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Influence of Wetness Periods on Infection of Celery by Septoria apiicola and Use in Timing Sprays for Control. M. L. LACY, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 78:975-979. Accepted for publication 11 July 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0975.

Conidia of Seploria apiicola collected from dried infected celery leaves began germinating 7-12 hr after plating or inoculation, depending on temperature and germination medium. Germination was 20% at 21C and 10% at 25C on water agar 8 hr after plating. Germination did not begin in distilled water or on celery leaves until 12 hr after plating or inoculation. Germination was complete (>95%) on water agar and reached 78-80% on leaves 36 hr after plating or inoculation at 21 or 25C. Lesions formed on celery leaves in significant numbers (one or more lesions per leaflet) only after 24 hr of continuous or interrupted (12 hr wet/12 hr dry/ 12 hr wet) dew within 15 days after inoculation at 21C. Lesions formed as early as 8 days after inoculation following wet periods of 36-48 hr and reached a maximum of 14 lesions per leaflet after 21 days at 21C. On celery leaves exposed to 36-48 hr of dew at 25C, a maximum of 2.5 lesions per leaflet formed after 21 days. With a wetness period of 12 hr (or longer) used as a conservative threshold value representing a risk of infection, chlorothalonil sprays were applied to inoculated field plots after ≥ 12 hr of wetness (provided that no sprays had been applied within the previous 7 days) or weekly. In 3 yr of trials, two fewer sprays were applied annually with the 12-hr wetness threshold than with the weekly schedule without any sacrifice in efficacy of disease control.

Keyword(s): celery late blight, disease forecasting