Clubroot Distribution,Pathotype Of Plasmodiophora Brassicae In Sichuan And Clubroot Resistance

Clubroot,caused by Plasmodiophora brassicae,is a soilborne disease which is a big threat to the cruciferous crops production.Now clubroot is distributed in 24 provinces of China and it's a big problem in Sichuan province.Using host resistance is an important strategy for clubrot control.Thus,researches into the pathotypes of P.brassicae,cultivar resistance evaluation and resistance mechanism are important since they are the bases of the resistant cultivar breeding program and cultivar resistance management.This thesis includes following researches:a.clubroot distribution and pathotype of P.brassicae in Sichuan,b.clubroot resistance evaluation on normal and clubroot-resistant cultivars based on multiple populations and pathotypes,c.occurrence,morphology and histopathology of small spheroid galls?SSG?,d.cortical colonization dynamic of P.brassicae in susceptible,partially resistant and resistant cabbage cultivars as well as the effect from the inoculum concentration on the colonization,e.the induced host defense by the avirulent populations and f.the application and mechanism of one clubroot suppressive humus soil.Major results are presented in the following sections.?Clubroot distribution and pathotypes of P.brassicae in SichuanIn order to investigate the clubroot distribution and pathotypes of P.brassicae in Sichuan,the clubroot distribution was surveyed in plant protection stations of each county of Sichuan from 2014 to 2016.Williams system was used to identify the pathotypes of 22 P.brassicae populations collected from different infested fields in Sichuan.The results showed that clubroot occurred in 89 counties of 19 cities in Sichuan,accounting for 50%of counties surveyed.These counties were classified as extreme,high,moderate,low,limited clubroot distribution density and no clubroot distribution with 23,7,11,16,32 and 0 counties fell into each category.11 out of 89counties had a relatively long disease history.Moreover,most counties with extreme and high clubroot distribution density or relatively long disease history were relatively concentrated geographically.Pathotype 2,4,7 and 11 were identified in 22populations.Pathotype 4 constituted for 77%,indicating it's the dominant pathotype in Sichuan.Geographic distribution of the pathotypes in Sichuan was observed obviously.?Clubroot resistance evaluation and the research into the small spheroid galls?SSG?Clubroot resistance evaluation on 9 normal cruciferous cultivars was conducted with 18 P.brassicae populations from Sichuan province with known pathotypes.In addition,clubrooot resistance evaluation on 7 clubroot-resistant Chinese cabbage cultivars was conducted with 12 populations from Chuanyu region with known pathotypes.The occurrence,morphology and histopathology of small spheroid galls?SSG?which were observed during the evaluation of the resistant cultivars were also investigated.Results showed that all the normal cultivars tested showed susceptible reactions to most of the populations tested.73%of the host-population combinations had disease severity index higher than?or equal to?75.What's more,while all seven clubroot-resistant Chinese cabbage cultivars exhibited high resistance levels to the major population tested,SSGs occurred in host-population combinations containing either the Chinese cabbage cultivar‘T1-145'or population‘FL'.This indicated that some clubroot-resistant cultivars or P.brassicae populations might be more likely to produce SSGs than others.Both fresh root weight and gall diameter of spindle galls were significant higher than SSGs,and histological observations indicated that delayed pathogen development in cortical infection might contribute to the restricted development of SSGs.Additionally,the number of SSGs formed per root was low?2.2 in average?.This observation further confirmed that some host-population combinations can not form large numbers of SSGs on single root.?Effect of inoculum concentrations on the cortical infection of P.brassicaePathogen development in the root cortex of three cabbage?Brassica oleracea L.var.capitata?cultivars that differed in clubroot reaction?susceptible‘Bronco',partially resistant‘B-2819',resistant‘Tekila'?was assessed over time at three inoculum concentrations in a controlled environment study.Cortical colonization and symptom development in‘Bronco'and‘B-2819'were affected by the inoculum concentration.When inoculated at 1×10⁶ and 1×10⁷ spores·mL^-1,clubroot incidence or severity was generally higher on‘Bronco'relative to‘B-2819'.However,there were no differences at 2×10⁸ spores·mL^-1.Similarly,the area colonized by P.brassicae?%??assessed on root cross-sections?were much higher on‘Bronco'than‘B-2819'at 1×10⁶,but differences on cortical colonization?if any?were small at higher inoculum concentrations.Also,the concentration of P.brassicae DNA in inoculated roots,assessed using qPCR,was higher in‘Bronco'relative to‘B-2819'at1×10⁶ spores·mL^-1,but there was no difference between these two cultivars at the two higher inoculum concentrations.In contrast,no cortical infection was observed in‘Tekila'at the two lower inoculum concentrations,and only a few spindle clubs developed at the highest concentration.These results demonstrate that one source of partial resistance was strongly affected by inoculum concentration,while strong resistance was much less affected.?Research into the induced host defense by the avirulent P.brassicae populationThis study investigated the effect of the induced host defense by the P.brassicae avirulent population on the development of the virulent population with both intact root and split root system.In addition,the expression of two defense-related genes at different time points after the avirulent population inoculation was assessed.The virulent population inoculation on the intact roots of rutabaga cultivar‘Wilhelmsburger'at 4 days after the avirulent population inoculation showed lower disease severity index than the control.This indicated that the induced host defense could restrict the virulent population development.Moreover,the virulent population inoculation on one side of the split roots of canola cultivar‘45H73'at 4 days after the avirulent population inoculation on the other side showed lower clubroot levels than the control,indicating that the induced host defense probably could spread systematically.Results of the expression of the two defense-related genes on canola cultivar‘45H29'indicated that the expression of BnOPCL on the inoculated treatments showed generally higher mean values than the non-inoculated treatments although no significant difference was identified.For another gene,BnCCR,significant higher expression level on the inoculated treatment was only observed at the third sampling date.?The application and the suppression mechanism of one clubroot suppressive humus soilTwo application methods of one clubroot suppressive humus soil were evaluated.In addition,its potential suppression mechanism was investigated with soil sterilization test,soil pH and chemical elements analysis and root hair infection observation.When the humus soil was mixed into the turfy soil?good for the clubroot development?at the ratio of 1:1,disease severity index?DSI?of this treatment reduced for 66.4 compared with that of the control.DSI reduction could increase to80.2 when the ratio increased to 3:1.In addition,when this humus soil was covered on the infested turfy soil at the depth of 6cm as the protection layer,DSI reduction for78.7 was observed.According to the soil sterilization test,no biotic clubroot suppressive factor was identified.Results from the soil pH and chemical elements analysis indicated that this humus soil had three clubroot suppressive factors:alkaline soil condition,high calcium content and high magnesium content.The former two could be the dominant factors.Moreover,this humus soil could suppress the root hair infection completely.This might be mainly due to the alkaline soil condition.