A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the International Society for Infectious Diseases <http://www.isid.org>

Date: 9 Jan 2009
Source: Western Farm Press [edited]
<http://westernfarmpress.com/vegetables/spinach-downy-0109/>

New strain of spinach downy mildew found in Salinas Valley

A novel strain of the downy mildew that attacks spinach appeared in Salinas Valley in July 2008, and if officially designated as a new race, it will bring the total to 11. The downy mildew, _Peronospora farinosa_ f. sp.
_spinaciae_, continued to be a problem during September and October [2008] on varieties previously resistant to it.

The disease was discussed by James C Correll, plant pathologist, University of Arkansas. "There's been a rapid increase in the number of races in recent years," he said. "We don't know exactly why this has occurred, but it most likely has much to do with production practices. Many changes have come in Salinas Valley to lettuce and spinach." These range from higher plant densities to sprinkler irrigation that splashes spores. Seed of organic spinach does not receive the typical treatment with fungicide, and that may have also helped the introduction of novel strains.

Correll works closely with the International Working Group Peronospora Committee which will meet in January 2009 to consider whether the new strain should be officially designated as race ll. Meanwhile, spinach breeders are working to incorporate as broad a resistance as possible in anticipation of additional new strains.

Since spinach is a year-round crop in California, Correll said, the downy mildew can easily find a new host after being carried by trucks, farm equipment, or airborne spores. Spores can survive for as long as 10 years.

[byline: Dan Bryant]

--
communicated by:
ProMED-mail
<promed@promedmail.org>

[Downy mildew caused by _Peronospora farinosa_ f.sp. _spinaciae_ (also called blue mold) is often considered the most widespread and destructive spinach disease worldwide. The fungus is specific to spinach and a few weeds of the genus _Chenopodium_. It survives between crops as spores in soil or mycelium on seeds. Symptoms include spots on the upper leaf surfaces which can become necrotic and lead to distortions, and 'downy'
growth on the leaf undersides. Infected plants are unmarketable, their taste is altered even when used as tinned food. Disease development is favoured by cool, humid conditions and mostly occurs in young plants.
Spread within a crop may occur by mechanical means, wind and splashing water.

Disease management includes fungicides, crop rotation, and cultural practices to minimise inoculum and reduce free moisture in the crop.
Previously, cultivars with single-gene resistance to a given pathogen race were used. However, these cultivars are increasingly succumbing to new races emerging in Europe, the Americas and Japan.

The downy mildew group of fungi includes many species in several genera which cause similar symptoms on a range of hosts, including many crops.
Individual species usually have a narrow host range affecting only one or few different hosts.

Maps
USA:
<http://www.mapsofworld.com/usa/usa-road-map-enlarge-view.html> and <http://healthmap.org/promed/en?g=5481136&v=34.5,-106.001,5>
US states:
<http://www.census.gov/geo/www/us_regdiv.pdf>
Pictures of spinach downy mildew symptoms:
<http://www.inra.fr/hyp3/images/6034441.jpg>,
<http://www.inra.fr/hyp3/images/6034442.jpg>  and
<http://plant-disease.ippc.orst.edu/plant_images/SpinachDownyMildewClose.jpg>
Links
Spinach downy mildew disease information:
<http://www.ipm.ucdavis.edu/PMG/r732100111.html>,
<http://scarab.msu.montana.edu/HpIPMSearch/Docs/DownyMildew-Spinach.pdf>,
<http://ag.arizona.edu/plp/plpext/diseases/vegetables/spinach/spindm.html>,
<http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=1014>  and <http://www.inra.fr/hyp3/pathogene/6perfar.htm>
Information on _P. f._ f. sp. _spinaciae_ races:
<http://www.uark.edu/ua/jcorrell/spinach%20diseases.htm>,
<http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-91-11-1392>,
<http://www.agrisupportonline.com/Articles/new_races_of_mildew_in_spinach.htm>, 

<http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=216021>
and
<http://www.worldseed.org/cms/medias/file/TradeIssues/PhytosanitaryMatters/PathogenCoding/
StrainDenomination/Spinach-downy_mildew.pdf> 

_Peronospora farinosa_ f.sp. _spinaciae_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=353025>  - Mod.DHA]

[see also in the archive:
2008
---
Downy mildews, maize, grapevine - India 20081110.3537 Downy mildew, cucumber - USA 20080711.2112 Downy mildew, cucurbits - India: (PU) 20080411.1322 Mildews, grapevine - USA: new strains 20080327.1152
2007
---
Downy mildew, cucurbits - USA (02): clarification 20071109.3633 Downy mildew, cucurbits - USA: (SC), new strain 20071023.3448 Fungal diseases, vegetable crops - Canada: cucumber, potato 20070730.2442 Downy and powdery mildews, grapevine - France (02) 20070724.2368 Downy & powdery mildews, grapevine - France 20070720.2323 Downy mildew, cucumber - USA (OH, MI), Canada (ON) 20070704.2120
2006
---
Downy mildew, cucumber - Canada (ON), USA (MI) 20060803.2149
2005
---
Downy mildew, cucurbits - Brazil (Minas Berais) 20050922.2803
2004
---
Downy mildew, physiological races, sunflower - Italy 20041202.3222
2003
---
Downy mildew, squash - Italy (Latium, Umbria) 20030401.0803 Downy mildew, pepper - Spain (Murcia) 20030108.0063 2000
---
Downy mildew, sunflower - India 20000219.0225
1998
---
Downy mildew disease, grapes - Australia (South West) 19981031.2133]