Citrus Mal Secco

The pathogen primarily infects citrus trees through wounds caused by pruning, wind damage, frost, or agricultural machinery. Once inside, the fungus colonizes the plant's xylem, obstructing water and nutrient transport, which inevitably leads to the decline and death of the affected tissues.

The life cycle of the pathogen involves the production of pycnidiospores within fruiting bodies called pycnidia. These structures develop on the bark of infected branches and release spores that are disseminated by wind, splashing rain, or pruning tools.

Mal Secco is considered one of the most destructive diseases in citrus-growing regions, especially in Mediterranean climates. It poses a significant threat to orchard sustainability and requires rigorous phytosanitary management to prevent outbreaks.

Early identification of the fungus is challenging because internal vascular damage often precedes external symptoms, making routine orchard inspections and diagnostic testing essential for early detection.

Citrus species are the primary hosts of the pathogen, with lemon trees showing the highest susceptibility. Other citrus types, including orange, citron, and bergamot, are also vulnerable, making the disease a widespread concern for commercial citrus production.

The disease leads to the progressive wilting of the tree canopy. Symptoms typically start in the younger branches and gradually spread throughout the entire tree, resulting in total crop loss and eventual tree death if left untreated.

Damaged trees suffer from rapid loss of foliage, dry and brittle branches, and a overall decline in vigor. In many cases, infected trees cannot be saved and must be removed and destroyed to prevent the spread of the pathogen to healthy plants.

Economic damage is severe, encompassing loss of revenue, costs of orchard sanitation, and restrictions on the movement of nursery stock. The presence of the disease in a region often leads to long-term quarantine measures that impact trade.

The pathogen affects not just the fruit production but also the long-term structural integrity of the trees, leading to the loss of investment in orchard development and maintenance.

Infection activity is most pronounced during periods of high humidity and moderate temperatures, typically in the spring and autumn months. Rain showers facilitate the release and dispersal of spores, increasing the risk of transmission within the orchard.

During the winter, the fungus remains dormant in the form of mycelium within the infected branches. As soon as temperatures rise and moisture becomes available in the spring, the pathogen reactivates and resumes the production of spores.

Spring is a critical window, as new vegetative growth is highly susceptible, and standard agricultural activities like pruning create fresh entry points for the pathogen if proper hygiene protocols are not followed.

The optimal temperature range for the development of Phoma tracheiphila is between 20°C and 25°C. While extreme summer heat may suppress the development of external symptoms, the fungus often continues to spread internally within the wood.

Autumn weather conditions, characterized by increased rainfall and cooling temperatures, often trigger a second peak in disease spread, potentially leading to new infections in trees before they enter winter dormancy.

Initial symptoms are characterized by chlorosis (yellowing) of the leaves on individual twigs, which quickly progress to leaf drop and the drying of the branches. This gives the tree a skeletal appearance as sections of the canopy die off.

A diagnostic sign is the internal discoloration of the wood under the bark. A cross-section of an infected branch reveals a characteristic reddish-brown or pinkish tint in the xylem, indicating the presence of fungal toxins.

Tiny black fruiting bodies, known as pycnidia, can often be found on the bark of infected branches. These are the source of secondary inoculum and are most visible after rainy periods when the bark is moist.

A sudden, rapid wilting known as an "apoplectic" attack can occur, where large sections of the canopy dry out over just a few days. This is particularly common when environmental conditions favor the rapid transport of toxins within the tree.

• Chlorosis and premature leaf fall.
• Pinkish or brownish discoloration of the inner wood.
• Appearance of black pycnidia on the bark surface.
• Rapid wilting and death of young shoots.
• Skeletalization of branches due to severe foliage loss.

Effective control starts with strict phytosanitary measures, including the use of disease-free certified planting material. Importing plants from unknown or infected sources is a major risk factor and should be prohibited.

Sanitation through pruning is vital. Infected branches must be pruned at least 30 cm below the point of visible internal wood discoloration. Tools must be disinfected with alcohol or other appropriate agents between every cut to prevent cross-contamination.

Chemical control usually involves the application of copper-based fungicides. These treatments are most effective when applied as a prophylactic measure after pruning or during periods of high risk for spore dispersal to protect open wounds.

The development and planting of resistant citrus varieties provide the most sustainable long-term solution. Research focuses on selecting rootstocks and scions that naturally limit the penetration and spread of Phoma tracheiphila.

Orchard management should prioritize the prevention of mechanical injuries to the trunks and roots during cultivation. Any wound is a potential entry site for the pathogen, emphasizing the need for careful handling of all machinery and equipment.