A study of the dynamics of Plasmopara viticola oospore maturation during 3 yr in the Bordeaux area of France showed significant differences among years. The date of oospore maturity and subsequent disease severity in spring appeared to be associated with the amount of rainfall after oospore formation. For example, heavy rainfall from September to February was generally associated with early oospore maturity and severe disease in May or June. To predict the date when most oospores are mature (DOM), a climate-based model called “Prediction of Oospore Maturity” (POM) was developed. The POM model, based on daily rainfall beginning in September, permits calculation of DOM as early as the end of January. POM calculations based on climatic data recorded since 1977 confirmed that the earlier the DOM, the more severe disease was in the spring. Four levels of predictable risk are proposed based on the time required for oospore maturation. Although the POM model needs validation, it already shows promise in grape disease management.