| Qinghai province is the origin place of YangTze River, Yellow River and Lancang-Mekong River. It is the natural barrier for the eastern ecological system in China, and has a strong effect on the ecological environment security for Qinghai-Tibet Plateau and the whole country. Dragon spruce (Picea asperata Mast) is the major tree species of natural forest and secondary forest in Qinghai province. However, the Arceuthobium sichuanense (H. S. Kiu) Hawksworth & Wiens causes severe damage to this tree species during recent years, and has become a devastating disease to Picea asperata. Moreover, this disease also causes economic losses, and poses threat to the ecological environment.In this study, the biological characteristics and the seed dynamics of A. sichuanense as well as its damage to the hosts were investigated. Meanwhile, the effects of chemicals on the A. sichuanense were also studied. The main results are listed below:1. The life cycle of A. sichuanense consists of four developmental stages, including spreading, plantation, incubation and reproduction. After maturation, the seeds of A. sichuanense are ejected out of the fruits by the internal pressure from watery sap, and stick to the stem of the hosts, in which the pathogen, in turn, accomplish the establishment of inner host-pathogen system by spreading their embryonic roots into the cortical layer of host stems. In spring of the next year, A. sichuanense endures the production of parasitic buds and flowers, and of the fruits and seeds, by which a new cycle of infection is caused. At the Maixiu forest stand in Huangnanzhou of Qinghai province, A. sichuanense starts the shooting in April and the production of female flowers in May, and then develops to the stage of full bloom in June. It accomplishes fruiting in July, and finally develops to mature fruits at the turn of August and September, during which the seeds are ejected out of the fruits, and starts a new cycle of infection.2. The damage severity of A. sichuanense was closely associated with the forest crown closure. It decreased along with the increase in the forest crown closure, and the disease index finally turned to zero when the forest crown closure was > 0.7. Interestingly, the disease index decreased along with the reduction in the forest crown closure when it was < 0.3.3. The experiment of seed rain was conducted to reveal the dynamic characteristics of seeds at distinct time-point and on various spatial positions. The results indicated that the ejection of seeds mainly took place during the period from the late August to the middle September, with the maximum rain density of 124 seeds per square meter a day. There were significant variations in the seed density and seed number at different developmental stages, and 90% of the seeds were ejected in the climax,and 70% of the seeds were distributed in the circle area of 3-7 m away from the trees. The area with the maximum seed density was 4~5 m away from the infected trees.4. Stem analysis was performed to identify the relationship between the disease index and the average annual growth in height, diameter and mass, and effects of disease index on the growth of dragon spruce. The grade III disease was severe enough to affect the trees at respective traits.5. Analyzing the effects of herbicide on the A. sichuanense showed the treatment with a 100-fold dilution harness solution (900 g/L) had the best activity against the pathogens, with an effective rate 100%. The analysis of effects of plant hormone on the abscission of fruits demonstrated that the treatment with a 200-fold dilution ethylene solution (40%) achieved the best the effects in enhancing the fruit abscission, and in controlling the A. sichuanense with an effective rate 100%.
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