Downy mildew caused by Pseudoperonospora cubensis is the main disease affecting melon (Cucumis melo) fruit during the rainy period in areas of Northeastern Brazil where the crop is produced. Up to ten fungicide sprays are used throughout the crop season. The establishment of risk analysis for forecast of damages caused by this disease requires an estimate of the losses under different epidemic situations. Three field experiments were carried out to determine the damage caused by downy mildew to the fruit yield of the melon crop. The epidemic was considered to begin in the phenological cycle of the host plant. Different intensities of epidemics were achieved by spraying one of the following fungicide mixtures: methyl thiophanate + chlorothalonil or metalaxyl + mancozeb. There was a significant reduction on the fruit yield when the disease started at 24 and 36 days after planting, but when the disease started on day 47, no effect in production was observed. It was concluded that the duration of the the epidemic and when it began in relation to the cycle of the host, should be considered when establishing prognostic models to describe the risks of yield losses.
