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: Wed 3 Jul 2024
Source: Vertical Farm Daily [summ. Mod.DHA, edited]
https://www.verticalfarmdaily.com/article/9641441/a-new-race-of-bremia-lactucae-bl-41eu-identified-and-
denominated-in-europe/


_Bremia lactucae_, the causal agent of downy mildew in lettuce, is genetically very variable. Even within one lettuce field, several races may be present. Monitoring the changes in the population is
important for breeders and growers.

More than 845 _Bremia_ isolates were collected from lettuce in 2023 and early 2024 in Europe. In 2023, 3 of the official races were found frequently, while a further 3 official races were found at very low
frequency.

However, one virulence pattern which had appeared for the first time in 2022 was found in 3% and 10% of the isolates in 2023 and 2024, respectively. Consistent test results of an isolate from France lead to its denomination as the type isolate of the new race Bl: 41EU. The new race was found repeatedly in France, Germany, Switzerland, the Netherlands and the United Kingdom. Bl: 41EU breaks many resistance host genes.

The importance of chemical control and hygiene measures in addition to plant resistance is emphasised. Prevention of prolonged leaf wetness will reduce the spread of downy mildew in lettuce crops.

--
Communicated by:
ProMED

[Downy mildew of lettuce, caused by the fungus-like organism (oomycete) _Bremia lactucae_, is a common disease of both glasshouse and field-grown crops. It affects all types of lettuce and can result
in severe losses by reducing both yield and quality of crops. Multiple pathotypes of the pathogen exist affecting different hosts, but lettuce strains are specific to lettuce and close relatives. Symptoms
on lettuce may include yellow patches on upper leaf surfaces and white growth on the corresponding underside, followed by browning and death of infected tissue. Systemic infections may also occur leading to internal rotting of stem and leaf veins. The disease is favoured by humid conditions.

Spores are spread within a crop mainly by splashing rain, wind or mechanical means. The pathogen survives between crops on infected plant material or in the soil as oospores which may infect the roots
of the next crop. Disease management may include cultural measures to reduce humidity within a crop, crop rotation, phytosanitation and preventative fungicide applications. The most effective means of
control is planting resistant varieties. 

Downy mildews include 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 a few different hosts.

Pictures
Downy mildew on lettuce:
https://live.staticflickr.com/4598/39450551752_f3b1a2ba86_b.jpg,
https://www.armuro.com/en/wp-content/uploads/sites/6/2022/01/Lettuce-downy-mildew-identify-and-control-treatments.jpg,

https://www.rhs.org.uk/getmedia/5854178C-469B-4F1F-A2E7-F3D84FA098B9/SCN0002223_337834,
https://www.seipasa.com/files/images/mildiu_lechuga_tratamientos.jpg
and
https://ucanr.edu/blogs/SalinasValleyAgriculture/blogfiles/3823.jpg
(systemic infection)

Links
Information on lettuce downy mildew:
https://www.rhs.org.uk/advice/profile?pid=216,
https://apps.lucidcentral.org/ppp_v9/text/web_full/entities/lettuce_downy_mildew_209.htm,
https://ipm.ucanr.edu/agriculture/lettuce/downy-mildew/#gsc.tab=0,
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=2735 and
https://www.armuro.com/en/lettuce-downy-mildew-identify-and-control/
_B. lactucae_ taxonomy and synonyms:
https://www.indexfungorum.org/names/NamesRecord.asp?RecordID=220062
and
https://www.speciesfungorum.org/GSD/GSDspecies.asp?RecordID=220062
- Mod.DHA]

[See Also:
2016
----
Downy mildew, lettuce - USA: (FL)
http://promedmail.org/post/20160225.4048228
2015
----
Downy mildew, lettuce - Europe: new strain
http://promedmail.org/post/20150527.3388689]

Website: http://www.isid.org

Published: July 8, 2024