| Bacterial canker is one of the diseases of Populus X euramericana. The pathogen of this disease was identified as Lonsdalea quercina subsp. populi that is new subspecies record in Chi na. Occurred in wider area, the disease is seriously harm to Populus X euramericana, but its i nfection cycle is not clear. The study for the rule of occurrence and development of the diseas e is in urgent need in order to better control the disease. In this paper, according to the pathog en characteristic of the disease, specific primers was designed and detection method was establi shed, and the studies were conducted for the pathogen number of bacterial canker of Populus×euramericana in early stage of disease occurrence, distribution in a tree after the pathogen in vasion, infection cycle and epidemic of the disease, and the results are as following.1. A pair of primers LqiR/LqiF was synthesized based on the DNA sequence of L. quercina subsp. populi N-5-1. Ten poplar canker pathogenic bacterial strains (N-5-1,4-4, A2210, A22I3, A2223, BX001, BX004, BX005, BZ008, BZ009),3type strains of different subspecies of Lonsdalea quercina (ATCC29281, LMG26264, LMG26267), other36bacterial isolates which have closer genetic relationship with the pathogen were tested with Real-Time PCR. The results showed that the specific primers detected10pathogen strains that were from poplar, with melt temperature86.05℃, but other strains had not been detected which showed the specificity of the primers. The lowest examination density was0.1pg DNA for this method. Accordingly, real-time PCR assays can be used to detect the pathogen by extracting DNA directly from infected plant tissues.2. Molecular quantitative method was established to assay relations between pathogen numbers and disease symptom and detect temporal dynamics of the pathogen population by using Real-Time PCR and histopathology. The results showed that bacterial accumulating number within inoculated branch bark tissue was detected after3days, but no disease symptoms appeared at28℃. However, the number of bacterial significantly increased and initial symptom occurred9dpi since cortical layer broken. Bacterial numbers reached a peak with a high population9-13dpi when the cell in the phloem was broken seriously, presenting severe disease symptoms. This shows close interrelations between the disease symptom and the quantity of the pathogen in the infected plant tissues, which suggests that Real-time PCR assay can be used to quantify the pathogen in infected plant tissues.3. The infection cycle of the pathogen o.n Populus X euramericana were studied by applying the molecular detection method established. The results showed that:(1) The pathogenic bacteria infecting the trees is mainly through wound of plant tissues, causing cell disassembly and forming a large cavity, with cell wall in xylem thicken and tylosis formed in vessel lumen. The pathogen are mainly distributed in cortical tissue, cambium and shallow xylem surrounding inoculation points. The results suggested that the pathogen of the bacterial canker locally infect Populus X euramericana trees.(2) The pathogen is overwintering in prior infected lesion.(3) Inoculating pathogen carring insects including Librodor japonicus and Protaetia brevitarsis in Populus X euramericana seedling planted in artifial net showed that the two insects can be a carrier of the pathogen.(4) The dynamics of pathogen population by unit weight in poplar trees presents a significant difference between spring and winter and it reached to a maximum in summer but a minimum in winter, and the content difference between the peak and low ebb reached100times, which accord with the investigation results in fields.4.The measurement of congregation indexes and analysis of Iwao’s regression and Taylor’s exponential law model show unanimously that the spatial distributing type of the diseased Populus×euramericana is aggregation distribution, with diseased center formed in poplar stands. The results showed that the spatial distribution patterns changed gradually from the aggregated distribution to the uniform distribution with the increase of the L. quercina subsp. populi infection density. The result of the average aggregation (λ) indicated that the aggregation distribution in early stage of the disease was caused by environmental factors. In the peak stage of the disease, with rise of density of diseased trees, the distribution appeared the average aggregation which may be caused by the combined effects of pathogen own aggregation behavior and environmental factors. |
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