Soil population of Helicotylenchus multicinctus under Pueraria phaseoloides, Flemingia macrophylla and natural bush fallows and their effect on plantain yield in the humid forest zone of Southern Cameroon

Soil population densities of Helicotylenchus multicinctus (Cobb) Golden were assessed under Flemingia macrophylla [(Willd.) Merrill] and Pueraria phaseoloides (Roxb.) Benth, and natural bush fallows over 24 months in three villages in southern Cameroon. Also assessed was the effect of H. multicinctus on yield of plantain. Trials were set up in four to five year-old Chromolaena odorata (L.) R. M. King & H. Rob. dominated natural bush fallow. Initial H. multicinctus population density at Ngoumou (512 100 cm–3 soil) was (p < 0.05) higher than at Mfou (241 100 cm–3 soil) and Nkometou (70 100 cm–3 soil). H. multicinctus population was correlated with (i) total annual rainfall (r = 0.79; p = 0.01) and (ii) clay content (r = 0.95; p < 0.001). At 12 months after planting (MAP), P. phaseoloides and F. macrophylla supported (p < 0.01) lower population densities of H. multicinctus (88 and 152 100 cm–3 soil, respectively) compared with the natural regrowth (287 100 cm–3 soil). Comparing the initial population with that at 12 MAP for each of the three fallow systems, both P. phaseoloides and F. macrophylla reduced density of H. multicinctus (p < 0.05). However, when the initial population density was compared with that at 24 MAP only under P. phaseoloides was the reduction maintained. Contrarily, population density under F. macrophylla was comparable between the initial and 24 MAP. This is the first report of the nematode reducing effect of P. phaseoloides. Plantain flowering and subsequent harvesting was earliest under P. phaseoloides, different from that under F. macrophylla (p < 0.05). Plant bunch weight was positively correlated with the length of the fruit filling period (r = 0.67) which in turn was positively correlated with the non damaged root index (NDRI) (r = 0.58). Plantain under P. phaseoloides had the highest NDRI and consequently recorded the greatest yield (p < 0.05). There was no difference in plantain yield between the natural regrowth and F. macrophylla. Pueraria phaseoloides could be used as one component for the biological control of H. multicinctus.