''Peronospora'' is a genus of

downy mildew 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 c ...

s that are obligate plant pathogens. They can cause severe damage to many different cultivated crops, as well as wild and ornamental plants. ''Peronospora'' is most closely related to '' Pseudoperonospora'', and together they form the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

s with coloured

conidia A conidium ( ; : conidia), sometimes termed an asexual chlamydospore or chlamydoconidium (: chlamydoconidia), is an asexual, non- motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also ...

. ''Peronospora'' has far more species than any other genus of the downy mildews. However, many species have been moved from this genus to other genera based on

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

evidence. Among these are the species now in '' Hyaloperonospora'', important pathogens of the

Brassicaceae Brassicaceae () or (the older but equally valid) Cruciferae () is a medium-sized and economically important Family (biology), family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous pla ...

. Now, the ''Peronospora'' species of most importance is likely '' 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.

History

''Peronospora'' was first described in 1837 by

August Carl Joseph Corda August Carl Joseph Corda (15 November 1809 – 1849) was a physician, botanist, and mycologist from Bohemia. This botanist is denoted by the List of botanists by author abbreviation, author abbreviation Corda when Author citation (botany), citing ...

, 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.

Habitat and ecology

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

in the genus is known as the floricolous downy mildews. These species produce conidiophores exclusively on the flowers of their hosts.

General form and structure

The first stage in the ''Peronospora'' life history is the sporangia. The sporangia are small spore-like structures about 65 um long that germinate a germ-tube when they are near a leaf stoma. A germ tube will come from the sporangium and penetrate the leaf cell where it will form a haustorium. The haustorium absorbs nutrients from the leaf, while hyphae invade the intercellular space, and the leaf will eventually develop a lesion. These lesions often start out yellow and then turn brown as the leaf starts to undergo necrosis. From here, ''Peronospora'' can undergo either asexual reproduction or sexual reproduction. Asexual reproduction occurs when the air outside is moist making for favourable conditions. During asexual reproduction, hyphae on the host plant will form sporangiophores, which will produce conidia. The conidia will be dispersed by the wind is able to infect other plants. The asexual cycle only takes five to seven days to complete. Sexual reproduction occurs when the conditions are unfavourable and it needs to withstand harsh environmental conditions. During sexual reproduction, the hyphae will undergo meiosis forming antheridia and oogonia, the only haploid structures in the ''Peronospora'' life history. The antheridia will fuse to the oogonia, initiating plasmogamy and then karyogamy, and will result in the production of many oospores. The oospores can then be dispersed by the wind to infect more plants. Both ''Peronospora'' and ''Pseudoperonospora'' are characterized by their ability to produce melanized sporangia, but ''Pseudoperonospora'' produces zoospores whereas ''Peronospora'' cannot.

Practical importance

The model oomycete pathogen, ''Peronospora parasitica'', used to be included in this genus, however it has been reclassified to the genus ''Hyaloperonospora''. Some species of ''Peronospora'' have been considered for their use as a bioweapon or have been classified as potential bioweapons. ''Peronspora somniferi'' was considered for its ability to devastate fields of the opium poppy, which could have targeted areas that depend on the crop. The United States has classified ''Peronospora tabacina'' as a possible bioweapon, because if it were used to target the US tobacco industry, it would lead to major economic loss.

Genomics and genetics

Only one species in the genus ''Peronospora'' has had its genome sequenced and assembled. In 2015, Derevnina et al. performed a ''de novo'' sequence assembly of the genome of two ''Peronospora tabacina'' isolates using Illumina sequencing. They estimated the genome size to be 68 Mb with a mitochondrial genome of 43 kb. The two assemblies had 61.8x and 128.9x coverage for the nuclear genomes and 6,824x and 43,225x coverage for the mitochondrial genomes. The mitochondrial genome only differed by seven single nucleotide polymorphisms, three small indels, and one copy number variant. Using a program to predict gene models, they found 18,000 potential protein coding genes.

List of species

The following species are placed in genus ''Peronospora'': *'' Peronospora aconiti'' *'' Peronospora aestivalis'' *'' Peronospora affinis'' *'' Peronospora agrestis'' *'' Peronospora agrimoniae'' *'' Peronospora alchemillae'' *'' Peronospora alpicola'' *'' Peronospora alsinearum'' *'' Peronospora alta'' Fuckel *'' Peronospora akatsukae'' Ito & Murayama *'' Peronospora anagallidis'' *'' Peronospora antirrhini'' *'' Peronospora aparines'' *'' Peronospora apula'' *'' Peronospora aquatica'' *'' Peronospora arborescens'' *'' Peronospora arenariae'' *'' Peronospora argemones'' *''

Peronospora arthurii ''Peronospora'' is a genus of downy mildews that are obligate plant pathogens. They can cause severe damage to many different cultivated crops, as well as wild and ornamental plants. ''Peronospora'' is most closely related to ''Pseudoperonospora' ...

References

{{Authority control Peronosporales Water mould plant pathogens and diseases Oomycete genera