Hendersonula dieback

Hendersonula dieback is a destructive fungal disease caused by the pathogen Hendersonula toruloidea (syn. Nattrassia mangiferae). This fungus is a serious threat to woody plants, capable of causing extensive necrosis of the bark and vascular tissues, which can lead to the rapid decline and eventual death of the affected tree.

The host range for this pathogen includes a variety of economically important trees, such as citrus species, walnut, almond, apples, and numerous tropical fruits. The disease is predominantly observed in arid and semi-arid environments where trees are subjected to high thermal stress and drought, making them highly susceptible to fungal penetration.

Symptoms typically manifest as cortical necrosis, longitudinal cracking, and the sloughing of bark. A characteristic sign of the infection is the presence of a black, powdery mass of spores beneath the dead bark. As the disease progresses, gumming may occur, and branches will show signs of wilting, eventually drying out completely as the vascular system becomes compromised.

The development of the fungus is strictly correlated with high temperatures, with an optimum range of 30°C to 35°C. Infection primarily occurs through bark injuries caused by sunburn, mechanical damage, pruning wounds, or wood-boring insects. Once established, the pathogen spreads rapidly via air-borne conidia, rainfall, and contaminated horticultural equipment.

Management strategies focus on prevention and sanitation:

• Remove and destroy infected wood immediately to reduce inoculum pressure.
• Protect tree trunks with white paint to prevent sunburn and heat-induced injuries.
• Sterilize pruning tools between cuts using alcohol or bleach solutions.
• Apply copper-based fungicides to pruning cuts and wounds to block infection sites.
• Ensure trees are properly irrigated to minimize drought stress, which is a major predisposing factor.