Septoria blight of celery (Apium graveolens var. dulce), induced by Septoria apiicola Speg., is a destructive disease that requires fungicide applications for its management. A reliable method of inoculum production was established. Best sporulation was obtained on celery agar (CA) at a predicted optimum temperature of 22.4$ sp circ$C. A moderate inoculum density within a range of 17-35 conidia/cm$ sp2$ of leaf surface provided non-coalescing lesions necessary for rapid enumeration. Initial blight incidence thresholds of about 0.6 and 0.5% to time the first fungicide application were established based on the proportion of maximum plant weight equivalent to the cost of one fungicide application calculated plant weight and petiole number per plant, respectively, in the summers of 1990 and 1991. The IBI levels of 0 and 2% to time the first fungicide application provided non-significant results in terms of the final amount of blight and yield. (Abstract shortened by UMI.)

Septoria blight of celery (Apium graveolens) caused by Septoria apiicola is a common disease. This work dealt with the development of a simple forecast model to time fungicide application to manage the blight. Two studies were conducted to examine the effect of relative humidity (65, 88, 92, 96%, and wet) temperature (15, 20, 25, and 28 C) and exposure period (6, 12, 24, 48, 72, and 96 hr) on spore production. Spores were produced at all levels of relative humidity and temperature tested. Overall spore production increased with increasing relative humidity. The greatest and the least were produced after a 48 hr wet period at 20 and 28 C, respectively. Two regression models were developed to predict spore production as functions of relative humidity and temperature. In order to validate the infection function model a field experiment was conducted in 1989 to quantify latent period. A mean latent period beginning of 12 days, a mean latent period 50% of 14 days, and a mean latent period ending of 19 days were established. Two regression models were developed to predict latent period beginning and latent period ending as functions of mean maximum and mean minimum temperatures. In order to develop a simple forecast model field experiments were conducted in 1989 and 1990. From these studies two simple forecast models are proposed to initiate fungicide applications: (1) Initial disease occurrence. (2) A cumulative disease severity value (CDSV) of 23. Using the forecasts proposed it is possible to reduce five to seven applications of fungicides during the celery growing season in Quebec.

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