INEFFECTIVENESS OF THE 1ST FUNGICIDE APPLICATION AT DIFFERENT INITIALDISEASE INCIDENCE LEVELS TO MANAGE SEPTORIA BLIGHT IN CELERY

Five initial (at transplanting stage) blight incidence levels (0, 2, 4 , 8, and 16%) were tested to select the optimum level for beginning fu ngicide application to manage Septoria blight incited by Septoria apii cola in celery (Apium graveolens var. dulce). The experiment was condu cted on muck soil at the Agriculture Canada Research farm at Sainte-Cl otilde, Quebec, in the summers of 1990 and 1991. An increase in the in itial blight incidence levels raised the proportion of area under the disease progress curves. The proportion of yield loss also rose signif icantly with increased initial blight incidence. A logistic model esta blished to describe yield loss as a function of initial blight inciden ce 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 fungici de application could be established. Nevertheless, an analysis of vari ance indicated no significant difference in yield loss between 0 and 2 % initial blight incidence levels. Because the disease increases rapid ly, leading to significant loss in yield at very low blight incidence levels at the transplanting stage, fungicide applications must be init iated as soon as the disease is observed. If the disease appears later in the season, however, initiation of fungicide applications can be d elayed until a higher disease incidence threshold is reached.