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  Citrus Decline

Introduction > Factors Responsible For Citrus Decline > Symptoms > Control Measures >


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Introduction

  • There seems to be less enthusiasm and more discouragement for growing Nagpur orange crop.
  • One of the main reasons for this steady decline appears to be "Die-back".
  • Citrus decline, also often referred to as citrus "die-back" is not a specific disease but is a symptomic expression of many disorders in the plant.
  • The malady is also named as ‘frenching’. ‘chlorosis’, ‘neglectosis’ etc.
  • Terms like die-back or decline are now used for certain specific disorders in other countries.
  • The term die-back is usually used as a synonym for exanthema of copper deficiency.
  • The term actually signifies the continuous dying of twigs.

 
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Factors responsible for citrus decline

Soil Factor

  • 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 plants.

Nutritional factors

  • Decline of citrus trees in South India is due to lack of adequate plant nutrition.
  • 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.

Rootstock factors

  • 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 and negligence.
  • Excessive intercropping, faulty, excess or no manuring, improper use of irrigation water, are some of the common causes leading to citrus decline.

Insect-Pests

  • 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.

Nematodes

  • 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.

Fungal diseases

  • 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

  • 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, exocortis etc.
  • 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.

 
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Symptoms

  • Symptoms vary with the cause of the malady.
  • 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.

 
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Conrol measures

  • 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 also useful.

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 fruit drop.
  • 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

Causes:

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.

Control: 

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.


 
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