| Clubroot, caused by Plasmodiophora brassicae Woron, is one of the important soil-borne diseases of worldwide cruciferous crops. In recent years, the clubroot occurrence area in Hunan since1984is obviously increasing, which has become a serious threat to the cruciferae crops. The most effective measure of controlling the clubroot is using resistant varieties. However, continuous planting clubroot-resistant varieties for many years in field would easily be broken down their resistance due to pathogenic differentiation in P. brassicae. Therefore, identification of the physiology-ical race of P. brassicae from the disease samples of different cruciferous crop in Hunan, PCR detection of P. brassicae in the naturally infested soil and the relationship between the physical and chemical characteristics of soil samples collected from clubroot areas in Hunan and disease severity were studied. The main results are as follows1. Identified by the Willimas System were the physiological races of P. brassicae from clubroot plants of Cruciferae crops:Chinese cabbage(Brassica campestris ssp. chinensis), tuber mustard(Brassica juncea var. tumida),red brassica sproutsis (Brassica campestris L. var. purpurea), rapeseed(Brassica napus) in4different disease areas in Hunan:Huaihua of southwest, Yiyang of north, and Zhuzhou and Changsha of the central area. The results showed that there were1st,4th,9th and13th race in Chinese cabbage,3rd and9th in tuber mustard,4th and9th in red brassica sproutsis, and4th and13th in rapeseed; in terms of area distribution in Hunan,13th and1st race separately distributed in Chinese cabbage and tuber mustard in Huaihua,1st race in red brassica sproutsis in Yiyang,4th and13th race in rapeseed in Zhuzhou, and9th race in Chinese cabbage and red brassica sproutsis in Changsha.2.12pairs of primers were used to amplify DNA from P. brassicae,8genome DNAs of clubroot galls and4genome DNAs of soil samples were chosen as the template DNAs. The results suggested that Primer1/Primer2in12pairs of primers could be used for general PCR as specifici primer for P. brassicae and GZ10/GZ11and GZ12/GZ13in the above12pairs of primers were specifici primers of nested-PCR as the pathogen. The results showed that3pairs of primers, Primer1/Primer2, GZ10/GZ11and GZ12/GZ13can be used for rapidly PCR detection of P. brassicae in Hunan. One specific segment of approximately500bp was obtained from the clubroot samples which were collected from different areas in Hunan by P. brassicae PCR detection with Primer1/Primer2, while there was no such segment obtained from the negative controls of healthy plants and non-DNA condition.4. Disease severity of soils with resting spores of the P. brassicae in step dilution have been detected by using1susceptible Chinese cabbage variety, as well as dilution of the inoculation of the threshold soils were detected by nested-PCR assay. The results suggested that the clubroot disease could happen when the soil with inoculation concentration of103-109spore/g soil and the concentration of resting spore was positive correlate with the disease severity. The nested-PCR could only detect the P. brassicae from infested soil samples whose disease index was above11.43.5.23naturally infected soil samples from different areas in Hunan province: Huaihua of southwest, Yiyang and Changde of north, and Zhuzhou and Changsha of the central area were detected by the nested-PCR assay with GZ10/GZ11and GZ12/GZ13. The result indicated that one specific segment of277bp was amplified from17soil samples among all the soil samples. The rest6soil samples disease indexes were all below5didn’t have specific segment.5. Based on6physical and chemical characteristics of soil (pH, total nitrogen content, organic matter content, cation exchange capacity, exchangeable Ca and exchangeable Mg content), the relationship between those characteristics and the occurence of cluboot was explored. The preliminary findings showed that:the acid condition was fit for clubroot disease to break out; soil total nitrogen content, CEC and exchangeable magnesium content were positively correlated with the occurrence of the clubroot; organic matter content of the soil and the exchangeable calcium content were not significantly correlated with the occurrence of the clubroot; the pH value of soil and exchangeable calcium content affect the occurrence of clubroot cooperatively. |
