| Summary: | The aim of this study was to determine the status of weed <i>Solanum nigrum</i> L. as a transitional host for <i>Meloidogyne enterolobii</i> and its effect on the population base of the nematodes in the next season. The nematode species infecting <i>S. nigrum</i> L. in a fallow field was identified by morphological identification and molecular diagnosis, and parasitic characteristics of the nematodes in <i>S. nigrum</i> L., including development of the nematode in <i>S. nigrum</i> L., the histopathological response of <i>S. nigrum</i> L. to <i>M. enterolobii</i>, and the host suitability of <i>S. nigrum</i> L., were studied. The <i>M. enterolobii</i> soil population density was evaluated before and after <i>S. nigrum</i> L. planting. Species identification revealed that it was <i>M. enterolobii</i> infection. Developmental observation indicated that juveniles of <i>M. enterolobii</i> developed fast in <i>S. nigrum</i> L., establishing feeding sites by 5 days after inoculation (DAI) and forming obvious egg masses on the root at 25 DAI. Histopathological observation showed the typical susceptible response of <i>S. nigrum</i> L., including giant cells with thick cell walls, uniformly dense cytoplasm, and less vacuolation, mainly inside the vascular cylinder. Host suitability assays suggested that <i>S. nigrum</i> L. is a good host for <i>M. enterolobii</i> with an average reproduction factor (RF) of 48.04 ± 14.71. Population densities assays revealed that <i>S. nigrum</i> L. increased the population density of <i>M. enterolobii</i> for two consecutive years from 0.48 ± 0.25 and 0.53 ± 0.31 J2/cm<sup>3</sup> to 1.33 ± 0.16 and 1.56 ± 0.43 J2/cm<sup>3</sup> of soil. These results indicated that <i>M. enterolobii</i> could reproduce well by infecting <i>S. nigrum</i> L. during the fallow season, and it increased the population base of <i>M. enterolobii</i> to the next season during vegetable production, which suggested a novel direction for the control of root-knot nematodes by controlling weeds as transitional hosts of <i>M. enterolobii</i> in the fallow season.
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