Root rot often occurred in the course of floating-bed producing tobacco seedling in tobacco areas of Honghe State in recent years. The pH value of water which blended the nutrient solution with fertilizer was alkaline in most places with the tobacco seedlings root rot. By using 4 M HC1 or 4 M NaOH solution, the nutrient solution in micro-pool was regulated to three pH values (5.50, 7.00 and 8. SO) to test the root rot of three tobacco varieties in Kunming and Mile. In the process of nursed seedlings, the water with three pH values was added into corresponding micro-pools respectively. The results showed that the root system damnification could easily induce the root rot because of moving floating-bed of seedlings, and the incidence rates of root rot in micro-pools with three pH values were correspondingly 25. 86 % , 45.31 % and 48.86 % in Mile respectively. The pH value of nutrient solution in micro-pools decreased down gradually during culturing seedlings after adding fertilizer, and the tobacco variety Honghuadajinyuan showed more susceptible to root rot in the comparison with the other two varieties.%红河州烟区近几年漂浮育苗中常发生根系腐变,多数发生根腐的育苗点,调配漂池液所用永源的pH值呈碱性.在昆明和弥勒两地,分别利用3个烤烟品种,调配漂池液所用水在加入瀑池前,用4 M HCl或4 M NaOH将其调节成3种不同pH值(5.50、7.00和8.50),结果表明:因搬动苗盘造成根系损伤、易导致根腐；弥勒试验点中对应于加入3种pH值的水,根腐发生率分别为25.86％、45.31％和48.86％.育苗池液中施入专用肥后,使漂池液的pH值下降,并随育苗进程持续下降.3个烤烟品种中,红花大金元对根腐较为敏感.
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