0 CARISSE Agriculture and Agri-Food Canada, 430 Gouin Blvd., St-Jean, QC, Canada J3B 3E6 Background and objectives Known for many years, Common leaf spot of strawberry, caused by Mycosphaerelia fragariae, was eliminated by the use of resistant cultivars [1]. The disease is now found in most strawberry fields because these cultivars were dropped in favor of new ones. It is a minor disease, attacking the fruit only under particularly severe epidemics. The disease is over treated resulting in waste of fungicide and unnecessary pollution. The objective of this study was to obtain quantitative information on host, pathogen, and environmental variables in relation to disease development. Materials and Methods Disease progression was monitored during two consecutive seasons in strawberry (cv. Kent) plots of 25 rows of 15 m long. Plots were treated according to commercial practices except that no fungicides were applied. From May to September, the number of lesions per leaflet, surface of leaflets, and apparition of new leaves were noted twice a week, on 30 marked plants. Weather data were monitored using a data logger (CR-1 0 Campbell Scientific) in the vicinity of the plots. Number of lesions per CM2 of leaf and leaf age were computed separately for each leaf. Infection days were identified based on the presence of new lesions and expected length of incubation period. Weather and host parameters were calculated for 3-day periods corresponding to the infection day + one day. Simple correlation analysis were done to determine the degree of association among variables and principal component analysis (PCA) was used to identify weather and host parameters that most influenced disease progress in the field. Results and conclusions Similar patterns of disease progression were observed in 1994 and 1995. Disease progress curves displayed three distinct phases which corresponded to phonological stages of the crop. The first phase was synchronized with the period of vegetative growth and thus characterized by the presence of young susceptible leaves and low levels of inoculum. During flowering and fruit production, disease progress almost stopped, but following plant renovation, disease increased rapidly due to the presence of new leaves and high levels of inoculum. From the principal component analysis, rain associated with high temperatures and presence of susceptible leaves (less than 7 day-old) were the most important factors influencing disease progress. Field data were also used to validate infection and sporulation models developed from controlled environment studies [2]. These models were used to identify Sporulation-infection days and expected disease level one incubation period later. For the 12 Sporulation-infection days observed, nine were properly predicted by the models. Results also indicated that the temperature was not limiting to infection under field conditions. However, duration of leaf wetness resulting from rain was highly correlated with the appearance of new lesions. The epidemiological information was used to formulate a management program for Common leaf spot of strawberry. References 1. Nemec S, 1971. Plant Disease Reporter 55, 573-576. 2. Carisse, 0., Brodeur, C. G. Bourgeois, G. and D. Dostaler, 1996. Phytopathology (supplement) 86 1 1.