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  Disease Management

Introduction > Algal Spot > Alternaria Rot > Anthracnose > Black Mould Rot > Bacterial Canker > Bark Eruption > Bud Union Crease > Citrus Mosaic > Dry Root Rot > Diplodia Gummosis > Dodder Laurel > Dodder > Felt Disease > Fusarium Rot > Gummosis > Ganoderma Root Rot > Greasy Spot > Greening Disease > Green Mould Rot > Loranthus > Preharvest Stem End Rot > Powdery Mildew > Post Harvest Rots > Psorosis > Sooty Mould > Scab > Sour Rot > Soft Rot > Tristeza > Twig Blight > Yellow Corky Vein >


Ikisan - disease management in citrus

Introduction

  • Citrus decline is a serious problem causing concern not only to growers but also to researchers and policy makers.
  • In the condition of decline, the affected trees do not die immediately but remain in a state of decadence and unproductiveness after having performed well initially.
  • Prominent among the various factors responsible for citrus decline are diseases caused by viruses, bacteria and fungi.
  • Symptoms caused by infectious agents resemble those of improper nutrition, incompatability, cultural abuses and physiological disorders.
  • It is the primary duty of the diagnostician to differentiate between the several agencies that cause similar symptoms, only then can proper control measures be applied and the disease cured.
  • In this chapter, an attempt is made to 1) indicate the various diseases commonly occuring in Andhra Pradesh that may lead to decline; (2) describe symptoms of each disease in sufficient detail to enable identification, (3) indicate the causal organisms and their mode of action; and (4) recommend remedial measures.
  • Diseases caused by viruses are dreaded, since there is no cure.
  • The only way to avoid such diseases is to use virus-free (nucellar) certified saplings or budlings for planting.
  • With this objective in view, a nucellar Sathgudi sweet orange bud bank has been developed at the Citrus improvement Project, Tirupati from which 4-5 lakh virus-free certified buds are supplied every year to registered nurserymen and government agencies.
  • Hence, Citrus growers are advised to use only material from well known private nurseries.

 
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Ikisan - Algal leaf spot of citrus

Algal spot : Cephaleuros virescence

Incidence and intensity of algal spot is more in rainy and winter seasons.

Symptoms


Algal spot
  • Greyish-green, velvety algal colonies 1 cm in diameter from on the trunk, limbs, branches and leaves.
  • The colonies sometimes coalesce and form a huge blotch.
  • With age, these colonies become raised and brick-red in colour.
  • Thickening and cracking of the affected bark into small, irregular-shaped platelets or shreds is common.
  • In severe cases of infection, terminal growth of branches is restricted and the leaves become chlorotic and drop off.
  • The alga forms lichens with the fungus Strigula complanata.

Control

  • Spraying 0.3% copper oxychloride (3 g of copper oxychloride/1 litre of water) at monthly intervals during every rainy season contains the disease.
  • Sandovit or Teepol should be mixed with the spray fluid 10 ml. per litre.
  • Thorough spraying of the trunk, limbs and branches must be ensured.

 
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Ikisan - Alternaria rot of citrus

Alternaria Rot : Alternaria citri

Symptoms


Brown spots on fruits

  • This is a common disease in sweet in Punjab. It also affects mandarin.
  • Infection by Alternaria citri causes stylarend rot in sweet orange.
  • Circular, brown spots develop at the stylar end of the fruit.
  • These spots enlarge, coalesce and cause rotting of the fruits.
  • In mandarin, circular brown spots develop on the rind of the fruit which enlarge and cause rotting of the fruits in the infected area.

 
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Ikisan - Anthracnose rot of citrus

Anthracnose Rot : Collectotrichum gloeosporioides

  • Acid lime and sweet orange are susceptible to this disease.

Symptoms
Symptoms at the stem end of sweet orange
  • It initiates at the stem end as a small brown area. As the area enlarges ,a soft pliable rot develops; it is dark brown in color.
  • The decay slowly advances along the core,inner rind and pulp of fruit.


 
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Ikisan - Black mould rot / aspergillus rot of citrus

Black Mould Rot (or) Aspergillus Rot : Aspergillus niger

  • All citrus species, viz. mandarin, sweet orange and acid lime are susceptible to this disease.

Symptoms


Powdery layer over affected surface
  • This rot develops as a light-coloured soft spot that can be punctured easily.
  • It gradually turns pale yellow and orange in colour with the tissue becoming wrinkled.
  • As the rot develops further, the decayed area sinks and mycelium and fruiting bodies of the fungus appear.
  • This forms a black, powdery layer over the affected surface.

 
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Ikisan - Citrus bacterial canker

Bacterial Canker : Xanthomonas campestris pv. Citri

  • In India, citrus canker is endemic and occurs in all the citrus growing areas. It is reported from Punjab, Tamil Nadu, Andhra Pradesh, Karnataka, Rajasthan, Madhya Pradesh, Assam, Uttar Pradesh.
  • This is more commonly observed on leaves, twigs and fruits of acid lime than on sweet orange.

Symptoms



Water soaked roundish yellow spots on leaf
Canker on twig

Canker on stem

Canker on acidlime
Canker on sweet orange fruit
  • Initially it shows up on leaves as small, raised translucent spots.
  • Over a time, the epidermis over the spot ruptures to expose a crater-like spongy, tan-coloured tissue.

  • A yellow halo surrounds the spot till it becomes old.
  • Old lesions are corky, irregular in shape and brown in colour.
  • The under surface shows greater involvement.
  • Cankered leaves are seldom retained to full maturity.
  • Lesions develop on petiole and midrib, resulting in early leaf fall.
  • On the twigs the lesions appear water-soaked and dark green first and later turn brown.
  • The pustule is corky, but without a crater-like depression.
  • In severe cases, the twigs dry up.
  • Canker has also been observed on acid lime roots as slightly raised corky, splotches.
  • On fruit, the infection appears as water-soaked lesions, similar to those on leaves except for the yellow halo and creater-like depressions, which appear only in the later state of infection.
  • Cankered fruit drop prematurely.
  • In sweet orange, the inner contents are affected while in acid lime the ifection is confined to the flabedo.
  • Oozing of gum from the cankered pustule, is common.

Etiology And Spread

  • The disease is caused by Xanthomonas citr.
  • Three strains of the pathogen have been distinguished by host reaction viz., (i) Cancrosis A, the common farm, which is extremely virulent on grape fruit, sweet oranges and sour oranges and to a lesser degree on others, (ii) Cancrosis B, which is virulent on lemons, but not on grape fruit and sweet oranges; and X.citri which attacks acid lime.
  • Canker-infected leaves, twigs etc, serve as the source of inoculum to spread the disease from season to season.
  • However, the cankered leaves drop off early and bacteria perish rapidly in the soil.
  • Infected leaves do not serve as the main source of infection and the carryover of the disease from season to season is mainly from cankerous lesions or twigs and branches.
  • The bacterium enters the host through stomata or wounds.
  • It multiplies in the intercellular spaces, dissolves the middle lemella and establishes in the cartex region.
  • Canker pustules develop and exude bacteria in the form of gummy substance.
  • They are freely disseminated, chiefly by wind and to a considerable extent by rains.
  • Citrus leaf miner (Phyllocnistis Citrella) helps dissemination and infection of citrus canker.
  • Leaves affected by miner and canker and get distorted and drop off early.
  • The injury to the leaf epiderins made by the burrowings of leaf miner serve as an easy opening to the canker bacterium and the canker lesions appear throughout in the zigzag manner.
  • Prevalence of 20-35° C temperature, high humidity and the presence of moisture on the host surface for 20 minutes or more favours incidence of disease.
  • A positive correlation exists between the rainfall and canker disease incidence.
  • The pathogen can survive in the unsterlized soil upto 9 days.
  • It is resistant to drying but direct exposure to sunlight kills the bacterium within 2 minutes.

Control

1. The only method of tackling the disease would be to prevent or reduce its incidence by

a) Pruning all the canker-infected twigs before monsoon and burning them.

b) Periodical spraying of suitable bactericides along with an insecticide.

c) By using canker resistant varieties.

2. In case of sweet orange, three sprayings at monthly interval should be carried out of, streptocycline 100 ppm (i.e. 100 parts per million, i.e. 1 g of chemical in 10 litres of water) or 0.2% Mancozeb (2 g of Mancozeb/litre of water) in combination with 0.3% copper oxychloride (3 g of oxychloride/litre of water). The first spray should be done when the fruits are marble-sized, irrespective of the appearance of the symptoms.

3. In case of acid lime, the spraying should be done four times at monthly interval in the rainy season and after the dead twigs have been pruned.

4. "Tenali Selection" acid lime was found to be tolerant to canker.


 
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Ikisan - Bark eruption in citrus

Bark eruption : Xanthomonas campestris pv. Citri

  • Acid lime is highly susceptible to this disease, while Rangpur lime and Rough lemon are resistant.
  • As the disease develops on the stem of seedlings, care should be taken to select nursery plants free from bark eruptions and to avoid injury to the stem during cultural operations.

Symptoms

  • Raised, brown and corky pustules develop on the stem of seedlings, which may encircle the entire stem.
  • Corky eruptions of varying sizes can form on the trunk and limbs of the tree.
  • Eruptions crack vertically and horizontally, forming roughly rectangular platelets.
Bark eruption

  • Gum may sometimes ooze from the cracks.
  • The leaf and fruit size is affected and the yield is low.
  • In severe cases, the twigs dry and the tree declines (dies).

Control

  • Scraping the erupted portion thoroughly and applying Bordeaux paste is a curative measure.

Bordeaux Paste Preparation

  • Dissolve 1 kg of Copper sulphate and 1 kg of Quicklime in 5 litres of water each.
  • Mix these two solutions in a third container, while agitating all the while.
  • Care should be taken to ensure that the substance of the container does not chemically react with copper sulphate, i.e. Copper, wooden or earthern vessels can be used for the final solution.

Prevention

  • Careful selection of the certified seedlings and careful handling of the plant during cultural operations to avoid injuries to the stem can prevent the disease.

 
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Ikisan - Bud union crease in citrus

Bud union crease

  • This disease is believed to be caused by a virus and appears at any time after 5 years of planting.

Symptoms

  • The most obvious symptom is the formation of small groove with honey-coloured gum encircling the bud joint on the wood with a corresponding projection on the bark.
  • Drying of the tree is common in severe cases.
Bud union crease

Control

  • Root stocks such as sweet orange, Cleopatra mandarin, Karhakatta and Rangpur lime do not develop the disease and should therefore be used at the time of propagation to avoid occurrence of this disease at a later stage.
  • Virus free certified buds should be used for propagation.

 
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Ikisan - Citrus Mosaic

Citrus mosaic

  • This disease is caused by a virus transmitted by diseased buds, aphids and the parasite Cassytha.

Symptoms


Mosaic on fruit
 
Mosaic on leaf
  • Affected leaves show irregular yellow or light-green patches alternating with normal green areas without reference to the veins.
  • The fruit drop prematurely and the quality is reduced by rugged surface.

Control

  • Aphids (Toxoptesa citricide) infesting the new flush should be controlled and the parasite Cassytha removed along with haustoria to prevent the spread of the disease.

Prevention

  • Virus-free certified buds must be used for propagation.

 
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Ikisan - Dry root rot of citrus

Dry root rot : Macrophomina phaseolina, Fusarium spp. and Diplodia natalensis Pole Evans.

  • Dry root rot is a major problem in all citrus growing areas of Andhra Pradesh in both sweet orange and acid lime.
  • Sweet orange trees on Jamberi root stock are more susceptible to this disease than those on the Rangpur lime stock.
  • The affected plant wilts in the early stages, resulting in ultimate death in the advanced stages which may occur in one month.

Symptoms

Dry root rot (dried young tree)
 
Dry root rot(dried roots)
  • The disease is characterised by wilting, drooping and yellowing of foliage.
  • Moist decay of root bark is seen in the early stages and a dry, shredded condition of the bark with hard dead wood underneath, is common in the later stages.
  • Affected roots emit a bad odour and there is some discolouration.
  • Excessive flowering, heavy crop of small-sized fruit and ultimate death of affected trees takes place in the last stages.

Etiology and Spred

  • The exact cause of the diseases is not clear.
  • Among the fungi, Macrophomina Phascolina, species of Diplodia and Fusarium are reported to be mainly responsible.
  • The disease has been ascribed to excessive moisture or drought irregular manuring, poor aeration, heavy soils, close planting and root injuries, inadequate supply of nitrogen besides the fungi.
  • The incidence of dry root rot is the cumulative effects of defective soil, age of plantation, attack of species of Diplodia, Fusarium and Rhizoctonia.

Control

  1. If only one or two roots are affected and the taproot and crown are still in good condition, the diseased roots may be cut and destroyed.
  2. The cut surface should be protected with Bordeaux paste.
  3. Deep ploughing or digging which are likely to injure roots should be avoided.
  4. Leaf mulch in the tree basins during the dry season helps in conserving the soil moisture and thereby reducing disease incidence.
  5. Foliar sprays with Urea (4.5 kg in 450 litres of water) may reduce the disease incidence.
  6. Application of 10 kg of neem cake followed by soil drenching with 0.5% Trichoderma vride formulation + 0.2% copper oxy chloride in the early stages of infection helps in reducing the disease intensity.
  7. The disease is effectively checked if the tree basin is drenched with 0.1% Carbendazim, (1 g of Carbendazim per litre of water) followed by another drenching of 0.25% Mancozeb (2.5 g of Mancozeb per litre of water) or 0.2% Chlorothalonil (2g of Chlorothalonil per litre of water), 12-24 hours after irrigation, at monthly intervals.
  8. One litre of each chemical fluid should be used per sq.m. area of the basin.

 
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Ikisan - Diplodia gummosis of citrus

Diplodia Gummosis : Diplodia natalensis

  • Diplodia gummosis ocurs commonly in Andhra Pradesh, Tamil Nadu and Uttar Pradesh.
  • It is common on Sathgudi and Batavian oranges, mandarins and lemosn in A.P. and Tamil Nadu.

Symptoms


Gum in the crotches
  • Profuse gumming on the upper portions of the trunk, branches and twigs is noticed.
  • Gumming may be common at the crotches or branches.
  • Sometimes, the branches break at the weakened crothches.
  • Gum comes out from the cracks developed on the diseased portion.
  • From bark, the infection spreads to wood which dries and becomes discoloured.
  • Black pycnidial bodies appear on the diseased bark which gets removed in flakes.
  • The disease spreads through dissemination of pycnidiospores by air, rain and insects.

Control

  • All factors which lead to weakening of the trees should be controlled.
  • Any wound or insect damage to the bark, especially on the limbs and forks, should be scraped, cleaned and painted with Bordeaux paste.
  • Gummed portions of affected bark should be removed and the cut surface protected.
  • Spraying with 0.1% carbendazim on the affected limbs and forks is useful to restrict the disease spread.

 
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Ikisan - Dodder laurel of citrus

Dodder laurel : Cassytha spp.

  • This is a phanerogamic parasite which transmits diseases like citrus mosaic, yellow corky vein and the Tristeza virus disease.

Symptoms


Dodder laurel
  • Dodder Laurel is a partial stem parasite and is leafless.
  • It twines around the branches of citrus with the help of haustoria and absorbs water and other nutrients from the host.
  • In severe infestation, the host becomes chlorotic, stunted, the twigs die and the yield is reduced substantially.

Control

  • The strands with haustoria must be destroyed completely wherever they appear on the citrus crop, fence plants and weeds in and around the orchard before seed set.

 
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Ikisan - Dodder of citrus

Dodder : Cuscuta spp.

  • This parasite transmits Tristeza and Psorosis virus diseases.

Symptoms


Dodder
  • This is leafless, complete stem-parasite with golden yellow strands, hence also called golden vine.
  • It twines around the branch with the help of haustoria and absorbs water and nutrients from the host.
  • In a severe attack, the host becomes pale and the twigs dry up leading to decline and resulting in poor yield and smaller fruit.

Control

  • The strands along with the haustoria must be removed and destroyed completely from the citrus plant, the fence plants and weeds in and around the orchard.

 
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Ikisan - Felt disease of citrus

Felt disease : Septobasidium pseudopedicellatum

  • Felt disease is observed in Guntur, Godavari and Cuddapah districts of Andhra Pradesh and Madurai and Tirunelveli districts of Tamilnadu. It is very severe in acid lime plantations around Tenali in Guntur district. This disease starts with the onset of monsoon in case of acid lime.

Symptoms

 
  • A soft, felt-like fungal growth encircles the twigs and branches.
  • The fungus grows over the bark and does not penetrate the surface.
  • The growth is light brown to grey in colour and colonies of scale insects can be seen underneath it.
  • This results in dying of stems.
Felt like leathery fungal growth on twig

Etiology and Spread

  • The disease usually starts with the onset of monsoon rains and continues throughout the rainy season. During winter the felty growth bursts and some of it gets peeled off.

Control

  • Pruning and destruction of the diseased twigs.
  • Spraying of monocrotophos 0.05% (1/2 ml of monocrotophos/litre of water) with Zineb 0.25% (2.5 g of zineb/litre of water) twice at monthly intervals with the onset of monsoon checks the disease.

 
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Ikisan - Fusarium rot of citrus

Fusarium Rot : Fusarium moniliforme

  • Both acid lime and sweet orange are susceptible to this disease.

Symptoms


White cottony fungal growth on fruit
  • The rot usually develops at the stem end and rarely at the stylar end.
  • The decay resembles the later stages of anthracnose rot to almost soft rot.
  • Fruit becomes pulpy and oozes a liquid with an undesirable odour.
  • Later, the rotted tissue is covered with a white, cottony fungal growth.

 
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Ikisan - Gummosis in citrus

Gummosis

  • Gummosis is wide spread in Punjab, and Assam.
  • In South India, it is common in the sweet orange.
  • This disease is reported from all citrus growing regions and it is also referred as collar-rot, trunk-rot, foot-rot and brown-rot gummosis.
  • This disease is caused by Phytophthora Palmivora, P.Citrophthora, P.nicotianae var Parasitica.

Symptoms

  • The first indication of the disease is exudation of gum from the bark of the trunk.
  • The bark cracks open and in the later stages dries up and falls off, exposing the wood for secondary infection by other organisms.
  • When scraped, the bark looks brown.
  • In severe cases, the bark is destroyed all round the trunk exhibiting a characteristic girdling and the tree eventually dies.
 

Gummosis symptoms at the base of the stem

Etiology and Spread

  • These fungi arre soil borne and the disease if favoured by rains.
  • Dampness in the soil, pH 5.4 to 7.5 and temperature of about 24°C are congenial for the growth of the fungus.
  • Incidence of gummosis depends on the abundance of moisture in the soil in contact with the bark over a sufficiently long period and favourable soil and air temperature.
  • Other pre disposing factors are heavy or ill-drained soils, high water table, low budding, deep planting, earthing up the trunk to cover the bud union and injuring the roots and base of the stem.
  • Infection takes place through bark.
  • Any injury to the bark aids infection favoured by excess raining or irrigation.
  • Sporangia are carried to trunks and fruit by water, rain and wind.

Control

  • Incidence of gummosis can be avoided by
  1. Selecting a site with proper drainage of water so that it does not stagnate;
  2. Planting the plants little higher than the ground level;
  3. Providing a double ring basin around the trunk; inner ring being 45 cms away from the trunk. This prevents the direct contact of water with the trunk.
  4. Avoiding excess irrigations.
  5. Painting the trunk with Bordeaux paste mixture up to the height of 50-75 cms above the ground level at least once a year.
  6. Scraping the diseased portions with a sharp knife.
  7. Protecting the cut surface with Bordeaux paste followed by spraying 0.3% fosetyl-Al reduce the spread.
  8. Soil drenching with 0.2% metalaxyl + 0.5% Trichodesma viride commercial formulation is also effective.

 
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Ikisan - Ganoderma root rot in citrus

Ganoderma root rot : Ganoderma lucidum

Ganoderma root rot is seen in old and neglected orchards of sweet orange, acid lime and mandarin. Also attacks mango, jackfruit, gulmohur, acasia, coconut and arcanut. Occurs commonly in sweet orange and acid lime.

Symptoms


Bracket at the base of the tree
  • The disease starts in the soil, in one or more of the lateral roots.
  • Whitish fungal strands spread along the surface of the bark of the root and gradually move to the bottom of the trunk.
  • The affected tissue becomes swollen, spongy and waterlogged and bracket like fungal fruiting bodies appear at the base of the trunk.
  • Wilting and death of the limb corresponding to the lateral root/roots is common.

Control

  1. Removal of dead or decaying old stumps from the orchard.
  2. Treating the basal portions of the stakes used as supports to the young seedlings with fungicide.
  3. Growing green manure crops and incorporating the same in Citrus orchards.
  4. Periodical collection and destruction of brackets found near the collars of other hosts.
  5. Treatment of the disease can be useful in early stages when only one or two roots are involved. The inferted roots should be cut and destroyed.
  6. Soil drenching with 0.5% Trichoderma viside formulation +m0.3% copper oxychloride solution is promising.

 
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Ikisan - Greasy spot/black malanose of citrus

Greasy spot/Black malanose: Mycosphaerella Citri,

  • Sweet orange, Jamberi (Rough lemon) and Rangpur lime are susceptible to this disease.
  • Rainy season and winter are the most congenial times for its occurance.

Symptoms


Spots on lower leaf surface
  • Small yellow or translucent spots appear on the lower surface of the leaves.
  • These gradually turn black or chestnut brown in colour and look like greasy spots.
  • Sometimes, many spots coalesce to form one huge spot.
  • They are rarely seen on the upper surface of the leaves. Leaves with many spots drop prematurely.

Etiology and Spread

  • The occurance of this disease is common under humid conditions.
  • The pathogen survives on dead leaves, twigs and fruits.
  • In moist weather the sparres ooze out and are disseminated by winds, birds, insects and other agents and reach fresh leaves, twigs and fruits. Fruits are infected only when the surface is wet for atleast 12 hours at 25°C.

Control

  • Spraying with copper oxychloride 0.3% (3 g of copper oxychloride/litre of water) or Zineb 0.25% (2.5 g of zineb/litre of water) once at new flush emergence is effective.
  • The lower surface of the leaves should be sprayed with the fungicide thoroughly.

 
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Ikisan - Greening disease in citrus

Greening disease : Obligate gram-negative bacterium

  • All three commercial species of citrus viz., sweet orange, acid lime and mandarin are affected by this disease.
  • The disease is caused by a Rickettsia-like organism.
  • It spreads through the use of diseased buds and the insect vector psyllid.

Symptoms


Green Chlorotic
areas on leaves
Fruits with lopsided growth
  • A characteristic symptom of the disease is the growth of small, leathery leaves with interveinal chlorosis and the distribution of green islands on a yellow area.
  • Leaves drop prematurely.
  • The symptoms are sectorial on the tree.
  • Other common symptoms include lopsided: fruit, with curved columella and aborted seeds.
  • Yield reduces and trees may become barren.
  • Die-back of twigs also occurs.

Control

  • Select certified disease-free seedlings.
  • Temporary suppression of the disease is possible by injecting infected plants with terracycline 500 ppm.

Prevention

  • Use of disease-free bud wood and protection of new flush from psyllids (Diaphorina citri) mitigates infection.

 
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Ikisan - Green mould rot of citrus

Green Mould Rot : Pencillium digitatum

  • All citrus varieties are susceptible to this disease.
  • This is also known as pinhole rot or blister rot.

Symptoms


Green mold on sweet orange
  • The disease is characterised by watersoaked, soft area which can easly be punctured by the push of a ginger.
  • On aging, the area enlarges rapidly and a white mycelium appears on the surface, followed by the development of olive-green coloured spore masses.
  • The spore masses are confined to the surface of the fruit.

 
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Ikisan - Loranthus in citrus

Loranthus : Loranthus longiflorus

Symptoms


Bulged haustoria of Loranthus
  • This is a partial stem parasite.
  • It gets attached to the host branches by means of bulged haustoria which serve as absorbing organs.
  • This in turn feeds on the host and leads to die-back and death of branches.

Control

  • Affected twigs and branches should be cut about 1 cm below the bottom-most haustorium and destroyed.

 
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Untitled Document

Pre-harvest stem-end rot : Gloeosporium limetticola

  • All the three commercial cultivars of sweet orange grown in Andhra Pradesh viz., Sathgudi, Mosambi and Batavian are susceptible to this disease.
  • The rot is most prevalent on fruit maturing in the rainy season (Angam crop) and seldom noticed in summer (Gyrangum and Edagaru crops).
  • It is not known to affect acid (Kagzi) lime.

Symptoms

  • This disease sets in on fruit nearing maturity as a small brown spot at the button and spreads from the stem end to stylar end.
  • The spot develops into a dark brown patch with a light brown periphery.
  • The entire fruit on the tree may rot and soften.
  • Some of them rot and fall off while others may dry up on the tree itself, and remain hanging.

Pre-harvest stem-end rot

Etiology and Spread

  • More than one pathogen is known to cause the disease viz.,Colletotcichum gloeosporioides and Gloeosporium species.
  • Trees damaged by gummosis, insects, drought, malnutrition etc are more vulnerable to infection by the pathogens. C.glocosporioides remains in a dormant or Quiescent conditions in the dead turing and branches.
  • Sparers of the fungus germinate on surface of citrus fruits forming appresoria.
  • Appresoria produce infection hyphae which remain latent within the upper layers of peel, even after the fruit matures.
  • Injuries on fruit peeling and over matured fruits of oranges and grape fruit function as sites of infection.
  • This disease is serious on limes, lemons and citrons.

Control

  • All the affected fruit which have fallen onto the ground and which have dried up along with the pedicel (stalk) must be collected and destroyed.
  • Spraying 0.05% Carbendazim (5 grams of Carbendazim in 10 litres of water) three times at monthly intervals during June, July and August will control the disease.

 
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Ikisan - Citrus powdery mildew

Powdery mildew : Acrosporium tingitaninum

  • Powdery mildew is common in Coorg, Nilgiris, Palnis, Wynad and Shevoy hills on mandarin oranges and some times on sweet oranges in the plains.
  • This disease occurs during the winter months and is seen in almost all varieties.

Symptoms


Powdery mildew on leaves
  • White, cottony or powdery growth appears on all parts of the affected plant.
  • The young leaves do not grow properly but crinckle, turn yellow and have distorted margins.
  • Prominent blisters, cupping and curling of the leaves takes place.
  • The cottony growth becomes black and in severe cases, the leaves dry up or drop off.
  • The symptoms are more severe on the upper surface of the leaf, but are also present on the lower surface.
  • Infected flowers start shedding and affected fruit also turn black and drop off.


Powdery mildew tree

Etiology and Spread

  • Comparatively cool and moist regions are prone to disease development.
  • Damp mornings with a few hours of sunshine favour onset of the disease.
  • The fungus is an ectoparasite and absorbs food materials from epidermal cells of leaf through haustoria.
  • It is a wind-borne disease.

Control

  • Spraying should take place immediately after the cottony growth is observed.
  • The disease can be contained with spraying of Sulfex 0.3% (3 g of Sulfex in 1 litre of water) or Carbendazim 0.05% (1/2 g of Carbendazim in 1 litre of water) three times at 20 days interval.

 
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Ikisan - post harvest rots in citrus

Post Harvest Rots

  • It is estimated that one fruit in every four harvested fruits is affected with post-harvest decays resulting in rotten fruit reaching the consumer.
  • Attack by micro-organisms is probably the most serious cause of post-harvest loss in perishables.
  • Pathogens inflicting perishable cause both qualitative and quantitative losses.
  • Quantitative pathogenic losses are due to the rapid and extensive breakdown of host tissue, by micro-organisms.
  • Qualitative pathogenic losses are thr result of discololuration.
  • Management of post-harvest diseases of citrus crop consists of careful handling of fruit during harvesting, transportation, packing and storage conditions at the market places (mandis).
  • However, storage life can be prolonged and rot prevented to a great extent, if fruits are dipped in a mixture of 3% wax emulsion and 0.05% Carbendazim (0.5 g of Carbendazim/litre of water), before packinag the fruit in boxes for transporting.
  • Injured or damaged portion of fruit is susceptible to all types of rots. Hence, to avoid the occurence of rots, careful handling and packing of the fruit is essential.
  • The fungus from one rotten fruit is easily passed on to the rest of the fruits, thus spoiling the complete lot in the box.

 
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Ikisan - Psorosis in citrus

Psorosis

  • Psorosis - A and Psorosis - B Viruses
  • Psorosis is generally used to designate a group of virus diseases, psorosis - A, psorosis - B, blind pocket, concave gum, crinkly leaf and infectious variegation.

Symptoms


Psorosis

Psorosis (dried tree)
  • Psorosis - A and Psorosis - B are observed in Andhra Pradesh.
  • The most conspicuous feature of psorosis - A is the scaling of the bark on the trunk or large limb.
  • Affected areas are almost circular.
  • In the beginning they are like small blisters eventually coalescing into large scaly patches girdling trunks and limbs.
  • The bark scales curl upwards.
  • Initially small amount of flecking of young leaves i.e. clearence of entire network of veins, and also vein banding of clarified tissue.

Control

  • Psorosis - B causes oozing of gum followed by bark scaling of the trunk starting from below stripwise.
  • Psorosis is spread mainly by infected buds, leaves and bark, occasionally by root grafting in the soil.

 
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Ikisan - Sooty mould of citrus

Sooty mould : Capnodium citri

  • In India, Sooty mould is common in all the citrus growing regions. However, the diseases is severe in South India, Assam and Kumaon. The Intensity of sooty mould is related to the infestation of scale and other insect pests.

Symptoms

Sooty mould on leaf
 
Sooty mould on fruit
  • It affects both acid lime and sweet orange.
  • Black, mouldy growth develops on honeydew secreted by insect pests like blackfly, aphids, scalses and mealy bugs.
  • No parasitism is observed in the citrus, but the presence of the black matting of mycelium may affect photosynthetic activity.
  • Affected young fruit do not develop because of the black coating. The trees look sickly.

Control

  1. Control of homopteran insects (honey dew secreting insects) by spraying monocrotophos or dimethoate combined with Copper oxy chloride or mancozeb helps in prevention of the disease.
  2. Spraying of diluted glue or a detergent can wash off the black coating.

 
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Ikisan - Citrus scab

Scab : Elsinoe fawcetti

  • In India, this disease is reported to be present in Bangal, Punjab, Assam, Karnataka, Uttar Pradesh and Andhra Pradesh.
  • In Andhra Pradesh, it is severe on Rangapur lime.

Symptoms


Lesions on fruit
  • The lesions on leaves in early stages consist of small semi-translucent dits, which become sharply defined pustular elevations usually on the underside, flat or somewhat depressed at the centre.
  • The opposite surface corresponding to the warty growth shows a circular leaves often become distorted, wrinkled, stunted and mis-shapen.
  • On the fruit, lesions consist of corky projections which often break into scab.
  • Humidity and rainfall play a very important role in the development of scab.

Control

  • Pruning and destruction of the diseased leaves, twigs, branches and fruits followed by spraying with 0.3% copper oxychloride, 1% Bordeaux mixture or 0.1% carbendazim is helpful in controlling the disease.

 
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Ikisan - sour rot of citrus

Sour Rot : Geotrichum candidum

  • Acid lime and sweet orange varieties are susceptible to this rot.

Symptoms


Water soaked hyphae over affected fruit
  • A putrified, sour smell is emitted by the fruit, hence the name.
  • Sour rot areas can easily be punctured as compared to green or blue mould areas.
  • The decaying areas first become creamy, then buff yellow and, finally a light buff colour, when the water-soaked hyphae show through the skin.

 
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Ikisan - Soft rot of citrus

Soft Rot : Rhizopus stolonifer

  • All citrus varieties are susceptible to this rot.

Symptoms


Soft rotted fruit in grey colour
  • The beginning of the decay is similar to that of Aspergillus rot.
  • The tissue softens and can be easily punctured and is later covered in a fluffy mass of mycelium with dark heads of spores which give colour.
  • Juice leaks from the affected fruit emitting the odour of fermenting sugar.

 
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Ikisan - Tristeza or quick decline desease of citrus

Tristeza (or) Quick decline

  • Quick Decline is a disease caused by the tristeza virus which spreads through the use of diseased budwood in nurseries, insect vectors, phanerogamic aphids and parasites like viz., Cassytha and Cuscuta. Wilting and death of the tree can occur within 2-3 days, hence, the name Quick Decline.

Symptoms

Tristeza (CTV) stem pit symptom
 
Tristeza tree
  • In acid lime, vein fleckings or clearing and stem pittings are observed.
  • Sweet orange varieties are symptom less carriers but their susceptible root stock of budlings show stem pits.
  • Some of the infected trees decline overnight and dry up in 2-3 days.

Vein clearing leaf

Etiology

  • Virus the causal agent of disease named as Carium Vialoris.
  • Two strains of tristeza, viz.,(i) the Virulent or fulminate and (ii) the ordinary type "avirulent" or mild have been recognized.
  • The former causes sudden wilting and drying of leaves, while the latter shows a gradual decadence in the condition of the tree.
  • Among the two strains of virus, one of which is an independent agent of stem-pitting, while the other, known as seedling yellows.
  • Tristeza virus is both vector and bud transmissible.
  • Insect-Vector transmission of the virus by several insects viz., by Aphis citricides, Toxoptera citricidus, T.aurantii, Aphis gossypii, A.Craccivora, A.spiraecola.
  • Among these Toxoptera Citricidus is widely distributed in India particularly in Southern India and Assam.
  • It is rather rare in northern India, where spread of the virus may be mainly through infected budwood.
  • Besides insects, the virus can be transmitted by budding, grafting and by means of dodder.
  • It is not transmitted through seed.

Control

  • The disease can be prevented from spreading by the removal of the parasites, and by the control of aphids (Toxoptera citricide) infesting the new flushes.

Prevention

  1. The use of certified sweet orange buildings on Rangpur lime rootstock offers tolerance to the disease.
  2. For Andhra Pradesh, Maharashtra and Karnataka, Rangapur lime is recommended as a root stock resistant to tristeza.
  3. For the Punjab region, Jatti khatti, Cleopatra mandarin and sweet orange are recommended as resistant root stocks.

 
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Ikisan - Twig blight in citrus

Twig blight : Diplodia natalensis, colletotrichum gloeosporioides, Fusarium spp.

  • The disease is common for both sweet orange and acid lime.

Symptoms


Twigblight on sweet orange
 
Die-back of twigs

  • Twigs and small branches die back from the top and gum can be seen at the base of dead twigs.
  • Due to nutritional disorders, the tree is susceptible to the blight.

Control

  • Spraying of carbendazim 0.05% (1/2 g of Carbendazim/ltre of water) thrice at monthly intervals should be done immediately after pruning of the dead twigs just before the onset of monsoon.

 
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Ikisan - Yellow corky vein disease of citrus

Yellow corky vein

  • Both acid lime and sweet orange are susceptible to this disease.
  • It is caused by a virus transmitted through budding and the phanerogamic parasite Cassytha

Symptoms


Yellow corking of midrib

Rough and variegated fruit
  • Initial symptoms are yellowing of the mid-rib and lateral veins, followed by distinct corking and browning of the midrib on the under surface of the leaf.
  • The surface of the affected fruit is rough and variegated.

Control

  • Use the certified buds and remove the Cassytha parasite promptly.

 
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