Club root is a disease that affects plants, particularly brassicas. It is caused by secondary zoospores that infect the roots and form galls, which retard growth of the plant. Soil conditions such as acidity, high organic matter, low calcium levels, poor drainage, low boron levels, low nitrogen levels, and high soil temperature can increase the likelihood of club root. Prevention measures include maintaining alkaline soil, practicing good garden hygiene, growing resistant varieties, implementing long crop rotations, using well-drained soil and/or raised beds, and adjusting planting times. Treatment options for home gardeners are limited, but soil amendments such as calcium carbonate, borax, calcium oxide, and nitrogen fertilizers can create an environment where club root is unable to thrive. Developing a strategy to reduce the spread and impact of club root, changing the types and timing of brassicas grown, and adjusting growing practices can help manage the disease.
Club root is an infection of the roots of brassicas and related plants by Plasmodiophora brassicae, a soil-dwelling micro-organism, leading to massive swelling, distortion and severely retarded growth. Badly infected roots can suffer from secondary rots that can cause the breakdown of the whole root system and plant death.
More importantly though, it’s probably the worst of the vegetable diseases, because it affects the huge brassica family, is very easy to spread, can seriously affect the harvest (sometimes completely) and stays in the soil for a great many years, probably indefinitely unless you take very disciplined precautions.
The only truly reliable solution is to stop growing anything from the brassica family for 20 years, but that’s not very useful advice. I think there are strategies that can be used to make it practical to succeed despite this very challenging disease.
Club root affects the whole brassica family, but it affects some more than others. The worst affected are Asian greens and brassicas that are in the ground for a long time like purple sprouting broccoli, kalettes and Brussels. However a few long lived brassicas like ball cabbages are less affected.
Quick growing crops like radish and turnips are normally out of the ground so quickly that the harvest is rarely compromised. I’ve also found that some Asian greens still give a good leaf harvest, despite their roots being badly affected. However in both cases these veggies can raise the number of resting spores in the ground.
Club root affects the integrity and functionality of roots, causing them to swell and ultimately decay away to almost nothing. You probably won’t notice you have a problem until their growth rate declines a little and then they start to wilt on hot days and don’t respond to watering.
Dig up a plant and you will see the swollen roots. At this point there’s very little you can do for plants that still have a lot of growing left to do. For plants that are close to harvest, you can try and keep them alive by keeping them well watered, helping out what’s left of the roots, but bear in mind that you are also helping the club root to thrive, so removal is probably best.
The most susceptible are brassicas that are planted out in early summer, when the ground is warm (above 15c) and then stay in the ground all through summer for a harvest in Autumn, winter or spring, by the end of summer, the damage is done.
I’ve noticed that some badly affected plants, like Asian greens, often show few symptoms above ground, despite their roots being a swollen mess. Other fast growing brassicas like radish and to a lesser extent turnips are out of the ground before their roots are affected, but can still perpetuate the existence of club root in the soil.
Resting spores germinate in late spring, when soil temperature exceeds about 12-15c, producing zoospores that swim very short distances in soil water to root hairs. These resting spores are exceptionally long-lived, with a half-life of about four years, they can survive in the soil - at high enough levels to cause infection - for up to 20 years.
<aside> 💡 There is conflicting evidence about the temperature at which resting spores germinate, the most quoted figure is 15c, some say 16c, others say germination has been seen in soil temperatures as low as 12c. One source even says that root hair infection (rather than just spore germination) doesn’t occur below 18c. All of these point to planting in winter, spring and late Autumn as being a good idea, as the optimum temperature for club root growth is a soil temperature of 20-24c, that means summer!
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The longevity of the resting spores is a crucial factor contributing to the seriousness of the disease. A substance secreted from the roots of host plants stimulates resting spore germination. After the initial infection through root hairs or wounds, the pathogen forms a plasmodium which divides to form many secondary zoospores, which are then released into the soil.
These second-generation zoospores re-infect the roots of the initial host or nearby plants. They then invade the cortex (interior) of the root. Once in the cortex, the amoeba-like cells multiply or join with others to form a secondary plasmodium. As this plasmodium develops, plant hormones are altered, which causes the infected cortical cells to swell. Clusters of these enlarged cells form “clubs” or galls.