''Plasmopara obducens'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

oomycete 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 o ...

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 (biology), f ...

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 ...

. It was first described on ''

Impatiens noli-tangere ''Impatiens noli-tangere'' (touch-me-not balsam; Latin ''impatiēns'' 'impatient' or 'not allowing', and ''nōlī tangere'' 'do not touch': literally 'be unwilling to touch') is an annual herbaceous plant in the family Balsaminaceae found in damp ...

'' in Germany in 1877. ''Plasmopara obducens'' is known from native species of ''Impatiens'' since the 1800s, but outbreaks on cultivated varieties of ''

Impatiens walleriana ''Impatiens walleriana'' ( syn. ''Impatiens sultanii''), also known as busy Lizzie (British Isles), balsam, sultana, or simply impatiens, is a species of the genus ''Impatiens'', native to eastern Africa from Kenya to Mozambique. The Latin s ...

'' 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 Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

and in 2004 in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. 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 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' ...

'' 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, and impatiens ''Impatiens'' is a genus of more than 1,000 species of flowering plants, widely dis ...

''. 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 monthly magazine devoted to articles about new scientific and technical developments, typically gleaned from recent scientific and technical journals. The periodical has been described as having a scop ...

Water mould plant pathogens and diseases Ornamental plant pathogens and diseases Peronosporales Protists described in 1886 Oomycete species {{plant-disease-stub