Yield decline of sweet potato in the humid lowlands of Papua New Guinea

Sweet potato (Ipomoca batatas (L,) Lam) is the major staple crop in Papua N ew Guinea and experiments were conducted investigating factors affecting yi eld decline. Yields; of unfertilized plots were related to rainfall and mea sured changes in soil properties, nematode (Meloidogyne sp., Rotylenchulus reniformis) and sweet potato weevil (Cylas formicarius) populations. The re search took place at two locations (Hobu and Unitech) on Eutropepts and Flu vents, respectively. Yields at Hobu decreased from 18 Mg ha(-1) in the firs t season to around 7 Mg ha(-1) in the third season, but no significant yiel d trend was observed at Unitech. Vine biomass was not affected by the numbe r of cropping seasons at Hobu brit it decreased at Unitech with time. Marke table tuber yield at both sites was significantly correlated to rainfall, i .e, the more rain the lower the yield. Significant changes in soil chemical properties included a decrease in pH and base saturation (Hobu) and a decr ease in CEC and exchangeable K (Unitech). No significant changes in soil bu lk density were found and no obvious pattern was found in the nutrient conc entrations of leaf samples with time. Nematode populations were high and tr ipled between the first and third season at Hobu. Half of the vines at Hobu and all of the vines at Unitech were damaged by sweet potato weevils, but tuber damage was higher in Hobu although the damage was only superficial. D espite the considerable variation in yield and yield determining factors, t he study showed that the decline in sweet potato yield may be attributed to the high nematode infestation, accompanied by an increase in vine damage b y weevils and a declining soil fertility. (C) 2000 Elsevier Science B.V. Al l rights reserved.