Downy mildew

Downy mildew refers to any of several types of oomycete that infect plants. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vine-type vegetables.

The prime example is Peronospora farinosaas featured in NCBI-Taxonomy and HYP3.

Hop Downy Mildew (Pseudoperonospora humuli) is specific to hops (Humulus lupulus). The disease is the single most devastating disease in Western United States hopyards, since the fungus thrives in moist climates. Infected young hop bines become stunted with thickened clusters of pale curled leaves. These spikes have a silvery upper surface, while the undersides of leaves become blackened with spores. These dwarfed spikes are called "basal spikes". 'Lateral' or 'terminal' spikes occur further up the bine. An entire hop crop could be devastated in only a few days.