Plant Diseases & Pests

Plant disease and garden pests can be tricky to pin down, but we’re here to help!

If you have noticed a problem with one of your plants, but are not sure what the exact cause might be, here are some steps to follow:

  1. Define the Problem
    Identify the plant and know what the normal characteristics of that plant are. Examine the entire plant to determine if there is truly a problem. If possible, compare it to healthy specimens.
  2. Look for Patterns
    Is the problem occurring on more than one plant or on more than one plant species? Is the problem uniformly distributed over the entire plant, or just on a few isolated areas?

    Scattered damage over the plant, or across several plant species indicates living factors like insects or pathogens. Uniform damage on a single plant or on several plants in the same pattern indicate nonliving factors such as physical or chemical damage.

  3. Determine a Timeline
    Does it appear that the problem started in an isolated spot and then spread to the rest of the plant? Or is the damage limited to one area of the plant and has remained there?

    Progressive spread of a problem indicates damage done by living organisms. If there is a clear delineation between damaged and undamaged parts of the plant that does not spread, these clues point to nonliving damaging factors.


Plant Problems in Detail

Fungal Diseases

FungusUsually dry and papery, fungal leaf spots are characterized by discolored concentric rings that look like a bull’s eye.

You may also be able to see the actual fungus growing inside these spots.

Bacterial Diseases

BacteriaBacteria usually enter through existing wounds in plant tissue.

Symptoms of a bacterial infection include a slimy texture on the affected area accompanied by a fishy or rotten-odor. It’s possible that the infection is initially confined between leaf veins. This can sometimes cause spots that have straight sides and appear angular.

Viral Diseases

VirusSymptoms of a viral infection can be defined in four categories:

1. Lack of chlorophyll formation in normally green area of the plant
2. Stunting or other abnormally slow growth
3. Distortion of leaves and flowers
4. Dead areas or lesions


NematodeThese microscopic roundworms feed on plant tissues, including roots and foliage. Root nematodes are the most common.

If your plant is infected with root nematodes, the entire plant is affected above-ground. Moisture and nutrients cannot get to the plant through the root system as they normally would, resulting in overall stress to the plant and perhaps even stunted growth.

Insects, Mites and Other Animals

AphidThe best way to identify garden pests is to locate them on the plant itself. But, if you’re unable to catch these critters in the act, there are some clues to help figure out who they are. The most important are:

1. The location of the feeding damage
2. The type of damage caused:

Type of Damage Possible Cause
Entire leaf blade consumed. Only tough midvein remains Caterpillars, canker worms, webworms
Distinct patterns of leaf missing Black vine weevil, leaf cutter bees, beetles, grasshoppers
Leaf surfaces damaged. “Skeletonization” of leaf Slugs, beetle larvae, pear slug, elm leaf beetle, thrips
Leaves “rolled” or tied together with silk Leafrollers or leaftiers
Discolored or swollen leaf tissue. Insects visible when held p to the light Leaf miners
Leaves falling off in early summer Petiole and leaf stalk borers
General decline of the plant, or of a specific branch Under bark borers such as the mountain pine beetle or the European elm bark beetle
General decline of the plant Root feeders such as the larval stages of weevils, beetles, moths sod webworms, and Japanese beetles.
Spotting or stippling Aphids, leaf hoppers, lygus bugs
Leaf curling or puckering Severe aphid or other sucking insect infestation
Uniform stipple, flecking or chlorotic pattern Lace bugs or adelgids
Random stipple pattern Leafhoppers, mites
Leaf and stem distortion / off-color foliage Aphids
Galls, swellings on leaf or stem tissue Aphids, wasps, midges
Damaged twigs are split Egg laying by sucking insects like tree hoppers and cicadas
General decline of entire plant or section of the plant Root, stem, or branch feeders, scales, mealybugs
Even rows of holes in trunk Birds such as the yellow-bellied sap-sucker
Branches torn or clean cut Cattle, deer, goats, horses
Chewing of bark with teeth marks Rodents like mice, squirrels or rabbits
Injury to low-growing foliage that resembles skeletonizing, distinct ‘silvering’ slime trail Slugs and snails


For More Information

Visit the Virginia Cooperative Extension’s page on Diagnosing Plant Problems.