Improved propagation methods to raise the productivity of yam (Dioscorea rotundata Poir.)
Improved propagation methods to raise the productivity of yam (Dioscorea rotundata Poir.)
B. A. Aighewi 0 1
R. Asiedu 0 1
N. Maroya 0 1
M. Balogun 0 1
0 University of Ibadan , Ibadan , Nigeria
1 International Institute of Tropical Agriculture , Ibadan , Nigeria
White Guinea yam (Dioscorea rotundata Poir.) is an important staple to millions of people in West Africa. Obtaining good quality planting material for yam cultivation is a major challenge. Multiplication ratios are low, and seed tubers are prone to contamination with pests and pathogens in the traditional systems of production. Some approaches to producing quality seed of yam are as follows: farmers select small whole tubers from a ware crop harvest; stimulate the production of seed tubers by 'milking' ware tubers while the leaves of the plant are still green (double harvest system); cut ware tubers into setts about the same sizes as regular seed tubers; or use the 'Anambra' system where smaller setts are cut and used to produce seed tubers. New methods that have been developed to address some of the challenges of quantity and quality of seed tubers are not yet widely applied, so farmers continue to use traditional methods and save seed from a previous harvest to plant the ware crop. This document presents an overview of traditional and modern methods of seed yam production and gives a perspective for the future. Among the modern methods of seed yam production, only the minisett technique, which uses 25-100 g tuber pieces, is currently used at farmer level, although on a limited scale. While tissue and organ culture techniques are the most rapid methods of multiplying disease free propagules, their limitations include high costs, need for skilled personnel and specialized equipment. The aeroponics and temporary immersion bioreactor methods of producing seed yam are relatively new, and still need more research. To build and sustain a viable seed yam production system, a multiplication scheme is required that combines two or more methods including tissue culture for cleaning the seed stock.
Seed yam; Dioscorea rotundata; Propagation methods
-
Of the 600 species of the genus Dioscorea, the most important
edible species are D. rotundata (white yam), D. alata (water
yam), D. cayenensis (yellow yam), D. dumetorum (bitter
yam), D. esculenta (Chinese yam), and D. bulbifera (aerial
yam). Among these, D. rotundata is the most preferred and
cultivated, accounting for a large proportion of yam
production in West Africa, a region which produces 93 % of the
world’s yam (FAO 2013).
Traditionally, ‘seed’ yams or setts (tuber portions) are used for
propagation. The yam tuber, which contains a deposit of starch,
does not have dormant buds as found on a typical tuber such as
potato (Solanum tuberosum). However, at the end of dormancy a
meristematic layer of cells beneath the skin of the tuber produces
sprouts, usually from the head (proximal) region (Onwueme
1973), indicating that the tuber is ready for planting. Most yam
farmers use seed tubers saved from a previous crop for
propagation. Due to short supply of quality seed tubers at affordable
prices, replacement of stocks of seed yams, which have been
infested by pests and diseases, is usually not possible and farmers
are forced to recycle poor quality seed yams with the risk of poor
yields. Damage from nematodes, viruses, tuber rotting fungi and
bacterial infections is a major contributor to the poor seed quality
and low yields in yam. A loss in yield of up to 50 % due to
viruses has been reported in western Nigeria (Craig 1964, cited
in Emehute et al. 1998), while storage losses due to nematodes
are estimated at 50 % and may be total, with Scutelonema bradys
and Meloidogyne spp., which are endemic in West Africa, being
the major species responsible (Amusa et al. 2003). Asala et al.
(2012) observed that the occurrence of Yam Mosaic Virus
(YMV), Cucumber Mosaic Virus (CMV), Yam Mild Mosaic
Virus (YMMV) and badnavirus as well as the mixed infections
observed in most of the five states and the Federal Capital
Territory (FCT) of Nigeria that they surveyed could be attributed
to the exchange of infected planting materials between States.
This situation is compounded by the low multiplication
ratio of 1:4 to 1:8 for yams (Alvarez and Hahn 1984), which
makes it difficult for ‘clean’ seed tubers to be multiplied
rapidly for distribution or sale to farmers and the absence of a
formal seed yam system in the entire yam zone of West Africa.
This review takes a critical look at various options available
for seed yam production, and the prospects for alleviating the
problems of poor quality seed yam and availability.
Traditional methods of seed yam production
In traditional yam cultivation there is no separation of seed
and ware yam production. The farmer plants for a ware crop
with the knowledge that seed tubers for the next crop will also
be produced. The yam crop stays in the field for 8 to 10 months
and the tubers a (...truncated)