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International Society for Infectious Diseases
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Date: Thu 9 Feb 2023
Source: Specialty Crop Industry [summ. Mod.DHA, edited]
https://specialtycropindustry.com/scala-fungicide-no-longer-viable-option-on-botrytis-for-vidalia-onion-producers/


Growers of Vidalia varieties of onion should be aware that pyrimethanil ["Scala"] fungicide appears not to be effective any more against botrytis leaf blight, an important disease to contend with every season.

University of Georgia Extension said that the general trend based on field observations and fungicide trials is suggesting that pyrimethanil has lost its efficacy. However, this has not been confirmed in laboratory analysis so far. As part of resistance management, farmers should think of including fungicides from other groups in their spray mixes.

[Byline: Clint Thompson]

--
Communicated by:
ProMED

[Botrytis leaf blight (BLB; also called blast) of onion is caused by the fungus _Botrytis squamosa_. BLB is a major disease of the crop in most growing areas worldwide. Yield losses in untreated plots of up to 30% have been reported. Symptoms may include elongated leaf spots, often with halos; leaf yellowing, necrosis, and tip dieback; and loss of plant vigour leading to reduced growth of bulbs. Disease development is favoured by high humidity and warm temperatures. Several other species in the genus also cause diseases of the crop (see links below).

The fungi are spread by water, wind, and mechanical means (including human and insect activities) and with plant debris. They can overwinter as mycelium in soil or plant debris, but sclerotia are formed for long-term survival. Disease management may include cultural practices (such as field sanitation, good crop ventilation, avoiding plant damage), use of clean planting material, and fungicides.

Pyrimethanil belongs to the class of aminopyrimidine fungicides. It is used widely to control botrytis species on a range of crops and has been designated an environmental contaminant. Mixing of fungicides from different classes is a vital tool to preserve their efficiency and reduce the risk of fungal strains with resistance to individual compounds developing. The report above may suggest the emergence of a new BLB strain.

Maps
USA (with states):
https://www.orangesmile.com/common/img_country_maps/usa-map-0.jpg

Pictures
BLB symptoms on onion:
http://www.omafra.gov.on.ca/IPM/images/onions/onion-botrytis5_zoom.jpg,
https://i1.wp.com/blog.pestprophet.com/wp-content/uploads/2018/02/f-late-blight-caused-by-botrytis.jpg,
http://vegetablemdonline.ppath.cornell.edu/Images/Onions/OnionBotrytis/Onion_BotrytisColl.jpg and
https://pnwhandbooks.org/sites/pnwhandbooks/files/plant/images/onion-allium-cepa-botrytis-leaf-blight/038.jpg

Links
Information on onion BLB:
http://vegetablemdonline.ppath.cornell.edu/factsheets/Onion_Botrytis.htm,
http://www.omafra.gov.on.ca/IPM/english/onions/diseases/botrytis_leaf_blight.html and
https://pnwhandbooks.org/plantdisease/host-disease/onion-allium-cepa-botrytis-leaf-blight
Botrytis diseases and pathogens of onion:
https://doi.org/10.1094/PHYTO-06-20-0258-IA (review),
https://www.mtvernon.wsu.edu/VSP/PDF/Onion%20Botrytis%20Diagnostic%20Guide%20-%20PHP%20Nov%202007.pdf,
https://blog.pestprophet.com/managing-botrytis-diseases-onions/ and via
https://www.apsnet.org/edcenter/resources/commonnames/Pages/OnionandGarlic.aspx
Fungal taxonomy via:
http://www.indexfungorum.org/Names/Names.asp
Information on pyrimethanil fungicide:
https://pubchem.ncbi.nlm.nih.gov/compound/Pyrimethanil
- Mod.DHA]

See Also

2009
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Fungal blight, onion - India: (MH) 20090103.0024
2008
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Bunch rots, grapevine - New Zealand 20080409.1298
Fungal diseases, grapevine - Australia, South Africa 20080310.0969
2005
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Gray mold, tomato - Mexico (Baja Calif): 1st report 20050419.1102
2002
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Botrytis sp., onion - USA (Washington) 20021005.5474

Website: http://www.isid.org

Published: February 17, 2023