Telling the difference between Botrytis Leaf Blight and Downy Mildew on onions

If you haven’t started already, it’s time to keep an eye out for botrytis leaf blight in onions. Especially with all the rain we have had so far this season! However, current identification of botrytis could be challenging in some areas due to pelting rain or hail injury (silver blotching without yellow necrotic spots). As well, with the frequent rain, winds and cooler night temperatures, the risk of the development and dispersal of downy mildew is becoming more likely. Below is a quick description of how to differentiate between these two diseases when scouting in onion fields.

Botrytis leaf blight is characterized by small (1 – 3 mm), discrete, gray/white leaf spots that turn light brown as the lesions age. Newly formed lesions are often surrounded by an area of silvery-white tissue, giving the appearance of a “halo”. The disease is generally distributed throughout the onion field (Fig.1).

Downy mildew, on the other hand, usually occurs in patches of the field. During early disease development, a violet/gray velvety looking growth develops that kills infected tissue (Fig 2). Affected leaves turn yellow and collapse.

Scout for both diseases at the same time. Walking in an established pattern (e.g. W, X or M), randomly sample 50 plants across the field. Count and record the number of lesions on three of the oldest green leaves or observe any velvety growth.

Note – it’s not unusual to find both diseases on a given onion plant (Fig. 3).

Fig 1. Botrytis leaf blight of onion

Fig 2. Downy mildew of onion

Fig 3. White lesions of botrytis leaf blight and purple sporulation of downy mildew of onion