tree. It does not catalog every possible disease; it routes you from what you can see to what is probably happening and what to do about it.
Start with what you see
Yellow leaves
The most common diagnostic search across the entire site. Yellow leaves can mean ten different things, but most of them are one of three: a watering imbalance, a nitrogen issue, or natural lower-leaf senescence. Disease is much further down the list than most people assume.
Quick check before reading further: if the yellowing starts at the bottom of the plant and works up, you are usually looking at a water or nitrogen issue. If it starts at the top or middle and the new growth is the most affected, it is more likely iron, manganese, or a virus.
By host plant
- Yellow leaves on tomato plants — water stress, nitrogen deficiency, septoria, or early blight; how to tell which
- Yellow leaves on cucumber plants — bacterial wilt, downy mildew, magnesium deficiency
- Yellow leaves on zucchini — borer damage, downy mildew, transplant shock
- Yellow leaves on peppers — overwatering vs nitrogen vs calcium issues
- Yellow leaves on hydrangea — chlorosis from alkaline soil, transplant stress, root rot
- Yellow leaves on roses — black spot defoliation pattern vs systemic issues
- Yellow leaves on tulips — normal post-bloom dieback vs early problem
General principles: Yellow leaves: complete guide covers the framework that applies to any plant. Read this if your plant is not in the list above.
Holes in leaves
Holes mean something is eating your plant. The shape, location, and pattern of the holes tells you what.
Decision framework:
- Ragged holes with shiny slime trails → slugs or snails (most common on hostas and seedlings)
- Round holes the size of a dime, evenly distributed → Japanese beetles or flea beetles
- Skeletonized leaves (veins remain, tissue is gone) → Japanese beetles or sawflies
- Notched leaf edges → weevils (vine, root, or strawberry root)
- Tiny pinprick holes throughout → flea beetles
- Whole sections missing → larger pests (rabbits, deer, woodchucks)
By host plant
- Holes in hosta leaves — slugs are the answer 90% of the time; iron phosphate is the safer fix
- Holes in rose leaves — sawflies, Japanese beetles, or rose chafer
- Holes in tomato leaves — hornworms, flea beetles, or stink bug damage
General principles: Holes in leaves: complete identification guide walks the full decision tree for any plant.
Brown or black spots
Defined spots are usually fungal or bacterial. The size, color, halo, and pattern are diagnostic.
Quick visual cues:
- Black spots with yellow halos, defoliation from the bottom up → black spot on roses or septoria on tomatoes
- Brown spots with concentric rings (target pattern) → early blight (tomato), alternaria (cucurbits)
- Greasy-looking dark spots that spread fast → bacterial speck or bacterial spot
- Tiny rust-orange pustules on leaf undersides → rust disease
- Powdery white coating → powdery mildew (not technically spots, but often grouped here)
By host plant
- Brown spots on tomatoes — early blight vs septoria vs late blight; how to tell the difference
- Brown spots on hydrangea — anthracnose, cercospora leaf spot, sunburn
Common diseases by host on the /problems/ index.
Curling or wilting leaves
Curling is one of the most over-diagnosed symptoms. Most tomato curl is not disease — it is heat or water stress, and the plant recovers. True viral curl is rare.
Decision framework:
- Curls up and inward, looks rolled like a cigar, leaves stay green and firm → physiological leaf roll (heat, water stress) — usually no action needed
- Curls and yellows from one branch → fusarium or verticillium wilt
- Curls and shows mosaic pattern or distorted new growth → virus (usually fatal, remove plant)
- Wilts in heat of day but recovers overnight → normal water stress
- Wilts and never recovers, even with water → soil-borne disease or root damage
By host plant
- Tomato leaves curling — physiological vs viral curl
- Pepper leaves curling — heat, aphids, broad mites
- Tomato plant wilting — water vs fusarium vs bacterial wilt
General principles: Wilting that is not from drought covers the diagnostic path when watering is not the cause.
General decline (no obvious cause)
The hardest symptom to diagnose. The plant just looks worse than it should. Stunted, off-color, slow, or fewer flowers than expected. Most of these cases trace back to one of four issues:
- Soil pH out of range for the species — common with rhododendrons, blueberries, hydrangeas, and roses. Test your soil before you treat anything else.
- Drainage problem you cannot see — sandy loam on top, hardpan or clay below. Plants do well for a year, then decline.
- Root competition from established trees — anything within the dripline of a large maple or oak is competing for water and nutrients.
- Slow-developing soil-borne disease — verticillium, fusarium, phytophthora. These often present as "the plant just is not happy" for an entire season before symptoms become specific.
If you can rule out the first three, read the disease pages on the /problems/ index for slow-onset soil diseases.
Fruit problems
Symptoms on the fruit itself, not the leaves. Different diagnostic path entirely.
- Blossom end rot — calcium-uptake issue in tomatoes, peppers, eggplant, squash
- Blossom drop on tomato — temperature stress, humidity, pollination failure
- Cat-facing on tomatoes — cool nights during flower formation
- Sunscald on peppers and tomatoes — exposed fruit after defoliation
- Cracking — uneven watering after drought
- Fruit rot in storage — fungal carryover from the garden
Browse the full problems index for fruit-specific diseases.
How to use this with a photo
If you have a photo of the symptom, take 30 seconds to log it in the garden journal with the date and what you did. Most diagnostic errors come from misremembering the timeline of when symptoms appeared. A photo with a date settles it.
You can also send a photo and short description to your local Cooperative Extension office — most offer free plant problem diagnostics. Find yours at the USDA NIFA Extension finder.
The honest tradeoff
This page is fast to scan, but it cannot do what a hands-on diagnosis with the actual plant can do. For high-value plants (mature trees, specimen shrubs, anything you cannot easily replace), get a confirmed diagnosis before you treat. Wrong treatment is often worse than no treatment — over-fertilizing a stressed plant or spraying a fungicide for a bacterial disease both make things worse.
For routine vegetable garden issues, this page should get you to the right answer in under a minute.