Jan 8, 2025
Downy Mildew of Onions (2025)
Last year we had many reports of downy mildew disease in onions. Seems like this year, the weather may not be as conducive to the disease, but it is still a good idea to scout the fields.
Downy mildew of onions is caused by the fungus-like Stramenopile Peronospora destructor. The pathogen produces abundant sporangia that germinate directly acting as spores. Reported hosts other than onions include shallot, leek, garlic and chives. The fungus overwinters in volunteer hosts, cull piles, and debris. Spores are wind borne. Spores of the fungus present in soil or bulbs germinate, infect roots or bulbs, and develop systemically in the plant. Spores that land on the leaf surface produce local lesions that can spread systemically to younger tissue. Infection can spread rapidly under cool (34– 82 °F), damp conditions generating new inoculum within the field. The pathogen requires cool temperatures and the presence of free moisture (rainfall, dew or overhead irrigation) to infect onion plants. Under those conditions repeated disease cycles may occur with devastating results.
Disease management: Regular scouting of field is important toassess disease severity and apply preventative fungicides. Disease management includesuse of disease free bulbs, transplants and seed. Crop rotation with non host(small grain, corn) for at least 3 years. Destroy volunteer plants, debris andcull pile. Better soil drainage, better circulation, preventative spray canreduce disease occurrence. Always rotate fungicide from different groups toavoid resistance build up.
![VIPM_01-08_02 VIPM_01-08_02](/images/default-source/agricultural-ipm/veg-ipm-update-images/2025/vipm_01-08_02.png?sfvrsn=2d953448_2&MaxWidth=600&MaxHeight=&ScaleUp=false&Quality=High&Method=ResizeFitToAreaArguments&Signature=9E716E97BB86BD343D1C581CA36195DE9DC6FA52)
Pics: https://www.omafra.gov.on.ca/IPM/english/onions/diseases/downy_mildew.html
Sources:
https://amarillo.tamu.edu/
https://barron.extension.wisc.edu/
To contact Bindu Poudel go to:
bpoudel@email.arizona.edu