Clubroot

Clubroot is an infection of the roots of brassicas – broccoli, Brussels sprouts, cabbage and cauliflower, including swede and turnip – causing swollen and distorted roots and stunted growth. It also attacks radishes and other plants closely related to brassicas, including wallflowers, aubrieta and stocks (Matthiola).

The swollen roots, and the lack of finer feeder roots, means that affected plants are unable to take up water and nutrients from the soil.

Clubroot is always worse on overly wet or waterlogged soils and on acidic (low pH) soils.

Clubroot infections start when the soil is moist and warm, and tend to occur from mid-summer to late autumn.

• Brassicas – broccoli, Brussels sprouts, cabbage and cauliflower, including turnip and swede
• Radishes
• Wallflowers
• Aubrieta
• Stocks (Matthiola)

Although often described as a fungus, Plasmodiophora brassicae is a soil-dwelling microorganism.

It produces resting spores that can remain viable in the soil for up to 20 years. These spores germinate when in the presence of susceptible plants and infect the roots. Here they develop and cause the characteristic distorted roots, producing more resting spores. When the roots rot, these are released back into the soil to start the lifecycle over again.

Try to keep plants growing as strongly as possible to help them fend off disease attacks. However, it may not be possible to do very much about large outbreaks of clubroot.

As clubroot prefers moist or wet soil, make sure soils are thoroughly cultivated and drain well. Growing brassicas in raised beds may help.

Similarly, as they prefer acidic soils, applying garden lime to acidic or neutral soils to raise the pH (make them more alkaline) may help reduce infection. Depending on the soil’s pH, you may need to apply between 250-500g per sq m (7¼-14½oz per sq yd).

Growing plants initially in 9-10cm (3½-4in) pots of compost until they’re well established and ready to plant out may give them a head start; as they’ll be bigger and stronger, they’re more likely to fight off attacks. Or grow them permanently in large containers, replacing the compost every year.

Be aware that you can spread contaminated soil around the garden or allotment on your shoes and tools, so spreading the disease.

Don’t grow brassicas or the other affected plants in soil known to contain clubroot for at least 10 years.

Some weeds are also affected by clubroot, including charlock, shepherd’s purse and wild radish, so remove these if growing close by.

Badly affected plants should be dug up and disposed of.

Unfortunately, there are currently no chemicals approved for home gardeners to use to treat clubroot.

Wherever possible, don’t buy brassica plants – grow your own from seed. If you have to buy them, try to check that they come from a clubroot-free source. This includes “free” plants from friends and neighbours

Some varieties have resistance to clubroot. Bear in mind that this resistance can break down. These include: Brussels sprouts Chronos, Crispus; cabbage Kilaxy, Kilaton; calabrese Monclano; cauliflower Clapton; swede Marian.

• Vegetable seeds
• Garden lime