Identifying Plant Pests and Diseases

Properly identifying plant pests and diseases is crucial for gardeners and farmers. Catching issues early allows for more effective treatment and prevention of spread. This guide will cover common plant afflictions and provide tips on identifying symptoms and causes.

Common Plant Pests

Several types of insects and other creatures can become problematic plant pests. Being able to recognize signs of an infestation is the first step in treating and eliminating the pests.


Aphids are small, soft-bodied insects that use their piercing mouthparts to feed on plant sap. Heavy aphid infestations can stunt plant growth and introduce viruses.

Signs of aphids:

  • Clustered colonies of green, yellow, black, white, or pink bugs on stems and undersides of leaves
  • Curled, twisted, or distorted leaves and shoots
  • Sticky honeydew residue on plants
  • Presence of ants, ladybugs, and other predators that feed on aphids

Spider Mites

These tiny pests look like moving dots to the naked eye. Spider mites suck nutrients through plant leaves, causing stippling, discoloration, and wilting.

Signs of spider mites:

  • Stippling – Small dots or speckles on leaves from mites removing cell contents
  • Fine webbing on undersides of leaves
  • Feeding damage visible on both sides of leaves
  • Bronzing, yellowing leaves that drop early
  • Thriving colonies of tiny red or black dots on undersides of leaves

Japanese Beetles

Metallic green and copper wing covers make Japanese beetles easy to identify. As adults, they devour leaves, flowers, and fruits of hundreds of plant species.

Signs of Japanese beetles:

  • Skeletonized leaves with only veins remaining
  • White grub larvae in soil that feed on roots
  • Clusters of metallic green beetles on plants and foliage
  • Presence of damaged, defoliated, or scarred plants

Scale Insects

Armored scale, soft scale, and mealybugs are common plant scale pests. They anchor themselves to stems and leaves to feed on sap. Heavy infestations can stunt plants and promote sooty mold growth.

Signs of scale insects:

  • Small bumps or lumps adhered to stems and leaves
  • Circular spots or dried resin-looking excretions on leaves
  • Sooty black fungus on leaves and stems due to honeydew secretions
  • Yellowing or wilting leaves despite adequate watering
  • Ant trails present due to honeydew secretions


Whiteflies resemble tiny white moths but are actually related to aphids and mealybugs. They weaken plants through sap feeding and promote growth of sooty mold.

Signs of whiteflies:

  • Clouds of tiny white bugs that fly up when foliage is disturbed
  • Yellowing or mottled leaves
  • Black sooty mold on leaves due to honeydew buildup
  • Sticky honeydew residue on affected plants
  • Presence of eggs, nymphs, pupa, and adults on undersides of leaves


Minute insects that rasp plant tissue and feed on released plant juices. Leaves become dotted with silvery-white feeding marks and turn brown.

Signs of thrips:

  • Silvery streaks or patches on flowers, leaves, buds and fruits
  • Distorted growth and damaged flower petals or leaves
  • Black dots of excrement on foliage, flowers and fruits
  • Populations of tiny (under 2mm), slender pale bugs visible inside flowers and on leaves


Caterpillars are the larval stage of moths and butterflies. Species like tomato hornworms, tent caterpillars, and cabbage worms can be serious plant pests. They chew through leaves, flowers, and tender shoots.

Signs of caterpillars:

  • Chewed or missing leaves, buds, and fruits, sometimes with frass visible
  • Presence of green, striped, or speckled caterpillars on or around plant
  • Clusters of eggs or cocoons on stems or undersides of leaves
  • Small black pellets of frass near feeding damage
  • Curled leaf shelters of tent caterpillars and webbed nests of fall webworms


Beetle and moth larvae (borers) that bore into stems, trunks, and woody parts of plants. They weaken and damage plants from the inside out.

Signs of borers:

  • Small round holes surrounded by sawdust or sap in trunks and stems
  • Swelling, cracking, or dying branches and twigs
  • Loose, split, or cracked bark from larval tunnels under bark
  • Wilting, stunted growth or dying branches
  • Presence of larvae inside tunnels or cavities in wood

Slugs & Snails

Slugs and snails ravage seedlings and plants with their rasping mouthparts. They leave behind telltale slime trails and mucus deposits as they feed.

Signs of slugs and snails:

  • Irregularly shaped holes in leaves, flowers and tender parts
  • Silvery slime trails visible on leaves, pavement and garden debris
  • Dried slime deposits on plants, giving leaves a shiny appearance
  • Damaged seedlings slashed off at soil line
  • Presence of gray/black slugs or snail shells around or on plants


Earwigs are nocturnal insects with distinctive pincers on their abdomen. They damage plants by chewing flower petals and holes in foliage.

Signs of earwigs:

  • Jagged holes in flower petals, leaves, fruits and tender plant parts
  • Small, dark brown insects visible around damaged plants or hiding in flower blossoms
  • Presence of shed exoskeletons around plant base

Common Plant Diseases

Identifying symptoms of diseases allows gardeners to take preventative measures and avoid spread. Learn to recognize common plant illnesses and disorders.

Powdery Mildew

One of the most widespread plant diseases, powdery mildew coats leaves and stems with white fungal growth that spreads rapidly in suitable conditions.

Signs of powdery mildew:

  • White or gray powdery coating over upper leaf surfaces and stems
  • Distorted, curled leaves with stunted growth
  • Dried, brown patches spreading across plants
  • Defoliation in severe infections as leaves yellow and shrivel

Downy Mildew

Unlike powdery mildew, downy mildew thrives in rainy, humid weather. It manifests as yellow splotches before coating plant undersides with white-gray fuzz.

Signs of downy mildew:

  • Pale yellow blotches on upper leaf surfaces
  • White-gray spore growth on undersides of leaves
  • Leaves curling, shriveling, browning, and falling off
  • Poor flowering and stunted growth
  • Damping off of seedlings

Botrytis Blight

Also called gray mold, it manifests as light brown lesions that develop fuzzy gray spore masses. Thrives in cool, persistently damp conditions.

Signs of botrytis blight:

  • Light brown, water-soaked spots on leaves, fruits, flowers and stems
  • Fluffy gray fungal growth in humid conditions
  • Shriveling, discoloration and rot as infection advances
  • Soft decay and collapse of affected plant tissues
  • Persistent dampness on plants


Fungal disease characterized by small, circular or angular dark lesions on leaves, shoots, and fruits. Especially common in warm, wet weather.

Signs of anthracnose:

  • Small dark spots with defined borders on leaves, fruits, stems or flowers
  • Expanding lesions that develop tan or pink spore masses
  • Infected leaves yellowing and falling off
  • Fruits with sunken lesions that rot and shrivel
  • Cankers on twigs and shoots
  • Presence of colorful fungal spore masses during wet weather

Bacterial Spot

Bacterial disease manifesting as small, water-soaked spots on leaves that grow into brown/black lesions surrounded by yellow haloes. More prevalent during wet conditions.

Signs of bacterial spot:

  • Small, water-soaked lesions appearing on leaves
  • Lesions turning brown/black with yellowing haloes
  • Circular spots resembling bullseyes when centers drop out
  • Severely infected leaves turning yellow and dropping
  • Fruit spots starting as water-soaked circles with greasy appearance

Blight Diseases

Blight diseases like early blight, late blight, and bacterial blight affect leaves, stems and fruits of plants like tomatoes and potatoes. They appear as rapidly enlarging brown/black lesions.

Signs of blight diseases:

  • Irregularly shaped brown spots with concentric rings on leaves
  • Blackened, dried, withered leaves still attached to stems
  • Dark streaks on stems and blackened shoots
  • Fruits with large, sunken brown spots and lesions
  • Rapid defoliation and dieback
  • Soft, watery rot in fruits, stems and leaves

Root & Stem Rot

Soil-borne fungal diseases that manifest as roots, stems, or crowns of plants turning brown or black. Entire plants wilt, yellow, and die.

Signs of root & stem rots:

  • Yellowing, drooping leaves starting at base of plants
  • Dark stems, crowns, and roots with rotting flesh
  • Stunted, slow growth and poor flowering/fruiting
  • Wilting and death of entire plants
  • Mushrooms or fungal growth around bases of affected plants


Fungal disease characterized by yellow, orange, reddish or brown powdery pustules on leaves and stems. Severely affected plants become defoliated.

Signs of rust:

  • Colored powdery pustules appearing on undersides of leaves or stems
  • Corresponding yellow spots on upper leaf surfaces
  • Leaves turning yellow or brown before falling off
  • Stunted growth and dieback on severely infected plants
  • Presence of spore horns protruding from pustules

Mosaic Virus

Viral infection causing mottled, streaked, and distorted leaves. Spread by insects, infected tools, and contact between plants. Stunts growth when severe.

Signs of mosaic virus:

  • Irregular yellow, light green, or white patterns on leaves
  • Distorted leaf shape and curling/twisting of foliage
  • Stunted growth habit
  • Fruits with abnormal coloring like streaking, mottling, or ring spots
  • Plant parts fusing together abnormally forming witches’ brooms
  • Self-sown seeds giving rise to infected seedlings

Wilt Diseases

Fungal diseases that invade plant vascular tissues and disrupt water transport, causing wilting. Include verticillium wilt, fusarium wilt, etc.

Signs of wilt diseases:

  • Leaves and stems wilting and drooping despite adequate moisture
  • Discolored streaking visible in stems when cut open
  • Browning inside lower stems near soil line
  • Premature leaf drop and dieback of branches
  • Stunted growth and reduced flowering/fruiting
  • Yellowing and death of part or all of affected plants

Preventing Plant Pest & Disease Issues

Prevention is critical for avoiding major pest and disease issues. Here are some key tips:

  • Choose resistant varieties – Select plant cultivars bred to resist common diseases.
  • Follow crop rotations – Don’t repeatedly grow the same plants in the same soil to prevent disease buildup.
  • Provide proper care – Address watering, light, and fertilization needs to keep plants vigorous and less stressed/susceptible.
  • Monitor plants – Make thorough inspections to catch issues early before they intensify.
  • Remove weeds – Eliminate alternate hosts that can harbor pests and diseases.
  • Clean up debris – Dispose of dropped leaves/fruits and dead plants to remove inoculum.
  • Use row covers – Keep out insect pests with lightweight fabric barriers.
  • Improve air circulation – Allow foliage to dry quickly by spacing plants appropriately.
  • Apply preventative fungicides/insecticides – Use integrated pest management techniques cautiously.

Treating Pest & Disease Problems

If prevention fails, pest and disease issues must be addressed promptly and properly for best results:

  • Remove heavily infected/infested plant parts- Prune affected leaves, stems, and fruits to eliminate sources of spread.
  • Apply insecticidal soaps – Use natural soaps to control soft-bodied pests like aphids without harming beneficials.
  • Apply selective insecticides – Choose insecticides targeted at the specific pest that minimize environmental impact.
  • Apply the appropriate fungicide – Select the ideal fungicide for the fungus involved and follow all label directions.
  • Improve growing conditions – Adjust watering, light levels, and fertilization to favor plant health over pests/disease.
  • Release natural predators – Employ beneficial insects that naturally prey on problem pests like ladybugs for aphids.
  • Apply organic sprays – Use plant-based sprays containing ingredients like neem oil for broad-acting pest/disease control.
  • Remove highly susceptible plants – Eliminate plants chronically plagued by issues to avoid infecting other vegetation.

Diagnosing Unknown Issues

When dealing with mysterious plant damage, employ some detective work:

  • Document all symptoms – Note the location, extent, appearance, and timing of unusual changes.
  • Research the symptoms – Use resources to match observations to possible causes. Consider common issues for the plants involved.
  • Inspect carefully – Check all plant parts for clues like insects, fungal growths, sticky deposits, etc using a hand lens.
  • Rule out other causes – Ensure proper care like watering isn’t the issue. Account for weather incidents like frost.
  • Compare affected and unaffected plants – Look for differences between normal and abnormal plants for insights into root causes.
  • Send samples to a diagnostic lab – If the issue evades identification, submit specimens to extension services for expert diagnosis.
  • Monitor spread – Track changes to determine whether the problem is intensifying or improving over time.
  • Test treatments – Apply targeted remedies for likely causes and monitor results. Adjust treatments based on plant response.

Identifying Common Garden Pests

Gardens can harbor a diverse range of plant pests that must be properly identified for effective management. Some of the most frequent garden offenders include:

Destructive Insects

Aphids – Small soft-bodied insects in colors like green, black, pink, white, or yellow. Leave sticky deposits on foliage. Cause distorted growth.

Thrips – Tiny elongated insects that rasp plant tissues. Leave silvery discoloration and scarring on leaves/flowers. Stunt growth.

Leafhoppers – Wedge-shaped bugs that hop rapidly when disturbed. Feed on plant sap, causing hopper burn. Spread diseases.

Japanese beetles – Metallic green beetles with coppery wings that skeletonize leaves. Larvae are white grubs in soil.

Sawflies – Caterpillar-like larva of wasp-related insects. Devour plant foliage. Easily confused with moth caterpillars.

Scale – Immobile insects that embed themselves on stems and leaves. Secrete sticky honeydew. Weaken plants and stunt growth.

Spider Mites – Tiny red or black spider-related pests. Colonize leaf undersides and cause stippling damage. Produce webs.

Chewing Pests

Caterpillars – Larval stage of butterflies and moths with voracious appetites. Chew holes in foliage and fruits.

Earwigs – Nocturnal insects with distinctive pincers. Tunnel into leaves, flowers, and tender shoots.

Beetles – Adults and larvae of leaf beetles, weevils, borers, and others feed on plant parts. Skeletonize leaves.

Slugs & Snails – Leaves shiny slime trails as they chew irregular holes in foliage and fruits. Damages seedlings.

Sucking Pests

Aphids – Crowded colonies use piercing mouthparts to feed on plant sap. Secrete sticky honeydew.

Whiteflies – Tiny white moth-like insects that take sap from undersides of leaves. Cause yellowing or death.

Mealybugs – Soft-bodied insects covered in powdery wax. Clustered colonies inject toxic saliva while feeding.

Thrips – Super small insects that rasp plant tissue and drink the exuded sap. Distort growth.

Soft Scales & Mealybugs – Immobile pests that attach to stems and leaves. Feed on sap. Secrete honeydew.

Leafhoppers – Small, mobile bugs that feed on sap. Cause foliage discoloration and foliage shedding.

Lygus Bugs – Also called tarnished plant bugs. Feed on buds and fruits. Cause cat-facing damage.

Organisms Causing Wilts, Rots & Decay

Fungi – Microscopic organisms like Fusarium, Verticillium, Phytophthora cause blights, wilts & root rots.

Bacteria – Bacterial pathogens like crown gall and soft rot bacteria create watery rot and wilt.

Nematodes – Microscopic worms transmit disease and feed on roots. Cause stunted growth.

Viruses – Systemic infections spread by insects/contact. Cause distorted leaves, ringspots, and mosaic symptoms.

Key Garden Pest Detection Strategies

Regular monitoring and targeted inspections are crucial for early identification of potential garden pests. Here are some tips for effective pest detection:

Conduct Regular Scouting

  • Walk through the garden at least once a week during the growing season.