If the cabbage, broccoli, or other cole crops in your garden turn yellow or wilt, or the plants are stunted, the cause might be clubroot. This soil-borne fungus Plasmodiophora brassicae, affects the roots of these vegetables. When you notice something is wrong, the disease might have already progressed to the point of a failed or reduced harvest. It is nonetheless crucial to control the disease because the pathogen survives in the soil for many years.
Vegetables Susceptible to Clubroot
Clubroot primarily affects members of the Brassica family, also known as cole or crucifer crops, which includes rapeseed (canola). Not all vegetables are equally susceptible. The most susceptible are cabbage, Napa cabbage (Chinese cabbage), and Brussels sprouts. Moderately susceptible are kohlrabi, kale, cauliflower, collards, broccoli, and rutabagas. In turnips and radishes, the susceptibility depends on the variety. Horseradish is highly resistant to the disease.
How to Identify Clubroot
Plants infected by clubroot may die off as seedlings. In infected plants that make it beyond the seedling stage, you might not notice any above-ground symptoms until the disease has progressed, as clubroot attacks only the roots.
Clubroot causes abnormally large, swollen, and distorted club-shaped roots. In broccoli, cabbage, and cauliflower, you’ll see spindle-shaped club-like galls on the fine fibrous roots. In radishes, rutabagas, and turnips, the galls are rounder and located on the taproot or the secondary roots. The galls may be small or large, depending on when the plant was infected.
Because the infected roots cannot absorb water and nutrients, growth will be stunted, or leaves will wilt and yellow then drop. Sometimes the plants look wilted during the day, especially in warm weather, then recover at night.
The malformed gall tissue lacks the protective outer layer of healthy roots and make them a target for soft-rot bacteria. The only way to diagnose clubroot is to pull an affected plant and take a look.
What Causes Clubroot
Clubroot is caused by Plasmodiophora brassicae, a fungus-like organism that does not form a true mycelium (the vegetative part of a fungus) and reproduces by resting spores. If susceptible roots are nearby, the resting spores germinate and produce zoospores. These are spores that can move around in water and infect the root hairs of susceptible plants.
In the infected roots, the cells of the fungus-like organism grow rapidly in number and size, forming the typical abnormal club-shaped growth. During that process, new zoospores are also produced, which then infect healthy tissue of the same plant or nearby plants. In the mature clubroot-affected root tissue, new resting spores are also formed. Once the roots disintegrate, these resting spores are released into the soil.
When looking at the disease cycle of clubroot and the conditions that favor it, two factors are at play: the resting spores, and the disease itself.
The disease requires a living host to develop and multiply but the pathogen survives in soil and plant debris. The resting spores can remain viable for ten years or even longer. The spores germinate in moist soil when temperatures are between 54 and 81 degrees F. The disease usually develops in cold, wet, and acidic soil with a pH lower than 7.0.
Clubroot can be spread by drainage or irrigation water, infested soil, infected transplants, gardening tools and equipment (with or without infected soil residue), footwear, and even by roaming animals.
How to Control and Prevent Clubroot
If you have detected clubroot in your garden, there is nothing you can do to save the affected plants. But you can take steps to make sure that you stop the disease from hitting your garden again. And if you want to plant cruciferous vegetables in your garden, it’s always a good idea to run through the following checklist of preventative and control measures:
- Make sure that the transplants you buy are disease-free. Don’t buy seedlings if any for sale look wilted or yellowed. Even if you select the healthy-looking ones, they might be infected and are not yet showing any symptoms.
- Don’t plant any members of the Brassica family in a garden bed or spot where clubroot has occurred in the past. Keep in mind that the spores remain viable in the soil for more than a decade.
- Rotate crops and wait five to seven years before planting susceptible cole crops in the same garden bed.
- Plant broccoli, kale, and other Brassicas in well-drained soil only. Remember that the zoospore pathogens of clubroot can move around in water, so plants sitting in wet soil create perfect conditions for the disease. If you have clay soil with poor drainage, improve it by adding organic matter.
- Check your soil pH and maintain a soil pH of 6.8 or higher, which can help prevent clubroot. Amend the soil with lime if necessary. While a high pH is recommended against clubroot, it can cause boron deficiency. Boron is a micronutrient that can be added as a foliar spray or in the transplanting water. A soil high in calcium and magnesium in the soil can also protect your crops against clubroot. Do a soil test to determine what nutrients your soil might need.
- Chose clubroot-resistant cabbage varieties. Clubroot can also survive on weeds that are members of the Brassica family. Make sure to remove all weeds from your garden and any bordering areas.
- If you suspect you might have clubroot in a garden bed, clean tools and equipment after exposure to infected plants and soil with a 10% bleach solution (one-part bleach to nine parts water).
- As part of your routine fall cleanup, discard all plant debris, including the roots, at the end of the growing season. Do not compost diseased plant material but safely dispose of it in the trash.