Clubroot caused by Plasmodiophora brassicae is a major disease of brassica crops worldwide and occurs on broccoli, Brussels sprouts, cabbage, cauliflower, turnip, rutabaga, and radish. It can also infect cruciferous weeds (mustard family) as well as some genera of grasses.
Infected roots enlarge to form galls that differ in size and shape depending on host plant. On crops with fleshy roots such as radish and turnip, galls form on the taproot or secondary roots. Crops with fibrous roots such as cabbage and broccoli produce club-like, spindle-shaped swellings on individual roots.
Plasmodiophora brassicae is a lower fungus that does not form true mycelium with cell walls. It produces a multinucleate mass of protoplasm called a plasmodium and reproduces by zoosporangia, motile zoospores and long lived resting spores that persist in soil and plant debris. When susceptible roots are present, resting spores of the fungus germinate to produce zoospores that swim in free water and infect root hairs. Here the fungus develops into a plasmodium and zoosporangia which release secondary zoospores to initiate new infections. It is the presence of plasmodia in the roots that causes root cells to divide repeatedly and enlarge into galls. Mature plasmodia develop into masses of resting spores which are released into the soil following invasion of the galls by secondary organisms. The disease is usually more severe on cold, wet, acidic soils and is spread by drainage water, infested soil on equipment, tools, or shoes, and infected transplants. Resting spores remain viable in the soil for up to eighteen years and the repeated production of brassica crops can lead to a rapid build-up of the pathogen. The disease is favored by soils with a pH less than 7.0.
The disease can be well established before above ground symptoms become evident. Infected roots are unable to absorb water and nutrients, top growth is stunted, and lower leaves may yellow and drop off. Affected plants may wilt during the day and recover at night. Root galls are often invaded by secondary organisms causing root decay and the death of the plants.
Cultural Controls & Prevention:
- Maintain a high pH by regular applications of lime. Calcitic lime is preferred over dolomitic, except where magnesium levels are low.
- Maintain high levels of calcium and magnesium. High pH can lead to boron deficiency in coarse soils. Apply boron as needed as a foliar spray or in the transplanting water.
- Finely ground lime alters the pH more quickly than coarse granules.
- Long (5-7 years) rotations between brassica crops.
- Improve soil drainage.
- Control brassica weeds.
- Avoid the movement of infested soil into clean areas.
- Produce transplants in clean soil or soilless media.
- Do not use irrigation water that has been contaminated with resting spores by run-off from infested fields into irrigation ponds.
Chemical Controls & Pesticides:
For Current information on disease recommendations ins specific crops including information on chemical control & pesticide management, please visit the New England Vegetable Management Guide website.