In this article we will discuss about the diseases and deficiencies of citrus fruit.
Diseases of Citrus:
i. Canker:
The bacterial Canker is caused by Xanthomonas axonopodis pv. citri (Hasse). The disease is highly infectious, spreads from tree to tree through the water splash. Disease appears on leaves, twigs and fruits. On leaves canker appears as yellowish spot, which gradually enlarge, turn rough and brownish and become raised on both the sides of the leaf. These spots are surrounded by a yellow halo. Fruit lesions become rough and croky and are confined only to the rind. Kagzilime and grapefruit are highly susceptible. It is one of the most serious diseases of acid lime prevalent all over the country.
The disease can be controlled by three spraying of 100 ppm streptocyclin, the first in October, the second in December and the third in February. Prepare solution by mixing 50 g Streptocyclin in 500 litres of water. Also add 25 g copper sulphate to increase the potency of the antibiotic in the mixture. Bordeaux mixture (2:2: 250) or 50 per cent copper oxychloride (0.3%) can also be sprayed.
ii. Scab:
The disease is caused due to the fungus Elsinoe fawcetti (Bittan Court and Jenkins). Small, dark brown, rough, irregular raised lesions appear mostly on under side of the leaves. Twigs and fruits are also infected.
Clipping of infected leaves should be done in July. Apply three sprays of ziram 27 SC (0.25%) or Dithane M-45 (0.25%) or Bordeaux mixture (2:2: 250) or 50% copper oxychloride (0.3%) during last week of June to August at 20 days interval.
iii. Gummosis (Foot Rot):
It is caused with fungus Phytophthora parasitica. Phytophthora diseases produce symptoms of decline through rotting of the rootlets, girdling of trunks and dropping of blighted leaves. Important symptoms above ground include dead areas of bark, exudation of small or large amounts of gum, a yellow gummosis zone at the cambium beyond the invaded, killed area and later drying and longitudinal cracking of bark.
Symptoms known as foot rot below ground include dead area with less noticeable gum, being soluble in soil water. Symptoms on leaves appear as dark, watery patches either on the edges or on the tops. The blighted leaves drop and complete defoliation may take place near ground. The gummy lesions may also appear on branches/limbs.
Treat foot rot, gummosis and cankers by decortication. The methods of treating gummosis is the removal of the diseased bark along with 5-10 cm strip of healthy bark beyond the diseased portion, disinfecting the wound with mercuric chloride (0.1%) or sodium hypochloride (0.1%) solutions. Cover the wound with Bordeaux paste. After the paste dries up in about a week, apply Bordeaux paint.
Afterwards, give a spraying of copper oxychloride (1.5 kg) in 500 litres of water or preferably Bordeaux mixture 2:2: 250 or give two application of Ridomil Gold 68 WP/Curzate M8 or Mateo 8-64 as paint (2g per 100 ml of Linseed oil) to the infected trunk and drench (25 g per 10 litres of water per tree) the soil at the base of the tree in February-March and again in July-August.
Give two sprays of Aliette 80 WP 0.25 percent (2.5 g/litre of water) in April and September in addition to soil drenching with Ridomil Gold 68 WP/Curzate M8/or Mateo 8-64 (25 g in 10 litres of water/acre) in February – March and July – August.
iv. Anthracnose or Die Back or Wither tip:
The diseases are caused due to the fungus Colletotrichum gloeosporioides (Penz.) or physiological causes. The disease manifests itself as light green spots which soon turn brown. If the spot has sufficient moisture, pinkish masses of spores ooze out of the surface. Sometimes fruiting bodies appear like black dots on pustules. The spots may be at the margins or tips of the leaf blades and sometimes near the midrib.
The lesions are usually surrounded by concentric rings. In advanced stages, lesions are found on the twigs which start dying back. Unfavourable conditions such as extreme temperature and moisture make the plant more vulnerable to the disease. Kinnow being early and prolific bearer is prone to this disease.
Regulation of the crop in early years, pruning of the diseased twigs and sprays with Bordeaux mixture (2:2:250) or 50% copper oxychloride (0.3%) during March, July and September help in reducing the disease.
v. Citrus Greening:
It is caused by a bacterium. The disease manifests as stiff, upright multiple twigs and buds. Leaves of infected trees show chlorotic pattern similar to zinc deficiency. The leaves may be completely yellow or yellow with green veins. Mature leaves show yellowing of midribs. Leaves become small, thick and upright. Pre-mature defoliation occurs. Die-back of branches and greening of fruits takes place. The disease is spread through the vector Diaphorina citri which is most effective in transmitting the disease.
For its control use disease free budwood and spray against the vector citrus psylla.
vi. Virus and Virus – Like Diseases:
(i) Tristeza:
It is caused due to virus (Citrus tristeza virus). The appearance of a tree infected with tristeza is that of a tree which roots have been injured. This is due to the sieve tube necrosis at the bud union which checks carbohydrate transportation from top to roots resulting in root starvation and heavy bearing which exhaust the tree. The first noticeable symptoms are chlorosis and bronzing of leaves, lack of new growth, off season flowering, heavy bearing, gradual defoliation and finally decline of the tree.
Under filed conditions, the only fairly specific symptom of tristeza is honey combing-a fine pitting of the inner face of the bark in the sour orange portion of the trunk below the bud union. Stem pitting in many citrus varieties is also caused by this disease. Although the disease is graft transmissible, spread is caused by vector (aphid). Its incidence has been found to be very high in Southern India.
For its controls use disease free bud wood and tolerant rootstocks such as rough lemon and Trifoliate orange. The vector (aphid) should be checked by giving timely spray.
(ii) Exocortis:
Exocortis is caused due to viroid (Citrus exocortis viroid): It is not common on kinnow and sweet orange budded on Jatti Khatti rootstock. Cracking and scaling of bark of trifoliate, citranges and Rangpur lime rootstocks is noted. Epinasty and curling of leaves, yellow blotches and cracks appears on shoots of some citrus species. Trees show stunted growth.
The disease spreads through contaminated budding knives and also through infected bud wood. No insect vectors are known. Use only resistant rootstock like Jatti Khatti. Sterilize the budding or cutting tools after each contact with infected plants to avoid spreading to healthy trees.
(iii) Ring Spot (Citrus Ring Spot Virus):
The disease appears as yellow rings on mature leaves. The ring number varies from one to several per leaf with variable diameter. These rings may coalesce to form bigger patches by covering the entire leaf. The severely infected plant shows die-back and decline symptoms with low fruit yield. The virus is transmitted through the infected bud wood. Use virus free plants for raising the citrus orchard.
vii. Melanose or Stem and Fruit-Rot:
Disease is caused due to Phomopsis citri. Dark circular depressions with yellowish margins appear on leaves, branches and fruits. Later, the spot become raised, rough and light brown and yellow margins disappear. Sand paper texture appears on the surface of leaves and fruits.
To check this disease, spray of Bordeaux mixture (2:2:250) or 50% Copper oxychloride (0.3%) during July-August and September is recommended.
viii. Citrus Sooty Mould:
The disease is caused by fungus capnodium citri. It produced black velvety superficial coating on leaves, twigs, stem and fruits. Spray Ziram 27 SC @ 1250 ml in combination with Ethion 50 EC (Phosmite 50 EC) @ 1000 ml in 500 litres of water per acre during August, September and October for the control of citrus sooty mould syndrome.
Deficiencies of Citrus:
A brief description of the deficiency symptoms of some of the essential elements is given below:
i. Nitrogen:
The deficiency symptoms of this element are easily recognized. The deficient trees show a general lightening of green part of the leaves. The leaves become distinctly yellowish green and some may even lose the green entirely when the deficiency persists for a longer time. The trees which remain severely deficient in nitrogen for long time, become stunted, have sparse foliage and produce dead-wood. Such trees bear very little fruit.
ii. Phosphorus:
The deficiency of phosphorous is rare in North India and is not easy to recognize in the field. The trees deficient in phosphorus have reduced growth and have more lack-lustre, smaller and narrower leaves than the normal trees. The fruit borne by such trees is rough in texture, having thick and coarse rind. The fruit has a hallow core and tends to drop prematurely.
iii. Potassium:
The symptom of potassium appears as reduced growth and smaller leaves. The new growth is very weak and the twigs break very easily. The fruits remain small and have a very thin, smooth and well-coloured rind.
iv. Magnesium:
The deficiency symptoms usually occur in old citrus leaves when the fruit is mature. The symptoms first appear as chlorotic spots at the tips and between the veins. The leaves become abnormally thin. The yellow spots enlarge until an inverted V-shaped area remains green near the base of the leaf. In acute deficiency, the entire leaves become yellow and later drop prematurely. In North India, soils have adequate magnesium content.
v. Zinc:
The symptoms of zinc deficiency, appears on fully mature new leaves as irregular interveinal chlorosis commonly known as “mottled leaf”. The terminal leaves become small and narrow referred to as “little leaf”. Fruit bud formation is severely reduced, twigs die back. The deficiency symptoms of zinc in citrus are widespread in north India.
vi. Manganese:
In the young citrus leaves, it is hard to distinguish it from zinc deficiency, but when the leaves are full grown, the deficiency can be easily recognised. The deficiency manifests itself by green veins on a light green background. In the mature leaves, bands of green develop along the mature leaves, with light-green tissue. In severe cases of deficiency, the dark- green leaf areas become dull green and the light-green areas turn grey or whitish. The leaves become thin and the die-back of the twigs starts.
vii. Iron:
With the iron deficiency, there is a general chlorotic condition of the areas between the veins and when the deficiency persists, the areas between the veins become yellow. In such cases, therefore, the entire leaves become like old ivory. There is premature defoliation and in severe cases, twigs start dying back.
viii. Copper:
The copper status of the North Indian soil is usually low, however, its deficiency in citrus has not been noticed. Grapefruit seems to be more sensitive to copper deficiency than sweet orange. The fruits develop symptoms of deficiency before the branches are affected. The fruits become misshapen, have dark- brown-gum-soaked eruptions, varying in size. There are gum pockets in the albedo. The leaves of deficient trees are deep green, over-sized and coarse. The shoots are long, vigorous and of S-shaped giving the appearance of excessive nitrogen fertilization.