Major Citrus diseases are given below:-
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.
- Infection extends to crown roots.
- Prolonged contact of the trunk with water as in flood irrigation; water logged areas and heavy soils.
- Soil inhabitants.
- Sporangia spread by splashing rain water, irrigation water and the wind.
- Irrigation water and wind.
- Injuries to crown roots or base of the stem during cultural operations should be avoided.
- If the lesion has girdled less than ½ the girth, remove the diseased bark with a knife along with ½” of uninvaded bark.
- The 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.
- The crest of these growth becomes covered with scabby corky tissue colour at first but later becomes dark olive with age.
- Lesions most common on the undersurface of the leaf. They penetrate leaf and are later visible on both sides.
- Infected areas run together and cover a large area.Leaves wrinkled, distorted and stunted.
- On twigs, similar lesions are produced.
- They form corky outgrowths.On fruits irregular scably spots or caked masses produced.
- Cream colour in young fruits; dark olive grey in old fruits.
- Fruits attacked when young become misshapen with prominent warty projections. They drop prematurely.
- Spray Carbendazim 0.1%
3.Citrus 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 grape fruit 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.
- Fine pitting of inner face of bark of sour orange stock.
- Grapefruit and acid lime are susceptible irrespective of root stock.
- Acid lime leaves show large number of vein flecks (elongated translucent area).
- Tree stunted and dies yield very much reduced. Fruits are small in size.
- Use of infected bud wood and Toxoptera citricida (aphid) is the important vector.
- For sweet orange and mandarin, avoid susceptible root stocks.
- For acid lime, use seedling preimmunised with mild strain of tristeza.
5.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.
- Fruits small, lopsided with curved columella. The side exposed to direct sunlight develops full orange colour but the other side remain dull olive green.
- Low in juice and soluble solids, high in acid. Worthless either as fresh fruit or for processing. Seeds poorly developed, dark coloured, aborted.
- Infected budwood; psyllid vector-Diaphorina citri
- Control psyllids with insecticides.
- Use pathogen free bud wood for propagation.
- 500 ppm tetracycline spray, requires fortnightly application.
- TamilNadu Agritech Portal