CHAPTER XII: DISEASES AND PESTS OF DATE PALM
Stalk cabbage secretive hunter: control measures
By A. Zaid, P. F. de Wet., M. Djerbi and A. Oihabi
This chapter is an attempt to provide basic information on major diseases and pests of the date palm. It should serve as a brief reference and a source of information for extension specialists, date growers and anyone interested in the date palm phytosanitary status.
2. Fungal diseases of date palm
2.1 Bayoud disease
Origin, distribution and economic importance
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. By 1940, it had already affected several date plantations and after one century, the disease has practically affected all Moroccan palm groves, as well as those of the western and central Algerian Sahara (Killian and Maire, 1930; Toutain, 1967).
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. Bayoud destroyed the world’s most renowned varieties that are susceptible to the disease and particularly those which produce high quality and quantity fruit (Medjool, Deglet Nour, BouFegouss). It also accelerated the phenomenon of desertifi cation (Figures 90a and b). The result is an infl ux of farmers who have abandoned their land and moved to large urban centres.
The continued spread of bayoud highlights the problem threatening the important plantations of Deglet Nour and Ghars in Oued Rhir, Zibans in Algeria and even in Tunisia, which is presently free of the disease, but has 70 % to 80 % of the date palm areas under varieties susceptible it..
The disease continues to advance relentlessly to the east, despite prophylactic measures and regular attempts at eradication undertaken in Algeria (Djerbi et al., 1985; Kellou and DuBost, 1947:Figure 91). 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.
The bayoud disease attacks mature and young palms alike, as well as offshoots at their base (Saaidi, 1979).
The first symptom of the disease appears on a palm leaf of the middle crown (Figure 92). This leaf takes on a leaden hue (ash grey colour) and then withens, from bottom to top, in a very particular way: some pinnae or spines situated on one side of the frond wither progressively from the base upward to the apex (Figure 93). After one side has been affected, the whitening begins on the other side, progressing this time in the opposite direction from the top of the frond to the base.
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. 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 palm can die at any time from several weeks to several months after the appearance of the first symptoms (Figures 94a and b). 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.
The causal organism responsible for bayoud is a microscopic fungus which belongs to the mycofl ora of the soil and is named Fusarium oxysporum forma specialis albedinis (Killian and Maire, 1930; Malencon, 1934 and 1936).
Biology and epidemiology
Fusarium oxysporum f. sp. albedinis is preserved in the form of chlamydospores in the dead tissues of infected palm, especially in the roots which have been killed by the disease and in the soil.
Spread of Bayoud in palm groves
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.
Many plants are often grown as intercrops in palm groves, notably lucerne ( Medicago sa-tiva L .; alfalfa), henna ( Lawsonia inermis L.) and vegetables. (Bult et al., 1967; Djerbi et al., 1985 and Louvet et al., 1973). These plants can harbour the bayoud organism without manifesting any symptoms (symptomless carriers).
Control of Bayoud disease
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.
Since the factors that favour high yield in date palms (irrigation, fertilisation, etc.) are the same that favour the growth of the fungus, cultural techniques are not advised. However, a signifi cant reduction in the amount of irrigation can retard the advance of infection,i.e. stopping irrigation between the months of May and October, during the hot season in the northern hemisphere (Pereau-LeRoy, 1958).
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. Under these conditions these palms can be protected for more than 10 years (Djerbi, 1983).
The essential task is to prevent the movement of contaminated plant material from an infected palm grove to a healthy one. This material, as has been previously mentioned, consists mainly of offshoots, palm fragments, manure and infected soil, and artifacts made from these materials. Legislation preventing the conveyance of contaminated vegetative material from one country to another, or from one region to another, has been passed by various countries such as Algeria, Egypt, Iraq, Libya, Mauritania, Saudi Arabia, Tunisia and USA.
The only productive means of controlling bayoud disease lies in continued research into resistant varieties. Many resistant cultivars have already been obtained in Morocco from three sources: selection of bayoud-resistant varieties from those already existing (local and introduced), selection of high-quality, resistant clones from the natural population of the date palm, and creation of resistant and high quality varieties through a hybridisation programme (Djerbi et al., 1986; Toutain, 1968).
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. Therefore, the following measures are imperative:
— Forbid the introduction of offshoots and all other plant material (palm fragments, artifacts made from date material, manure and infected soil) originating from bayoud infected countries or regions.
— Forbid the import of seeds and unprocessed products of symptomless carriers such as Alfalfa (Lucerne) and 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.
2.2 Black scorch disease
Black scorch, also called Medjnoon or Fool’s disease, is caused by Ceratocystis paradoxa (Hohn) which is the perfect form of Thielaviopsis paradoxa.
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. Typical lesions are dark brown to black, hard, carbonaceous, and, as a mass, give the petioles, fruit strands and fruit stalks a scorched, charcoal-like appearance (Figures 95a, b, c and d).
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.
According to Djerbi (1983), black scorch has been observed on 17 date varieties. Thoory, Hayani, Amhat, Saidy and Halawy varieties are highly susceptible. The disease has also been observed on Zahdi, Menakher, Baklany, Gantar, Halooa, Fteemy, Sukkar Nabat, Horra, Besser Haloo, Nakleh-Zianeh and Koroch varieties (Klotz and Fawcett, 1932). Medjool and Barhee varieties are also susceptible to the disease (Zaid’s own observations).
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. Under a severe attack, affected palms are to be removed and burnt.
2.3 Brown leaf spot
Brown leaf spot as with other common date palm diseases, has also been observed in North Africa and the Middle East (Rieuf, 1968). Dark lesions are clearly delimited on green leaves, and on dying leaves the margin of the lesion remains reddish/brown as the centre becomes pale. Lesions also occur on the rachis, pinnae and spines (Figures 96a, b, c). Brown leaf spot is caused by Mycosphaerella tassiana (De Not) Johns.
Because it is a minor disease, no treatment is recommended. However, annual pruning of old infected leaves and their immediate burning is advised.
2.4 Diplodia disease
Diplodia disease, caused by Diplodia phoenicum (Sacc), has been recorded on 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. The fungus may infect the outside leaves and fi nally kill younger leaves and the terminal bud;, or the central cluster may be infected and die before the older leaves. Yellowish-brown streaks extend along the leaf base (Figure 97).
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. Dipping or spraying the offshoots with various chemicals (benomyl, Bordeaux mixture, methylthiophanate, thiram and other copper-based fungicides), has been found effective against the disease.
2.5 Graphiola leaf spot
Graphiola leaf spot is caused by Graphiola phoenicis (Moug) Poit., which is a smut fungus. It develops sub-epidermal, in small spots on both sides of the pinnae leaves, on the rachis and on the leaf base (Figure 98). The numerous fruiting structures emerge as small-yellow/brown to black sori, 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. This is because of the 10 — 11 month incubation cycle for this pathogen. On a leaf, sori are abundant on apical pinnae, less abundant on the middle section becoming even less on the basal section.
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.
Graphiola leaf spot disease is most common in Egypt (Delta region and Fayum) but absent in the less humid oases. 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. Around the world it is the most widely spread disease and occurs wherever the date palm is cultivated under humid conditions — mostly marginal date growing areas (Mediterranean coast) but also in the southern most humid regions of Mali, Mauritania, Niger and Senegal.
Control measures include leaf pruning coupled with treatment with Bordeaux mixture or any large spectrum fungicide (mancozeb, cupric hydroxide, cupric hydroxide + maneb, or copper oxychloride + maneb + zineb; 3 to 4 applications on a 15-day schedule after, sporulation, have been recommended). Genetic tolerance has been found in some varieties (Barhee, Adbad, Rahman, Gizaz, Iteema, Khastawy, Jouzi and Tadala).
2.6 Khamedj disease
Khamedj or infl orescencKe rot is a serious disease affecting most date growing areas of the old world. 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 can reappear each year on the same palm with the same intensity and it is estimated that, in serious cases, 30 — 40 kg of fruits are lost annually (Chabrolin, 1928).
During 1948 — 1949 and 1977 — 1978 severe outbreaks occurred in Iraq at Basrah, affecting male and female palms and destroying 80 % of the harvest (Al Hassan and Waleed, 1977). Serious damage was also recognised in Katif in the Kingdom of Saudi Arabia in 1983, with losses ranging from 50 to 70 %.
The disease is caused by Mauginiella scattae Cav., which is always found in a pure state in affected tissues (Figure 99). However, Fusarium moniliforme and Thielaviopsis paradoxa may rarely cause inflorescence rot.
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. The inflorescences become dry and covered with powdery fructifi cations of the fungus.
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. The collection and burning of all infected inflorescences and spathes should be followed by treating the diseased palms with the following fungicides after the harvest and one month before the emergence of spathes: a bordeaux mixture or a copper (1/3), sulphate-lime (2/3) mixture or a 3 % dichlone spray or a 4 % thirame spray at the rate of 8 litres per palm or with benonyl and tuzet at the rate of 125 g/hl (Al Hassan et al., 1977).
Some varieties are particularly susceptible to Khamedj disease: Medjool, Ghars, Khadrawy and Sayer. Others manifest a good capacity for resistance: Hallawi, Zahdi, Hamrain and Takermest (Laville, 1973).
2.7 Omphalia root rot
Omphalia root rot was recorded in California, USA and in Mauritania by Fawcett and Klotz (1932) and Bliss (1944), respectively. It is also called a decline disease because of its association with declining date palms.
Four Mauritanian varieties (Ahmar, Marsij, Mrizigueg and Tinterguel) were found to be susceptible to this disease by Sachs (1967). Unlike other date varieties planted in California, Deglet Nour was found to have the lower infection rate.
Two species of Omphalia ( O. tralucida Bliss and O. pigmentata Bliss) cause the disease and are widely spread in date plantations of Coachella Valley, CA-USA and in Kankossa (Mauritania) (Djerbi, 1983).
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.
The use of Brestan or Dexon at the rate of one spray every two weeks for eight weeks was recommended by Sachs (1967) as a chemical control measure.
2.8 Belâat disease
Belâat disease was reported by several authors and from several North African countries (Algeria, Morocco, Tunisia, etc.) (Maire, 1935; Monciero, 1947; Calcat, 1959 and Toutain, 1967). The entire cluster of young fronds will whiten and die as a result of the attack, followed by the infection and death of the terminal bud (Figures 100 and 101). Accompanied by secondary organisms, the infection will progress downward in the trunk as a conical wet heart rot form, releasing an odour of acetic and butyric fermentation.
Belâat disease is caused by Phytophtora sp. similar to P. palmivora (Djerbi, 1983). Effi cient maintenance of date plantations is highly recommended to avoid attacks by this disease. Spraying with maneb or Bordeaux mixture at the rate of 8 litres/palm could control the disease at its early stages. Offshoots of affected palms usually remain healthy.
Fruit rot damage varies from one year to another depending on humidity and rain and also on the time of these factors from the Khalal stage until fruit maturation (Figure 102). Even though losses vary from one country to another and from one variety to another, they can be easily estimated to be between 10 % and 50 % of the harvest (Darley and Wilbur, 1955; Calcat, 1959; Djerbi et al ., 1986). Table 67 summarises these damage prevalent in different countries.
Estimates of loss caused by fruit rot
Medjool, Deglet Nour
Covering with paper wraps
Dusting with Fungicides
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.
Lowering the humidity inside the bunch, by the use of wire rings, and/or by removing a few fruit strands from the centre of the bunch, will facilitate ventilation and drying of wet fruit. Protection from rain or dew is reached by using paper covers in the early Khalal stage to cover the fruit bunch. Fungus spoilage could also be limited by dusting the fruit bunches during the Khalal stage with 5 % ferbam, 5 % malathion, 50 % sulphur and an inert carrier (40 %) (Djerbi, 1983).
3. Phytoplasmic diseases of date palm
3.1 Lethal yellowing
Lethal yellowing destroyed about 300,000 coconut palms in Miami (Florida, USA) in less than fi ve years (McCoy, 1976). Previously, the disease killed more than 15,000 coconut palms in Florida, (USA).
The host list of palm species attacked by lethal yellowing is large and includes Phoenix dactylifera L.; P. canariensis Hort., and P. reclinata Jacq. (Thomas, 1974).
Developing fruits of the coconut start dropping from the palm followed by the formation of new inflorescences which rapidly become necrotic. These first symptoms are followed by a rapid and generalised yellowing, leading to the death of the palm (Figure 103).
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.
Nixon (1954) observed this disease in Al Hassa (Saudi Arabia). 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. Newly formed leaves are reduced in size and marked by a faint narrow, yellow longitudinal line on the midribs (Figure 104). Leaves become chloritic and their life span is reduced. Death of leaves starts from the distal end and extends towards the base. Diseased spathes split open before their complete emergence and are reduced in size. The number and size of the bunches produced are also reduced year after year till the diseased palm fails to produce and dies.
Positive amplifi cation bands were obtained from DNA templates extracted from diseased tissue of date palm using the Polymerase Chain Reaction (PCR). These DNA tests offer basic support to the hypothesis that the cause of Al-Wijam disease is a Mycoplasma- like organism (Djerbi, 1999; personal communication).
3.3 Brittle leaves disease
Brittle leaves disease, also called «Maladie des Feuilles Cassantes» in French, was first observed in Nefta, Tozeur and Degache date plantations (Tunisia) and in Adrar, M’zab and Biskra (Algeria) (Djerbi, 1983).
Both adult and young palms including offshoots are attacked alike. A broad chlorotic striping of the pinnae followed by drying of the tip of the frond is the first symptom of this disease (Figures 105 and 106).
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. However, recent investigations with PCR showed that the causal agent seems to be a Mycoplasma-like Organism (Djerbi, 1999; personal communication).
Chemical analysis of date palm leaves and soils showed that concentrations of all nutrients in the tissue were higher in leaves of unhealthy palms. The exception was the concentration of manganese, which was ten times lower in the unhealthy palms (Djerbi, 1983). In addition, the conductivity and the phosphorus concentrations of the soil with diseased palms are higher than that of healthy ones. These results suggest that the areas affected by the disease have a build-up of major nutrients and salts as a result of irrigation, which have contributed to the high electrical conductivity. High pH and conductivity may have caused lack of manganese in the soil.
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. Djerbi (1983) found a gradient of susceptibility within Tunisian varieties even though they all seemed to be equally attacked.
4. Diseases of unknown cause of date palm
4.1 Bending head
Also called «Le Coeur qui penche» in French, the bending head is a minor disease observed in Algeria, Egypt, Mauritania and Tunisia (Munier, 1955). The central cluster of fronds takes the form of an erect fascicle with a bent tip. The trunk bends and may even break.
Thielaviopsis paradoxa and Botryodiplodia theobromae Pat are fungi commonly isolated from declining palms (Brun and Laville, 1965). 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.
Originally this disease was first reported by Fawcett and Klotz (1932) in USA. Other cases were found in Algeria, Egypt and Tunisia (Djerbi, 1983). 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. Lesions, from one to several centimetres, involve only the epidermis and a thin layer of subjacent tissue.
According to Fawcett and Klotz (1932), a bacterium is commonly found associated with the lesions, and certain palms are more susceptible than others.
4.3 Faroun disease
Laville and Sachs (1967) reported this disease of unknown cause from Mauritania. 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. According to Djerbi (1983) no varietal resistance has been observed.
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. According to Djerbi (1983), no varietal resistance has been observed.
5. Physiological disorders of date palm
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 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 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.
5.4 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, 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).
5.6 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.
5.7 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).
5.8 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.
5.9 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:
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.
5.10 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).
6. Major pests of date palm
The date palm and its fruits are subject to attacks by several pests that are, in most cases, well adapted to the oasis environment. Damage caused by pests is considerable and leads to heavy economic losses.
White scale, caused by Parlatoria blanchardii Targ., is widely present in most date palm growing areas of the world except in USA, where it was eradicated in 1936, and in some countries of the southern hemisphere (Namibia and RSA).
It is considered a serious pest in Algeria, Kuwait, Libya, Mauritania, Morocco and Tunisia. Iraq, Oman, Saudi Arabia and Sudan consider this pest a moderate one, while Egypt, Jordan, UAE and Yemen consider it a minor pest.
Damage by white scale is very serious on young palms between two to eight years of age, but even under severe attacks, the palm and its offshoots do not die.
Nymphs and adults suck the sap from the leafl et, midribs and the dates. Under each scale insect, a discoloured area appears on the leafl et. Heavy infestation causes leafl ets to turn yellow and contributes to the premature death of the fronds (Figures 112, 113 and 114).
Respiration and photosynthesis are almost stopped resulting in early death of the infested leaf. Damage on fruits is easily noticeable and the production is not marketable. The cycle of Parlatoria blanchardii Targ. is summarised in Figure 115. The number of generations developed during one year varies from three to four depending on temperature.
The natural enemies of Parlatoria blanchardii are: Hemisarcoptes malus, Chrysoperla vulgaris, Cardiastethus nazarenus, Coccinellidae (29 species), Nitidulidae (5 species), Mycetaeidae (1 species), Aphytis mytilaspidis, Cybocephalus nigriceps, Cybocephalus rufi frones, Chilocorus bipustulatus var. iraniensis and Chilocorus sp. (FAO, 1995) (Figure 116).
Natural enemies and pruning normally keep pest populations at tolerable levels. In the 1970s the coccinellid Chilocorus bibustulatus var. iraniensis was introduced into Mauritania and Morocco, but permanent establishment failed and efforts were discontinued. In the 1980s, attempts were made to introduce the coccinellids into northern Sudan, but they were not successful either. In 1993 the coccinellids were released in Oman, but there is
no information on their establishment. The introduction of coccinellids is currently being investigated in Tunisia.
Chemical control appears to be conducted occasionally in young plantations. Mineral oils are used (Djerbi, 1994).
Red scale, Phoenicococcus marlatti . cockerell, is exclusively a pest of palms, particularly date palms, with other palms as host plants (e.g.: Doupalm, Canary Island palm and the California fan palm). It is probably found wherever date palm is cultivated, but with no great threat (Dowson, 1982). The extent of its damage is known to be less than that caused by the Parlatoria scale.
Leaves of date palm are often found to be clotted over with thin, minute, greyish scales with darker centres (Figures 117a and b). The darker spot is oval in outline and is the body of the insect itself. The individual scale is seldom larger than a small pinhead, roundish in shape, and deep pink to dark red in colour, but partly or entirely covered with a white waxy secretion that forms a cottony mass (Nixon and Carpenter, 1978).
All exposed portions of the palm can be attacked by the pest. Heavy infestations could cause complete coverage of the leaf surfaces by scales, which will result in interference with the metabolic functions of the plant. Attacked leaves and underlying tissues may be damaged to a depth of a few millimetres and will consequently be killed in severe cases.
The red date scale usually stays out of the light and is found massed on the white tissues at the bases of the leaves and fruitstalks, where it is protected by fibre and other leaf bases. Frequently, the scale is found on roots underground. The red scale is not as easily detectable as most other scales because of its natural tendency to hide. Red scale is not suspected until the base of the green leaf is cut and subsequently observed. Stickney et. al. (1950) provided a comprehensive study of the insect’s biology.
P. marlatti . passes its lifecycle in a protective covering of wax that it secrets. The female produces numerous eggs under the protective scale. After the eggs hatch, the nymphs crawl out and move about freely, feeding at various positions. Once a suitable location on the host plant is selected, nymph’s will insert their needle-like mouth parts to suck the sap. When they start to feed, layers of wax, forming the covering of the scale over the body, are secreted.
Soon after beginning to feed, adults will moult. Later on, males are incapable of feeding and will mate with the females and die. The female, once fertilised, increases rapidly in size and produces eggs before dying within the scale.
The pest breeds actively during the summer months and hibernation starts in early winter. A complete life-cycle takes approximately 55 days during summer and 158 days during winter. Three to fi ve generations could be found annually.
It is worth mentioning that the scale appears to cause considerable damage to plants growing under favourable conditions. Areas where the climate is milder or more humid may also face severe scale attacks.
Even though this scale insect is regarded insignifi cant, and with no economic impact, the first measure is to cut away all attacked leaves and burn them in order to stop the spread of the pest. Infested palms, offshoots or even tissue culture-derived plants, which are still at the hardening phase, must be sprayed with malathion 370 — 450 g or with parathion 120 g a.m. dissolved in 450 litres of water.
Since the scale is a sucking insect, the use of ultracide or dimenthoate when the pest is mobile is also recommended (Djerbi, 1994). Infested offshoots could also be subjected to a temperature of 50°C for 65 hours in an insulated room. General predators, such as Pharoscymnus anchorago (Fairmaire), are considered as active predators.
Bou Faroua, also called Goubar or Old World date mite, is caused by Oligonychus afrasiaticus McGregor, and O. pratensis Banks. This mite is present in all date growing areas, and damage is severe in neglected plantations.
Immediately after fruit set (Hababouk stage), mite eggs are deposited to produce larvae which will feed on the fruits and later cover these with a web retaining sand particles. The cycle length is about ten to fi fteen days depending on temperature. Mites will rapidly multiply causing the drop-off of the fruits. Affected mature fruits are of no commercial value (Figures 118 and 119).
Chemical analysis of infested and fully matured dates shows that the water soluble substances such as sugar are less in infested dates (Hussain, 1974). Under Iraq’s climate, the Old World date mite has six overlapping generations during the fruiting season of palms (Hussain, 1974). The mite population on dates reaches its peak during the middle of July. The first appearance of mite on immature dates is during the first week of July. Even though they are found on all parts of the date, the majority of mites congregate near the calyx area, where most of the eggs are laid. Mite and eggs are also found on fruit stalks. The mites migrate to the palm crown during the last week of August. Hussain (1974) states that the fi bres and frond bases taken from infested palms during the winter months show adult and nymph mites. This mite does not hibernate on the leafl ets, date palm seedlings, offshoots or on the many species of vegetation in the plantation.
Dusting date bunches early in July with sulphur at the rate of about 100 — 150 g per palm is effective (Djerbi, 1994). The Iraqi variety «Sayer» is relatively resistant to mite attack.
Caroub moth, also called «Ver de la Datte» in French, is caused by Ectomyelois ceratoniae . Zeller, and is found in all date growing areas. The larva of the Caroub moth attacks dates in plantations, packing houses and stores. Eggs are laid on the dates and hatching begins four days later. The larval period is about three weeks in warm months and eight weeks in colder months. The pupal period is about fi ve days.
Taking into account the moth’s life cycle, it is recommended to protect the fruit bunches, to clean the plantation from wind-fallen fruits and to fumigate harvested and stored dates. The use of pheromone traps will not only help to determine the emergence of moths but also to estimate the population level. The rate of infestation could be lowered by spraying the infested fruits with Bacillus thuringiensis (Djerbi, 1994).
6.5 Rhinoceros beetle (Oryctes rhinoceros Linné)
The adult beetle is a stoutly-built insect about fi ve centimetres in body length and shiny black in colour with a reddish under-surface covered with short, fi ne hair. Its tibiae are furnished with thorn-like spines. This insect has earned the name of rhinoceros beetle because of the presence on its head of a horn-like structure, which is conspicuously longer in the male (Figure 120).
The adults feed on tender leaves, inflorescences and fruit stalk of the fruit bunches of date palm, (Figures 121a, b and c) whereas the grubs thrive on decomposing dung and decaying vegetable matter like stumps and trunks of palms. This insect is also a pest of coconut and other palms.
Within a week of the emergence of the females they start laying eggs. The whitish-brown eggs are laid singly in dung heaps and decomposing vegetable matter. The eggs hatch out into fat soft-bodied pale-yellowish curled larvae in about 10 to12 days. The larvae become full-grown in about 4 or 5 months and they take another 6 to 7 months in hibernation before they transform themselves into pupae. The full-grown larva is a stout fl eshy creature measuring about 7 cm in length with brownish head and dirty white appearance. The full-fed grub pupates in the dung heaps, etc., in a specially prepared oval chamber made of soil or excretory matter. The adult beetles emerge from the pupae in about 3 to 4 weeks and fl y to nearby palms and start feeding on them causing damage. There is only one life-cycle during the year.
Contrary to other pests, only the adult beetles are responsible for causing damage to the palms. The pest has been found to be more destructive to young plants. They remain hidden during the daytime and become active at night, when they fl y about and reach the tops of date palms. They drill large holes close to the base of the growing heart-leaf and enter the stem. They feed on the softer tissues of the growing heart-leaf and cut right through it, with the result that further growth stops and the palm ultimately dies. The beetle also causes damage by boring into tender fronds, chewing tissues and throwing them out as a fi brous dry mass (Figure 122). Fronds may hence break and if the growing point is bored the plant dies off. Most of the damage occurs during the rainy season.
The adult beetles should be attracted and destroyed by putting up mercury-vapour light traps at regular intervals in infested plantations.
The light trap is based on the fact that some insects are very active at night and are attracted by the light. This method of mechanical control is presently included in Integrated Pest Management.
The degree to which insects are attracted varies according to the type of traps as well as to the nature and power of light. It was shown that the mercury-vapour light is the best tool to attract insects.
The advantages of using light traps are::
— to obtain information on the number of captured species;
— to predict the occurrence of an outbreak of an insect-pest; and
— to use it as a mechanical control method since it can reduce the number of insects as well as production losses.
The insect collector (D) should be half filled with diesel, kerosene or paraffin; (Figure 123).
6.6 Red palm weevil and African palm weevil
The red palm weevil (RPW), Rhynchophorus ferrugineus Oliv., also called the Indian palm weevil, is well known in the Middle East where it causes severe damage on date palms (Table 68). The RPW was first noted in the Arabian Peninsula in the mid 1980’s and in Egypt in 1992 (Figure 124). The weevil was first observed in Rass El Khaima, United Arab Emirates in 1985. Approximately, 5 to 6 % of palms in the Middle East region are infested with the RPW with an annual rate of infection of about 1.9 (Table 69).
Distribution of red palm weevil in the Near East