Late blight of celery

The fungus produces small, flask-shaped reproductive structures called pycnidia, which appear as dark specks within the infected necrotic leaf tissues.

As a pycnidial fungus, it spreads primarily through rain splashes and irrigation, which carry the conidia from infected leaves to healthy parts of the plant.

Classification-wise, this pathogen is highly specialized, meaning it does not typically jump to non-related plant families, focusing its evolutionary energy on celery.

Accurate identification is possible by observing these black pycnidia under a magnifying glass, which confirms the presence of the Septoria infection.

The primary damage caused by this pathogen is the degradation of leaf quality, which renders leaf celery unsalable for the fresh produce market.

In root celery production, the loss of photosynthetic leaf area leads to stunted bulb growth and reduced yields, negatively impacting the total harvest weight.

The disease also affects the post-harvest storage potential, as infected tissues provide entry points for secondary bacterial and fungal rot organisms.

Severe infestations can lead to total crop loss, especially in fields where humidity remains high and no fungicide program is implemented.

Financial losses occur not only through yield reduction but also due to the high cost of intensive chemical control required to mitigate the pathogen's impact.

Late blight of celery is most prevalent during periods of frequent rainfall, high humidity, and moderate temperatures typical of late summer and early autumn.

The pathogen thrives when moisture remains on the foliage for extended hours, as the spores require a film of water to germinate and penetrate the tissue.

Greenhouse environments are particularly susceptible if irrigation methods are inefficient, leading to constant leaf wetness and poor airflow between rows.

The disease cycle often starts from infected seeds or overwintered crop debris in the soil, which serve as the primary source of inoculum in the spring.

As the season progresses, secondary cycles of infection can occur every two weeks under optimal conditions, leading to rapid spread throughout the field.

Symptoms initially appear as small, yellowish-brown spots on the leaves, which gradually darken into brown or grey lesions with defined borders.

The most diagnostic feature is the presence of numerous tiny black dots within these spots, which are the visible pycnidia of the fungus.

As the infection advances, the leaves turn yellow, wilt, and eventually dry out, starting from the oldest leaves at the base of the celery plant.

On stems and leaf stalks, the lesions become elongated, sometimes causing the stalks to bend or break, which significantly weakens the entire plant structure.

In humid conditions, the lesions can merge together, resulting in a scorched appearance of the entire plant canopy before the plant dies prematurely.

The use of pathogen-free seeds or treated seeds is the most effective way to prevent the introduction of Septoria apiicola into new production areas.

Crop rotation remains a fundamental strategy, with a break of at least three years between celery crops to allow the inoculum in the soil to naturally decline.

Sanitation practices, including the removal or deep incorporation of crop residues, help break the cycle and reduce the volume of overwintering spores.

• Apply protective fungicides at the first sign of symptoms to prevent spread.
• Improve field drainage to reduce overall humidity levels.
• Use wider plant spacing to enhance airflow and speed up foliage drying.

Integrated pest management (IPM) focusing on early detection and the use of resistant varieties offers the most sustainable path for long-term control.