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Plant Pests

Microscopic and sub-microscopic pathogens and insects in the environment can cause significant damage to plants. Find out how you can manage these plant pests in the sections below.

Identify the causal agents

Managing plant health problems effectively involves identifying causal agents accurately. For growers needing laboratory services, identifying causal agents helps determine the appropriate tests to be conducted to get information on the possible causal agents and final diagnosis. Diagnosis based on symptoms alone is risky and inaccurate.

To identify the causal agent accurately, you will need to consider and analyse the following information in relation to the plant’s growth.

  • Signs and symptoms of unhealthy plants
  • Presence of possible agents responsible for the problem
  • Growing conditions
  • Growing practices

You should consider the following questions when a plant health problem arises.

On healthy plants 

  • What is the genus and species of the plant in question?
  • Is the plant sensitive to certain environmental factors (e.g. sunlight, soil moisture, and humidity)?
  • Are there any other Information to note about the appearance and growth habits of the healthy plant grown outdoors or indoors?

On unhealthy plants

  • What symptoms of poor health are exhibited by the affected plant? What were the initial symptoms? When did the symptoms first occur?
  • Which plant parts are affected?
  • Are symptoms present only on exposed plant surfaces, or also on protected, covered tissues such as unopened flowers?
  • Where do the symptoms appear (e.g. only on one side of the plant, only on old leaves, only one plant affected, etc.)?
  • How rapidly do early symptoms change into advanced ones?
  • How long have the symptoms been present?
  • Is the problem associated with a particular growth stage? 

On the surrounding environment 

  • Have there been any unusual weather patterns or weather changes in the past few weeks or months?
  • Is there any evidence of environmental stress factors, such as extreme temperatures, water stress, pollution, wind, or other mechanical damage?
  • Has the soil been exposed to these stress factors as well?
  • What type of fertilisers have you applied?
  • How often do you water the plant? How much water do you use to water the plant? How does the water drain off the site?
  • Did the onset of symptoms correspond with any new cultural practice?

 

Common plant pests

Insects are a type of plant pest. They damage plants by feeding on them during the course of completing their life cycle. Good sanitation and horticultural best management practices can prevent or reduce insect infestation.

Some examples of good horticultural practices are:

  • Removing weeds
  • Removing affected plant parts
  • Improving basic environmental and operational conditions

Types of plant pests

Different plant pests cause different symptoms on your plants. Knowing what these pests are and their symptoms are important to help you remedy the infestation. The common types of plant pests you might find among your plants include:

  • Bacteria (including phytoplasmas)
  • Fungi
  • Viruses
  • Caterpillars, especially in their larval stages
  • Moths
  • Beetles
  • Thrips
  • Mites
  • Nematodes
  • Weeds

Sucking insects 

Food source Plant sap
Symptoms of infestation 
  • White or yellow spots
  • Brown patches on leaves
  • Silver or bronze stippling
  • Leaf-curling
Examples
  • Aphids
  • Mealybugs
  • Whiteflies
  • Scale insects
  • Mites
  • Thrips
  • Leafhoppers
  • Vectors of viral plant diseases

Scales 

Spider mites 

Thrips 

Mealy bugs 

Whiteflies 

Leaf hoppers 



Chewing insects

Food source
  • Plant leaves
  • Twigs
  • Branches
  • Tree trunks
Symptoms of infestation

Caterpillar feeding results in: 

  • Leaves with only veins left
  • Shoot dieback
  • Defoliation

Bagworm feeding results in:

  • Circular holes on leaves
Examples
  • Caterpillars (the larval stage of moths and butterflies)
  • Bagworms (moth larvae which bind leaf and twig debris with silk to make a bag around them)
caterpillars caterpillars2  bagworms

Caterpillars

Bagworms


Mining insects

Food source
  • Leaf surfaces
Symptoms of infestation
  • Mine trails in leaf tissues
  • Severe defoliation and dieback
  • Serpentine, winding trail, and blotch tunnelling patterns
Examples
  • Larvae of moths, flies, or beetles

Serpentine tunnelling pattern

Serpentine tunnelling pattern

 Blotching

Blotching



Boring insects

Food source
  • Bark
  • Stems
  • Branches
  • Tree trunks
  • Buds
  • Roots
Symptoms of infestation
  • Borer holes
  • Insect frass
  • Sap exudation
  • Tunnels
  • Plant part dieback
Examples
  • Adults or larvae of beetles, weevils, and moths.

Borer hole 

Weevils


Plant parasitic nematodes

Food source
  • Root tissues. Nematodes puncture root tissues to withdraw nutrients and inject enzymes that inhibit plant growth
Symptoms of infestation
  • Yellowing
  • Stunting
  • Wilting
  • Root gall
Examples
  • Microscopic, worm-like organisms found in the soil around plant roots

Root gall

Yellowing

Signs of plant pest infestation

If you observe damage on your plants, look out for the following pests which might be responsible.

 Type of damage  Pests responsible
Chewed foliage, blossoms, or fruits
  • Moth and butterfly larvae/caterpillars
  • Beetle larvae or adults
  • Grasshoppers
  • Crickets
  • Snails
  • Slugs
Bleach, bronze, or silver stippling (flecked) or streaking on mined leaves
  • Leafhoppers
  • Plant bugs
  • Thrips
  • Aphids
  • Psyllids
  • Spider mites
  • Leaf miners
Distortion (swelling, twisting, cupping) of plant parts
  • Thrips
  • Aphids
  • Eriophyid mites
  • Gall-makers
  • Psyllids
  • Nematodes
Twig or plant shoot dieback (frass may issue from holes)
  • Borers, such as beetles or moths
  • Scales
  • Gall-makers

Presence of the following insects or insect-related products on plants:

  • Honeydew and sooty mould
  • Faecal specks on leaves
  • Webs and rolled leaves
    Cottony fibrous materials
  • Aphids
  • Soft scales
  • Leafhoppers
  • Mealybugs
  • Psyllids
  • Whiteflies
  • Lace bugs
  • Thrips
  • Some leaf beetles

Common plant diseases

 Plant disease

Description 

Leaf spots

 Caused by:

  • Bacteria
  • Fungi

Appearance of bacterial spots:

  • Water-soaked with yellow halos
  • Halos form v-shaped lesions

Appearance of fungal spots:

  • Dry, small spots of dead cells
  • Cells join to form larger dead patches

   

  Fungal leaf spots                               Bacterial leaf spots

Wilting
(loss of leaf or shoot rigidity)

Caused by:

  • Bacteria
  • Fungal pathogens

Resulting in:

  • Infection, or destruction of plants' vascular systems
  • Loss of rigidity in leaves or shoots

           

Fusarium wilt                                           Angsana wilt

Damping off
(death of seedlings before or just after their emergence from the ground)

Caused by:

  • Fungi


Resulting in:

  • Death of seedlings before or just after emergence from ground

Prevent damping off by:

  • Sowing seeds at the right density in clean media
  • Not over-watering seeds

 

Damping off 

Sooty moulds
(refers to the fungi that grow on leaf surfaces)

   
Resulting in: 

  • Black sooty coatings on leaf surfaces
  • Sunlight being blocked by the coating, preventing leaves from making food for plants

  

Sooty mould on leaves

Ganoderma rots

(attack on trunks and butts of trees and palms)

Resulting in:

  • (Advanced stage) woody, semi-circular brackets or conks formed by fruiting bodies of the fungi

  

Ganoderma brackets

Twig dieback and blight 
(death of all twigs and branches)

Caused by:

  • Fungus attacking and killing growing tips

Result of twig dieback:

  • Defoliation

Result of twig blight:

  • Death and browning of leaves and needles

    

Twig blight                                          Twig dieback

Viral diseases

Caused by:

  • Non-cellular virus particles

Resulting in:

  • Chlorotic and yellow patterns of mosaics, mottles, blotches, ringspots, vein-banding on leaves and leaf curls
  • Flower malformations
  • Abnormal shoot branching, bunching, and severe stunting of plant parts

   

  Ring spots                                                 Mosaic

Control measures for plant diseases

The methods for controlling pests vary depending on the plant, and the pests and causal agents involved.

Control measures should be applied to entire plant beds or garden patches rather than just an individual plant. Damages can be significant if the entire plant population is not protected. In most incidents, managing the disease or stopping the infestation is difficult once it has spread. Thus, almost all methods are aimed at preventing pest attacks and pest outbreaks.

Integrated pest control strategy

Integrated pest management (IPM) is an integrated pest control strategy involving a combination of various available control methods. In IPM, proliferation of beneficial insects is encouraged, and pesticides are used only as the last resort.

Step 1

Set up physical barriers (e.g. a netted structure over your plantings) to keep pests out.

Step 2

Follow good cultural and agronomic practices to keep your plants in good health. This reduces the plants’ susceptibility to diseases and pest attacks.

Step 3

Install sticky traps (yellow or blue sticky boards). These traps are non-chemical means of trapping incidental pests.

Last updated on 29 March 2019

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