A Preliminary Study On Alternaria Leaf Spot Of Cotton In Xinjiang

Alternaria leaf spot of cotton was prevalent in major cotton planting areas of the world.It is a worldwide disease,is also a common disease of cotton in China.However,there are great differences in the kinds of pathogens that cause Alternaria leaf spot of cotton in different regions,and there is a lack of systematic research on it.Especially Xinjiang is the largest cotton region in China,and it is also the cotton region where leaf spot disease occurs more seriously in China.However,there is no systematic study on the pathogenic species of Xinjiang.From late April to the end of May 2016,continuous rainfall and low temperature in the cotton area of north Xinjiang resulted in the outbreak of cotton leaf spot disease,and the incidence of cotton leaf spot was as high as 100% in many places.It is very important to find out the pathogenic species of cotton leaf spot in many places as soon as possible.Therefore,this study was carried out.This study mainly includes the following 4 aspects: Firstly,to identify the pathogen of Alternaria leaf spot of cotton in Xinjiang,to identify the similarities and differences between Xinjiang and other regions;Secondly,the study of inoculation method of cotton leaf spot pathogen laid a foundation for the further study of the disease;Thirdly,the effects of environmental conditions on the growth of cotton leaf spot pathogen were determined to provide the basis for the scientific control of the disease.Fourthly,The RAPD analysis of the pathogen of Alternaria leaf spot of cotton in Xinjiang was conducted to reveal the genetic diversity among the strains,and the correlation between the population and the source of the strains and the time of collection.The results were as follows:1.After investigation,sampling,isolation,purification and identification,Alternaria leaf spot of cotton occurred in 30 cotton planting areas in Shihezi,Changji,Borle,Altay,Aksu,Kashi and Hotan in 2016 and 2017,which were identified based on their morphological characteristics and the sequence analysis of both rDNA internal transcribed spacer and histone3 gene,was mainly caused by A.tenuissima,A.alternata and A.macrospora.It is not exactly the same as other areas outside Xinjiang.The disease occurs from seedling stage to adult stage.It is easy to spread in seedling stage and adult stage because of continuous low temperature and high humidity,so it should be paid great attention to in the important cotton planting area of Xinjiang.In order to reduce the damage and loss caused by the disease,the key control was carried out at seedling stage and adult stage.2.All of the 3 pathogens were inoculated with injury or non-injury,The incidence of injury inoculation is earlier than that of non-injury inoculation.Inoculation at different temperature conditions showed that inoculation was easy to succeed under low temperature condition(15? in the daytime/10? at night).Not only the incubation period of the disease isshort,but also the disease is more serious.Under the condition of no injury inoculation,it also showed the same result.It can be seen that temperature is the decisive factor for the success of inoculation.The field investigation showed that the incidence of long staple cotton(island cotton)was more serious than that of upland cotton(Gossypium hirsutum).In seedling stage inoculation test,long staple cotton(island cotton)was more susceptible to disease than upland cotton,and the incidence of A.macrospora inoculation was higher than that of other 2pathogens.2 inoculation methods,leaf inoculation in vitro are more easy to control the environmental conditions,easy to operate,save time and labor.Therefore,leaf inoculation in vitro can replace the whole plant inoculation in general basic research.3.The 3 pathogens,A.alternata,A.tenuissima and A.macrospora,had the fastest growth rate on PDA medium.The temperature can be grown at 5?~35?.The optimum growth temperature was 25? ~ 30?.Hyphal growth above 30? or below 10? was significantly inhibited,and hyphal growth rate was positively correlated with humidity.The higher the humidity was,the faster the pathogen growth rate was,the faster the hyphal growth rate was,and the faster the hyphal growth rate was when the humidity exceeded 90%.Under the conditions of 12 h illumination and 12 h darkness,the mycelium grew fastest under 24 h illumination and 24 h darkness.The 3 pathogens adapted to a wide pH range of 5~10,and the hyphae could all grow,but when pH was between 7~8,the mycelium growth was the best.4.Random amplified polymorphic DNA(RAPD)analysis was carried out on 54 strains of Alternaria spp.Cluster analysis showed that 54 strains of Alternaria spp could be divided into 2 groups when the genetic similarity coefficient was between 0.54 and 0.61.The first group is small-spored species which contains A.alternata and A.tenuissima,and the second group is large-spored species which is A.macrospora,when the genetic similarity coefficient is between 0.63 and 0.70.The first small-spored species group of 48 strains can be divided into two groups: A.alternata and A.tenuissima.This result is basically consistent with the result of morphological identification of the strains.It is indicated that there is a rich genetic diversity among the 54 Alternaria spp.strains,and their phylogenetic relationship was correlated to the source of the strains and the time of collection.