Ringspot, or black blight, is a fungal disease of Brussels sprouts.
If you have opted to grow Brussels sprouts, your are in good company. Brussels sprouts, fresh from the garden, 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!]
Impressive and delicious, your Brussels sprouts plant may develop light brown or black leaf spots, often with a yellow halo. The spots are usually limited by 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, the spots begin to look more like the concentric rings of a target. If you look closely, with a hand lens, or using a microscope, you can actually see the fruiting structures. Complete leaf loss can occur.
Ringspot life cycle
Since the infected leaf is going to be lost anyway, remove it and throw it in the trash. The pathogen that causes black blight, Mycosphaerella brassiciola, is already in the soil, and it travels on the wind and via splashing rain. It prefers the cool, moist conditions that commonly occur in California autumns - just as your Brussels sprouts are really growing. In other regions, this disease commonly infects broccoli, cabbage, cauliflower, and weeds in the brassica family, such as mustard. It generally only causes significant problems in the Bay Area on Brussels sprouts. I don’t know why.
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 ringspot has occurred in your garden, try a 2 to 3 year crop rotation and remove weeds that may host the disease.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.