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.
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.
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.
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.
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.
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
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.
The cut surface should be protected with Bordeaux paste.
Deep ploughing or digging which are likely to injure
roots should be avoided.
Leaf mulch in the tree basins during the dry season helps
in conserving the soil moisture and thereby reducing disease incidence.
Foliar sprays with Urea (4.5 kg in 450 litres of water)
may reduce the disease incidence.
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.
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.
One litre of each chemical fluid should be used per
sq.m. area of the basin.
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.
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.
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.
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
Selecting a site with proper drainage of water so that it does not
stagnate;
Planting the plants little higher than the ground level;
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.
Avoiding excess irrigations.
Painting the trunk with Bordeaux paste mixture up to the height of
50-75 cms above the ground level at least once a year.
Scraping the diseased portions with a sharp knife.
Protecting the cut surface with Bordeaux paste followed by spraying
0.3% fosetyl-Al reduce the spread.
Soil drenching with 0.2% metalaxyl + 0.5% Trichodesma viride
commercial formulation is also effective.
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
Removal of dead or decaying old stumps from the orchard.
Treating the basal portions of the stakes used as supports to the
young seedlings with fungicide.
Growing green manure crops and incorporating the same in Citrus orchards.
Periodical collection and destruction of brackets found near the collars
of other hosts.
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.
Soil drenching with 0.5% Trichoderma viside formulation +m0.3% copper
oxychloride solution is promising.
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.
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.
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.
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.
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.
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.
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
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.
Spraying of diluted glue or a detergent can wash off
the black coating.
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.
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
The use of certified sweet orange buildings on Rangpur
lime rootstock offers tolerance to the disease.
For Andhra Pradesh, Maharashtra and Karnataka, Rangapur
lime is recommended as a root stock resistant to tristeza.
For the Punjab region, Jatti khatti, Cleopatra mandarin
and sweet orange are recommended as resistant root stocks.
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.
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.
Disease Management
