Objective   Occurrence of the root-lesion disease on yams caused by nematodes on continuously cropped fields and the parasite community in soil were investigated for the disease control.

  Method  Soil specimens at the continuous cropping fields cultivating early-maturing Yongfeng yam using the traditional method, late-maturing Zhugaoshu yam using the directional shallow trough method and the traditional farming in Taihe county were collected for analysis. Nematodes in the soils were isolated by Baermann funnel method, identified to genus, and counted separately based on their morphology. Variations on the nematode community during the growth of yams were observed.

  Result  The population of the nematodes that caused the root-lesions on the fields grown Yongfeng yams increased significantly beginning from June, and that grown Zhugaoshu, from August. Both peaked in October. The 9, 13, and 12 genera of largely bacterivorous and herbivorous nematodes were isolated from the fields of different varieties of yams grown by using different cultivation methods. In October, when the parasite abundance peaked, the population of Acrobeloides decreased significantly by 93.55% in the soil at the Yongfeng yam field, those of Rhabditis by 52.80% at the Zhugaoshu field cultivated by directional shallow trough method and by 75.99% at the field farmed by the traditional method, while that of Acrobeloides at the Zhugaoshu traditionally cultivated field by 96.01%. The ecological indexes on the nematodes varied depending upon the associated yam varieties, cultivation methods, as well as sampling time. For instance, the indexes of H', J', and WI were significantly lower in October than other times in a year.

  Conclusion  October appeared to be the critical month in a year for controlling the nematodes in yam fields. A significant rise in the parasite population would disrupt the ecological balance resulting in a considerable reduction on bacterivores that ill-effects health of the farm soil.