Potato Virus Y (PVY) infection and nitrogen (N) deficiency cause similar symptoms (chlorosis and stunting) on potato foliage. While conventional methods, including ELISA and petiole testing, require destructive sampling and a longer time to diagnose, spectral analysis can be non-destructive, rapid and efficient. Spectral reflectance for potato cultivars representing three market types, chip processing, red-skinned fresh, and fresh and processing russets, were assessed in separate greenhouse trials in response to three N rates (90, 200, and 290 kg/ha) and two PVYN:O infection levels (clean and infected) at 4, 6, and 8 weeks after inoculation (WAI). Normalized Difference Vegetation Index (NDVI) was able to differentiate clean and PVYN:O infected samples of red-skinned and chip processing cultivars, at 4 and 8 WAI, respectively. Overall, cultivars differed in their spectral responses, indicating the importance of studying cultivar-specific spectral responses against PVY infection in future.

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Oil palm is the largest national crop produced in Papua New Guinea. It is grown on over 80,000 ha of young volcanic soils in five Provinces, employs over 12,000 workers and uses >12,000 tonnes of fertiliser to offset nitrogen deficiency which is the most limiting factor to production. Oil palms strip out 160 - 200 kg N ha-1 yr-1 from the soil. Nitrogen fertilisers account for 60-70 % of all variable production costs but 40-60 % of applied fertiliser cannot be accounted for. Few studies have investigated the amounts of nitrogen lost via leaching, denitrification, volatilisation or as surface runoff in tropical soils and none have been done in Papua New Guinea. Oil palm soils typically have extremely high infiltrabilities (80-8,500 mm hr-1) and receive high annual rainfall which throughfall makes spatially non-uniform. The objective of this study was to assess and quantify nitrogen losses and suggest strategies that might assist in reducing them and their impact on the environment. The modest facilities available at the two research sites, West New Britain (Dami) and Oro (Sangara) Provinces, meant that no analytical work could be done on-site, so simple but appropriate methods were used to evaluate losses, with samples collected, preserved and sent off-shore for analysis. Large four-palm plots were used to evaluate runoff; a gas trap was used to collect evolved nitrous oxide, and lysimeters, suction cups and finally an in situ destructive soil sampling procedure were all used to assess leaching losses and the rate of nitrification of ammonium fertiliser. Results suggest that under the extreme total annual rainfall at Dami (3,500-4,000 mm) and to a lesser extent at Sangara (2,500-3,000 mm), leaching is the dominant loss pathway, with the rate of loss depending, to some extent, on the rate of nitrate formation and the retentivity of the soil for ammonium, but mainly on the rate at which drainage water is generated. A leaching model was developed that indicated that the average residence time of nitrogen fertiliser in the root zone (0-50 cm) varied from 21 days in February, at Dami, to 190 days in May, at Sangara.