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Hosts

Celery (Apium graveolens) and celeriac (Apium graveolens var. rapaceum).

Distribution

Temperate regions in the world. In Hawaii, this disease occurs in Kamuela where celery is grown. Distribution of the pathogen is restricted by the cooler temperatures needed by the host.

Symptoms

Leaf spots caused by Septoria are dark, circular to irregular in shape, and 3-10 mm in diameter. Dark colored fruiting bodies of the fungus which form in the center of leaf spots give the spots a grainy appearance. As spots expand, some are restricted by leaf venation and become partially angular in shape. Lesions are surrounded by yellow or chlorotic tissue. A large number of spots can form on leaves and reduce yield. The stalk or the petiole of the celery is also attacked. Small black spots occur in large numbers on stalks.

Biology

Taxonomy

KINGDOM: Fungi 

Phylum: Ascomycota (teleomorph or sexual stage). 

Order: Dothideales; Family: Dothioraceae; 

Genus: Mycosphaerella 

Traditional: Coelomycetes (the imperfect fungi or mitosporic fungi) 

Asexual spores are formed in a fungal cavity e.g. pycnidia; 

Anamorphic state. 

Genus: Septoria

Biology

Septoria produces huge amounts of asexual spores in dark fruiting bodies, called pycnidia. The spores are hyaline (clear), thin, multicellular (2 - 4 cells), and about 20 to 60 um long (Walker). Pycnidia are formed on the older lesions and their development is encouraged by moist weather.

Epidemiology

Septoria is seed borne and fruiting bodies can be found on the seed coat of the celery seed. The first appearance of the disease is thus in the seedbeds. Low levels of the disease occurs throughout the growing season. Cool wet weather favors disease development by Septoria. Temperatures below 24°C or 75°F are conducive to disease formation. High humidity allows abundant production of spores and epidemics are initiated by splashing spores or by movement of spores by contact. 

Spores that are splashed to healthy leaves germinate when moisture is available and produce a small fungal thread called a germ tube. This grows on the hosts epidermis (skin) of the celery plant for a short distance then enters the leaf. The fungus proliferates internally within the leaf tissue and causes yellowing, then browning of the host. These are visible as leaf spots. Spores are formed on the surface of diseased areas and the disease cycle continues as new spores are splashed to other healthy leaves. 

In the Northeastern U. S., Septoria attacks the celery crop in the wet months which are in the later part of the growing season. During the warmer season, another leaf spot caused by the fungus Cercospora occurs. Since the Cercospora leaf spots are more common during the early part of the growing season, it is called early blight while the Septoria disease is called late blight. In other parts of the country, including Hawaii, Septoria will appear during any part of the growing season, depending on the weather. Cooler weather early in the crop will promote late blight.

Management

Every effort should be made to acquire clean seeds. In the past, hot water treatment of seeds (48° - 49°C for 30 mins) was used to effectively free seeds of this pathogen. Diseased fields should be cleaned following harvest and host material should not be left in the field. Septoria will survive in soil in decomposing celery tissue for months. 

New crops should not be planted adjacent to diseased crops to prevent rapid disease spread. Fields should be rotated to reduce early infection from the pathogen left from the previous crop. 

Fungicides are available to reduce disease levels. Check label directions and follow carefully.

References

Hawksworth, D. L., P. M. Kirk, B. C. Sutton and D. N. Pegler. 1992. Ainsworth & Bisby’s Dictionary of the Fungi. Eighth Ed. CAB international. University Press, Cambridge, United Kingdom. 

Walker, J. C. 1952. Diseases of vegetable crops. McGraw-Hill Book Co Inc. New York USA

FILE: BS-APII

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