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Observations from a site-fixed field trial of 12 years in Jilin Province show that potassium chloride (KC1)application has a significant positive influence on co stalk rot incidence.Incubation experiments were conducted to study the effects of KC1 and soil extracts on the growth of Fusarium graminearum,the most common stalk rot fungi in this area,and the population changes in rhizosphere fungi,bacteria and actinomyces at different growth stages of co.The results show that KC1 addition to the potato dextrose agar (PDA) medium could not directly suppress Fusarium graminearum development.Soil extracts from soil samples taken from the field plots with and without KC1 application affected Fusarium graminearum development,with soil extracts with KC1 treatments suppressing Fusarium graminearum development more significantly,compared with that from the KC1-free treatment.These results indicate that soil extracts play a role in the interaction between co and Fusarium graminearum.Long-term KC1 application may increase the populations of rhizosphere fungi and actinomyces in the early growth stages,while there is no significant difference in the number of bacteria in rhizosphere among the treatments.Also,the populations of rhizosphere fungi are negatively correlated with the incidence of stalk rot in the early growth stages of co.The sensitive infection stages of pathogen to co consist of the stages when there is significant difference in the populations of rhizosphere fungi and actinomyces.The change of microorganism populations (especially fungi) in soil may be associated with the incidence decrease and is one of the mechanisms of KC1 suppressing co stalk rot.