Scouting Onions for Botrytis Leaf Blight Halo Lesions
Christy Hoepting, Extension Vegetable Specialist
Cornell Vegetable Program
Botrytis Leaf Blight (BLB) halo lesions are the tiny pin-prick to pin-head sized yellow necrotic spots surrounded by silvery halos (Fig. 1, lesions 5 and 6). The silvery halo is often blotchy in shape.
Sometimes the necrotic spot is barely visible, which can make identification of such versions of these lesions tricky to identify (Fig. 1, lesions 2 and 3).
When BLB halo lesions get old, the center becomes sunken and often splits, it is still yellowish in color and remnants of the silvery halo can usually still be seen (Fig. 1, lesion 9).
Various nicks and dings caused by blowing debris or herbicide injury may be confused with BLB halo lesions (Fig. 2).
BLB halo lesions can be distinguished by their ghostly appearance not penetrating the leaf surface. BLB halo lesions are most abundant on the outer leaves, usually on the underside of the leaf, and are distributed anywhere along the leaf. All of these lesions are counted when scouting to use for spray thresholds for BLB. Count the number of BLB halo lesions on the outer three leaves of 20-30 plants per field. Numbers can be highly variable among plants, so it is good to take a look at several plants. The number of lesions per leaf is the number of lesions per plant divided by 3. The spray threshold is 1.0 BLB halo lesion per leaf.
Figure 2. BLB halo lesion amongst other necrotic spots on onion leaf. Photo: C. Hoepting, CCE Cornell Vegetable ProgramScouting Onions for Botrytis Leaf Blight Halo Lesions (pdf; 234KB)
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Upcoming Events
Small Vegetable Workshop
August 13, 2026
Fulton, NY
Identify vegetables that make sense to grow on small acreage, including under high and low tunnel season extension. Bill Ingersoll, owner of Ingersoll Farms, will provide a short tour and discuss other important factors of running a mixed vegetable farm, including a farm stand and selling at farmers' markets.
Chipping Potato and Field Crops Twilight Meeting
August 27, 2026
Dansville, NY
This year, our field meeting is bringing together field crops and chipping potatoes. SWNY Dairy, Livestock, and Field Crops Specialist Katelyn Miller will lead a field crops management discussion. Hear the latest updates in potato disease control and see the Cornell chipping potato variety trial run by Walter De Jong. Dinner follows the educational event.
1.5 DEC credits available in 10, 1a, 21, and 23.
IPM Field Day for Urban Growers
September 10, 2026
Buffalo, NY
Join us for a hands-on field day exploring integrated pest management and season extension tools for urban growing spaces and the opportunity to build community with local growers!
This event will focus on best practices for using row covers and rat mitigation. Check out our CCE demonstration trial using insect exclusion netting to manage swede midge, an invasive fly that causes damage to brassica crops. Explore how to identify rat activity, reduce attractants, and strengthen site management. Hear from local growers on how they manage pests with a focus on organic management and how they extend the season on their farms and gardens.