Plants

General Information

Microscopic pathogens and submicroscopic pathogens and insects in the environment can cause significant damage to plants. Managing these plant pests requires a multi-pronged approach.

To manage plant health problems, it is important to identify the causal agents accurately. This helps determine the appropriate laboratory 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, sufficient information from the grower is needed on:

  • healthy versus unhealthy plants
  • presence of possible agents responsible for problem
  • growing conditions
  • growing practices

Observations for Growers

Growers can take note of these questions when a plant health problem arises:

On Healthy Plants
  • What is the genus, species names of the plant in question?
  • Is the plant sensitive to certain environmental factors (e.g. sunlight, soil salinity and humidity)?
  • What are the characteristics, appearance and growth habits of the healthy plant?
  • How does the plant normally appear when grown under various conditions (indoor vs outdoor)

On Unhealthy Plants
  • What are the symptoms of the affected plant? What were the initial symptoms? When did the symptoms first occur?
  • What are the plant parts affected?
  • Are symptoms present only on exposed plant surfaces or also on protected, covered tissues such as unexpanded or unopened flowers?
  • Where is the distribution of the symptoms (only one side of the plant, only on older or new leaves, only one plant affected)?
  • How rapidly do early symptoms change into advanced ones?
  • How long have the symptoms been present?
  • Is a particular growth stage associated with the problem?

On the Surrounding Environment
  • Have there been any unusual weather patterns, changes or developments recently (past weeks or months)?
  • Is there any evidence of stress factors (temperature extremes, water stress or excess, salt buildup, 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 & how much do you 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 pests. They cause damage to plants from their feeding or during the course of completing their life cycles on the plants. Farm sanitation and good agricultural practices can prevent or reduce the spread of infestation.

Some examples of good agricultural practices are:

  • removing weeds
  • removing affected plant parts
  • improving ventilation

Types of Plant Pests
  • Bacteria (including Phytoplasmas)
  • Fungi
  • Viruses
  • The larval stages (caterpillars)
  • Adults of moths
  • Beetles
  • Thrips
  • Mites
  • Nematodes
  • Weeds
Sucking Insects
Feeds on Plant Sap
Symptoms on Plants Feeding results in:
  • White or yellow leaf spots
  • Silver or bonze stippling
  • Brown patches and leaf curling
Examples
  • aphid, mealy bugs, whiteflies, scale insects, mites, thrips and hoppers have piercing mouthparts
  • vectors of plant viral diseases

scalesspider-mitesthrips
                      Scales                                     Spider mites                                       Thrips

mealy-bugswhitefliesleaf-hoppers
                Mealy bugs                                   Whiteflies                                     Leaf hoppers

Chewing Insects
Feeds on
  • Plant leaves
  • Twigs
  • Branches
  • Tree Trunks
Symptoms on Plants Caterpillar feeding results in:
  • Leaves with only veins left
  • Shoot die-back
  • Defoliation

Bagworm feeding results in:
  • Circular holes on leaves
Examples
  • Caterpillars (larval stage of moths and butterflies)
  • Bagworms (moth larvae with bind leaf and twig debris with silk to make a bag around them)

caterpillars1caterpillars2bagworms2
                                            Caterpillars                                                               Bagworms

Mining Insects
Feeds on
  • Upper and lower leaf surfaces
Symptoms on Plants Feeding results in:
  • Mine trails in leaf tissues
  • Defoliation and dieback (severe attacks)
 Examples
  • Larvae of moths, flies or beetles

Tunneling patterns:
  • Serpentine
  • Winding trails
  • Blotches 

serpentine-minesBlotchmines
         Serpentime Mines                      Blotching


Boring Insects
Feeds on
  • Barks
  • Stems
  • Branches
  • Tree Trunks
  • Stems
  • Buds
  • Roots
Symptoms on Plants
  • Bored holes
  • Insect frass
  • Sap exudation
  • Tunnels and die-back of plant parts 
Examples
  • Larvae or adults of beetles, weevils and moths

 borer-holeweevils
                  Borer hole                                        Weevils


Plant Parasitic Nematodes
Feeds on
  • Physically injuring root tissues by punturing them to withdraw plant nutrients and injecting plant enzymes that affect plant growth
Symptoms on Plants
  • Yellowing
  • Stunting
  • Wilting
  • Root Gall 
Examples
  • Microscopic, worm-like organisms found in the soil around plant roots 

root-galls Yellowing
                   Root galls                                Yellowing 

Symptoms & Signs of Plant Pests



Type of Damage Pests Responsible
Chewed Foliage, Blossoms or Fruits
  • larvae of moths and butterflies (caterpillars)
  • beetle larvae or adults
  • grasshoppers and crickets
  • snails and slugs
Bleached, Bronzed, Silvered, Stippled (flecked), Streaked on Mined Leaves
  • leafhoppers
  • plant bugs
  • thrips
  • aphids
  • psyllids
  • spider mites
  • leaf miners
Distortiion (swelling, twisting, cupping) of plant parts
  • thrips
  • aphids
  • eriophyid mites
  • Gall Makers
  • psyllids
  • nermatodes (root galls)
Dieback of twigs, shoot of entire plant (frass may issue from holes)
  • borers (beetles or moths)
  • scales
  • gall makers
Presence of 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, plant bugs
  • mealybugs, some aphids, plant bugs, some scales, some whiteflies

Common Plant Diseases



Plant Disease Description
Leaf Spots Caused by:
  • Bacterial infections
  • Fungal infections

Bacterial spots:
  • Appear as water-soaked with yellow halos
  • Turn into v-shaped lesions

Fungal spots:
  • Drier, small spots of dead cells
  • Joined to form larger dead patches

fungal-leaf-spotsbacterial-leaf-spots
                  Fungal leaf spots                               Bacterial leaf spots

 Wilts
(refers to the loss of rigidity of leaves or shoots)
Caused by:
  • Bacterial
  • Fungal pathogens

Resulting in:
  • Infections or destruction of plants' vascular systems
  • Loss of rigidity of leaves or shoots

fusarium-wiltangsana-wilt
                   Fusarium wilt                                           Angsana wilt

Damping-off
(refers to the 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 right density in clean media
  • Not overwatering them

damping-off-1damping-off-2
                                                      Damping off                          

 Sooty Moulds
(refers to the fungi that grow on the surfaces of leaves)
  •  Forms black sooty coatings on the surfaces of leaves
  • Extensive coating will block off sunlight preventing leaves from efficiently making food for plants

sooty-mould-on-leaves-1sooty-mould-on-leaves-2
                                             Sooty mould on leaves

 Ganoderma Rots
  •  Attack trunks and butts of trees and palms

Advanced stage of infection:
  • Woody, semi-circular brackets or conks growing from infections are fruiting bodies of the fungi

ganoderma-brackets-1ganoderma-brackets-2
                                               Ganoderma brackets

 Twig dieback and blight (death of all twigs and branches) Caused by:
  • Fungus attacking and killing the growing tips

Result of twig-dieback:
  • Defoliation

Result of twig blight
  • Death and browing of leaves and needles

twig-blight twig-dieback
                        Twig Blight                                          Twig dieback

Viral Diseases Caused by:
  •  non-cellular virus particles

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

ring-spotsmosaic
                        Ring Spots                                                 Mosaic

Control Measures for Plant Diseases


Methods for control of plant diseases and pests vary depending on:

  • plants
  • pests
  • causal agents

Control measures should be applied to entire plant beds or garden patch rather than just an individual plant. Damage and loss to one or a few plants are insignificant compared to the population. In most infectious disease incidents, curing the disease is difficult once it has set in. Thus almost all methods are aimed at preventing and protecting against attacks.

Integrated Pest Control Strategy

The Integrated Pest Management (IPM) is a an integrated pest and disease control strategy that combines the appropriate use of various control methods. In IPM, pesticides are used as the last resort and the proliferation of beneficial insects is encouraged.

Step 1

Physical Barriers of planting within a netted structure to exclude pests

Step 2

Follow good cultural and agronomic practices to keep vegetables and plants in good health. This ensures that they would not be susceptible to diseases and pest attacks.

Step 3

Add sticky traps (yellow or blue sticky boards). They are non-chemical means of reducing the incidental pests within the structures.

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