The presence of any impregnated layers of Calcium carbonate,
or clay pan affect the permeability and aeration of soils as well as
root growth and drainage. The depth of Kankar or clay pan in the soil
determines the depth of active root system and also the age at which
the decline starts. Decline is delayed by the increasing depth of hard
pan. Bad drainage causes poor aeration. The problem of poor drainage
exists in many parts of the country. Under the conditions of poor aeration
the root development is arrested, leading to less absorption of nutrients
and ultimately to decline of the top.
Citrus trees are quite sensitive to excess of salts and are injured
if the total salt concentration in the soil exceeds 1000 ppm. The presence
of excessive salts in any of the horizons up to a depth of 180 cm adversely
affects the health of citrus plants.
High CaCO3 content in soils is an important contribution
factor for the decreased availability of iron and zinc in soils. Deficiencies
of zinc, iron and manganese are quite common on calcareous soils.
A high pH not only affects the availability of micro-nutrients, but
a high sodium content of such soils may have direct toxic effects in
the case of sensitive crops like citrus. A low pH allows the leaching
of nutrients. Excessive irrigation is injurious for the health of citrus
Decline of citrus trees in South India is due to lack of adequate
Sweet oranges in South India suffer from deficiencies, the most common
on the plants being of zinc; in some cases lack of copper may cause
trouble and in acid sandy soils other elements may also be lacking.
The excess of iron and salinity and high uptake of phosphorous and
manganese as the causes of die-back have been pointed out.
Rough lemon as rootstock has shown incompatibility with Mosambi (sweet
orange) in Maharashtra state, with Sathgudi in Andhra Pradesh, Blood
Red and Mosambi in Punjab.
In many instances failure of citrus trees may be traced to incompatibility
between stock and scion.
In Maharashtra Rangapur lime as the rootstock for Nagpur oranges was
found to be better than Jamberi.
Susceptibility of rootstocks to root rots, salinity water logging,
cold, viruses etc. also leads to citrus decline.
Orchard management factors
In many cases, decline of citrus trees can be traced to mismanagement
Excessive intercropping, faulty, excess or no manuring, improper use
of irrigation water, are some of the common causes leading to citrus
A large number of insects attack citrus trees. Insects cause damage
to citrus in two ways viz. (i) by direct feeding and (ii) by acting
as vectors for the transmission of viruses.
These are the principal contributing factors of citrus decline.
Incidence of diseases in citrusis high.
Several diseases of citrus contribute towards citrus decline.
The incidence of nematodes has been recognized as one of the contributing
factors of citrus decline. They cause damage to citrus trees by feeding
on roots. Nematodes were found to associate with declining trees.
Two important nematodes which have world wide distribution are (i)
citrus nematode (Tylenchulus semipenetrans Cobb) which causes a disease
called ‘slow decline’ in California and ii) the burrowing nematode (Rodopholus
similes Cobb Thorne) which causes a diseases called ‘spreading decline’.
Severe nematode infestation leads to citrus decline.
Incidence of diseases in citrus is high. Several diseases of citrus
contribute towards citrus decline.
Survey of the citrus orchards in different parts of the country revealed
that Colletotrichum gloeosporioides to be the cause predominantly associated
with the die-back disease.
Majority of the isolations from diseased and apparently healthy twigs
yielded this fungus only.
Viruses are the most potent factors leading to citrus decline. Quick
decline in America was due to tristeza. Tristeza is universally present.
The presence of tristeza reported from every state in India. Other
viruses contributing to citrus decline are psorosis, xyloporosis, stubborn,
Virus infected trees do not put on normal healthy growth, become stunted,
show dieback of twigs and branches.
The productivity is substantially reduced.
Another important contributing factor is greening mycoplasma. It is
reported from all over the country.
All the citrus species and varieties are susceptible, but the sweet
and mandarin oranges are worst affected.
Budded, as well as, seedling trees suffer.
The trees do well in the first few years of their orchard life, bearing
very little crop and ultimately become uneconomical.
They show symptoms of ill health and decline.
The affected trees do not always die completely, but remain in a state
for decandance and unproductive for a number of years.
Some-times they may suddenly wilt and die in a day or two.
In early stages, symptoms are restricted to a few limbs, but eventually
the whole tree is involved.
Trees show sparse mottling leaves, stunted growth, sickly appearance.
Midrib and lateral veins of old, mature leaves turn yellow with interveinal
areas along the veins showing diffuse yellowing.
Leaves may turn yellow and are shed with the onset of summer or autumn
and the die-back of twigs starts.
Dead shoots stand out prominently and may be found dead right down
to the main trunk.
The entire tree bears short twigs carrying narrow small leaves on
their lower portion. Subsequent secondary growth consists of short,
upright small, weak shoots showing a variety of discolouration of leaves.
Often these leaves have green veins of green blotches. Occasionally,
small, circular, green spots appear on yellow tissue on leaves. The
die-back of weak shoots continues.
There is excessive flowering, but the fruits are not carried to maturity.
The fruits show distinct sun-blotching.
The feeder root system becomes depleted, roots turn black and sometimes
are covered with rotting bark.
Either only a few trees or entire orchard may be affected.
Until more is known about etiology, good cultural practices,
improvement in soil fertility and drainage, control of insect pests,
nematodes, etc. may be useful to minimize the incidence of decline.
Use of resistant rootstocks and certified budwood for propagation is
Control of Fruit drop
The main cause of fruit abscission is disturbance of auxins across
the stalk, Use of plant growth regulators for preventing the abscission
has been attempted. The most effective auxin in controlling the fruit
drop is 2, 4-D (2,4-dichlorophenoxy acetic acid). The effective concentration
ranges from 4 to 25 ppm for checking fruit drop.
The treatment of 2,4-D 20 PPM plus Benomyl 1000 PPM controlls the
drop of pea sized fruits. Two sprays of GA3, 15 PPM plus
Benomyl 1000 PPM plus urea 1% controlls the drop of marble size fruits.
Another one spray of above treatment further reduces the pre-harvest
The fruit drop is caused by stem-end and stylar-ends can be checked
with sprays of 20 ppm Aureo-fungin (an antibiotic) in March, April,
July and September, alternated with Bordeaux mixture (5:5:50) sprays.
Irregular flowering in Nagpur mandarin
The flowering in Nagpur mandarin in the absence of low temperature
is mediated through soil water deficit stress, the success of which is
dependent upon the nature of surface and sub-surface soil.
The soil properties such as high clay content, mainly concentrated
in sub-surface, low free lime content, coarse fragments and sand content
were found conducive for irregular flowering behavior of Nagpur mandarin.
These properties account for lesser moisture depletion during
water stress period, with the result, the required quantum of stress is
seldom obtained and plants flower erratically.
Use of growth retardants in irregular flowering orchards
The only remedy possible in irregular flowering
orchards is to stop the vegetative growth with the help of growth retardants
which otherwise continue due to constant moisture supply from sub-surface
layers. Application of Paclobutrazol @ 18g/plant at the time of imposition
of water stress is found very effective in inducing good floral response.