Belonging to the kingdom of chromalveolates, Peronospora is a genus of plant pathogens (Eumycota). There are around 75 species in the genus, mostly specific to genera or families of herbaceous dicotyledonous plants. One of the best known Peronospora species (Peronospora parasitica, on Brassicaceae) is a species complex. Diseases caused by Peronospora are generally referred to as downy mildew. Downy mildew is considered a highly aggressive disease, capable of destroying crops in a very short space of time. There are numerous species of Peronospora that can attack a wide range of plants. Downy mildew can develop from soil-borne oospores and can also be seed borne. The most important source of inoculum is probably the dispersion of air-borne spores from nearby crops or volunteer plants. Cool, moist conditions (approx. 15 ºC) are conducive to infection. Just a short period (four hours) with a wet surface is enough to enable infection. The disease is extremely difficult to control, both outside and inside greenhouses. Severe attacks of downy mildew result in crops that cannot be sold. This poses a serious problem for growers. Partially resistant varieties, a good disease management system and good crop rotation can help reduce the disease burden. The risk of selecting fungicide-resistant stocks is high. Due to the extremely rapid rate of dispersion and aggressive damage, seed treatment will prove ineffective in many cases. In some fast-growing crops such as spinach and lamb's lettuce, the active substance Metalaxyl-M can successfully prevent early damage.