Diseases

Plasmodiophora brassicae Woronin - Clubroot of crucifers.

Systematic position.

Department Plasmodiophoromycota, order Plasmodiophorales, family Plasmodiophoraceae, genus Plasmodiophora.

Biological group.

Obligate endoparasite of crucifers.

Morphology and biology.

The parasite causes formation of fusiform, ball-shaped, or improper swellings or galls on roots. The appearance of sprouts damaged by Clubroot does not usually change. Only at very early and strong infection are growth inhibition and yellowing of leaves observed. Damage of roots in adult plants causes supressed development. The parasite infects the roots of wild and cultivated crucifers, and also infects root hairs of species in other plant families, forming zoosporangia and zoospores. Cysts are almost spherical or spherical, 1.6-4.3 (3.9 on the average) microns in diameter, ellipsoidal, ovoid (4.0-6.0 microns), sometimes oblate, oblong or irregular (2.5-6.9 microns in length), with colorless verruculose wall, usually forming one or many zoospores at germination. Primary zoospores pyriform, subspherical to oval, 2.5-3.5 microns in diameter, with 2 apical flagellums; short flagellum obtuse at the end, lengthy used as whip. Primary (sporangiogenous) plasmodia polynucleate, varying in size. Host cells may contain some plasmodia. A few or numerous zoosporangia locating freely or in compact accumulations; they are 6.0-6.5 um in diameter, angular or slightly oblong, ovoid to subspherical and spherical, forming 4-8 secondary zoospores being 1.9-3.0 um in diameter. Short flagellum 3.4 microns in length, obtuse at the end, lengthy one as a whip of 11.8 microns length. Secondary (cystogenous) plasmodia 100-200 microns in diameter, colorless or pale-gray, amoebiform. Cysts germinate, giving primary zoospores. Zoospores penetrate into cells of root hairs and turn into amoeboids. Infected host cell can contain a lot of uninucleate amoeboids. Some of them may fuse and give rise to young polynucleate primary (sporangiogenous) plasmodia, which enlarge, increasing the number of nuclei by multiple synchronous mitoses. Mature plasmodium cleaves into zoosporangia. Nuclei in a zoosporangium undergo two or three mitoses, forming four to eight zoospores in each zoosporangium. After copulation and plasmogamy the zoospores penetrate into young epidermal root cells and give rise to secondary binucleate amoeboids, which give rise to secondary (cystogenouse) plasmodia. The young secondary ameboids may coalesce within a host cell. There is fast growth and multiple mitotic divisions of the plasmodial nuclei. At the stage of secondary plasmodium the pathogen causes pathologic changes in a plant. The parasite causes abnormal fast growth and divisions of cells. During the division of a host cell the parasite is also divided. At the end of secondary plasmodium development the meiotic nuclear divisions occur following binary karyogamy. The plasmodium cleaves into uninucleate haploid cysts.

Ecology.

Dormant spores (cysts) remain viable in soil more than 3 years. Their germination and plant infection are observed at soil humidity of 60-90% (minimum: 50%, optimum: 75-80%); pH 5.4-6.5 is most favorable for infection. Minimum temperature changes for infection of plants on acidic soils is 10-12.C, maximum 30-35 .C, and optimum 20-25.C. The extra water supply at optimal temperatures promote stronger damage to plants.

Distribution.

Clubroot of crucifers occurs in Europe, Northern Africa, Northern and Southern America, Australia, New Zealand, Japan, Hawaiian Islands (Mazin, Protsenko, 1976). As oilseed rape it is registered in Leningrad Region and Central Chernozem region. In Leningrad Region the Clubroot causes high harm (Vakhrusheva, 1983). In Central Chernozem region the disease has no economic significance (Nikonorenkov et al., 1997).

Economic significance.

Disease results in a drop of crop yield by 10-60% or more. Control measures include seed dressing, adherence to crop rotation, control of cruciferous weeds, optimal times of planting, correct application of organic and mineral fertilizers, use of resistant cultivars, liming of soil.

Reference citations:

Mazin V.V., Protsenko E.P. 1976. The pathogen of Club Root of crucifers Plasmodiophora brassicae Woron. Moscow: Nauka. 192 p. (In Russian)
Nikonorenkov V.A., Portenko L.G., Karpachev V.V. 1997. Diseases of oilseed rape. Kormoproizvodstvo, 5-6: 42-44. (In Russian)
Vakhrusheva T.E. 1983. Oilseed rape diseases in Leningrad Region. Bulletin VIR, 127: 40-44. (In Russian)

© Gasich E.L.