Peronospora hyoscyami

Most of the Peronospora species are highly specific to their hosts and can generally be found anywhere the host plant grows, or is being cultivated. A large portion of their life cycle is spent inside their host plant. Many species of Peronospora are seedborne pathogens, so the worldwide spread of Peronospora crop-plant pathogens is likely to be a result of unknowingly trading infected seeds to new areas. There are also many Peronospora species that are spread by wind currents, which allows them to disperse over large distances. Peronospora species prefer humid air and cool temperatures. One clade in the genus is known as the floricolous downy mildews. These species produce conidiophores exclusively on the flowers of their hosts.

Peronospora was first described in 1837 by August Carl Joseph Corda, a Czech mycologist and physician, in his first of six volumes of his Icones fungorum hucusque cognitorum. Since then, many of the species originally placed in Peronospora have been allocated to other genera or given rise to new genera based on new techniques such as molecular genetics. There was an epidemic in 1960 of Peronospora tabacina affecting tobacco plants leading to $25 million in losses across eleven countries, which was about 30 percent of the tobacco plants. Another epidemic that was caused by Peronospora destructor reduced the yield of sweet onions by 25 percent in Georgia, USA in 2012, and led to an estimated $18.2 million in losses.