| Clubroot, caused by the fungus Plasmodiophora brassicae Wor., is a soil-borne disease of cabbage which has been proved difficult to control. In recent years, it became more and more serious and resulted in considerable losses to vegetable production. P brassicae, an obligate endoparasite, is not culturable on synthetic medium. And therefore it is more difficult to study than other plant pathogen. Presently, only a few were reported about clubroot in China and mainly concentrated on the incidence and control of this disease. Systematic studies on the biological characteristics of P brassicae and the relationship between root hair infection and disease incidence were first carried out in this paper. Some preliminary studies on the disease developing conditions were also determined. The results indicated that: 1.Biological characteristics of P brassicae The optimum temperature for the germination of resting spores of P brassicae was 240C and the optimum pH was 6.O梸6.7. The lethal temperature was 450(12 for the pathogen, and the visible light caused remarkable inhibiting effect on the germination of spores. It was helpful to enhance the germination rate significantly for the treatment of rotted clubs. The highest germination rate for the resting spores was 75% in the root exudates of susceptible host. The root exudates of resistant hosts or non-crucifer (tomato) stimulated the germination of spores. 2.Structure of resting spores and primary zoospores of P brassicae under scanning I transmission electron microscope 3 The section of clubroot gall was examined by scanning electron microscope. The resting spores were nearly spherical with mammil on the wall of resting spores. Their diameters ranged from 2.1 to 11pm (the mean 2.5 pm). The resting spores did not arrange closely in the infected host cells and very few of them had some catkin-like materials between two spores. Using transmission electron microscope, the primary zoospores with unequal length biflagellate were in nearly spherical or oval shape and their size varied widely from 1.6 to 3.6 pm. 3.Disease developing conditions of P brassicae The study compared with three inoculation techniques, which were dipping the root of seedlings with spore suspension, mixture of resting spores and soil, addition clubroot slice along with health roots. The results indicated that the disease of cabbage seedlings using the soil inoculation occurred more seriously than other two methods. Their disease index was 53.9% and disease percentage was 86.9%. The pH values on clubroot growth were also tested in the greenhouse, and the optimum pH value of the clubroot growth was pH5 (the disease index 49.8%, the disease percentage 87.0%). 4.Correlation between the number of root hair infection and the disease resistance of plant variety The root hair infection with P brassicae was frequently in 1 梸3cm of the top root below the hypocotyle of plants. Five days after inoculation, obviously differentiated zoosporangia were first found in the infected root hair cells, and the number increased gradually to the high peak after inoculation 13 days, then the germinating rate decreased. Zoosporangial clusters partial or completely evacuated were found in some root hairs in 8-day-old seedlings, the cells of the root cortex infection were observed in these seedlings during the same infectious... |
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