| Clubroot, which is caused by Plasmodiophora brassicae Woronin, is one of the most destructive diseases in cruciferous vegetables production in the world. Recently, the best control measures was the use of resistant cruciferous vegetables cultivars, but newly developed resistant cruciferous vegetable cultivars were prone to lose their resistance after a new pathogenic formation. Thus, the control of the clubroot is still a durable problem, and the realization of the P. brassicae’s life cycle and its infection progress would be of great importance for preventing the occurrence of the disease. In this study, we optimized the resting sporangium’s germination condition, and then investigate P. brassicae’s asexual life cycle on non-heading Chinese cabbage and the pathogenicity of primary zoospore and secondary zoospore; and in the last, we observed the resting sporangium and secondary cyst’s internal structure by TEM. The results were as follows:1. Optimization of resting sporangium germinationWith the method of shallow liquid culture, it was found that ryegrass’s root exudation act stronger than hosts’root exudation on resting sporangium germination; the ryegrass’s root exudation’s promoting effect weakened as diluted, there was no promoting effect when diluted to 10% or lower; the germination activity became weaker by cold-storage, the germination rate was 0.34% after cold-stored 20 d. The germination process was observed at the optimal condition, the highest germination rate was 2.16% at 7d, and the accumulated germination rate was 5.10%.2. Study on the optimum infection condition and asexual life cycle of P. brassicae on non-heading Chinese cabbageWith the method of solution culture (Luo et al,2014), the optimal infection method was investigated. Infection efficiency was obviously higher in fresh nutrient, maximal root hair infection was observed at 25℃, pH 5.5, and 10 spores/mL. Under optimum infection conditions, three groups of non-heading Chinese cabbage were cultured, verified that P. brassicae’s secondary zoospore could reinfect non-heading Chinese cabbage’s root hairs or epidermis after released from root hairs, the asexual life cycle exists when Plasmodiophora brassicae Woron. infect non-heading Chinese cabbage.3. Study on the role of primary zoospore and secondary zoospore in pathogenesisIn the research of primary zoospore’s pathogenic role, fluazinam was used as inhibitor, the non-heading Chinese cabbage seedlings which root hairs were infected and secondary zoospores were not released, were calculated in the culture solution with fluazinam, the group without inhibitor was as a control, the incidence of experimental group and control group were observed; the research of secondary zoospore’s pathogenic role used the method of asexual life cycle’s study, each group was calculated 25 d in the solution, and 40 d in the sand, then the incidence were observed; the diseased plant’s swollen root were paraffin sectioned to examine the roots’cells were infected or not. The results indicated that primary zoospore and secondary zoospore were both could infect non-heading Chinese cabbage and cause clubroot, resting sporangium were observed in the paraffin sections.4. Ultrastructure of P.brassicae’s resting sporangium and secondary cyst.The mustard seedlings were calculated and inoculated P. brassicae in the soil, after 10 days, the seedlings were pulled out, the roots were rinsed thoroughly with water, secondary cysts were collected by shaking the infected roots in the triangular flask with distilled water, resting sporangium and secondary cyst’s internal structure were observed by TEM. Resting sporangium’s diameter was 2.5～3.1 μn, which wall was not smooth, two-layered and mononuclear, with a spike; there were some osmiophilic bodies. Secondary cyst was circular, which diameter was almost 3.5μm, binuclear; the cyst’s diameter was almost 3.0 μm, rohr was observed obviously in the internal, the stachel which top was sharp lied in the rohr. |
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