Clubroot: Common Brassica Root DiseaseJanuary 15, 2022
Fans of Harry Potter are no doubt familiar with the mandrake in that universe, a magical plant with the screaming gnarly roots and whose cry can be fatal for anyone who hears it. In our muggle world, some plants can develop a condition called clubroot, which gives the roots of the plants a similarly engorged appearance but is only fatal to the host plant. Infected plants may not show significant signs of damage, such as yellowing and wilted leaves, until much later in the disease progression because the symptoms first manifest underground.
Clubroot is a devastating crop disease that impacts many cruciferous vegetables including cabbage, brussels sprouts, rutabaga, and other brassica plants. This large and diverse product family has over 3000 species, out of which around 300 are particularly susceptible to this disease. In addition to the crops that we consume directly, processed plants such as canola crops used to make oil are also threatened.
Unfortunately, clubroot is a persistent soil pathogen. If it is found in your garden or growing site, it is extremely difficult to eradicate and its resting spores will likely remain indefinitely, poised to strike suitable future hosts. Crop rotation, selecting resistant cultivars, and good soil management are key to reducing the spread of this disease.
What Is Clubroot?
Clubroot is a plant disease found in brassica crops as a result of the eukaryotic protist pathogen Plasmodiophora brassicae. This type of soil-borne disease is distinct from fungal, bacterial, and viral plant diseases and is relatively rare. However, there are several notable protist plant pathogens in addition to P. brassicae that impact other economically important crops such as potatoes, beets, and grains.
P. brassicae specifically grows within the root cells of infected plants and causes the roots to form distinctive galls or clubs shaped deformations. This parasite reprograms the cellular functions of its host cells to divert nutrients away from the plant in order to help with its own spore formation and reproduction. Eventually, the roots of the infected plants will die, disintegrate and release these spores into the soil, where the resting spores can stay for up to 20 years! P. brassicae is an obligate parasite that must feed on living plant tissues and cannot live without a host.
Life Cycle Of Plasmodiophora brassicae
Plasmodiophorids have a complex life cycle that’s not entirely understood by scientists. In fact, the genome of P. brassicae was only recently sequenced within the past decade. What researchers do know about its disease cycle is that it involves three processes: zoosporic stages, formation of plasmodia inside host cells, and resting spore formation.
The cycle starts with resting spores releasing primary zoospores that infect the root hairs of host plants. These zoospores penetrate the cell walls of root hairs cells and form the primary plasmodia or a mass of cells. The primary plasmodia produce zoospores and then release these secondary zoospores. When the primary or secondary zoospores reach the root cellular cortex, they develop into a secondary plasmodium which triggers swollen roots in diseased plants. After further growth, the secondary plasmodium eventually divides into multinuclear plasmodia and finally becomes resting spores which get released into the soil during plant decay.
Symptoms Of Clubroot
Above-ground symptoms of clubroot disease will be similar to many other diseases and ailments. Affected plants may show signs of yellowing leaves, wilting leaves even at the slightest water stress, reduced production, stunted growth, and premature plant death. P. brassicae disrupts the root system of plants so the roots are not able to effectively take up water and nutrients to support the rest of the plant. Infected plant roots will show visible signs of galls or swollen, club-like structures. During the early stages of disease development, the root galls may be small but will expand to the entire root system. Infected plants later in the season may have very little root system remaining as the roots become brittle; P. brassicae zoospores are then released into the soil.
If you suspect that your plants may have clubroot from contaminated soil, farm machinery, or infected transplants, carefully dig up the plant to make sure the root hairs and root system stay intact for close inspection.
Do not compost diseased plants in the regular compost pile since clubroot is a soil pathogen. Remove any susceptible plants or cabbage family weeds, like wild mustard, that are in the vicinity as they might also be host plants. While infected soil should not be used to grow any plants in the cabbage family for at least 5-7 years, it is still suitable to grow nonhost crops. In order to prevent contaminated equipment or tools from spreading the pathogen, you must take care to sanitize them with a 1:9 solution of bleach and water or with thorough spraying of undiluted Lysol. While clubroot is typically transmitted through farm equipment, spores can also be transported by strong winds and soil movement through water erosion.
Prevention is absolutely key for clubroot because it is very difficult to eradicate from infested soil. Long-term control and mitigation are needed for this disease. High soil moisture, low soil pH, and soil temperatures between 68-77 degrees Fahrenheit can all exacerbate the impact of clubroot. One way to remedy acid soils and help prevent clubroot is to add lime to raise the soil pH above 7, out of the acid soil range. Another method is to improve the overall soil draining because plants in soil with poor drainage are more susceptible to root damages in general.
Using good garden sanitation and IPM measures to control root-feeding pests is always a good preventative measure. There are also clubroot resilient hybrids of broccoli, cauliflower, Chinese cabbage, and rutabaga that are currently available. Finally, if using transplants, grow them in sanitized soil or a soilless medium to prevent transmission of soil diseases including clubroot.