A rapid LAMP (loop-mediated isothermal amplification) detection method was developed on the basis of the ITS sequence of P. viticola, the major causal agent of grape downy mildew. Among the 38 fungal and oomycete species tested, DNA isolated exclusively from P. viticola resulted in a specific product after LAMP amplification. This assay had high sensitivity and was able to detect the presence of less than 33 fg of genomic DNA per 25-μL reaction within 30 min. The infected leaves may produce sporangia that serve as a secondary inoculum. The developed LAMP assay is efficient for estimating the latent infection of grape leaves by P. viticola. When combined with the rapid and simple DNA extraction method, this assay’s total detection time is shortened to approximately one hour; therefore it is suitable for on-site detection of latent infection in the field. The sporangia levels in the air are strongly associated with disease severity. The LAMP method was also demonstrated to be able to estimate the level of sporangia released in the air in a certain period. This assay should make disease forecasting more accurate and rapid and should be helpful in decision-making regarding the control of grape downy mildew.