| Clubroot, caused by the soil-borne obligate biotroph Plasmodiophora brassicae Woronin,is one of the most important diseases of cruciferous crops worldwide(Buczacki, 1983).The disease is known to occur in more than 60 countries in Europe, Asia and North America, and results in a 10–15% yield reduction on a global scale(Dixon, 2009). In China, clubroot affects 3.2–4.0 million ha of cruciferous crops annually, leading to20–30% yield losses on average(Wang et al., 2011a). Canola(oilseed rape, Brassica napus) is one of the most important oil-crops in China, and in its main production region,i.e. the Yangtze River region, the risk of clubroot on its yield is increasing year after year.Traditional agricultural practices such as the application of lime to increase the soil p H value may reduce its damage to crops, but their effects are often insufficient to keep the plant healthy. Breeding of resistant canola cultivars is an effective approach to suppress the disease impact and to minimize the yield loss. The aim of this study was to identify new germplasms for clubroot resistance and to clarify the interactions between the host and the pathogen. Furthermore, effects of sowing date and fungicides application on clubroot severity and yield loss were also discussed. The results may provide a better understanding of the pathogenesis of P. brassicae, the breeding of resistant canola and the integrated management strategy.1. Effects of medium nutrients and resting spores concentration on the infection of P. brassicae to its host plantSix nutritious levels of the inoculation medium and seven concentrations of resting spores of P. brassicae were designed to investigate their effects on infecting a chosen susceptible B. napus cultivar, Suosheng No.2. The experiment was implemented in an artificial climate chamber(with day 24℃/16 h and night 16℃/8h). Results showed that plants grown in quartz sand(medium treatment 2) and in peat-vermiculite-pearlite(medium treatment 6)developed into typical clubroot galls after artificial P. brassicae inoculation. Symptoms of clubroot were not observed on plants grown in the other 4 tested media(i.e. treatments 1, 3,4 and 5). Disease incidence of plants in treatment 6 was 100% with its disease index of69.6, highlighting its potentials in clubroot host-pathogen interaction studies in artificial inoculation conditions. Whereas in treatment 2, plants appeared larger galls than any other treatment, making it a better medium to yield more P. brassicae spores. Disease incidence and index were both positively correlated to the concentration of inoculated P. brassicae resting spores. High concentrations of inoculating spores at 1×106, 1×107 and 1×108 spores/m L could easily caused the host infection, leading to a disease incidence of 100% at50 days after inoculation(d.a.i). However, even susceptible hosts could not be successfully infected by resting spores at low concentrations.2. Screening clubroot resistance in B. napus germplasms188 canola germplasms were selected to evaluate their resistance to the predominant P.brassicae race in China(pathotype 4) by both disease nursery identification and artificial inoculation. Twelve resistant canola lines introduced from Canada were used as the resistant control. Results showed that 22 out of the 188 tested germplasms were resistant,including 2 high resistant germplasms, to P. brassicae pathotype 4. The observed resistant cultivars/germplasms were mainly from the most severely affected provinces such as Sichuan, Hubei and Guizhou. A significant negative correlation was observed between the clubroot disease index and the content of erucic acid and glucosinolates in rapeseed.However, the infection was significantly and positively correlated to the content of stearic acid(C18:0). In artificial inoculation experiment, all the 12 Canadian resistant canola lines showed various levels of resistance to three isolates of P. brassicae from Xiuning and Yixian of Anhui, Guanghan of Sichuan. Line CR5 showed immunity to all the three isolates.3. Differences of infection and gene expression of P. brassicae in resistant and susceptible canola cultivarsA resistant canola cultivar(cv.45H29) and a susceptible canola cultivar(cv.45H26) were selected to study their differences in infection after artificial inoculation of P. brassicae.Primary infection was observed in more than 90% of root hairs for both cultivars at 7 d.a.i.Thereafter, the infection was decreased to less than 20% at 14-35 d.a.i. For both cultivars,the amount of primary infection was similar at each assessing time point. Contrarily,secondary infections were rare for either cultivar at 5 d.a.i or 7 d.a.i., but became apparent after 14 d.a.i. At 14-28 d.a.i., the level of secondary infection was more obvious in the resistant cultivar than in the susceptible one. The in planta expression of 12 selected candidate P. brassicae genes was investigated by reverse transcription quantitative PCR.All these genes were up-regulated at 5 or 7 d.a.i. in the resistant cultivar. For the susceptible cultivar, 12 candidate genes could be classified into three groups according to their expression patterns: two genes showed their expression peak at 14 d.a.i., three genes gave two expression peaks at 14 and 35 d.a.i., and the others showed an expression peak at35 d.a.i. Results from this study will be useful in breeding for resistant canola cultivars and in selecting candidate pathogenicity genes for further studies.4. Influences of sowing date on clubroot severity and seed yield in winter oilseed rapeTo develop the efficient clubroot control method by different seeding date and seedling age of winter canola in Anhui. Winter oilseed rape(cv.Heyou56) was sown at nine periodical dates to understand the potential infection of clubroot under field conditions for two seasons(2012-2014). Results showed that young seedlings were easier to be infected by P.brassicae than older seedlings. Delayed sowing date led to a significantly lowered disease incidence in comparison with the early sown crop. The yield of canola was in decreasing along with the delayed sowing; however, such yield reduction could be compensated due to the impact of clubroot which leading to only an insignificant difference between early and late sowing. Therefore, operations of using oilseed rape cultivars with improved tolerance to late sowing and increasing planting density were recommended to canola growers to sustain their production. Furthermore, decreased disease incidence led to a reduced number of P. brassicae resting spores in the field, which favored a sustainable management of clubroot.5. Effects of fungicides on clubroot severity and seed yield in winter oilseed rapeFive fungicides and one micro-elements fertilizer were applied by root-drenching method during rapeseed seedling stage(2-3 true leaves) to evaluate their effects on controlling P.brassicae induced clubroot. Their effects were also compared with the traditional control method, i.e. lime treatment. Results showed there were significant differences(P<0.01) on disease incidence, disease index and rapeseed yield among treatments. All the six treatments gave varied controlling effects, as the application of 50% Carbendazim WP gave the highest control of 90.82%. Chemicals, e.g. 10% Cyazofamid SC, 50%Carbendazim WP, 75% Chlorothalonil WP and 60% Sulfur & fenaminosulf WP, could significantly decrease the yield loss caused by clubroot. There was significantly negative correlation between rapeseed production and clubroot disease index(r =-0.7962) with the regression of yield(y) to disease index(x) as y =-32.65 x +2372. |
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