Citrus mal secco

Taxonomic classification and nature of the pathogen
Phoma tracheiphila (also identified as Plenodomus tracheiphilus) is a fungal pathogen classified within the Ascomycota phylum. It is a highly specialized vascular pathogen that colonizes the xylem of host plants. Due to its systemic nature and destructive impact on citrus orchards globally, it is strictly monitored as a quarantine pathogen in many agricultural regions.

Diseases and susceptible crops
The primary disease associated with this pathogen is known as "mal secco" (meaning "dry disease"). While it primarily devastates lemon trees, other citrus species including citron, bergamot, and certain varieties of orange and mandarin are also vulnerable. The infection leads to progressive wilting, dieback of twigs and branches, and eventually the death of the entire tree.

Biology and life cycle
The pathogen enters the plant through wounds, pruning cuts, or natural plant openings. Once inside, the fungus invades the vascular tissues, impeding the transport of water and nutrients. Its life cycle involves the production of pycnidia, which release conidia. These spores are disseminated via wind, rain splashes, and human activity, particularly through contaminated pruning tools.

Developmental conditions and impact
Phoma tracheiphila thrives in cool, humid environments. The severity of the damage is significant, as the fungus creates internal blockages that cause the foliage to wilt and die suddenly. A diagnostic hallmark of this disease is the internal reddish discoloration of the wood within affected branches, which distinguishes it from other forms of citrus dieback.

Protection and control measures
Managing mal secco requires a combination of strict quarantine procedures and cultural practices to prevent the spread of the fungus. Key control strategies include:

• strict prohibition of movement of infected plant material into non-infested zones;
• immediate removal and incineration of symptomatic branches;
• frequent disinfection of pruning equipment using appropriate chemical agents;
• utilization of resistant or tolerant citrus rootstocks and scions;
• application of copper-based fungicides to protect vulnerable tissue and pruned surfaces.