This serious fungal disease causes the roots of Brassica crops to swell up (like clubs) and become unable to function properly. It may be found wherever Brassicas are grown (which means pretty much anywhere), though its severity varies considerable. A serious infection will often kill a plant, while a milder one will reduce the harvest. The most susceptible crops are Cabbage, Chinese Cabbage and Brussels Sprouts, though all Brassicas are vulnerable to some degree. An infected plant won’t show symptoms above ground until the root damage becomes serious enough that it affects growth. You will then see wilting and eventual yellowing of leaves as they can’t get enough water and nutrients (wilting may also be caused by the even more common Cabbage Root Maggot, which also damages roots). Inspect for Clubroot by uprooting a plant and inspecting the roots to see if they are swollen and misshapen (the severity of this will vary considerably). If you find seriously infected plants you should remove them. The best strategy against Clubroot is to avoid importing it into your garden, which means not importing soil or any Brassica seedlings (it’s easy enough to grow your own). If this disease gets into your soil it can stay there for 10 years or more, even without any Brassicas to infect. Fortunately there is a good strategy for dealing with this potentially serious pest. Clubroot can only thrive in acid soil, so if you raise the pH of your soil it won’t be a problem. You have to get it up to neutral or slightly alkaline (pH 7.2 is ideal).
Image: David B. Langston, University of Georgia, Bugwood.org