Pseudoperonospora

Pseudoperonospora is a genus of plant-pathogenic oomycetes in the family Peronosporaceae. Species within the genus cause downy mildews on a range of host plants. The best known is Pseudoperonospora cubensis, which causes cucurbit downy mildew on cucumber, melon, squash, and related crops. Other species infect different hosts, including Pseudoperonospora humuli, which is associated with hop downy mildew, illustrating the genus’s agricultural relevance.

Pseudoperonospora spp. are oomycetes rather than true fungi. They propagate asexually by sporangia that release motile zoospores in moist conditions and sexually by oospores that serve as overwintering structures. Infections are favored by leaf wetness and cool to moderately warm temperatures. The pathogen disperses via water splash and wind-driven sporangia, infecting the lower leaf surface and producing a characteristic downy growth of sporangia that can be seen as a gray to purplish coating.

Symptoms typically begin as yellow, angular lesions on the upper leaf surfaces, with a dense, pale to dark gray or purple sporulation on the undersides. Over time, infection can cause early leaf drop and substantial yield losses in susceptible cucurbits, and similar damage can occur on other hosts attacked by different Pseudoperonospora species.

Management relies on integrated strategies. Cultural measures include sanitation and removal of infected plant debris, crop rotation, and use of resistant cultivars where available. Chemical control targets oomycetes and often employs products with alternating modes of action to delay resistance; timing is crucial to protect new growth. Quarantine and monitoring help prevent spread in susceptible regions.