Alternaria leaf spot of ornamental plants

The causative agent of this disease is the microscopic fungus Alternaria ethzedia, classified within the Hyphomycetes group. It is a known phytopathogen that primarily targets the foliage and stems of various ornamental plant species, often emerging as a significant issue in greenhouse environments.

Ornamental plants are the primary hosts for Alternaria ethzedia. The pathogen thrives particularly well on crops grown in high-density conditions where stagnant air and high humidity provide the necessary environment for the fungus to establish and colonize plant tissues.

The symptoms of the disease are characterized by the appearance of dark brown or black spots, which frequently exhibit a concentric ring pattern (bull's-eye lesions). As the infection advances, these spots coalesce, leading to extensive tissue necrosis, yellowing of the leaves, and eventual defoliation of the affected specimens.

The development and spread of Alternaria ethzedia are heavily dependent on warm temperatures (20–25°C) and high relative humidity. The conidia of the fungus are dispersed through air currents, water splashes during irrigation, and handling by contaminated tools, making hygiene a critical aspect of disease management.

The economic impact of this disease is significant, as it degrades the aesthetic value of ornamental plants, often rendering them unsellable. To protect the crops, growers should implement a robust integrated pest management (IPM) program:

• Remove and destroy all infected plant debris to reduce inoculum sources.
• Improve greenhouse ventilation to minimize leaf surface wetness duration.
• Implement proper spacing to encourage air circulation between plants.
• Apply preventative fungicide treatments, such as copper-based or systemic azole compounds, following local label guidelines.