Mal Secco disease in citrus fruits

Mal Secco is one of the most dreaded diseases of lemon trees. It causes a progressive drying of the branches, then of the entire trunk. Of fungal origin, this disease often takes hold following an injury or a period of water stress, before spreading throughout the tree’s tissues. While initially subtle, it can significantly compromise the orchard’s productivity and the trees’ health. Let’s explore the causes of Mal Secco, the damage it causes, and prevention methods adapted to the organic management of Mediterranean orchards.

Mal Secco: a fungal disease

Mal Secco takes its name from Italian, literally meaning “dry disease”. This disease, first observed in late 19th-century Greece, has since become one of the major scourges of lemon trees in Mediterranean areas.

Its causative agent, Phoma tracheiphila (also known by the synonyms Plenodomus tracheiphilus or Deuterophoma tracheiphila ), belongs to the order Pleosporales. This pathogenic fungus attacks the vascular system of citrus trees, progressively blocking the flow of sap. The lemon tree ( Citrus limon ) is its primary victim, although other species such as bitter orange, citron, bergamot, and Citrus jambhiri can also be affected. Orange, mandarin, clementine, and grapefruit trees, on the other hand, are much more resistant.

A typically Mediterranean distribution

The Mal Secco thrives in mild, humid climates , where citrus fruits naturally flourish. It is found throughout almost the entire Mediterranean basin: Greece, Italy, Turkey, Tunisia, Algeria, Cyprus, and Israel. Its presence even extends to the Caucasus, Georgia, Iraq, and Yemen.

A fungus that colonizes the sap

Mal Secco is a tracheomycosis, that is, a fungal disease that attacks the vascular system of wood (the xylem). Once established, the fungus colonizes these channels and blocks the ascent of sap, thus depriving the upper tissues of water and nutrients.

Infection occurs primarily through wounds : poorly healed pruning, wind damage, friction, or even a crack caused by cold. Unlike other diseases, Phoma tracheiphila spores do not enter through the stomata of leaves, but through these accidental openings in the tissues.

The conidia , the reproductive spores of the fungus, develop in tiny black structures called pycnidia , visible on the surface of infected branches. These conidia are dispersed by water—rain, irrigation, runoff—and can contaminate other nearby trees.

The development of the fungus is highly dependent on climatic conditions.

• Between 14 and 28 °C , the infection progresses actively, with an optimum between 20 and 25 °C .
• Above 30°C , the mycelium stops growing.

These parameters explain why the disease is most prevalent during cool and wet periods , often between September and April .

In contaminated wood left on the ground, the fungus can survive for up to four months , and remain infectious for almost a year in an affected orchard.

Symptoms of Mal Secco

The development of Mal Secco often begins at the tips of the branches before gradually spreading to the main branches and then the trunk.

In spring, the leaves of young shoots discolor , taking on a characteristic yellowish hue. The veins lighten, the leaves curl up, and eventually fall off. This yellowing, or chlorosis , precedes the complete drying out of the branches.

On affected branches, the wood changes color : initially orange, it turns gray or reddish-orange. In cross-section, this internal tint is a typical sign of the fungus’s presence. When the wood dries out completely, it becomes blackish.

Infected tissues often show gum production in the xylem, a result of the tree’s defense reaction to canal obstruction.

Different forms of Mal Secco

Mal Secco does not always manifest itself in the same way. Depending on the vigor of the tree, soil conditions, or climate, three main forms are observed:

• Classic mal secco : it develops slowly, starting from the branches towards the trunk. The drying out is gradual, often over several months.
• The fulminant disease : it acts brutally, affecting the root system and leading to the rapid death of the tree.
• Black disease : a chronic and persistent form, characterized by continuous infection of the wood and slow but lasting weakening.

Progression and severity of the disease

Mal Secco is considered the most serious fungal disease of lemon trees . The fungus progresses through the internal tissues, gradually blocking all circulation. Complete dieback of the tree can occur in a few months in the most severe cases, or over several years if the progression is slow.

Affected trees quickly become unproductive . Even when they still appear alive, flowering and fruiting drop drastically. Some branches die, while others send up new shoots at the base or on the rootstock, giving a deceptive appearance of regeneration. These shoots, however, also become infected, perpetuating the cycle of decline.

Economic losses can be heavy in unmonitored orchards: reduced yield, cost of replacing trees, loss of fruit quality, not to mention the biosecurity measures to maintain to prevent any spread of the pathogen.

A parasite under regulatory surveillance

Given its devastating impact on Mediterranean orchards, Mal Secco is subject to mandatory control measures in several bordering countries. This classification reflects the severity of the threat and the difficulty of eradicating it once the fungus has been introduced.

Plant health services often impose strict protocols in contaminated areas: elimination of affected plant material, restriction of movement of citrus plants, sanitary control of grafts and regular inspection of at-risk plots.

This increased vigilance aims to limit the spread of the fungus , whose survival in dead wood and pruning cuts makes management particularly delicate.

How to prevent Mal Secco?

Prevention remains the best way to protect a citrus orchard from Mal Secco . 

• Choose certified and healthy plants : source only from nurseries located in disease-free areas.
• Avoid replanting in sensitive areas of orchards or soils where infected trees have been uprooted.
• Disinfect pruning tools between each tree using pure bleach, letting them soak for at least one minute. Have several pairs of pruning shears or saws on hand to ensure continuous disinfection.
• Do not prune in wet or rainy weather: these conditions facilitate the spread of the fungus.
• Limit soil work in autumn and winter, when humidity is high.
• Remove affected parts immediately by pruning back to healthy wood. In case of advanced infection, cut back the main affected branch.