Sweet potato (Ipomoea batatas (L.) Lam) is the major staple crop in Papua New Guinea and experiments were conducted investigating factors affecting yield decline. Yields of unfertilized plots were related to rainfall and measured changes in soil properties, nematode (Meloidogyne sp., Rotylenchulus reniformis) and sweet potato weevil (Cylas formicarius) populations. The research took place at two locations (Hobu and Unitech) on Eutropepts and Fluvents, respectively. Yields at Hobu decreased from 18 Mg ha −1 in the first season to around 7 Mg ha −1 in the third season, but no significant yield trend was observed at Unitech. Vine biomass was not affected by the number of cropping seasons at Hobu but it decreased at Unitech with time. Marketable 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 decrease in CEC and exchangeable K (Unitech). No significant changes in soil bulk density were found and no obvious pattern was found in the nutrient concentrations of leaf samples with time. Nematode populations were high and tripled 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. Despite the considerable variation in yield and yield determining factors, the study showed that the decline in sweet potato yield may be attributed to the high nematode infestation, accompanied by an increase in vine damage by weevils and a declining soil fertility.

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Trichoderma's plant growth stimulating effect is well-recognized besides its well-known role as a biocontrol agent against plant diseases. Two greenhouse and field studies were conducted at the Papua New Guinea University of Technology (PNGUOT) to test the effects of Trichoderma harzianum on the growth of tomato and bean plants. Greenhouse potted plants were inoculated with three T. harzianum strains at 106 CFUmL-1, whereas, the field experiments were inoculated with T. harzianum strain LIPIMCO548 in rice bran at 0, 25, 50 and 75g/m2 soil. In the greenhouse experiments, Trichoderma inoculation increased the root and shoot length, and total fresh weight of bean plant up to 16.57, 20.79 and 21.37%, respectively over the un-inoculated control. For tomato plants, significantly higher (p ≤0.05, LSD) average root growth was observed when inoculated with T. harzianum strain CE262 over the control. On average, Trichoderma inoculation increased the root and shoot length, and the fresh weight up to 26.4, 9.6 and 18.8%, respectively over the control. Similar growth patterns were also observed in the field experiments. These results will be of immense value to vegetable growers in Papua New Guinea (PNG), especially to the intensive vegetable growing areas in the highland regions of the country.

Field experiments were conducted in 2012 and 2013 to study the effects of inoculum densities of Meloidogyne incognita applied at 10, 100, 1000, 5000, and 10000 juveniles/eggs per 500 cm3 along with un-inoculated control treatments, on a susceptible sweetpotato variety, K9. At the highest initial inoculum (Pi), an average marketable tuber yield of 991 kg ha-1 was recorded compared to 3495.5 kg ha-1 from un-inoculated control, accounting for 72% relative yield reduction. Significant (p≤0.05) yield reduction of 37% was also recorded from the lowest Pi of 10 juveniles/500 cm3. Probit analysis projected that 50% loss to marketable tubers are likely to occur at populations as low as 40 nematodes/500 cm3.There was a high positive correlation (r = 0.87**) between percentage relative yield loss and initial inoculum densities. There was significant (p≤0.01) impairment in marketable tuber quality with 4% cracking at 10 M. incognita per 500cm3, increasing to 37% at the highest Pi of 10,000. Highly significant positive correlations were also observed for cracked tubers (r = 0.93**) and non-marketable tubers (r = 0.96**), when tested against the Pi densities of M. incognita. Nematode population was monitored during the cropping season revealed an overall significant increase (p≤0.05) in juvenile populations at mid-season (60 days after inoculation) from the initial inoculum populations. A reverse in trend of juvenile numbers was observed at harvest (120DAI).

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