Septoria late blight is a worldwide disease of celery plants. First reported in Italy in 1890, late blight was then reported in North America in 1921, leading to losses of 25-50%. The disease is explosive – a half billion spores can be produced on a single celery plant. Each spore can start an infection resembling a dark spot, which can grow large enough to cause leaf death. Celery growers spray fungicides to prevent septoria infections. Another option is to use laborers to trim off the infected parts of the celery; however, this is not practical.

“Septoria late blight is an important disease of celery worldwide. Yield loss ≤ 70% can occur. … Effective management of septoria late blight is essential for the production of a marketable crop of celery. The disease threshold for celery is effectively zero because plants with noticeable lesions on leaves and petioles [stalks] are unmarketable, so diseased petioles must be removed by hand. … Septoria late blight is difficult to control once present in a field, and celery growers must rely on application of foliar fungicides to manage this disease. The labour requirements to trim fresh-market celery with lesions are high, and loads of processing celery showing disease symptoms can be rejected entirely.”

Author: C.L. Trueman, et al.
Affiliation: Department of Plant Agriculture, University of Guelph, Ontario
Title: Evaluation of disease forecasting programs for management of septoria late blight (Septoria apiicola) on celery.
Publication: Canadian Journal of Plant Pathology. 2007. 29:330-339.