Studies On Primary Infection Sources, Disease Development, Biological Characteristics And Chemical Control Of Maize Gray Leaf Spot

Gray leaf spot (Cercospora zeaemaydis Tehno and Dainels), also known as Cercospora leaf spot fungus, since it outbroke in Dandong City of Liaoning Province in 1991, the disease caused great harm to maize growing in northern China. Since 2007, the disease occurred in Yaan,Sichuan Province,it became a catastrophic diseases which needed to be resolved after large spot and small spot disease in maize production. Because of the differences in ecological conditions between north and south, the incidence and prevalence of the disease may make different rules. Therefore, this paper, on the basis of the ecological conditions of Yaan.made a systematic study of the gray leaf spot disease in primary infection source, occurrence and prevalence rules,biological characteristics, and filtered out the best chemical agents controlling the disease, the results were as follows:1. The results of the study on primary infection source of gray leaf spot disease showed that:victims of the seeds'surface was not attached to the gray leaf spot pathogen's spores, and the pathogen was not isolated within the seeds, which could determine the surface and the internal of seeds were not the hibernation sites of the pathogen, the victim maize seeds were not the primary infection source of gray leaf spot, and the seeds would not cause the disease in the following year.In the ecological conditions of Sichuan Yaan, the bacteria dormant mycelium of the invalid body stored indoors,hanged outdoors and collected from corn stalk stacks as in dry conditions survived well, therefore, pathogens and spores could be eligible in the various separation,and spores could maintain a certain germination rate, but the germination rate decreased with time gradually increasing. The invalid body buckle with a pot and buried in the soil at 10cm were unable to isolate the pathogen and obtain spores.These results showed that:under the ecological conditions in Sichuan Yaan, dry climate and environmental conditions were conducive to gray leaf spot pathogen' survival; and in the wet environmental conditions, the invalid body could perish easily, so that the mycelial and spores losed their living conditions, the pathogen could not survive the winter. Thus, gray leaf spot pathogen'mycelial and spores on the invalid body stored indoors,hanged outdoors and collected from corn stalk stacks were several major primary infection source of this disease2. The results of the study on the occurrence and dynamics of gray leaf spot disease in the field showed that:under the ecological conditions in Sichuan Yaan, gray leaf spot disease occurred in the late growth of maize, it started in early July. Make diseased plant rate,daily growth rate of strain, lesion height and disease index as the indicators of horizontal and vertical expansion of the disease,we found that there was not obvious stagnant period in the changes of strain rate and lesion height,from the disease started, it presented linear rising trend until the strain rate was up to 100% and the entire plant was infected. And the changes of daily growth rate of strain and disease index both presented slow-fast-slow trend,and it can be divided into three stages:the first stage was from the disease starting to the filling stage, at this phase, disease index and daily growth rate of strain changed slowly; the second stage was the plant coming into filling stage, at this phase, disease index and daily growth rate of strain changed fast:the third stage was the plant going after milky stage, disease index and daily growth rate of strain changed slowly again until the illness development was up to the top and the strain rate was up to 100%.The findings also showed that the occurrence and development of this disease was influenced by corn growth and the meteorological factors. In the filling stage, this disease developped fast.and from the beginning to the filling stage and after milky stage,this disease changed at a slower speed.Climate condition was a major factor affecting the incidence and development of this disease.Low temperature,high humidity and rate of rainfall days as well as more rainfall could be conducive to the development of this disease.3. The results of the study on Gray leaf spot pathogen's biological characteristics showed that:the most suitable medium for mycelial'growth were: corn leaf fried juice medium, PSA medium, cornmeal medium, PDA medium, followed by V8 medium,Czapek medium, the least suitable medium were peanut leaf spot Cercospora fungus medium and Richard medium; mycelial'growth on the temperature requirements were not strict, between 10 and 35℃the mycelial could grow, the most appropriate temperature was 25℃; mycelial' growth on the pH value requirements were not strict,either. pH value between 4 and 12, the mycelium could grow, PH value between 5 and 10 was suitable for its growth, the most suitable PH value for mycelial'growth was 6 to 8; there were no significant differences of mycelial' growth in the light, alternating light and dark conditions, but in comparison, the light condition was more conducive to the growth of mycelium; different carbon sources had different effect on the mycelial'growth. The pathogen liked glucose, maltose and lactose best, followed by sucrose, fructose, xylose and glycerol, the worst was starch. Nitrogen requirements of mycelial' growth showed significant differences. The pathogen liked KNO3, beef extract and yeast extract the best, and the worst were ammonium sulfate and ammonium chloride. Spores could germinate between 10 and 35℃, the opproriate temperature was 15～30℃, the optimum temperature was 25℃; Spores could germinate in the pH value between 3 and 12, pH value 4 to 10 was better, and the optimum pH value was 7; spores could germinate in the relative humidity (RH) higher than 80%, and with the humidity increasing, spores' germination rate also increased; spores germinated requiring little nutrition and nutrition would inhibit the germination of spores.4. The results of the study on different fungicides' prevention and treatment effect to gray leaf spot disease suggested that:Fuxing, Shengshi,Cuisheng, thiophanate-methyl, carbendazim had a certain controlling effect on this disease, but different types of fungicides had different controlling efficacy. Fuxing had the highest controlling efficacy,76.48; followed by Cuisheng,68.65; and the controlling effect of carbendazim was the lowest,46.05. Through the significant differences analysis, there were differences in the controlling efficiency of the five agents, except Cuisheng and Shengshi, there were significant differences in the controlling efficiency among the others. Which showed that in the field, pharmaceutical Fuxing could have the best controlling efficiency on gray leaf spot disease, followed by Cuisheng and Shengshi,then thiophanate-methyl, and carbendazim had the worst controlling effect.