ProMED
A ProMED-mail post

Source: The Scottish Farmer [summ. Mod.DHA, edited]
https://www.thescottishfarmer.co.uk/news/23260134.new-blight-found-denmark-resistant-50-uk-chemistry/
Another strain of potato late blight with resistance to a widely used fungicide has emerged. Researchers from the University of Aarhus, Denmark, have shown that 5 geographically diverse isolates from 2022 Danish crops belonging to this genotype, EU_43_A1, were completely resistant to a member of the carboxylic acid amide (CAA) fungicide group (mandipropamid).

EU_43_A1 was first identified in Denmark in 2018 and subsequently accounted for 21% of samples in 2021 and 45% in 2022. It was also detected in the Netherlands and Belgium during 2022. This genotype is the 1st strain of _Phytophthora infestans_ reported to have resistance to a CAA fungicide. Cross resistance to other CAA fungicides is likely, which has a bearing on 50% of fungicides used in the UK against late blight.

In Denmark, there are fewer fungicide groups approved for late blight, which may be a reason that resistance has developed to one of them. This is an exact repeat of what happened with fluazinam [see comment below] earlier.

While fungicides with different modes of action are still effective, it is critical that over-reliance on single substances does not increase the risk of resistance developing to the remaining fungicides. Mixing products from groups with different modes of action is essential to preserve efficacy for future use.

[Byline: Ken Fletcher]
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Communicated by:
ProMED

[Late blight of potato (PLB) and tomato is caused by the fungus-like organism (oomycete) _Phytophthora infestans_ and can cause 100% crop loss. The pathogen can also affect some other solanaceous crops. In potato, it affects leaves as well as tubers; in tomato, it causes lesions and rotting of leaves, stems, and fruits. The disease is favoured by cool, moist conditions. It can spread rapidly within a crop and destroy it within a few days. Under favourable conditions, epidemics in tomatoes may be even more rapid than in potatoes.

The pathogen is spread by plant material (including potato seed tubers, tomato transplants, plant debris, volunteer crop plants), mechanical means (including human and insect activities), wind, and water. Disease management requires an integrated approach; it may include removal of pathogen reservoirs, crop rotation, preventative fungicide treatments of planting material, as well as fungicide sprays of crops. Farm-saved or uncertified seed tubers have often been reported as sources of PLB outbreaks. Certified clean planting stock and management strategies for fungicide resistance of the pathogen are considered vital to control late blight outbreaks. Commercial crop cultivars vary in susceptibility to late blight. Development of resistant cultivars is being counteracted by the adaptability of the pathogen.

Late blight is considered an increasing problem worldwide. Considerable variation in aggressiveness between different pathogen strains has been observed, but more virulent strains are emerging frequently. The presence of both A1 and A2 mating types of the pathogen increases the chances of strains with additional fungicide resistances and increased yield losses developing. In Europe, regional differences in pathogen population diversity no longer exist; increasingly virulent strains have been reported to emerge mainly on potato (see previous ProMED posts in the archives), but recently also on tomato (ProMED post 20180611.5850443). EU_43_A1 was named in 2020 and is spreading rapidly in Europe (see links below). It made up 40% of the 2020 UK plb population (ProMED post 20210507.8349166).

Fluazinam is a pyridinamine fungicide introduced in the 1990s for PLB control. It has a multi-site mode of action and strong persistence of effect against the pathogen. EU_33_A2 (Green 33) was the 1st PLB strain with resistance to fluazinam (ProMED post 20200103.6868372). For information on CAA fungicides see links below.

Maps
UK:
https://www.worldometers.info/img/maps/unitedkingdom_physical_map.gif
Europe, overview:
https://mapofeurope.com/wp-content/uploads/2018/03/europe-political-map.jpg

Pictures
Late blight on potato:
https://onvegetables.com/wp-content/uploads/2018/08/Potato-late-blight-No-halo.jpg,
https://www.thompson-morgan.com/static-images/tandm/static-articles/how-to-stop-potato-blight/what-is-potato-blight.jpg,
http://farm1.static.flickr.com/81/281344513_74bbffe5fe.jpg and
http://cdn.phys.org/newman/gfx/news/2013/keepingpacew.jpg (resistant vs. susceptible cultivars)
Late blight on tomato:
http://ipm.illinois.edu/ifvn/volume15/images/tomato_late_blight.jpg
Microscopy of PLB infected cells:
http://micro.magnet.fsu.edu/optics/olympusmicd/galleries/brightfield/images/potatoblight.jpg

Links
Additional news story:
https://thefarmingforum.co.uk/index.php?threads/new-late-blight-fungicide-for-irish-potato-growers.381186/
Information on late blight:
https://gd.eppo.int/taxon/PHYTIN (with photo gallery),
https://doi.org/10.1079/cabicompendium.40970,
https://cropscience.bayer.co.uk/threats/diseases/potato-diseases/late-blight/,
https://cipotato.org/press_room/blogs/combating-late-blight/,
http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm,
https://www.gov.mb.ca/agriculture/crops/plant-diseases/print,mgmt-late-blight-potatoes.html and
https://www.hutton.ac.uk/research/departments/cell-and-molecular-sciences/epidemiology
PLB epidemiology and strains in Europe:
https://agro.au.dk/forskning/internationale-platforme/euroblight/pathogen-monitoring/about-pathogen-monitoring (incl. EU_43_A1 spread),
https://agro.au.dk/forskning/internationale-platforme/euroblight/pathogen-monitoring/genotype-frequency-chart/ (current populations),
https://agro.au.dk/forskning/internationale-platforme/euroblight/pathogen-monitoring/genotype-map/,
http://agro.au.dk/forskning/internationale-platforme/euroblight/currently/news/nyhed/artikel/new-emerging-clones-of-phytophthora-infestans-in-europe/ and
https://doi.org/10.1007/s11540-011-9187-0 (review and history)
_P. infestans_ taxonomy and synonyms:
http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=232148 and
http://www.speciesfungorum.org/GSD/GSDspecies.asp?RecordID=232148
EuroBlight:
http://euroblight.net/
Information on CAA fungicides:
https://www.frac.info/frac-teams/working-groups/caa-fungicides/information,
https://doi.org/10.1079/9781845939052.0096 and
https://doi.org/10.1002/9783527699261.ch20
- Mod.DHA]