|Crop Knowledge Master||Fungi|
|club root of crucifers (Plant Disease Pathogen)|
Stephen A. Ferreira, Extension Plant Pathologist
Rebecca A. Boley, Educational Specialist
Department of Plant Pathology,CTAHR
University of Hawaii at Manoa
Clubroot can cause significant losses to turnip, rutabaga, cabbage, cauliflower and broccoli, Brussels sprouts, kohlrabi, Chinese cabbage, and radish. In addition, most cruciferous weeds are susceptible and can serve as reservoirs of the pathogen from one year to another.
Occurs worldwide, but most frequently in Europe and North America.
Above ground symptoms include wilted and yellowish leaves and stunted plants. Lower leaves may drop off. Affected plants do not grow acceptable produce. The distinctive symptom is abnormal enlargement (clubs) of roots, including fine roots, secondary roots, or the taproot, or even on the underground stem. The clubs usually are thickest at the center and tapered toward the ends. Clubs may be isolated and cover only part of some roots or they may coalesce and cover the entire root system of the plant. Some affected plants may not exhibit above ground symptoms until they are pulled and the tiny clubs are noticed on the roots (Sherf and MacNab, 1986; Agrios, 1978).
P. brassicae overwinters as resting spores. When environmental conditions are favorable, a resting spore germinates and produces one zoospore which infects a host root hair and produces a plasmodium within the root. After a few days, the plasmodium cleaves into multinucleate portions, each of which develops into a zoosporangium. The zoosporangia are discharged from the host, where each releases four to eight zoospores (sometimes referred to as secondary zoospores). Secondary zoospores enter roots, tuber tissues or underground stems. Each zoospore develops into a plasmodium which spreads through host cells, producing the clubbed root effect. The plasmodium forms resting spores which are released into the soil as the host tissue decays. Resting spores can survive in soil indefinitely, though they only grow and multiply in a limited number of hosts.
Cool, wet and acidic soils provide the most favorable environment for the pathogen. In acid soils, the temperature range for infection is between 50 and 95 F, the optimum being between 68 and 77 F. In alkaline soils, the temperature range is lower (Sherf and MacNab, 1986).
At least nine physiologic races or pathotypes of P. brassicae are known to exist and have been reported from Germany, Canada, England, the Netherlands, and the United States. The races can be identified from disease reactions observed crucifer species (Sherf and MacNab, 1986).
Resting spores of P. brassicae can be disseminated through the transport of infested soil by means of tools, equipment, animals, and humans. Clean fields can be infested with diseased transplant material, and runoff from infected fields. Datnof et al. (1984) demonstrated that resting spores can be present in pond water sediment, especially those fed by runoff, where that water is used for irrigation. Pumping water from ponds with resting spores present is likely to spread the pathogen into pathogen-free seedbeds and fields.
A number of cultural practices should be followed to help curb P. brassicae infestation including buying or using disease free transplants, using well drained and pathogen free soil, eliminating nearby crucifer weeds, incorporating a 7-year rotation of non-cruciferous crops, adjusting soil pH to 7.2 or higher, and using resistant crop varieties. Resistant varieties are exist for radishes, rutabagas (swedes), and turnips. There are no acceptable resistant varieties of cabbage, cauliflower and broccoli, and Brussels sprouts.
Raising soil pH can provide good control of P. brassicae because spores germinate poorly or not at all and secondary zoospores are not produced at all, thus no clubs develop. Soil pH should be adjusted at least 6 weeks before planting the crop. Hydrated lime (1500 pounds per acre) provides good control in heavy soils and additional lime, if needed, can be added in the form of ground limestone or air-slaked lime. Rotation crops should be selected with the alkaline soil in mind as some crops may be adversely affected by a high pH. There are reports that both calcium and magnesium affect P. brassicae, but that their effects are dependent on soil pH (Myers and Campbell 1985). While pH is likely the most important factor, high concentrations of calcium and magnesium may give adequate control at pH < 7.2.
In addition to cultural control measures, control of P. brassicae has been shown with the use of S-H mixture, a biological control agent, in both greenhouse and field tests. S-H mixture is believed to inhibit zoosporangium formation, and it probably raises the pH of the soil a little since the mixture has a pH value of 8.0 (Lin et al., 1990)
If there is any question about soil contamination, treat the seedbed with chloropicrin, methyl bromide, metham-sodium, di-trapex, or dazomet 14 to 21 days before planting. When used properly these agents kill weed seeds as well as the clubroot organism (Sherf and MacNab, 1986).
PCNB is an effetive, nonphytotoxic, and economically feasible control measure for use in large fields. It is effective when applied in transplant water as a suspension of 6 lb 75% WP in 100 gallons water using about 1/3 pint per plant. This is approximately 400 gallons per acre for cabbage (Sherf and MacNab, 1986).
Benomyl and trichlamide provided good protection against P. brassicae on Chinese cabbage seedlings (Naiki and Dixon, 1987). As little as 100 mg a.i./kg was needed to elicit a 50% decrease in disease development, and 250 mg a.i./kg was needed to inhibit root hair infections. Calcium cyanamide also provided adequate protection, though required higher concentrations of fungicide (1000-2000 mg a.i./kg to elicit a 50% decrease in disease development and to inhibit root hair infections).
There is evidence (Datnoff et al., 1987) that chlorinating irrigation water significantly reduces clubroot incidence. Laboratory studies showed that resting spores suspended in as little as 2 mg chlorine per liter distilled water for 24 hr., rinsed then added to seedbeds reduced disease incidence to seedlings; unwashed spores caused stunting and interveinal chlorosis to cabbage seedlings. In field tests only 200 mg chlorine per liter significantly reduced clubroot incidence, which was statistically similar to control treatments, however phytotoxicity to the host occurred in the form of reduced plant height, fresh weight, and stand count. It is possible that the addition of one or more organic compounds may neutralize the phytotoxicity effects if applied before chloronated irrigation water, but further investigations into this area are necessary.
Agrios, G.N., ed. 1978. Plant Pathology, 2nd ed. Pp. 195-200. Academic Press. New York. 703 pp.
Datnoff, L.E., T.K. Kroll, and J.A. Fox. 1984. Occurrence and population of Plasmodiophora brassicae in sediments of irrigation water sources. Plant Disease 68:200-203.
Datnoff, L.E., T.K. Kroll, and G.H. Lacy. 1987. Efficacy of chlorine for decontaminating water infested with resting spores of Plasmodiophora brassicae. Plant Disease 71:734-736.
Dixon, G.R., and D.L. Robinson. 1986. The susceptibility of Brassica oleracea cultivars to Plasmodiophora brassicae (clubroot). Plant Pathology 35:101-107.
Lin, Y.S., S.K. Sun, S.T. Hsu, and W.H. Hsieh. 1990. Mechanisms involved in the control of soil-borne plant pathogens by S-H mixture. Pp. 249-259 In: Biological Control fo Soil-borne Plant Pathogens. D. Hornby, Ed. CAB International, Wallingford UK.
Myers, D.F. and R.N. Campbell. 1985. Lime and the control of clubroot of crucifers: Effects of pH, calcium, magnesium, and their interactions. Phytopathology 75:670-673.
Naiki, T. and G.R. Dixon. 1987. The effects of chemicals on developmental stages of Plasmodiophora brassicae (clubroot). Plant Pathology 36:316-327.
Sherf, A.F. and A.A. MacNab. 1986. Vegetable diseases and their control, second edition. Pp. 256-260. John Wiley & Sons, Inc. New York. 728 pp.