Peronospora


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.

History

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.

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 in 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.

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 ''Peronospora'' species