PESTS AND DISEASES IN ORANGES

Oranges

PESTS AND DISEASES

  1. Leaf miner
  • These are very serious pests of oranges right from the nursery to the field.
  • The adult moth deposits tiny eggs on the lower surface of the leaf.
  • Upon hatching the larva enters the leaf where it remains until maturity.
  • It then comes out of the leaf and forms a pupation cell by folding over a part of the leaf margin.
  • The larvae sometimes mine in between the upper and lower surfaces,
  • The serpentine mine is characteristic and conspicuous.
  • As a result of the leaf mining, the lamina weakens and curls.

Control

The pest is resistant to most available insecticides. Bulldock alternated with Folimat can control leaf mines effectively.

  1. Citrus psyllid
  • It’s a common but minor pest of mature orange.
  • It is more evident in the cooler areas of the tropics.
  • The adult is an aphid-like insect about 2mm long with transparent wings almost twice the length of the body.

Damage

  • The leaves are conspicuously pitted, the pits opening the lower leaf surface.
  • In severe attacks, young blades are cupped or otherwise distorted and yellow in color.
  • Leaves with many picks tend to curl upwards.
  • Citrus psyllid causes serious problems by transmitting a bacterium causing greening disease.
  • Leaves get interveinal yellowing, narrowing of leaves and leaves stand upright with plant dieback.
  • The plants become less productive and fruits are underdeveloped.
  • The disease is mainly spread by scion/bud materials.

Control

  • Pruning off infected braches can slow disease development.
  • General insecticides like sumithion, salut, decis, dimethoate and dursban can be used to control insects.
  1. Aphids

These are shiny black brown insects and may be winged or wingless.

The main injuries caused by aphids on citrus are

  • Severe curling and deformation of young leaves
  • Stunted growth of leaves and twigs
  • Some branches and twigs show dieback symptoms
  • There is impairment of leaf functionality from the presence of sooty mould fungus which grows in copious quantities of honey dew excreted by the aphids

Control

  • Spray should only be applied in periods of flush growth and before populations build up (before much curling of leaves).
  • Spray using dimethoate, fenitrothion or pyrethroids.
  • Some contact pesticides like dursban may be used during heavy infestation.
  1. Orange dog
  • Adult is a dark brown butterfly with numerous pale yellow markings.
  • The caterpillar has 5 stages and all are harmful.
  • The first 3 are dark brown with white markings and resemble bird droppings.
  • The fourth and fifth stages are pale green caterpillars with black, brown and grey markings.
  • If the caterpillars are disturbed, they secrete a pink Y shape organ from right behind the head.

Control

  • Remove and kill the caterpillars found on young trees.
  • Where infestation is high, spray using dimethoate.

Other pests include

  • Citrus woody white fly
  • Scales
  • Thrips
  • Mites
  • Mealybugs
  • Nematodes

Diseases

Oranges are attacked by many fungal diseases especially when grown under hot, humid conditions.

These include;

  • Bacterial citrus canker
  • Leaf and fruit spot
  • Melatose
  • Scab
  • Stem and end rots
  • Phytophthora
  • Penicillium

Control

  • Sprays with fungicides are recommended to control fungal diseases using such chemicals like copper oxychloride, kocide, benlate or rodomil.
  • These chemicals may have to be used in combination depending on the diseases present.
  • Viruses also attack orange plants where the most deadly is Ctv-citrus tristeza virus transmitted by aphids.
  • The aphids are controlled using insecticides mentioned above.
  • Whereas quick decline is caused by aphids, spreading decline is caused by nematodes.
  • The nematodes can be controlled using furadan, nemacur or temik.
  • For bacterial citrus fruit canker, spray with a copper based bactericide.
  • Also defoliating of crops affected by bacteria canker and destroying the leaves reduces diseases incidences.
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