By WILFRED WAU
SWEET potato or kaukau, scientifically known as Ipomoea batatas, is one staple root crop in Papua New Guinea, especially in the Highlands region.
It is increasingly becoming important in coastal areas due to its agronomic superiority over other crops, such as taro and yams.
Sweetpotato provides the primary source of dietary energy for 60 per cent of the population.
Production in the country is limited by pest and disease. Viruses are a major constraint.
Viruses are among the smallest organisms known and can be seen only using a very powerful electronic microscope.
They are very simple organisms and only have the capacity to live and multiply inside every part of their hosts/victims.
Most viruses are moved from one plant to another by an insect which feeds on plant sap, such as aphids or whiteflies.
Virus diseases can often be diagnosed by mosaic patterns on leaves, stunting of the plant, leaf malformations, and tuber malformations.
Symptoms are not always expressed due to interactions between the virus and the sweet potato plant, growing conditions such as fertility and the weather, or the age of the plant when it is infected.
In most cases, the infected plants are usually symptomless and only appeared during stress period especially in dry periods.
With the practice of propagating sweet potato vegetatively using vines, sprouts or roots obtained from existing materials are prone to accumulate pathogens.
This is so when viruses are transmitted with the propagated materials to newly-established fields, leading to crop decline and poor root quality.
Currently, more than 20 viruses are recognised as pathogens of sweet potato worldwide.
Of these, six have been recorded in PNG, although very little information is available on their occurrence and distribution.
These viruses include sweet potato feathery mottle virus (SPFMV), sweet potato virus G (SPVG), sweet potato mild mottle virus (SPMMV), sweet potato ringspot virus (SPRSV), sweet potato chlorotic fleck virus (SPCFV) and sweet potato caulimo-like virus (SPCaLV).
These viruses exist as single or mixed infections.
Their impact on yield and quality vary, depending on the region, virus species and cultural practices.
Research by the International Potato Centre (CIP) in Peru showed that virus diseases caused by single virus infections or virus combinations can result in 18–100 per cent reduction of yield in sweet potato.
From the final report of an ACIAR project Validating and documenting a strategy for producing virus-free sweet potato planting materials in Papua New Guinea, trials on local highlands commercial sweet potato varieties showed that PT/virus-free planting materials consistently out-yielded non-PT vines of the same variety by 52 per cent and 66 per cent respectively.
These have showed the severity of the viral infection if it does not properly diagnosed and managed.
Under a European Union funded project – “Generation and adaptation of improved agricultural technologies” – implemented by Nari, a survey was conducted on the viruses at seven target sites in PNG.
In conjunction with other project activities, the survey was aimed at establishing information on the prevalence and distribution of sweet potato viruses in new and old gardens.
In this study, symptomatic sweet potato leaf samples were collected from farmers’ old and new gardens in selected communities in 2013. They were Kopafo (Eastern Highlands), Kiripia and Alkena (Western Highlands), Murukanam and Derin (Madang), and Hisiu and Yule Island (Central).
Using a Nitrocellulose Membrane Enzyme-Linked Immunosorbent Assay (NCM-ELISA) kit, developed by CIP, the samples were tested for SPFMV, sweet potato chlorotic stunt virus (SPCSV), SPMMV, SPCFV, sweet potato latent virus (SPLV), SPCa-LV, Cucumber mosaic virus (CMV), C-6, SPVG and sweet potato mild speckling virus (SPMSV).
The results revealed preliminary detection of additional viruses – SPMSV and SPCSV – and three other known viruses (SPFMV, SPVG and SPCaLV). The most prevalent virus detected was SPCaLV, which was present in all sites sampled; followed by SPFMV, SPSMV and SPVG, and finally SPCSV.
Incidences of virus-like symptoms are high in the low soil temperature sites compared to the high soil temperature sites in the coastal lowlands.
There is a positive coloration, cultivation, intensity, and distribution of infested (‘dirty’) planting material, besides the use of planting materials from old gardens to plant new gardens.
In terms of virus disease management, Nari’s Highlands Regional Centre at Aiyura has been active in propagating Pathogen Tested (PT) or virus-free planting materials of farmer preferred sweet potato varieties.
The cleaned materials have been made available to the sweet potato farming communities in the Highlands of PNG.
With the devastating effects of prolong drought and frost experienced mid last year (2015), communities are still into recovery agriculturally.
As such, organisations such as the International Organization of Migration, World Vision, Oxfam and Care International are presently ordering large quantities of virus-free materials and distributing them throughout the Highlands region. – Nari
By WILFRED WAU