''Plasmopara obducens'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

oomycete Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...

that causes

Impatiens ''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'' (one species), ''Impatiens'' make up the family Balsaminaceae. ...

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 crucif ...

. It was first described on '' Impatiens noli-tangere'' in Germany in 1877. ''Plasmopara obducens'' is known from native species of ''Impatiens'' since the 1800s, but outbreaks on cultivated varieties of '' Impatiens walleriana'' started in 2003 in the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

and in 2004 in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. The outbreak spread worldwide by 2016.

Morphology

Sporangiophores are an average of 373 (260-484) micrometers with the first branch appearing at 197 (120-271) micrometers. Sporangia are 16 (13-18) micrometers by 13 (11-16) micrometers. Resting spores are globose with a diameter of 28 micrometers

Disease symptoms

Initially, leaves are stippled or yellowed. Infected leaves curl downward. A white down is often present on the underside of the leaves; buds and stems may also develop a down. Eventually, the leaves fall off the plant leaving just stems. In about a week, the plants are dead.

Taxonomy

J. Schröter originally placed the species in the genus ''

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 ornamenta ...

'' but later moved it to the genus ''

Plasmopara ''Plasmopara'' is a genus of Oomycota. ''Plasmopara'' species are plant pathogens, causing downy mildew on carrot, parsley, parsnip, chervil Chervil (; ''Anthriscus cerefolium''), sometimes called French parsley or garden chervil (to disting ...

''. Some authors believe the outbreaks on ''I. walleriana'' and ''I. balsamina'' are not caused by ''P. obducens'' but two new species ''P. destructor'' and ''P. velutina''.

References

External links

''Disease threatens garden impatiens; Surprising scientists and horticulturalists, once-mild downy mildew disease has struck the popular blooms in 33 states''
March 20, 2013 Vol.183 #8

Science News ''Science News (SN)'' is an American bi-weekly magazine devoted to articles about new scientific and technical developments, typically gleaned from recent scientific and technical journals. History ''Science News'' has been published since 1 ...

Water mould plant pathogens and diseases Ornamental plant pathogens and diseases Peronosporales Species described in 1886 {{plant-disease-stub