Diseases of Citrus
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 trunk with water as in flood irrigation; water logged areas and heavy soils.
Mode of spread and survival
Soil inhabitants. Sporangia spread by splashing rain water, irrigation water and wind. Irrigation water and wind.
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.
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 coloured; 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.
Free moisture for 20 minutes, 20-30°C.
Mode of survival and spread
Wind and rain splashes. Survives in infected leaves for 6 months. Injury caused by leaf miner helps the entry of the bacterium.
Control leaf miner when young flush is produced. Prune badly infected twigs before the onset of monsoon.
Tristeza or quick decline (Virus)
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. Fine pitting of inner face of bark of sour orange stock. Grapefruit and acid lime are susceptible irrespective of root stock. Tree stunted and dies yield very much reduced. Fruits are small in size.
Mode of spread
Use of infected bud wood 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.
Greening (Liberobactor asiaticum) (Phloem limited Bactria- like- organism)
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.
Mode of spread
Infected budwood; psyllid vector-Diaphorina citri
Control psyllids with insecticides. Use pathogen free bud wood for propagation.