The Watering Can 2: Club Root Disease of Brassicas

Clubroot is a common disease of all members of the cabbage family (Cruciferae) and can be a debilitating disease for those trying to grow any of the plants from this very large group. It is a difficult, if not impossible disease to get rid of and is very common on allotment sites.
Historically, reports date back to the 13th century in Europe. In the late 19th century, a severe epidemic destroyed large proportions of the cabbage crop in St. Petersburg. The Russian scientist Mikhail Woronin eventually identified the cause as a "plasmodiophorous organism" in 1875, and gave it the name Plasmodiophora brassicae. Originally thought to be a slime mold it has now been reassessed as a fungus like microorganism It is a complex organism and there are reports that there are as many as nine different strains worldwide. It has had many names and in 18th, 19th and early 20th century Britain it was sometimes called finger and toe, fingers and toes, anbury, or ambury, these last two also meaning a soft tumour on a horse.

Cool, wet (70 to 80% water holding capacity), and acidic soils provide the most favouable environment and part of the process of growing a reasonable crop in infected soils is to mitigate these conditions by the opening up of the soil to allow better drainage, and the application of lime to raise the pH of the soil, as this has been shown to reduce the effects of this disease.

The dormant cysts of clubroot will germinate in reponse to exudations from the roots of suitable plants and produce spores. These spores enter the plant via the root hairs and eventually affect the whole of the root system and produces hormones which causes the host cells to enlarge up to twenty times their normal size. Clusters of these enlarged cells are responsible for the clubbing of the roots and also infects the surrounding soil by producing resting cysts.

Once infected, it has been reported that soils can contain cysts for as long as sixteen to eighteen years even without a host plant, so it is likely that once it is in an allotment plot it will remain there, as there are a wide range of crops that can be affected as well as a number of weeds. The disease can also be transported easily on tools, equipment, and footwear as well as moving infected soil from one area to another on tools, e.g. when hoeing. The run off of rain water can also affect previously unaffected areas.

The disease attacks all members of the cabbage and related families of plants. i.e: cabbage, sprouts, broccolli, swede, kale, pacchoi, mustard, turnip, radish, cauliflower and kohlrabi. Other plants and weeds that are infected include;, shepherds purse, wallflowers, horse radish, stocks, and all other plants belonging to the Cruciferae family.

There are unfortunatly no visible signs that the plot contains this disease and the only way of knowing is when plants of the susceptable crops are grown and symptoms appear. Affected seedlings will not show any root swellings until about 3 weeks after infection. When susceptible plants are attacked in the seedling stage, they can die. When plants are attacked at a later stage, the disease rarely kills, but roots that are severely distorted have a reduced capacity to absorb minerals and water from soil. Plants wilt in hot weather but partly recover at night. Consequently, top growth may be stunted, yellowish, and likely to prematurely bolt or to wilt in hot weather.

But even with extensive root clubbing, top growth may be nearly normal. If clubroot is suspected then one way of finding out is to sow a quick growing crop, such as radish, in order to establish it’s presence.(see photograph). Steps can then be taken to lessen the problem. The potential of cultural practices to reduce crop losses due to clubroot are limited, and chemical treatments to control the fungus are either banned due to environmental regulations, are not cost effective, or unavailable.

The breeding of resistant cultivars therefore is a promising alternative. Some cultivars show some levels of resistance:  Calabrese ‘Trixie’; Swede ‘Marian’ and Kale ‘Tall Green Curled’. Cabbages ‘Kilaxy’ and ‘Kilaton’ and Cauliflower ‘Clapton’ all show some resistance.

Cultural methods can also be employed to provide a better chance of procuring a reasonable crop when clubroot is known to be present.

The main priority is to provide a free draining soil with a suitable level of lime in order to reduce the activity of the disease as it produces more damage on acid soils. Winter digging will help to open up the soil by allowing better drainage, especially on clay and heavy soils and a dressing of lime should be given to raise the acidity level above neutral (pH 7.0), a simple soil testing kit is useful to establish a rough idea of how much is needed. Do not apply manure or compost at the same time as lime as the two will interact and all nutrients will be lost from the compost and the lime neutralised.

It is generally easier to make soils more alkaline than it is to make them more acid. Because different soil types react in different ways to the application of lime you will have to add more lime to clay soils and peaty soils than you will in sandy soils to achieve the same result.

To increase your pH by 1.0 point and make your soil more alkaline.

Remember to always read and follow the manufactures recommendations when using any chemical product especially in windy weather. However I have found that this is often slow to work and that lime application should be carried out as soon as you know you are going to grow brassicas on that part of the plot. Remember also that plant rotation will help to a degree in keeping this disease at bay.

When sowing seed of susceptible crops, do not sow them direct into the allotment, but use a soil less compost and sow in pots or a seed tray. Pot the plants on when large enough and, if possible, do not plant out until the have been grown in at least as 4” pot. This will give you good plants to start off with as they will have a good root system. On planting, excavate a hole larger than required and plant in some more soil less compost and a dusting of lime. Water plants well in, preferably with clean water and keep well fed throughout their period of growth. Remember also to keep down all weed growth on your plot and do not accept plants from anyone just in case they are contaminated.

Finally, please remember that while it is perfectly O.K. to compost the leaves and stems of affected plants, the root systems should not be included but disposed of in household waste.