Brown Blotches on My Onion Leaves — What Is It?

Distinct brown spots, blotches, or lesions on onion leaves can have several different causes, each with a different visual signature and appropriate response. The most common culprits in UK and northern European gardens are purple blotch (Alternaria porri), Botrytis leaf blight, and — when early-season wet conditions persist — downy mildew progressing to leaf collapse. Identifying which you have guides the correct response and helps you avoid making things worse.

Purple blotch (Alternaria porri)

Purple blotch causes small, water-soaked spots that develop into large, oval, purple-brown lesions with a distinct yellow halo. The lesions often have concentric rings within the brown area, giving a target-board appearance similar to early blight on potatoes. In humid conditions, dark fungal sporulation appears in the centre of older lesions. It is most common in warm, wet or humid weather (July–August) and thrives in beds with poor air circulation. Remove affected leaves and improve airflow by clearing dense growth around plants. Avoid overhead watering in the evening. Copper-based sprays (copper hydroxide or copper oxychloride) applied at the first sign of infection can slow progression; organic growers routinely use these within certified organic programmes.

Botrytis leaf blight

Botrytis squamosa causes small, whitish lesions with a pale halo, often distributed across the leaf rather than concentrated at tips. In contrast to neck rot (which affects stored onions from harvest onwards), this form of Botrytis attacks living leaf tissue in the field. Lesions expand and coalesce in cool, humid conditions, eventually causing tip dieback and overall leaf death. It is most problematic in cool, wet seasons with poor air circulation. Management is similar to purple blotch: remove affected tissue, improve airflow, and avoid evening overhead watering. In severe infections, copper sprays slow progression but are rarely necessary in home gardens.

Downy mildew progressing to brown leaves

Downy mildew (Peronospora destructor) initially shows as pale green or greyish patches with a fuzzy undersurface growth. As the infection advances, affected tissue collapses and turns brown, creating large dead patches. This is distinct from the well-defined lesions of Alternaria or Botrytis. Downy mildew progresses rapidly in cool, wet, still weather. Improve air circulation, remove affected leaves, and avoid wetting foliage. Once downy mildew is systemic in a plant, recovery is unlikely — harvest promptly if the majority of leaves are affected.

Does leaf blotch affect the bulb?

In most cases, moderate leaf blotching reduces photosynthesis slightly but does not directly infect the bulb. If the majority of leaves remain functional and the disease does not progress to total leaf collapse, bulb development is only modestly affected. The exception is severe, late-season infection that kills the majority of foliage before the bulb has reached full size — in that case, harvest promptly rather than waiting and hope for recovery.