Celery phoma rot

Celery phoma rot is caused by the fungus Phoma apiicola (often synonymous with Phoma apii), an Ascomycete pathogen. It is a serious threat to celery crops, leading to significant yield losses in both the field and storage.

The pathogen survives in the soil, on crop residues, and within seed lots. Seed transmission is a primary pathway for the introduction of the disease into new fields, making seed sanitation a critical control point for farmers.

The fungus develops pycnidia, which are small black fruiting bodies visible to the naked eye. These pycnidia contain masses of spores that are released during high humidity and dispersed by wind, water splashes, and field equipment.

Phoma apiicola is known for its ability to persist in the soil for several years. Even in the absence of a host, the fungus can remain viable in its dormant state, waiting for favorable environmental conditions to emerge.

Once the infection enters the host tissues, the fungal mycelium colonizes the plant, disrupting water and nutrient transport. The infection is often latent, meaning symptoms may not appear until the plant reaches a more mature stage.

The pathogen primarily affects celery, including root, leaf, and stalk types. It attacks various plant parts, including roots, stems, petioles, and the mature root bulb, causing severe tissue degradation.

The most devastating economic damage occurs during storage. The fungus causes a dry rot, which renders the celery roots unsuitable for sale. Affected areas can spread rapidly through crates if storage conditions are poor.

In the field, infected seedlings often collapse and die before or shortly after emergence, leading to reduced plant stands and the need for costly replanting or lower density crops.

The degradation of the plant structure reduces the aesthetic quality of the product. Even if the plant survives, it is often stunted, with lower overall growth potential compared to healthy specimens.

By compromising the plant's structural integrity, the disease facilitates the secondary invasion of bacteria and other fungi, which can accelerate the rotting process significantly.

Seedling blight is a common early sign, characterized by dark brown lesions on the hypocotyl. These lesions can girdle the stem, causing the seedling to wilt and eventually collapse.

On leaves and petioles, the disease manifests as irregular brown or grayish spots. As the lesions expand, small, black, pin-sized pycnidia appear in the center of the necrotic tissue.

Root bulbs exhibit sunken, dark brown, or black lesions. Internally, the affected tissue becomes dry, crumbly, and dark, distinguishing it from bacterial soft rots which are typically slimy.

Petioles may become brittle and deformed. In severe cases, the entire crown of the celery plant may show signs of decay, leading to the yellowing and wilting of the outer leaves.

In humid conditions, a sparse mycelial growth may be visible on the surface of the lesions. These areas eventually become necrotic, leading to the death of the specific plant part or the whole plant.

The primary control measure is the use of disease-free, certified seeds. Treating seeds with appropriate fungicides before planting is essential to eliminate internal and surface-borne inoculum.

Implementing a long crop rotation (at least 3–4 years) is vital. Celery should not follow other Apiaceae crops, as they may also serve as hosts for the pathogen, sustaining the infection cycle.

Field hygiene is critical. Deep plowing of crop residues helps bury the fungus, accelerating its decomposition and reducing the initial spore load for the next season.

Fungicide applications are effective during the growing season. Protective measures, including copper-based products or systemic fungicides, should be applied preventively if the risk is high.

• Ensure proper irrigation to avoid water stagnation in the field.
• Maintain optimal storage conditions (low temperature and low humidity).
• Remove and destroy symptomatic plants promptly.
• Disinfect tools and storage equipment to prevent spore spread.