Black Blight

Black blight, or ringspot, is a fungal disease of broccoli, cabbage, cauliflower, and Brussels sprouts. Weeds in the brassica family, such as mustard, are also susceptible.

Black blight spot (AfroBrazilian) CC BY-SA 3.0

If you grow Brussels sprouts, you are in good company. Brussels sprouts look like an impressive Medieval weapon and taste far sweeter than the frozen, store-bought variety. [Show up at Grandma’s house on Thanksgiving with a freshly cut stalk of Brussels sprouts, and I can guarantee you will be the talk of the day!]

Black blight symptoms

Impressive and delicious, your Brussels sprouts plant may develop light brown or black leaf spots, often with a yellow halo. These spots usually stop at leaf veins and can have an angular shape. These leaf spots are fungal population explosions. In severe cases, these lesions can also occur on the sprouts. Over time, they begin to look more like the concentric rings of a target. If you look closely, with a hand lens or microscope, you can see the fruiting structures. Complete leaf loss can occur.

Black blight life cycle

Since infected leaves fall anyway, take them off and throw them in the trash. The pathogen that causes black blight, Mycosphaerella brassicicola, is in the soil. It also travels on the wind and via splashing rain. It prefers cool temperatures and moist conditions common in autumn - just as your Brussels sprouts are growing.

Black blight control
 
Since the pathogen is probably in your soil, the best control measure is to plant resistant varieties. Also, monitor plants for signs of black blight. Infected plants, or plant parts, should not be added to compost. If black blight has occurred in your garden, try a 2- to 3-year crop rotation and remove weeds that may host the disease.