Phoma leaf spot (Boeremia)

Boeremia exigua is a destructive fungal pathogen belonging to the Ascomycota division. It is responsible for causing Phoma leaf spot, stem cankers, and root rot in a broad spectrum of plants, including significant agricultural crops such as potatoes, legumes, and various ornamental species found across temperate and subtropical regions.

The primary symptom of infection is the development of necrotic, often circular, lesions on leaves and stems. As the disease progresses, these spots expand, often displaying a target-like appearance. In advanced stages, the fungus produces pycnidia—small, black, fruiting bodies visible to the naked eye—that release spores, allowing for rapid disease progression.

The life cycle of the fungus is persistent, as it survives the winter on crop debris, in the soil, and on contaminated seeds. Primary infection occurs during early spring when environmental conditions become suitable. Spores are then dispersed via rain splash or mechanical movement, leading to secondary cycles of infection throughout the growing season.

High relative humidity and moderate temperatures are the key triggers for disease outbreaks. The pathogen thrives when plants are under stress, particularly due to nutrient imbalances or mechanical injuries, which create entry points for the mycelium to invade healthy tissues, leading to severe defoliation and yield reduction.

Management of Boeremia exigua requires an integrated pest management (IPM) approach to minimize crop loss and prevent inoculum buildup in the fields:

• The use of pathogen-free seeds and certified planting material.
• Implementation of crop rotation cycles to disrupt the pathogen's lifecycle.
• Sanitation practices such as the removal or deep burial of plant residues.
• Strategic application of foliar fungicides during periods of high humidity and rainfall.