FIELD VALIDATION OF A FORECAST MODEL FOR DOWNY MILDEW OF LETTUCE CAUSED BY BREMIA LACTUCEA V PHILION 1, 0 CARISSE 1 and MR MCDONALD 2 1CRDH, St-Jean, Qc. J3B 3E6 Canada. 2 HRIO, R.R. I Kettleby, Ont, Canada Background and objectives Lettuce production in Quebec is important, representing over 60% of the total Canadian production. The climatic conditions of this region are conducive to the development of several diseases including Downy mildew of lettuce caused by Bremia lactucea (Regel) which can be devastating during cool and wet weather. Control is achieved by systematic applications of EBDC fungicides and/or metalaxyl which are both costly and of environmental concern. Furthermore, current EBDC schedules are not always effective at controlling Downy mildew and metalaxyl use can lead to the spread of resistant strains of the pathogen. Moreover, breeding programs are challenged by a rapid spread of virulent races of the pathogen .These considerations have lead to a need for alternative control strategies, including a more efficient use of available fungicidal tools. This study was undertaken to evaluate a weather based forecasting model for pre-symptom applications of EBDC, post-symptom applications of metalaxyl, and a furrow application of granular metalaxyl at transplant to control Downy mildew of lettuce. Materials and methods Plots of lettuce were transplanted with seedlings (cv Ithaca). Current agricultural practices were used except for fungicides. Five fungicide schedules were compared (Weekly protectant, Pre-symptom protectant, Post-symptom systemic, Granular metalaxyl at transplant, and Control) in a CRBD with 3 blocks. Symptom development was forecasted following a sporulation infection period (SIP) defined by leaf wetness starting before 3h00 AM and ending after lOhOO AM at temperatures between 5 and 20C and a 115 dd latent period . Symptoms of a SIP were assumed to appear within 32dd. Pre-symptom sprays were applied just before forecasted symptom development dates with a maximum of one spray per week or 25 mm of rain in case of overlapping latent periods originating from consecutive SIP days. Post-symptom sprays were applied at the end of the 32dd symptom apparition period for a maximum of I spray per 14 days. Results and conclusions The weekly schedule received 6 sprays. A total of 15 SIP days were recorded for which 4 protectant sprays were applied in the pre-symptom schedule and 2 systemic sprays in the post-symptom schedule. Except for the control, lettuce yield was the same in all treatments. All treatments significantly reduced disease severity as compared to the control. The two metalaxyl based schedules were equally effective (disease rating of 1,9 for post-symptom and 1,8 for the granular). Similarly for the two EBDC schedules (2,9 for the weekly and 2,7 for the pre symptom). At harvest, metalaxyl was better than EBDC for controlling the disease. On the last field survey two days prior harvest, disease severity was lower in the weekly, pre-symptom and post-symptom schedules than for the granular metalaxyl treatment. Granular metalaxyl at transplant could be used as an alternative or complementary control strategy. For similar disease control level, it was possible to reduce the number of protectant sprays from 6 to 4 by applying them according to the model. The cheaper and non resistant prone EBDCs can thus be used reliably instead of metalaxyl for equivalent yields. References 1. Crute IR, Johnson AG, 1976. Annals of Applied Biology 83, 125-137. 2. van Bruggen AHC, 1992. California Iceberg Lettuce Advisory Board Research Program 110-117.