Garden

Diagnosing plant problems

A plant looks wrong and you want to know why. Extension diagnosticians don't start by guessing a disease. They gather facts in a set order, starting with what the plant is and what is normal for it. This guide follows NC State's and Colorado State's steps. The Plant Problem Diagnosis Key walks you to a likely cause one question at a time.

Updated October 2026 6 min read By Connor Schmitz

The plant

Extension diagnosticians work through a plant problem in a set order, gathering facts before naming a cause. NC State's version has ten steps, and Colorado State's has the same core. Both start with what the plant is.

Colorado State explains why. Hundreds of pests and diseases attack plants in any region, but once you know the plant, the list of likely ones gets much shorter. NC State adds that some plants are normally a color that would mean a deficiency or disease on another plant.

Normal or not

Colorado State notes that gardeners often notice something normal for the first time and blame it on a pest or disease. Some common examples from Colorado State and NC State:

Yellowing and dropping of older needles on the inside of an evergreen in fall. A white pine sheds about half its needles each fall.
Male squash flowers that never make fruit.
June drop, when apple trees and other fruit trees shed some young fruit.
Aerial roots on tomato and corn stems.
Spore spots under fern fronds, and pollen cones on pines and spruces.
Fuzz under leaves, and variegated leaves.

Colorado State also warns not to assume a thing is normal. Older needles dropping early can still be a sign of stress or disease.

Signs and symptoms

Colorado State separates two kinds of evidence:

Symptoms The plant Changes in how the plant grows or looks, such as cupped leaves, wilting or galls.

Signs The cause The organism itself or direct evidence of it, such as insects, their droppings (frass) or fungal threads.

Many problems share symptoms, so a symptom alone rarely settles it. Colorado State lists non-living signs too: girdling roots, no visible root flare, mower damage on the trunk, standing water, plugged drip emitters and a soil test showing high salts.

Describe the problem in detail. NC State's questions: Is the change on the whole plant or just some parts? On new growth or old? The whole leaf, or just between the veins? In the center of the leaf or at the edges? Round or irregular? Colorado State says that with only a vague description, a diagnosis is impossible.

Patterns

Where the damage shows up is one of the best clues. NC State starts with how many kinds of plants are affected. Pests and diseases often go after one plant family. If several unrelated plants in one spot show the same symptom, a non-living cause such as heat, cold, drought, salt, air pollution or a herbicide is more likely.

Among plants of the same kind, NC State reads the spread like this:

Spread	Likely cause
Uniform	Non-living
Random	Often a living pathogen
Hot spots	Usually a living pathogen
In a line	Likely man-made
High ground	Too little water
Low ground	Too much water

NC State Extension Gardener Handbook, Chapter 7. A uniform pattern points even more to a non-living cause when all parts of the plants were hit within a short time.

Then look at the single plant. NC State's rules of thumb: if the whole plant is affected, suspect the environment or something attacking the roots, lower stem or water-conducting tissue. If random parts above ground are affected, suspect a pest or an airborne pathogen. If only the lower parts are affected, consider shade or temperature.

Try it

Weigh the pattern clues

Spread Plants hit Onset

LeansLiving

Clues3 of 3

A random spread often means a living pathogen. One kind of plant fits a pest or disease. Symptoms that keep spreading fit a pest or disease.

Living means a pest or disease. Non-living means weather, water, salt, chemicals or care. The rules are NC State's and Colorado State's. They point a direction and don't make a diagnosis.

Timing

Colorado State uses timing to split the two kinds of cause:

Non-living Sudden Symptoms appear at once and don't progress, or show up on several plant species.

Living Spreading Symptoms get worse over time, are uneven on the plant, and affect one or a few related species.

Woody plants keep a record. NC State suggests measuring the distance between bud scale scars on a branch. Each scar marks the start of a year's growth, and short gaps show years the plant was under stress, such as a drought.

Quick check

A row of shrubs and the perennials in front of them all browned along the edge of a driveway in the same week. What is the more likely kind of cause?

Answer: something non-living. Several unrelated plants with the same symptom, in a line, all at once, all point away from a pest or disease. NC State reads a linear pattern as likely man-made, and lists salt and herbicides among the common non-living causes.

Plant Problem Diagnosis Key One question at a time, from what you see to a likely pest, disease, nutrient problem or growing condition, with how to confirm it.

Open the key

Next steps

Colorado State's next question is whether the problem needs any action. Many insect and disease problems are only cosmetic on a healthy plant. Stressed plants are less resilient. Colorado State's rule of thumb is that a healthy deciduous tree can lose a third of its leaf area before stress becomes a concern. Evergreens tolerate much less, since their needles last several years.

Colorado State also notes that non-living stress often sets a plant up for insects and disease. When a pest shows up on a stressed plant, finding the stress matters as much as treating the pest.

If you can't pin it down, your county Extension office can help. Most states also have a university plant diagnostic lab that will identify a disease or insect from a sample you mail in. NC State's tips for a sample:

Bring both healthy and damaged parts, and both old and new growth if you can.
Bring whole stems with the leaves attached, not loose leaves.
Bring it in right away. If it has to wait, keep it in a dry plastic bag in the refrigerator.
For insects, collect adults and young, in a closed jar of rubbing alcohol or frozen for a few hours.

Colorado State asks for photos as both wide shots of the whole plant and its surroundings and close-ups of the damage.

Plant Diagnostic Labs The university lab in your state, the tests it runs, and whether it takes samples from out of state.

Find a lab

Summary

Know the plant first. It narrows the list and tells you what is normal for it.
Rule out normal changes, like fall needle drop or June fruit drop.
Look for signs, the pest or its traces, not just symptoms.
Several kinds of plants, a uniform or linear pattern, or a sudden onset point to a non-living cause.
One kind of plant, a random or hot-spot pattern, and damage that spreads point to a pest or disease.
If you still can't name it, send a sample to your state's plant diagnostic lab.

Sources

University extension services.

NC State Extension, Extension Gardener Handbook, Chapter 7, "Diagnostics" ↗. The ten steps, normal features, patterns in the landscape and on the plant, bud scale scars and samples.
Whiting, D., O'Meara, C. and Small, M., Colorado State University Extension, "Diagnosing Plant Disorders," GardenNotes ↗. Identifying the plant, normal features, signs and symptoms, timing, and when action is warranted.

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