The Oomycetes, or Oomycota, form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms within the Stramenopiles. They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the result of contact between hyphae of male antheridia and female oogonia; these spores can overwinter and are known as resting spores. Asexual reproduction involves the formation of chlamydospores and sporangia, producing motile zoospores. Oomycetes occupy both saprophytic and pathogenic lifestyles, and include some of the most notorious pathogens of plants, causing devastating diseases such as late blight of potato and sudden oak death. One oomycete, the mycoparasite Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi. The oomycetes are also often referred to as water molds, although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens.
Impatiens is a genus of more than 1,000 species of flowering plants, widely distributed throughout the Northern Hemisphere and the tropics. Together with the genus Hydrocera, Impatiens make up the family Balsaminaceae.
Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables that grow on vines. The prime example is Peronospora farinosa featured in NCBI-Taxonomy and HYP3. This pathogen does not produce survival structures in the northern states of the United States, and overwinters as live mildew colonies in Gulf Coast states. It progresses northward with cucurbit production each spring. Yield loss associated with downy mildew is most likely related to soft rots that occur after plant canopies collapse and sunburn occurs on fruit. Cucurbit downy mildew only affects leaves of cucurbit plants.
Impatiens irvingii is a species of flowering plant in the family Balsaminaceae. It is native to tropical Africa.
Peronosporaceae are a family of water moulds that contains 21 genera, comprising more than 600 species. Most of them are called downy mildews.
Hyaloperonospora parasitica is an oomycete from the family Peronosporaceae. It has been considered for a long time to cause downy mildew of a variety of species within the Brassicaceae, on which the disease can cause economically important damage by killing seedlings or affecting the quality of produce intended for freezing. Hyaloperonospora parasitica causes downy mildew on a wide range of many different plants. It belongs to the Kingdom Chromista, the phylum Oomycota, and the family Peronosporaceae. The former name for H. parasitica was Peronospora parasitica until it was reclassified and put in the genus Hyaloperonospora. It is an especially vicious disease on crops of the family Brassicaceae. It is most famous for being a model pathogen of Arabidopsis thaliana which is a model organism used for experimental purposes. Accordingly, the former Hyaloperonospora parasitica has been split into a large number of species. For instance, the taxonomically correct name of the parasite of the well-known model organism Arabidopsis thaliana is Hyaloperonospora arabidopsidis, not H. parasitica, whereas the pathogen of Brassica has to be called Hyaloperonospora brassicae.
Peronospora sparsa is an oomycete plant pathogen that causes downy mildew in berry producing plants; especially in the genus's Rubus and Rosa. Downy mildew plant pathogens are often host specific and cause problems in cloudberries, blackberries, boysenberries, strawberries, and arctic bramble. Since they are host specific, Peronospora sparsa will not cause downy mildew in grapes because a different plant pathogen causes downy mildew in grapes; Plasmopara viticola. Although it depends on the cultivar, symptoms do not normally start until later stages of disease and can look different on different plants. The most common symptoms include red lesions in the veins of leaves, with dry and deformed berries.
Peronospora trifoliorum, commonly known as downy mildew of alfalfa, is an oomycete plant pathogen infecting alfalfa.
Plasmopara halstedii is a plant pathogen infecting sunflowers. The species is one of many pathogens commonly referred to as downy mildew. P. halstedii originated in North America.
Plasmopara viticola, the causal agent of grapevine downy mildew, is a heterothallic oomycete that overwinters as oospores in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet condition release zoospores. Zoospores are splashed by rain into the canopy, where they swim to and infect through stomata. After 7–10 days, yellow lesions appear on foliage. During favorable weather the lesions sporulate and new secondary infections occur.
Pseudoperonospora cannabina is a plant pathogen that causes downy mildew, which is a fungal-like disease caused by an oomycete.
Pseudoperonospora humuli is a plant pathogen that causes downy mildew on hops.
Pseudoperonospora cubensis is a species of water mould known for causing downy mildew on cucurbits such as cantaloupe, cucumber, pumpkin, squash and watermelon. This water mould is an important pathogen of all these crops, especially in areas with high humidity and rainfall, such as the eastern United States. In most years the disease is an annual, late-season problem on squash and pumpkin in the eastern and central United States, however, since 2004 it has become one of the most important diseases in cucumber production. Considered a highly destructive foliar disease of cucurbits, successful breeding in the mid-twentieth century provided adequate control of downy mildew in cucumber without the use of fungicides. The resurgence in virulence has caused growers great concern and substantial economic losses, while downy mildew in other cucurbit crops continues to be a yearly hindrance.
Peronospora is a genus of oomycetes that are obligate plant pathogens of many eudicots. Most species in this group produce a downy mildew disease, which can cause severe damage to many different cultivated crops, as well as wild and ornamental plants. There are 19 genera that produce downy mildew, and Peronospora has been placed alongside Pseudoperonospora in the group of downy mildews with coloured conidia. Peronospora has far more species than any other genus of the downy mildews. However, many species have been moved from this genus to be reclassified to other or new genera. Among these was the most famous Peronospora species, formerly known as Peronospora parasitica, and now known as Hyaloperonospora parasitica. Now, the Peronospora species of most importance is likely the Peronospora tabacina. Peronospora tabacina causes blue mold on tobacco plants and can severely reduce yields of this economically important crop to the point where it has been classified as a bioweapon.
Peronospora destructor is a plant pathogen. It causes downy mildew on leaves of cultivated and wild Allium. Allium cepa is most often affected, while Allium schoenoprasum (chives) and Allium porrum (leek) are only occasionally affected.
Plasmopara is a genus of Oomycota. Plasmopara species are plant pathogens, causing downy mildew on carrot, parsley, parsnip, chervil, and impatiens.
Commonly known as Philippine downy mildew, this disease is caused by the species Peronosclerospora philippinensis of the fungal-like protist class Oomycetes, which also has members such as water molds and Phytophthora infestans, which caused the potato blight that led to the Great Irish famine.
Ovidiu Constantinescu was a Romanian mycologist known for his work on the taxonomy of the Peronosporales.
Basidiophora is a genus of oomycetes belonging to the family Peronosporaceae. It is a water mold that causes downy mildew disease on plants such as the New England aster.
Peronospora aquilegiicola is a species of oomycete in the family Peronosporaceae, first described in 2019. It is a plant pathogen: it can infect susceptible plants belonging to the genus Aquilegia, causing the plant disease Aquilegia downy mildew, as well as plants belonging to the genus Semiaquilegia. Peronospora aquilegiicola is native to East Asia, however it has a widespread distribution in the United Kingdom, and has since been detected in the German states of Lower Saxony, North Rhine-Westphalia, and Bavaria.
