Exobasidium lauri

The causative agent of this disease is the basidiomycete fungus Exobasidium lauri. It is a highly specialized parasite that exclusively targets the tissues of the bay laurel (Laurus nobilis). The fungus triggers cellular proliferation within the plant tissues, leading to the formation of abnormal, gall-like swellings on the leaves and tender stems.

The primary host plant is the bay laurel. The disease is most prevalent in nursery environments and commercial plantations where bay laurel is cultivated for its aromatic leaves or as an ornamental plant. It thrives in temperate to subtropical climates where humidity levels are frequently high enough to support fungal development.

Symptoms are characterized by distinct hypertrophic swellings on the leaves. Initially, these galls may appear yellowish or light green, transitioning to pinkish or reddish hues. In humid conditions, a characteristic white powdery coating emerges on the underside or surface of the galls; this is the fungal sporulation layer, indicating active reproduction and high risk of secondary infection.

Environmental conditions are the main drivers of the disease outbreak. High humidity, heavy rainfall, and poor air circulation are critical for the survival and dispersal of Exobasidium lauri spores. When plants are crowded or improperly pruned, the microclimate within the canopy remains damp, allowing the fungus to germinate and penetrate the host tissue with high efficiency.

The economic impact of this disease is significant, as it affects both the health of the plant and the quality of the harvest. Heavy infestations reduce the photosynthetic capacity of the foliage, leading to premature leaf drop, stunted growth, and decreased yield of aromatic leaves. If left untreated, young plants can suffer severe physiological stress or even death in nursery conditions.

Integrated management and control measures should include:

• Sanitation, involving the removal and destruction of all infected leaves and shoots.
• Improving air circulation through proper spacing and regular thinning of the canopy.
• Application of copper-based fungicides during the early stages of the growing season or before predicted wet periods.
• Avoidance of overhead irrigation to minimize leaf wetness.
• Ensuring proper soil drainage to maintain plant vigor and overall resistance to infection.