CHAPTER XII: DISEASES AND PESTS OF DATE PALMBy A. It should serve as a brief reference and a source of information for extension specialists, date growers and anyone interested in the date palm zaid Ammari status.
The name bayoud comes from the Arabic word, “abiadh”, meaning white which refers to the whitening of the fronds of diseased palms. This disease was first reported in 1870 in Zagora-Morocco. Bayoud disease causes considerable damage that can sometimes take on spectacular proportions when the disease presents its violent epidemic aspect. Bayoud has destroyed in one century more than twelve million palms in Morocco and three million in Algeria. It is evident therefore, that Bayoud constitutes a plague to Saharan agriculture and at the present expansion rate, it will certainly pose serious problems of human, social and economic nature to other date-producing areas of the world. A brown stain appears lengthwise on the dorsal side of the rachis and advances from the base to the tip of the frond, corresponding to the passage of the mycelium in the vascular bundles of the rachis.
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Afterwards, the frond exhibits a characteristic arch, resembling a wet feather and hangs down along the trunk. This whitening and dying process of the pinnae may take from a few days to several weeks. The same succession of symptoms then begins to appear on adjacent leaves. The disease advances ineluctably and the palm dies when the terminal bud is affected. The rapid evolution of the symptoms depends mainly on planting conditions and on variety.
A small number of disease infected roots, reddish in colour, are revealed when an affected palm is uprooted. The spots are large and numerous towards the base of the stipe. As they advance towards the upper parts of the palm, the coloured conducting fascicles separate and their complicated path inside the healthy tissues can be followed. Palm fronds manifesting external symptoms exhibit a reddish brown colour when cut, showing highly coloured conducting fascicles. There is, therefore, a continuity of vascular symptoms that exist from the roots of the palm to the tips of the palm fronds. The observation of symptoms is necessary to recognise the bayoud, but to identify this disease with certainty, samples of affected fronds must be analysed by a specialised laboratory.
Contamination occurs regularly from palm to palm and more rapidly as the amount of irrigation increases. The appearance of the disease in locations far from the original infected area is caused primarily by the transport of infected offshoots or palm fragments harbouring the fungus. Soil treatment of this type of disease is destined, a priori, to fail and should therefore be avoided. Chemical control can, however, be feasible in the event of the discovery of primary sources of infection in a healthy area. In this case eradication techniques should be used: palms are uprooted and incinerated on the spot. The soil is then treated with methyl bromide or chloropicrin and the area closed off with replanting prohibited until further notice.
However, a signifi cant reduction in the amount of irrigation can retard the advance of infection,i. Since the contamination occurs mainly by root contact, disease-free palms can be isolated by digging a trench of 2 m deep around them. Water should be provided by a trough bridging the rest of the grove to this isolated plot. The essential task is to prevent the movement of contaminated plant material from an infected palm grove to a healthy one.
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This material, as has been previously mentioned, consists mainly of offshoots, palm fragments, manure and infected soil, and artifacts made from these materials. The only productive means of controlling bayoud disease lies in continued research into resistant varieties. In addition, the present success of date palm propagation by in vitro culture will make it possible to rehabilitate the Moroccan and Algerian palm groves that have been destroyed by bayoud. It will also be possible to reconstitute the palm groves presently threatened by Bayoud and create new date-growing areas with the help of high quality, resistant varieties.
In conclusion, bayoud disease is an epiphytic disease for which there is no known cure at present. Only preventive measures could protect healthy date plantations from this disease. Henna from bayoud-infected countries or regions. Adopt legislation preventing the conveyance of the above plant material.
Immediately report cases where symptoms similar to the ones caused by the bayoud appear. Information on bayoud and other major diseases and pests is necessary for the success of all above actions and must be available to all date growers. Black scorch has been observed on date palm in all date growing areas of the world. Symptoms are usually expressed in four distinct forms: black scorch on the leaves, inflorescence blight, heart or trunk rot and bud rot on palms of all ages. Infections are all characterised by partial to complete necrosis of the tissues.
Decay is most serious when it attacks the terminal bud and heart leading to the death of the palm. Some palms recover, probably by development of a lateral bud from the uninjured portions of meristematic tissue. These palms show a characteristic bend in the region of infection. This is why it is called Medjnoon. They set normal growth back by several years.
Thoory, Hayani, Amhat, Saidy and Halawy varieties are highly susceptible. Good sanitation is the first step in the control of black scorch. The affected fronds, leaf bases and inflorescences should be pruned, collected and immediately burned. The pruning cuts and surrounding tissues should be protected by spraying with Bordeaux mixture, lime-sulphur solution, copper sulphate lime mixture, dichlone, thiram or any new copper-based fungicides.
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Under a severe attack, affected palms are to be removed and burnt. Because it is a minor disease, no treatment is recommended. However, annual pruning of old infected leaves and their immediate burning is advised. 20 date varieties all around the world, although it appears to be most common to Deglet Nour. Symptoms are severe on offshoots and are characterised by death either while they are still attached to the mother palm or after they have been detached and planted out. On the leaves of older palms, the ventral mid-portion of the stalks is commonly affected, showing yellowish brown streaks, 15 cm to over one meter in length, extending along the leaf base and rachis.
The upper part of the leaves however, may still appear green and unaffected. Since the fungus usually enters the palm through wounds made during pruning or cutting when removing the offshoots, one precaution is to disinfect all tools and cut surfaces. 1 to 3 mm in diameter, with two layers. These sori are abundant on three year-old leaves, conspicuous on two year-old, but absent or infrequent on one year-old leaves. The normal 6 – 8 year life of date palm fronds will be reduced to 3 years by Graphiola disease and heavily infected leaves die prematurely which consequently reduce yield of the palm.
In Saudi Arabia, it is abundant in Kattif, Demam and Jeddah, but absent in Iraq. Reports of this disease also originate from Algeria and USA. It causes damage on inflorescences in neglected palm groves in hot and humid regions, or in areas with prolonged periods of heavy rain, 2 to 3 months before emergence of spathes. The disease is caused by Mauginiella scattae Cav. However, Fusarium moniliforme and Thielaviopsis paradoxa may rarely cause inflorescence rot.
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The first visible symptom of the disease appears on the external surface of unopened spathes and is in the form of a brownish or rusty-coloured area. It is most apparent on the internal face of the spathe where the fungus has already begun to infect the infl orescence. When the infected spathes split, they reveal partial or complete destruction of the flowers and strands. Severely damaged spathes may remain closed and their internal contents may be completely infected. Transmission of the disease from one palm to the next occurs through the contamination of male inflorescences during the pollination period. The infection of the young inflorescence occurs early and happens when the spathe is still hidden in the leaf bases.
The fungus penetrates directly into the spathe and then reaches the inflorescences where the fungus sporulates abundantly. The frequent appearance of the disease in neglected date plantations indicates that good sanitation and effi cient maintenance is the first step in the control of Khamedj disease. Some varieties are particularly susceptible to Khamedj disease: Medjool, Ghars, Khadrawy and Sayer. It is also called a decline disease because of its association with declining date palms. Unlike other date varieties planted in California, Deglet Nour was found to have the lower infection rate. The premature death of fronds followed by retardation and cessation of growth are the main disease characteristics followed by necrosis and destruction of the roots. A completely non- productive stage is the result of the attack.
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Belâat disease is caused by Phytophtora sp. Effi cient maintenance of date plantations is highly recommended to avoid attacks by this disease. Offshoots of affected palms usually remain healthy. Table 67 summarises these damage prevalent in different countries. The most common fungi causing fruit spoilage are the calyx-end rot caused by Aspergillus niger and the side spot decay caused by Alternaria sp. Protection from rain or dew is reached by using paper covers in the early Khalal stage to cover the fruit bunch.
The host list of palm species attacked by lethal yellowing is large and includes Phoenix dactylifera L. Developing fruits of the coconut start dropping from the palm followed by the formation of new inflorescences which rapidly become necrotic. In date palm the fronds become desiccated and grey-brown instead of becoming yellow. A soft rot of the growing point occurs, converting the meristematic area into a putrid, slimy mass.
The crown topples from the palm, leaving a naked trunk. The causal agent is a mycoplasma-like organism. It is believed that the pathogen is disseminated by wind-born arthropod vectors. Removal of diseased palms and their offshoots, quarantine measures, the use of tolerant types of palms and the treatment with antibiotics are the main control measures.
In Arabic, Al Wijam means poor or unfruitful. The disease is characterised by a retardation in terminal bud growth,and the whole crown of leaves formed after the occurrence of the disease have the rosetting symptoms. Leaves become chloritic and their life span is reduced. Both adult and young palms including offshoots are attacked alike.
Yields drop signifi cantly as more fronds are affected and the retardation in terminal bud’s growth becomes evident. Leaves are shorter and of irregular size. The causal agent remains unknown and no fungi or other pathogens were isolated. Chemical analysis of date palm leaves and soils showed that concentrations of all nutrients in the tissue were higher in leaves of unhealthy palms.
In addition, the conductivity and the phosphorus concentrations of the soil with diseased palms are higher than that of healthy ones. Quarantine measures seem to be the only means of limiting the spread of the disease. Since manganese is defi cient in unhealthy palms, this nutrient could be brought to these palms either by spraying or by injection. Tunisian varieties even though they all seemed to be equally attacked. Diseases of unknown cause of date palm4. The central cluster of fronds takes the form of an erect fascicle with a bent tip. The trunk bends and may even break.
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Effi cient maintenance and appropriate sanitation of the date plantation is the first control measure. Diseased parts of infested palms are to be collected and burnt in order to limit the spread of the disease. According to Djerbi, the disease is characterised by whitish, irregular blotches and streaks on the leaf stalks, midribs and pinnae that become outlined by reddish brown margins. The name “dry bone” comes from the drying out of the surface of the leaf stalk with a hard, smooth and white appearance.
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Affected palms, present a parasol form produced by the old and mid-level fronds, while new fronds present a short rachis with an irregular arrangement of pinnae and spines. Leaves remain green during the first stages and then decline and become yellow. The terminal bud assumes a conical form and becomes a stunted rosette. All these symptoms are accompanied by the abortion of the axillary buds, resulting in failure of fl owering for one or two seasons before foliage symptoms appear. Two to four years is the average duration of the disease from the appearance of the symptoms to the death of the palm. Also called “Rapid decline”, rhizosis is a minor but fatal disease of unknown cause.
The first symptom is premature falling off of fruits. However, if the attack is sometime after fruit development, the fruit withers and shrivels on the bunch. A reddish-brown discolouration of pinnae appears on mature fronds and the disease progresses from the bottom to the top of the fronds which rapidly die. Offshoots die with the diseased mother palm and the disease is hence self-limiting.
Blacknose applies to the abnormally shrivelled and darkened tip of a date. 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. 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. 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. Crop losses may be avoided by using non-susceptible varieties, or by reducing the number of fruit stalks in susceptible varieties. 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 is characterised by an unusual bending of the crown of Barhee variety. Affected palms were found to bend mostly to the south and sometimes to the south-west. In Israel this bending disorder is also found with Dayri variety. Neither the cause nor the control of this disorder is known. It seems that within 2 to 3 years, the bending is corrected. 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. It may also be due to reduction in growth caused by an inequilibrium of growth regulators. 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. 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.