By Elick Guaf
Sweet potato or kaukau is the most important staple food crop in PNG.
Kaukau contributes up to 64% of staple food crops by weight and 63% by food energy production.
It is estimated that 2.87 million tonnes is produced in PNG every year.
Kaukau is a crop that naturally grows for more than one year but is normally grown as a yearly crop.
In most parts of the tropics, it is planted from vine tip cuttings and in temperate regions, it is grown from rooted sprouts or slips plucked from a nursery where the storage roots are placed in the soil to sprout.
Apical tip cuttings, 30-45cm long are planted by inserting into the soil at an angle.
In other places, the tip cuttings are put down flat in soil and covered.
Yet other countries where people also like eating kaukau like in East Africa, cuttings may be wilted or left in the shade for a few days before planting.
Others bury the central portion of a cutting in the soil, leaving a leaf each exposed at both ends.
PNG is also a centre of diversity for kaukau in the Pacific.
There are close to 5,000 varieties cultivated by farmers throughout PNG.
This diversity is made possible with the range of isolated ecological locations with people living in these places planting and living on kaukau as their major staple food.
Besides, a large number of cultivars is found within a small area is an indication of the number of different cultivars in PNG.
Other factors influencing the diversity for kaukau is the conductive natural environment like the good soil fertility, the good level and frequency of rain, the consistent temperature and humidity which ensure the good level of water vapour in the air, the intensity of sunlight and the consistent number of hours in a day that provide the energy for plant growth; are all positive factors that contribute to promote natural flowering and true seed setting in sweet potato plant.
These factors also promote and support the natural sprouting of these true seeds and survival of these plants to bearing root tubers.
This natural occurrence combined with the curiosity of rural kaukau farmers, who plant the cuttings from the new plants they observe in their gardens, when removing vines from the old garden to prepare land to plant a new garden, recognises the different kaukau from that they have planted in that garden.
The tip cutting from the new kaukau plant and in a way the tuber yield is evaluated against his or her existing cultivars on characteristics like appeal, number of tubers and size of tubers and taste.
The farmers also look out for unique tuber characteristics like colour of the flesh and skin and if the new kaukau variety can yield well during wet or dry seasons or both.
This new plant is a result of nature crossing between the kaukau varieties planted in that garden.
Most often, the tuber yield from the true seed will yield plenty of good looking and better tasting tubers than the parent cultivars.
Where this is the case, the farmer plant a larger area to the new-found variety of kaukau adding one more cultivar to the collection with those he or she already have in the garden.
There are close to 1,700 kaukau cultivars maintained in the NARI collections – 1,200 in Aiyura, 1,100 at Keravat and 380 at Laloki with duplication between stations.
The collections were made in the late 1950s through to the early 1960s.
However, we need to understand the genetic diversity of our kaukau varieties to rationalise the local and the regional kaukau gene pool assembled and maintained at the different NARI locations throughout PNG.
Through a scientific method of identifying genetic differences between the kaukau varieties called, genetic figure printing, it was observed that the diversity in PNG cultivars was smaller compared to that from South American from where kaukau ancestors originate.
This finding is paramount and raises an alarm for critical work on broadening the sweet potato genotyping and increasing the genetic diversity in PNG to ensure food and nutrition security.
At NARI, the kaukau gene pool assembled and maintained are put through an evaluation process.
This is done by planting the different varieties together in field trials and by collecting information on the growth vigour, how they varieties are affected by common pests like the sweet potato weevil and common diseases, one of which is the leaf and vine scab, the varieties are compared and the good performing varieties are selected for further evaluation in multi-location trials.
High number of tubers with more than 300gm, good shape that is easy to peel with a potato peeler and good taste which also include the texture of the tuber flashes after cooking is usually the main criteria for the selection of good varieties.
A lot of work on evaluation of kaukau cultivars in PNG was done at Keravat.
This evaluation covered eight countries – PNG, Solomon Islands, Vanuatu, Fiji, Tuvalu, Kiribati, Tonga and Western Samoa – under a project titled “Selection, trial and dissemination of sweet potato cultivars” – which is part of the European Union funded project “Pacific regional agricultural programme” between 1990 and 2000.
Through this project, 79 kaukau cultivars were selected and further evaluated in replicated and multi-location trials in the eight participating countries.
The kaukau cultivars selected through this project are suitable for the lowlands and planting materials are available at NARI centres like Bubia, Laloki and Keravat.