Exploring the association between latent Plasmopara viticola infection and downy mildew epidemic in commercial vineyards: Application of qPCR assay

It is critical to quantify latent Plasmopara viticola infections in commercial vineyards rapidly and accurately to estimate the potential for grapevine downy mildew (GDM) epidemics. In this study, a well‐established quantitative PCR (qPCR) assay was employed to quantify the amount of P. viticola in asymptomatic grapevine leaves from various commercial vineyards in the Ningxia Hui Autonomous Region (Ningxia), China. The qPCR revealed efficient detection of P. viticola, with 56.6% and 95.8% of the asymptomatic leaves testing positive to the pathogen in 2018 and 2019, respectively. The molecular disease index (MDI), which offers insights into the progression of latent infection preceding the appearance of visible GDM symptoms, showed relatively modest values in the initial phases. However, it increased variably as grapevines grew. The disease index progress curve and the relative area under the disease progress curve revealed that all assessed vineyards experienced different degrees of epidemics after the initial GDM observation, with most vineyards experiencing more severe GDM epidemics in 2019 than in 2018. Bayesian and non‐linear regression analyses demonstrated that MDI data matched well with occurrence of GDM at early, mid and late stages of disease, and the overall accuracy of predictions exceeded 50%, 64% and 70%, respectively. Furthermore, latent infection levels were positively correlated with subsequent assessments of disease progression (p