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DISEASES AND PESTS OF DATE PALM: Physiological disorders of date palm




  • Blacknose

    Blacknose applies to the abnormally shrivelled and darkened tip of a date. Deglet Nour and Hayani seem to be the most susceptible varieties to this physiological disorder (Fawcett and Klotz, 1932).

    Blacknose results from excessive checking of the epidermis, especially in the form of numerous small, transverse checks or breaks at the stylar end of the fruit. Pronounced shrivelling and darkening occur in proportion to the abundance of the checks and are related to humid weather at the Khalaal stage.

    Given the fact that checking is induced by high humidity and rainfall, it follows that measures to avoid conditions that tend to increase humidity are to be taken. The conditions to be avoided include excessive soil moisture and the presence of intercrops and weeds, especially at the susceptible stage of fruit development. According to Nixon (1932), bagging the fruits in brown wrapping paper was found to inhibit the occurrence of blacknose checking. Over thinning can also increase the incidence of checking and subsequent development of blacknose.

    Crosscuts

    Crosscuts is a physiological disorder of fruit stalks and fronds reported from the United States, Pakistan and a few Middle East date growing countries such as Israel and Iraq (Bliss, 1937; Djerbi, 1980). In the United States more than 1,000 fruit bunches were damaged in a single plantation in 1934, up to a quarter of the crop was lost.

    Crosscuts, or V- cuts, are clean breaks in the tissues of the fruit stalk bases and on fronds (Figures 107a and b). It consists of a slight to deep notch, similar to a cut artifi cially done by a knife. Fruits borne on strands in line with the break wither and fail to mature properly. Crosscuts result from an anatomical defect in the fruit stalks and fronds involving internal, sterile cavities leading to mechanical breaks during elongation of the stalk or the fronds. Crosscuts are commonly found in varieties having crowded leaf bases and its incidence increases as the palms get older. Sayer and Khadrawy varieties are especially susceptible to this disorder, and are no longer propagated in some countries (Carpenter, 1975).

    Crop losses may be avoided by using non-susceptible varieties, or by reducing the number of fruit stalks in susceptible varieties.

    Whitenose

    Whitenose disease is commonly found in Iraq, Libya and Morocco (Hussain, 1974; Djerbi, 1983). Dry and prolonged wind in the early Rutab stage causes rapid maturation and desiccation of the fruit resulting in whitish drying at the calyx end of the fruit. The affected fruit becomes very dry, hard and has a high sugar content. Hydration may correct this condition in harvested fruits.

    Barhee disorder

    Barhee disorder is characterised by an unusual bending of the crown of Barhee variety. The disease was first reported in California (USA) by Darley et al. (1960) and later in Al Basra (Iraq) by Hussain (1974). It was also found at the Kibbutz YOTVATA (Israel) by Zaid (1996). Affected palms were found to bend mostly to the south and sometimes to the south-west.

    At the Kibbutz Kineret (Israel), this phenomenon is severe and bending could reach an angle of about 90°. In Israel this bending disorder is also found with Dayri variety. Literature shows that it also affects Jahla and Aguellid varieties (Djerbi, 1983).

    Neither the cause nor the control of this disorder is known. However, at Yotvata Kibbutz (Israel), growers are correcting this situation by fi xing a heavy iron bar to the opposite side of the bending (Figure 108); fruit bunches from the opposite side are tied to this bar in order to move the actual weight against the bending side. It seems that within 2 to 3 years, the bending is corrected. Bunch handling is also proposed to correct such an abnormality (Yost, 1968).

    Black scald

    Black scald, different from blacknose, is a minor disorder of unknown cause occurring in the United States (Djerbi, 1983). It consists of a blackened and sunken area with a defi nite line of demarcation. The disease usually appears on the tip or the sides of the fruit, and affected tissues have a bitter taste. The appearance of the disorder suggests exposure to high temperature, but the exact cause is not defi nitely known (Nixon, 1951).

    Bastard offshoot

    This is a deformed growth of date palm vegetative buds especially of offshoots fronds (Figures 109a and b). Mohamed and Al-Haidari (1965) stated that the bastard condition is due to infestation by the date palm bud mite Makiella phoenicis K. It may also be due to reduction in growth caused by an inequilibrium of growth regulators.

    Leaf apical drying

    This is not a disease but a physiological reaction to transplantation of adult palms (injury of their root system). All palms with these symptoms recover within two to three years after their transplanting (Figure 110).

    Fertilisation injury

    As shown in Figure 60, this type of injury is present only with young tissue culture-derived palm plants (first two years after fi eld planting) and when fertilisers (N, P, K) are applied too close to the palm’s stipe. The nature of fertilisers is not the cause, but rather how close to the stipe the fertiliser was applied. If the damage is severe, it could cause the death of the young palm.

    Frost damage

    As stated in Chapter IV the date palm resists large temperature variations (-5 to 50°C) with a growth optimum between 32 and 38°C and a zero of vegetation of about 7°C. The vegetative activity will also decrease above 40°C and ceases around 45°C.

    When temperature falls below 0°C, it causes serious metabolic disorders with some injury to date palm leaves characterised by a partial or total desiccation. Water of protoplasma freezes after coming out from the cells. During defrost, water invaded inter-cellular spaces and affected leaves turn brown and desiccated. The severity of damage is related to the intensity and duration of frost:

    – At -6°C, leafl et ends become yellowish and dry up;

    – At -12°C, leaves of external crown desiccate; and

    – From -15°C, leaves of middle crown freeze and if low temperatures are suffi ciently prolonged, the central crown is reached and all foliage desiccates and the palm seems to be completely burned.

    The relative stable temperature of terminal bud and trunk allows the date palm to resist frost in winter, and high temperature in summer. In fact, the terminal bud is protected by the fi brillium and the leaf bases; the internal temperatures of the trunk and terminal bud undergo less big variations than those of atmosphere; the difference is round 14°C less in summer and 12°C more in winter.

    Frost injury to the date palm groves is not in direct loss of fruit on the palm but in freezing and loss of leaves so that the palm cannot support and mature the fruit crop the following year. Serious damage caused by frost was observed in plantations in Morocco (Guir, 1952; Tinghir, Tinjdad, 1965) and in USA (1873, 1940 and 1950) where temperatures of approximately -15°C occurred and frost caused a complete desiccation of leaves. In Morocco, palms were considered lost and the damage looked like a disaster to the local population. However, in spring, terminal buds started to grow although they were severely affected, and a good bloom was obtained (Djerbi, 1983).

    The most practical and available protection for the date growers is to turn on the water and keep the date plantation wet when the temperature begins to get low enough (-5°C and below). A date plantation just irrigated or being irrigated when the temperature falls, has some heat stored, which gives protection.

    Data are also available on principal date varieties and their susceptibility to cold:

    Resistant: Zahdi.

    Moderatly susceptible: Bentamoda, Bentkbala, Besser Halou, Hayani, Itima, Jouze, Khastawi, Mesh Degla, Sayer, Tadala, Tazizot and Thoury.

    Susceptible: Ammari, Amri, Arechti, Barhee, Beid Hmam, Dayri, Deglet Nour, Horra, Khadrawy, Maktoum, Medjool, Menakher and Saidy.

    Highly susceptible: Brain, Fursi, Hallawy, Hilali, Khlass, Khush Zebda and Ghars.

    Lack or excess of water

    The growth of the date palm is highly affected by variations in water availability and the water content of the soil. A decrease in yield, or complete failure in fruit production could result from these water variations.

    To compensate for high evapotranspiration, the date palm requires a quantity of water from 1,500 to 2,800 mm/year. Prolonged water stress will signifi cantly decrease growth and yield, and if the drought continues for several years, date palm can dry up and die.

    On the other hand, when the water table is high and drainage is inadequate and/or the leaching and transport of soluble salts is not complete, high evaporation rates tend to increase the concentration of salts in soil and in surface water. However, there are limits of salt tolerance and the date palm will not grow when soluble salt of the soil is above 6 percent. As stated in Chapter IV, the following shows the relationship between salts, growth and yield:

    – irrigation with water of salinity up to 3.5 mmhos/cm (i.e. 2240 ppm) will not affect the yield, provided that the leaching requirement of 7 % is provided for.

    – With an irrigation water of 5.3 mmhos/cm salt content and a leaching requirement of 11 %, yield reduction is only 10 %.

    – When the salt content of the irrigation water reaches 10 mmhos (i.e. 6400 ppm) and a leaching requirement of 21 %, the reduction in yield is around 50 %.

    The timing of leaching must be adjusted in each case, according to the quantities of soil and water, conditions of drainage, and characteristics of rainfall.

    Although date palms are resistant to fl ooding, healthy growth of palms requires a well-drained soil, and it is clear that irrigation must always go hand in hand with drainage.

    Serious losses, sometimes irreversible may occur in neglected date plantations (Figure 111). In such cases signs of decline appear on palms favoured by root penetration of numerous saprophytes and parasites that could lead to the death of palms (Djerbi, 1983).


    Figure 108. Barhee disorder. Note the iron bar fi xed to the opposite side of bending

     

    Figure 109.

    A – Bastard offshoots on a tissue culture-derived Barhee palm;

     

    B – A close-up on the same palm

    Figure 110. Symptoms of leaf apical drying caused by transplanting adult palms

     

    Figure 111. Salt stress shown on a seedling date palm at Guanikontes (Namibia)

    Courtesy FAO

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