Clubroot—Plasmodiophora brassicae
Clubroot is a problem on plants such as cabbage, broccoli, alyssum, and nasturtium, as well as many weeds in the mustard family. During initial stages of clubroot, aboveground symptoms may be absent. Foliar symptoms include stunting, yellowing, and wilting. Extensive galling, swelling, and distortion of the roots and hypocotyl are the main symptoms of the disease. Clubroot is common in soils where Brassica spp. plants have previously grown.
Identification
Clubroot may often be confused with nematode damage. Aboveground symptoms of both disorders are similar--wilting or stunting of leaves. Digging up the plants and observing the roots is the only way to distinguish the two disorders. Roots with clubroot are heavily clubbed and may appear spindle shaped. Multiple infections of the same root cause extreme swelling and distortion. Nematodes cause distinctive galls or swellings to form on the roots, but they are not clubbed or spindle shaped. In some cases, small white or brown structures (bodies or eggs of some nematode species) may be seen.
Life cycle
The fungus that causes clubroot persists in soil for many years. Infection is favored by acid soils with adequate moisture, but infections can occur above pH 7.0. In the presence of host plant roots, spores germinate and release swimming spores, called zoospores. These zoospores infect and colonize root hairs. Later, a second type of zoospore appears that can infect the main roots. Infection and colonization by this second zoospore causes the galling and clubbing of roots. Additional spores are formed inside the galled roots and are released into the soil when roots decay. The fungus is dispersed by the movement of infected plants, especially transplants, and the movement of soil.
Solutions
Clubroot is most common in acid soils. Add lime annually to affected soils below pH 7.2. Provide good drainage. Minimize the spread of the pathogen by using pathogen-free transplants. Avoid planting plants where other infested plants in the mustard family have grown, such as broccoli and cabbage. Solarization will also give control.
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Clubroot on broccoli infected at early seedling stage.
Plants infected with clubroot are stunted.
Infected plants show significant root clubbing.
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