| Tobacco brown spot caused by Alternaria,is one of the most important diseases in tobacco.Tobacco is considered as an important economic support in Sichuan Province,especially in the relatively poor mountainous areas.However,recently tobacco brown spot has brought huge economic losses to the tobacco industry.In order to identify the population structure of tobacco brown spot and to improve the epidemic dynamics of tobacco brown spot in Sichuan Province,to screen out new fungicides,so as to understand the scientific background for the diagnosis and possible control measures of the disease may be provided.The establishment of integrated control system of tobacco brown spot in Sichuan Province laid a theoretical foundation.A total of 258 samples of tobacco brown spot were collected from 2016 to 2017,and172 isolates of Alternaria were isolated by tissue separation and single-spore isolation techniques.Based on morphological characteristics and molecular biological identification,and verified by Koch’s postulates,all pathogens causing tobacco brown spot were identified as 2 species,A.alternata(68%)and A.longipes(32%);however,A.alternata was the dominant specie.The result of current study on overwintering showed that the pathogens of tobacco brown spot could overwinter in fields such as deciduous leaves,tobacco stems and dead weeds.The main overwintering places of A.alternata and A.longipes were deciduous leaves and tobacco stems,respectively.The result with reference to epidemic dynamics of monitoring nurseries for two years showed that the plain tobacco growing region entered initial period of tobacco brown spot at the beginning of August;while peak incidence period was at the end of August and declined period was at the beginning of September.The mountain tobacco growing region entered in initial period in late July,peak incidence period was at early August and decline period was at late August.Based on that,the critical periods for prevention in plain and mountainous tobacco growing regions have been determined.Meanwhile,after epidemic dynamics curve of time having been fitted,the optimal model of disease index with time in mountainous tobacco growing region was established preliminarily for epidemic surveillance of tobacco brown spot.The result showed that the reciprocal model had the highest fitting degree,with R~2 of 0.99 and F value of269.380,and the variance test of the model reached the extremely significant level,which could be further used for predicting disease occurrence and epidemic.Eleven commonly used fungicides were selected to test indoor toxicity of A.alternata with the method of mycelial growth rate,and six corresponding fungicides which had been selected indoor were selected against tobacco brown spot in the field control efficacy.Finally,the result of the control efficiency was validated;Fluazinam and Cyprodinil would be recommended to control tobacco brown spot.Meanwhile,Fluazinam and Carvacrol compound were selected to test indoor toxicity of A.alternata with the method of mycelial growth rate and field control efficacy against tobacco brown spot.The result showed that the prevention effect approached to the best and had obvious synergistic effect at a ratio of 1:4. |
