Advances in diagnostics of downy mildews: Lessons learned from other oomycetes and future challenges

Downy mildews are plant pathogens that damage crop quality and yield worldwide. Among the most severe and notorious crop epidemics of downy mildew occurred on grapes in the mid-1880s, which almost destroyed the wine industry in France (Gessler et al. 2011). Since then, there have been multiple outbreaks on sorghum and millet in Africa (Jeger et al. 1998), tobacco in Europe (Zipper et al. 2009), and recent widespread epidemics on lettuce, basil, cucurbits, and spinach throughout North America (Cohen and Ben-Naim 2016; Correll et al. 2011; Parra et al. 2016). In the mid-1970s, loss of corn to downy mildew in the Philippines was estimated at US$23 million (USDA 2013). Today, crops that are susceptible to downy mildews are worth at least $7.5 billion of the United States’ economy (USDA 2015). Although downy mildews cause devastating economic losses in the United States and globally, this pathogen group remains understudied because they are difficult to culture and accurately identify.