Bayoud disease is caused by the fungal Fusarium oxysporum f. sp. albedinis in date palms

Bayoud definition

 Bayoud is a fungal disease affecting the date palm, having been observed for the first time around 1870 in the Draa Valley, north of Zagora in Morocco. Subsequently, this disease progressed towards the west and, significantly, towards the east, following the palm groves. It is worth noting that the palm groves located along the Draa wadi were particularly affected by Bayoud during this period. In Morocco, losses in palm trees have been estimated at more than 10 million individuals.

Causative agent of Bayoud disease

The parasite causing Bayoud disease was first isolated in 1921 and identified in 1934 by Malençon. It is a microscopic fungus belonging to the soil mycoflora. It is part of the group of ascomycete fungi, classified in the order Moniliales. It has been designated as Fusarium oxysporum f.sp. albedinis, a species that affects both young and adult palms.

Fusarium oxysporum characteristics morphology

Fusarium oxysporum albedinis strains isolated from the rachis of infected palms exhibit a distinct morphology, characterized by a salmon-pink thallus and a curly mycelium. In contrast, samples from soil, palm roots, and some intercropped plants, such as alfalfa and henna, which are considered healthy carriers, exhibit a diverse morphology. Fusarium oxysporum reproduction occurs exclusively asexually, involving the formation of microphialides and macrophialides, which generate microconidia and macroconidia, respectively. The bottle-shaped microphialides are arranged perpendicular to the mycelium and endogenously generate several microconidia, which are usually unicellular but can also be bicellular. These microconidia are characterized by their hyaline, globular or elongated appearance, with dimensions varying from 3 to 15 µm in length and 3 to 5 µm in width. Macrophialides, which are significantly larger than microphialides, produce macroconidia that are characterized by a falcate shape, are relatively few in number, have 3 to 5 septa and display dimensions of 20 to 35 x 3 to 5 µm. In old cultures and in soil, Fusarium oxysporum is distinguished by the presence of mycelium or macroconidia, as well as by chlamydospores that can be spherical, intercalary or terminal, with a diameter varying from 6 to 20 µm.

Symptoms and damage caused by Bayoud's disease in date palms

When date palms have Bayoud fusarium wilt disease, the first sign that they have it is palms that look pale and dead in the middle crown of the tree. An experienced observer must identify this subtle symptom.
The root system of the date palm is affected by conidia; it is therefore the mycelium, coming from the surrounding diseased roots through the soil, that carries out the attack. It concentrates on the very young tissues located near the caps of the rootlets or on areas altered by various biological or mechanical causes (states of suffering, insect bites, etc.) and penetrates them by intercellular and then intracellular routes until reaching the palm. The drying out of the palm does not manifest itself immediately over its entire surface, neither from top to bottom nor from bottom to top; it occurs laterally, extending from one side to the other through the top. Initially, on the affected side, yellowing and folding of the lower leaflets will be observed, followed by those of the immediately upper leaflets. This phenomenon will spread gradually, reaching the top of the palm, but it will remain unilateral throughout this process. It will thus be possible to observe organs with parts that are both dry and green, referred to as "hemiplegic." Desiccation will then continue in the area that remains green, this time from the top to the base. Subsequently, the palm will be completely dead, the mycelium having completed its crossing. Apply the same procedure to the remaining palms until the plant succumbs. When a date palm showing symptoms of Bayoud is carefully uprooted, areas of reddish-brown coloration can be observed at the roots, trunk, and palms. These areas indicate the points of entry and passage for the parasites within the tree.
In Morocco, the repercussions of the Bayoud attack have had deleterious effects on the oases, leading to a significant negative impact on the palm heritage. Thus, ten million palm trees have been wiped out, representing two-thirds of the total population, and several cultivars have disappeared, such as Berni and Idrar. It should be noted that the most successful commercial cultivars, such as Mejhoul, Boufeggous, Bouskri, Jihel, Bourar, and Bouitoub, are those that suffer the most pronounced effects of this disease. Indeed, the palm grove has recorded a decrease of more than 50% of its palm trees among the productive and commercial cultivars, thus leading to a significant loss of an essential source of income for the inhabitants of the oases.

Method of combating Bayoud disease of date palms

Prophylactic measures

Prophylactic measures are implemented in areas or countries that remain free of Bayoud with the aim of ensuring their protection. This mostly means not letting contaminated plants move from an infected palm grove to a healthy palm grove. It also means not letting diseased people or seeds and products from plants like alfalfa and henna (which could potentially spread the fungus to healthy areas) come into the country. It is also necessary to establish legislation to prohibit the transport of plant material.

Cultural struggle

Cultural techniques to combat the disease are based on avoiding conditions conducive to the proliferation of the pathogen. It is imperative to avoid henna and alfalfa crops, which require abundant irrigation conducive to the spread of the disease and are hosts of the pathogen. Irrigate palm trees from the time they are planted at a young age using a "drip" system. Enrichment of the soil with potash must be carried out with caution in order to avoid any excess since it is recognized as a mineral element likely to harm the development of vascular fusarium.

Genetic struggle

The adoption of resistant varieties is the only truly effective strategy to counter Fusarium wilt. Among the 32 Moroccan varieties evaluated in the experimental stations of Errachidia and Zagora, only six of them demonstrated complete resistance to Fusarium oxysporum. The species concerned include Bousthami noire, Bousthami blanche, Iklane, Tadment, Sayre Layalet, as well as Boufaggousse, also known as Moussa. Twenty years later, researchers identified another resistant variety, Boukhanni. However, it should be noted that all these varieties have a lower date quality, which is an obstacle to their large-scale transplantation. Researchers have done more work on carefully chosen clones like Ennajda, Al-Amal, Al-Fayda, Bourihane, and Mabrouk that have high resistance and quality. These clones came from natural seedlings or controlled crosses. The Ennajda clone, cultivated on a significant scale as part of the reconstitution of the Moroccan palm grove, produces high-quality fruits, recognized and accepted by palm growers.