What's wrong? Your broccoli, cabbage, and cauliflower have leaves that are wilting and turning a sickly yellow. Younger plants show signs of decay, and mature plants haven't grown much since you planted them. Dig one up, and you discover bulbous masses of golf-ball-size galls instead of a healthy, fibrous root system.
How did this happen? Club root (Plasmodiophora brassicae), a nasty fungus, infected the roots, causing them to swell asymmetrically. The swollen roots cannot absorb water and nutrients properly—before long, you'll be reading the plant its last rites.
What must you do right away? This is no time to be patient. Club root is a persistent and damaging disease that afflicts all the members of the brassica family, which also includes Brussels sprouts and kale. You need an aggressive plan to stop it from spreading through your soil.
Dig up infected plants. Once you've identified a plant with club root, carefully dig around the infected area and gently remove the entire root system from the soil so as to prevent the clubs from breaking up and potentially releasing thousands of spores.
Discard (don't compost) the infected plants.
How can you prevent it from returning?
Sow a cover crop of winter rye in early spring and till it into your soil 2 to 3 weeks after germination, suggests Paul Williams, Ph.D., plant pathologist at the University of Wisconsin. In reported research, this substantially reduced viable club root spores in the soil.
Club root thrives in moist, cool, acid soils. Raise your garden soil's pH to a more alkaline 7.2 by mixing hydrated lime into it in fall. Add plenty of organic matter to improve soil drainage, and remember to check the soil pH regularly.
Also, because club root develops in cool, wet environments, you can cook it out of the soil using a technique known as solarization. Simply spread clear, construction-grade plastic over fungus-infested soil and leave it in place for four to six weeks. The sun's radiant energy kills the fungus. Thinner sheets of plastic (1 to 2 mils) heat the soil more quickly and absorb more solar rays than thicker sheets. Soil solarization works best during warmer months, generally between May and September, depending on your geographic location.
You can try the organic farmer's trick of rotating crops by planting broccoli and its kin brassicas in a different area of your garden each year for 5 to 10 years before reintroducing them to that original plot. But be aware that this is one tenacious pest: Club root spores persist for many years by feeding off decaying organic matter while they lie in wait for the next broccoli or cabbage seedling to appear.
Finally, to avoid introducing club root to your soil from an outside source, start your own plants from seed in a sterile potting mix.