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New strains of light leaf spot on oilseed rape in the United Kingdom

A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases

Date: Mon 9 Aug 2021
Source: Rothamsted Research [summ., edited]

Light leaf spot (LLS) is becoming increasingly resistant to azole fungicides but not QoI or SDHI fungicides in laboratory tests. Both azole and non-azole products are still performing similarly to control LLS in the field. Continued azole sensitivity monitoring is needed. Use of a mixture of an azole plus a QoI/SDHI is recommended for good disease control, together with integrated approaches such as use of LLS resistant crop varieties. LLS has become the UK's most important disease of brassicas and can potentially reduce yields considerably if left untreated. It is also a major headache for brassica growers across Europe, Australia, New Zealand and parts of the US.

The fungus shows especially high genetic diversity, which means it has a considerable ability to evolve fungicide resistances. In the populations throughout Europe, increasingly complex variants of the gene targeted by azole fungicides are now widespread which, in lab testing, are far less sensitive to azoles. Within the UK, northern populations are less sensitive than southern ones. There are also differences between European populations, e.g. Danish populations are much more sensitive than UK or German ones. In the US, the populations are still highly sensitive to azole, QoI and SDHI fungicides.

[Ref: https://bsppjournals.onlinelibrary.wiley.com/doi/10.1111/ppa.13441]

Communicated by:

[Light leaf spot (LLS) of _Brassica_ species is caused by the fungus _Pyrenopeziza brassicae_. It is considered one of the main diseases of oilseed rape in northern Europe and is also a major disease affecting vegetable brassicas in cool, humid climates. In oilseed rape (_Brassica napus_), leaves become infected soon after sowing but remain symptomless until lesions appear on older leaves. In severe cases, developing leaves and buds can be affected causing leaf distortion and stunting of plants. Stems may split and break off. Developing pods are often infected, leading to premature ripening of seed and pod shatter. Different vegetable brassicas vary in susceptibility to the fungus; they generally show spots and discolourations on leaves and stems affecting quality and yield.

LLS is polycyclic and survives the summer on crop debris, volunteer oilseed rape, and vegetable brassicas. Spores are spread by wind, rain splash and with infected plant material. Disease management may include varieties with decreased levels of susceptibility, removal of infected stubble, disease monitoring, and timely fungicide applications.

In the UK, LLS has caused considerable yield losses of oilseed rape in the past. Reduced sensitivity of the fungus to some of the fungicides used for LLS control has been observed. An alert for emerging azole resistance was issued previously (ProMED post 20150123.3112436). Research on host resistance genes and development of LLS resistant crop varieties is being carried out. LLS has become another example of pathogens extending their range due to climate change (for some recent examples, see ProMED posts 20200811.7665227, 20200909.7763574, 20141024.2895414).

Azole (demethylase inhibitor, DMI, also called sterol biosynthesis inhibitors) fungicides are the largest class of fungicides. They were introduced in the 1970s and new products continue to be introduced. QoI fungicides (quinone outside inhibitors, strobilurins) are strongly antifungal agents produced by fungi. Being derived from natural products, they are considered environmentally safe but, like several other fungicide classes, they have single-site activity and therefore pathogen resistance is of major concern worldwide. SDHI (succinate dehydrogenase inhibitor) fungicides are not new, but have previously had limited use so that resistance remained limited to a few pathogens and crops. A "2nd generation" of SDHIs is currently being used to combat fungal diseases that have developed resistance to other groups of fungicides. Integrated disease management, including varying crops or crop cultivars in time and space, as well as rotating or mixing chemical classes of fungicides is vital to extend the useful life of host resistances and agrochemical compounds.

Europe, overview:

LLS symptoms on oilseed rape:
http://www.blackthornarable.co.uk/uploads/images/2D1A9844.jpg and
LLS on vegetable brassicas:
http://cached.imagescaler.hbpl.co.uk/resize/scaleWidth/620/offlinehbpl.hbpl.co.uk/news/WOH/broccoli-Pixabay-20170210113908984.jpg (broccoli) and
http://hortnews.com/wp-content/uploads/2015/07/Light-leaf-spot-on-buttons.jpg (brussels sprouts)

Additional news story (subscr.):
Information on LLS of oilseed rape:
https://ahdb.org.uk/lightleafspot, https://croprotect.com/diseases/light-leaf-spot,
https://cropscience.bayer.co.uk/threats/diseases/oilseed-rape-diseases/light-leaf-spot/, http://www.adlib.ac.uk/resources/000/099/850/TN512.pdf and http://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO.2003.93.6.657
LLS on brassica hosts:
LLS disease cycle:
_P. brassicae_ taxonomy and synonyms:
http://www.indexfungorum.org/names/NamesRecord.asp?RecordID=322140 and
Information on DMI fungicides:
http://www.greenhousemag.com/article/gmpro-0610-dmi-fungicides-mode-of-action/ and
Information on strobilurins:
https://www.ncbi.nlm.nih.gov/pubmed/17474024 and
Information on SDHI fungicides:
http://www.nzpps.org/journal/64/nzpp_641190.pdf and
History and review of agricultural fungicides:
https://www.apsnet.org/edcenter/apsnetfeatures/Pages/Fungicides.aspx. - Mod.DHA]

See Also

Light leaf spot, oilseed rape - UK (02) 20201123.7964200
Light leaf spot, oilseed rape - UK 20200317.7099953
Blackleg, oilseed rape - UK: host resistance break down 20180503.5782524
Light leaf spot, oilseed rape & vegetable brassicas - UK: alert 20170213.4836896
Fungal diseases, wheat & oilseed rape - UK: alert 20160128.3971413
Light leaf spot, oilseed rape - UK: spread 20150928.3675076
Light leaf spot, oilseed rape - UK: alert 20150123.3112436
Light leaf spot & aster yellows, oilseed rape - UK, Canada 20121022.1356899
Light leaf spot, oilseed rape - UK: increase 20100826.3015
and older items in the archives

More news from: ISID (International Society for Infectious Diseases)

Website: http://www.isid.org

Published: August 26, 2021

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