INFLUENCE OF WETNESS PERIODS ON INFECTION OF CELERY BY SEPTORIA-APIICOLA AND USE IN TIMING SPRAY FOR CONTROL

Conidia of Septoria apiicola collected from dried infected celery leav es began germinating 7-12 hr after plating or inoculation, depending o n temperature and germination medium. Germination was 20% at 21 C and 10% at 25 C on water agar 8 hr after plating. Germination did not begi n 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 25 C. Le sions formed on celery leaves in significant numbers (one or more lesi ons per leaflet) only after 24 hr of continuous or interrupted (I 2 hr wet/ 12 hr dry/12 hr wet) dew within 15 days after inoculation at 21 C. Lesions formed as early as 8 days after inoculation following wet p eriods of 36-48 hr and reached a maximum of 14 lesions per leaflet aft er 21 days at 21 C. On celery leaves exposed to 36-48 hr of dew at 25 C, a maximum of 2.5 lesions per leaflet formed after 21 days. With a w etness period of 12 hr (or longer) used as a conservative threshold va lue representing a risk of infection, chlorothalonil sprays were appli ed to inoculated field plots after greater-than-or-equal-to 12 hr of w etness (provided that no sprays had been applied within the previous 7 days) or weekly. In 3 yr of trials, two fewer sprays were applied ann ually with the 12-hr wetness threshold than with the weekly schedule w ithout any sacrifice in efficacy of disease control.