1.Gummosis : Phytophthora parasitica, P. palmivora,P. citrophthora
First symptoms are dark staining of bark which progresses into the wood.
Bark at the base is destroyed resulting in girdling and finally death of the tree.
Bark in such parts dries, shrinks and cracks and shreds in lengthwise vertical strips.
Later profuse exudation of gum from the bark of the trunk.
Injuries to crown roots or base of stem during cultural operations should be avoided.
If lesion has girdled less than ½ the girth, remove the diseased bark with a knife along with ½” of uninvaded bark.
Bark of trunk should be coated with Bordeaux paste
2.Scab/Verucosis : Elsinoe fawcetti
- Attacks leaves, twigs and fruits of mandarin.
- Sour orange, lemon, mandarin, tangelos extremely susceptible Grapefruit, sweet oranges and acid lime highly resistant. Severe in rainy seasons.
- On the leaves the disease starts as small pale orange coloured spots.
- The leaf tissue is distorted to firm hollow conical growths with the lesion at the apex.
3.Canker: Xanthomonas campestris pv citri
Acid lime, lemon and grapefruit are affected. Rare on sweet oranges and mandarins.
Affects leaf, twig and fruits. In canker, leaves are not distorted.
Lesions are typically circular with yellow halo; appear on both sides of leaf, severe in acid lime (difference from scab) When lesions are produced on twigs, they are girdled and die.
On fruits, canker lesions reduce market value.
Streptomycin sulphate 500-1000 ppm; or Phytomycin 2500 ppm or Copper oxychloride 0.2% at fortnight intervals.
Control leaf miner when young flush is produced.
Prune badly infected twigs before the onset of monsoon
4.Tristeza or quick decline: Citrus tristeza virus (CTV)
Lime is susceptible both as seedling or buddling on any root stock.
But mandarin and sweet orange seedlings or on rough lemon, trifoliate orange, citrange; Rangpur lime root stocks tolerant; susceptible root stocks are grapefruit and sour orange.
In sweet orange or mandarin on susceptible root stocks, leaves develop deficiency symptoms and absise.
Roots decay, twigs die back. Fruit set diminishes; only skeleton remains.
For sweet orange and mandarin, avoid susceptible root stocks.
For acid lime, use seedling preimmunised with mild strain of tristeza.
4.Greening : Liberobactor asiaticum ( Phloem limited bacteria)
This disease affects almost all citrus varieties irrespective of root stock.
Stunting of leaf, sparse foliation, twig die back, poor crop of predominantly greened, worthless fruits.
Sometimes only a portion of tree is affected.A diversity of foliar chlorosis.
A type of mottling resembling zinc deficiency often predominates.
Young leaves appear normal but soon assume on outright position, become leathery and develop prominant veins and dull olive green colour. Green circular dots on leaves.
Many twigs become upright and produce smaller leaves.
Infected budwood; psyllid vector-Diaphorina citri
Control psyllids with insecticides.
Use pathogen free bud wood for propagation.
500 ppm tetracycline spray, requires fortnightly application