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Scouting Onions for Botrytis Leaf Blight Halo Lesions

Christy Hoepting, Extension Vegetable Specialist
Cornell Vegetable Program

May 21, 2024

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. 



BLB Lesions on an onion leaf

Figure 1. Ten Botrytis leaf blight (BLB) halo lesions on an onion leaf. Lesion No. 1, 4, 5, 6 and 7 have tiny yellow necrotic centers. In lesion No. 2, 3 and 10, the necrotic center is not visible. Lesion No. 8 does not have a distinct yellow center and blends into No. 7. Lesion No. 9 is an old lesion with a sunken center and silvery halo still visible. Photo: C. Hoepting, CCE Cornell Vegetable Program









BLB lesion among other nicksFigure 2. BLB halo lesion amongst other necrotic spots on onion leaf. Photo: C. Hoepting, CCE Cornell Vegetable Program


Scouting Onions for Botrytis Leaf Blight Halo Lesions (pdf; 234KB)

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Upcoming Events

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December 16, 2025
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Come hear the latest on insect pest control and fertility management in potatoes from Cornell University experts. Potato variety trial updates will be shared too. After lunch will be the Empire State Potato Grower's Meeting. 1.5 DEC credits in 10, 1a, and 23.

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Announcements

Cornell Crop and Pest Management Guidelines Paused

From Cornell IPM:

Highlighting its commitment to quality and long-term sustainability, Cornell Integrated Pest Management's Pesticide Safety Education Program has announced
a pause in production of the 2026 Cornell Crop and Pest Management Guidelines.

Cornell IPM Director Alejandro Calixto said this temporary measure is the result of a comprehensive reimagining process facilitated by Illume Projects of Ithaca, which included end-user interviews, internal surveys and sales data analytics.

"It became clear to us that we cannot continue producing the guidelines and fully restructure them at the same time," Calixto said. "By pausing production, we can dedicate all available resources and time to rebuilding the production process, ensuring that when we re-launch in 2027, the guidelines will reflect a modernized approach built around the grower and other user experience."

A series of annually updated reference manuals produced by Cornell IPM and the College of Agriculture and Life Sciences (CALS), the Cornell Crop and Pest Management Guidelines are widely used by farmers, agronomists, crop consultants and extension educators in New York and throughout the Northeast. They include information about current IPM recommendations, pesticide options, cultural practices, nutrient management, disease, insect and weed identification and resistance-management strategies.

Calixto and Pesticide Safety Education Program Lead Mike Helms will spearhead efforts to restructure the guideline process over the next 12 months, with a goal of launching a more streamlined, user-friendly version in 2027.

Limited copies of the 2025 guidelines remain available for purchase while supplies last through The Cornell Store. The 2026-2027 Greenhouse Guidelines will be available.

For more information contact Helms at mjh14@cornell.edu.


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