Ineffectiveness of the First Fungicide Application at Different Initial Disease Incidence Levels to Manage Septoria Blight in Celery. I. W. Mudita, Graduate Student, Department of Plant Science, Macdonald Campus of McGill University, 21,111 Lakeshore Road, Sainte-Anne-de-Bellevue, Quebec, Canada H9X 3V9. A. C. Kushalappa, Associate Professor, Department of Plant Science, Macdonald Campus of McGill University, 21,111 Lakeshore Road, Sainte-Anne-de-Bellevue, Quebec, Canada H9X 3V9. Plant Dis. 77:1081-1084. Accepted for publication 28 June 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-1081.

Five initial (at transplanting stage) blight incidence levels (0, 2, 4, 8, and 16%) were tested to select the optimum level for beginning fungicide application to manage Septoria blight incited by Septoria apiicola in celery (Apium graveolens var. dulce). The experiment was conducted on muck soil at the Agriculture Canada Research farm at Sainte-Clotilde, Quebec, in the summers of 1990 and 1991. An increase in the initial blight incidence levels raised the proportion of area under the disease progress curves. The proportion of yield loss also rose significantly with increased initial blight incidence. A logistic model established to describe yield loss as a function of initial blight incidence levels predicted loss in yield exceeding the cost of one fungicide application at the 0% initial blight incidence level. On the basis of this model, no initial disease incidence threshold to initiate fungicide application could be established. Nevertheless, an analysis of variance indicated no significant difference in yield loss between 0 and 2% initial blight incidence levels. Because the disease increases rapidly, leading to significant loss in yield at very low blight incidence levels at the transplanting stage, fungicide applications must be initiated as soon as the disease is observed. If the disease appears later in the season, however, initiation of fungicide applications can be delayed until a higher disease incidence threshold is reached.