Don’t Let Your Cruciferous Crops Get Beaten By Clubroot

Photo by Gerald Holmes

Photo by Gerald Holmes

Crops Affected

The disease occurs worldwide and impacts cabbage, cauliflower, Brussels sprouts, broccoli, and Chinese cabbage.

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Identification

Clubroot is caused by a soilborne fungus Plasmodiophora brassicae. This relatively simple fungus forms no mycelia (numerous threadlike branches) and is an obligate parasite, meaning that it can develop and multiply only in living host cells. Infected plants may appear normal at first. Clubroot may develop extensively on plant roots before the initial signs of infection: abnormal wilting and yellowing of leaves, especially on warm days) is noticed above ground. If soils are moist, symptoms may not become apparent until water stress occurs. When affected plants are pulled from the soil, the roots appear swollen and distorted — hence the name clubroot. Root malformation may vary in size from very small swellings on tap and lateral roots to large club-shaped roots, depending on when the plants became infected. In addition to reducing the plant’s ability to take up water, affected tissue is susceptible to invasion by soft rotting bacteria.
Clubroot can cause drastic yield reduction and occasionally total losses in crucifers. When infection occurs at an early stage of growth, young plants are stunted and may die, whereas plants infected at later stages fail to make marketable heads or growth.

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Survival And Spread

Clubroot can survive in the soil for seven to 10 years or longer as resting spores. These spores can be spread from field to field by infested soil, contaminated water supplies, infected transplants, infested soil on farm machinery, and even by animals such as cattle.
When soil conditions are conducive, the resting spores germinate to produce zoospores, which “swim” by means of flagella to infect susceptible plant root hairs. Germination requires moist, acidic soil and can occur over a wide temperature range.
Disease development is favored by high soil moisture and soil temperatures. Although clubroot has been found in soils exhibiting a wide pH range, the disease is primarily associated with acidic soils.

During the development of the disease, new zoospores are produced. These can infect the same plant or adjacent plants, repeating the cycle. Eventually, resting spores are formed within the diseased plant tissue, and these are released into the soil when the plant roots disintegrate.

Management Methods

It is important to avoid the introduction of the pathogen. Once the disease is present in a field, control measures are difficult, expensive, and not very effective. Infected transplants are an important source for the spread of clubroot. Growers should only plant healthy seedlings grown using clean media, trays, and equipment.

Long rotations can help prevent a pathogen buildup and can reduce disease if weedy hosts are controlled. If a field is known to be infested, the soil pH should be adjusted to pH 6.8. However, it is important to note that liming alone will not control clubroot.
Preplant soil treatment and transplant drenches with approved chemicals can provide some control.

Consult UF/IFAS recommendations for currently labeled fungicides for clubroot control in Florida crucifers.

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