Physiological disorders of mandarins

Physiological disorders

Fruit drop

         In spite of very high initial flowering and fruiting in mandarins, the ultimate yield is often low primarily owing to heavy fruit drop. However, all fruits that fail to mature do not drop at one time but at different times. There are more or less definite periods or stages when extensive dropping occurs. In mandarins, the shedding of flowers and fruits come in more or less in 3 distinct waves. The first wave occurs soon after fruit setting, second during May–June known as June drop and third one known as pre-harvest drop, i.e. the drop of mature fruits before harvesting. Fluctuating temperature, low atmospheric humidity, imbalance of soil moisture, lack of proper nutrition, hormonal imbalance, incidence of insect-pests and diseases are some factors causing fruit drop. Accordingly, maintenance of appropriate soil moisture level during fruit development and application of growth regulators—2, 4-D (10ppm), NAA (5ppm), 2, 4, 5-T (5ppm)—check fruit drop quite effectively. Further, application of Aureofungin @ 20 ppm helps in better retention of fruits through control of fungal diseases.

Granulation

        It is a physiological disorder of juice sacs of citrus including mandarins wherein they become comparatively hard, assume a greyish colour and become somewhat enlarged. The concentration of pectic substances increases, whereas there is reduction in juice content, TSS and acid content. Because of low sugar and acid content, the granulated vesicles become rather tasteless and colourless. Young, vigrous trees are more likely to develop granulated fruits than older ones. Similarly, large fruits have more granulation than small ones. In addition, granulation increases as the picking season advances. The incidence of granulation is highly specific to the type of the mandarin being cultivated. It is favoured by high relative humidity and temperature during spring.

      Spraying of lime reduces the extent of granulation. Reduction in irrigation also lessens its incidence. The application of 2, 4-D (12ppm), zinc and copper reduces the incidence of granulation considerably.

Decline

        After fruitful production for about 15 years, mandarin orchards start bearing little crop and become uneconomical. They show symptoms of ill health and decline. The affected trees do not die completely but remain in state of decadence and unproductiveness for a number of years.

         Initially, only a few limbs of the plants are involved but later whole tree is affected. Plants show sparse foliage, stunted growth, sickly appearance and in leaves, mid-rib, lateral veins and interveinal area show diffused yellow colour leading to ultimate shedding of leaves. As a result of dieback, twigs become short and bear only a few narrow leaves at their basal ends. Such plants are also characterized through excessive flowering and very poor fruit set. Unfavourable soil conditions (presence of hard pan, high pH, poor drainage and high salts), malnutrition, poor orchard management, indiscriminate use of fertilizers, intercropping, incidence of insect-pests and diseases are major factors contributing to it.

 
         Good cultural practices, improvement in soil fertility and drainage, control of insect-pests, nematodes and diseases may be useful to minimize its incidence. Use of resistant and compatible rootstocks and certified budwood for propagation are strongly recommended for a healthy and productive mandarin orchard.