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 Program