Addition of Plasmopara obducens to the EPPO Alert List
Considering the fact that new outbreaks of Impatiens downy mildew caused by Plasmopara obducens were reported at least in two EPPO countries, and in other parts of the world (e.g. Québec, California, Michigan), the EPPO Secretariat decided that to add this pathogen to the EPPO Alert List.
Plasmopara obducens (a downy mildew of Impatiens)
Why: New outbreaks of Impatiens downy mildew caused by Plasmopara obducens have been reported almost simultaneously (2003/2004) in European countries (Germany and United Kingdom) as well as in North America (e.g. Québec, California, Michigan).
EPPO region: Germany, United Kingdom. In UK, it is suspected that the origin of the outbreak observed in 2004 was a nursery in Guatemala. According to records of specimens conserved at the US National Fungus Collection (BPI), P. obducens had been found in the past on wild balsam (I. noli-tangere) in Bulgaria, Czech Republic, Denmark, Finland, Germany, Romania, Russia. Its presence on I. noli-tangere was confirmed recently in Lithuania. So far in UK, P. obducens has not been found on wild I. noli-tangere.
Asia: China, India, Korea Republic.
North America: Canada (Manitoba, Québec), USA (California, Indiana, Michigan, Minnesota, Mississippi, Missouri, Montana, West Virginia, Wisconsin). In USA, the disease was first reported in 1942, and since then it has occurred sporadically. In 2004, it was detected for the first time in California and Michigan. In Canada, it is mentioned that P. obducens was found for the first time in Québec in April 2004 on I. walleriana.
South and Central America: Costa Rica, Guatemala.
On which plants: Cultivated and wild species of Impatiens (e.g. I. balsamina, I. noli-tangere, I. walleriana). There are unconfirmed reports of New Guinea hybrids being hosts of P. obducens.
Damage: The upper surface of affected leaves becomes pale green to yellow with no distinct lesions. White to greyish ‘fungal’ growth is observed on the underside of leaves. Premature leaf fall is commonly observed. Affected plants become stunted with distorted leaves. Number of flower buds can be reduced. When young plants and seedlings are infected they generally do not survive. Another downy mildew has been described on Impatiens (Bremiella sphaerosperma) but it causes slightly different symptoms. With P. obducens, leaves are mottled with no distinct leaf lesions whereas Bremiella sphaerosperma causes distinct leaf spots. The pathogens also present morphological differences. Impatiens downy mildew is favoured by cool and humid conditions.
Pictures can be viewed on Internet
Dissemination: Spores can be disseminated by air and water splash. It is reported that P. obducens can survive for a long time in the soil and infected plant debris (but without any further details). Data is lacking on possible seed transmission. Over long distances, trade of infested plants can spread the disease.
Pathway: Plants for planting (including cuttings), soil.
Possible risks: Impatiens are commonly grown in the EPPO region for ornamental purposes (indoors and outdoors). Control measures can be applied (chemicals, hygiene measures), but may not be fully effective (in particular, chemical treatments may mask symptoms for a while). In UK, as serious damage was observed during the 2003/2004 outbreaks, emergency phytosanitary measures were taken (‘notifiable pest’) and a cost-benefit analysis was conducted. Trade of cuttings and young plants is important in Europe, and there is an obvious risk of moving infected plants. However, data is lacking on biology, epidemiology and economic impact of the disease. The situation of P. obducens on wild I. noli-tangere would also need to be clarified in Europe, as these plants may act as reservoirs for the disease.
EPPO RS 2005/108
Panel review date - Entry date 2005-07
Wegulo SN, Koike ST, Vilchez M, Santos P (2004) First report of downy mildew caused by Plasmopara obducens on impatiens in California. Plant Disease 88(8) p 909.
DEFRA, Plant health – Downy mildew.
Michigan State University – Department of Plant Pathology. Extension. Dr Hausbeck publications on Impatiens downy mildew. http://www.plantpathology.msu.edu/labs/hausbeck/hausbeck.htm
Ministerio de Agricultura y Ganadería. Servicio Fitosanitario del Estado (Costa Rica). Lista de enfermedades de los cultivos agricolas de Costa Rica. http://www.protecnet.go.cr/plagas/LISTA
USDA-ARS. Systematic Botany and Mycology Laboratory
Invasive Fungi. Plasmopara obducens by ME Palm. http://nt.ars-grin.gov/sbmlweb/OnlineResources/FungiOnline.cfm