Phoma leaf spot of wasabi

Phoma leaf spot, caused by the fungal pathogen Plenodomus wasabiae (formerly Phoma wasabiae), is a significant disease affecting wasabi crops. This fungus acts as a severe threat to cultivated Eutrema japonicum, causing necrotic lesions that compromise both the health of the plant and the quality of the commercial rhizome.

The host range is primarily limited to members of the Brassicaceae family, particularly wasabi. The fungus infiltrates the plant tissues, leading to vascular breakdown and tissue necrosis, which can rapidly spread across a planting bed if environmental conditions remain favorable for fungal growth.

Initial symptoms are characterized by water-soaked, dark lesions on the leaf petioles and the upper parts of the rhizome. As the disease progresses, these lesions expand and darken, frequently developing visible black fruiting bodies known as pycnidia on the surface. These structures serve as the primary source of secondary inoculum, facilitating the spread of the pathogen.

The development and spread of Plenodomus wasabiae are heavily dictated by environmental factors, particularly prolonged periods of high humidity and moderate temperatures. The spores are typically dispersed by splashing water during irrigation or heavy rainfall, making greenhouse management and careful water application critical for limiting the spread of the disease.

Integrated disease management (IDM) is the most effective approach for control. This includes using certified disease-free seedlings, maintaining proper sanitation by removing infected plant debris, and implementing strict hygiene protocols for tools. Fungicide applications may be utilized as a preventative measure, specifically targeting conditions conducive to pathogen development to ensure crop longevity.