DISEASES AND PESTS OF DATE PALM: Fungal diseases of date palm

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.

Disease symptoms

The bayoud disease attacks mature and young palms alike, as well as offshoots at their base (Saaidi, 1979).

External symptoms:

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.

Internal symptoms:

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.

Pathogen

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

Survival

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.

Host plants

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

Chemical control

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.

Cultural control

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).

Prophylactic measures

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.

Genetic control

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.  

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.

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.  

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.

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).

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).

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.

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

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.

TABLE 67

Estimates of loss caused by fruit rot

Source: Djerbi, 1983.

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).

Figure 90. Spread of Bayoud disease in Moroccan date plantation

A – During early years of attack

B – Later when most palms die and desertifi cation takes over

Figure 91. Spread and distribution of Bayoud in Algeria (1982) (Source: Djerbi, 1983)

Figure 92. Bayoud symptoms appear on one or more leaves of the middle crown.

Figure 93. Unilateral progression of the whitening and dying process on one side of the frond.

Figure 94.

A – Bayoud symptoms advance to the central cluster;

B – The palm dies when the terminal bud is affected

Figure 95.

A – Black scorch (Thielaviopsis paradoxa) symptoms on the attacked young frond;

B – See dwarfi ng effect on a young frond of one year old tissue culture-derived Medjool palm at Naute (Namibia)

C – Effect on four year old tissue culture-derived Medjool plant;

D – Late stage of attack.

Figure 96. Brown leaf spot caused by Mycosphaerella tassiana (De Note) John at three different stages of attack:

A – early;

B – medium;

C – late.

Figure 97. Diplodia disease caused by Diplodia phoenicum. Note the characteristic of the symptoms at an early stage of infection.

Figure 98. Fruiting structures called sori of the Graphiola leaf spot. Note it is on both sides of the pinnae.

Figure 99. An open spathe showing the attack by Mauginiella scaettae Figure 100. An adult date palm with a dead terminal bud fully destroyed by Belâat Disease

Figure 101. Conical wet heart rot of the terminal bud caused by Phytophtora sp. (Belaât)

Figure 102. Early stage of checking – Fruit rot caused by the high humidity around the bunch

Courtesy FAO