Potato (Solanum tuberosum) is a worldwide cultivated tuber-bearing plant which is the fourth main food crop in the world after rice (Oryza sativa), maize (Zea mays) and wheat (Triticum aestivum), in terms of both areas cultivated and total production (Douches et al. 1996). The potato does not require special growth conditions and is a major field crop in temperate regions and increasingly in warmer regions (Haverkort 1990). Potatoes are grown as a major crop in countries with very large populations, in different climatological zones including temperate regions, the sub-tropics and tropics, under very different agro-ecological conditions, lowlands as well as highlands, and in very different socio-economic environments (Struik and Wiersema 1999). The potato can be considered as one of the most important food crops in many African countries. The potential of this crop is reflected in the large increase in area of production where Africa showed the highest rate of growth within the developing world over the past twenty years (Hesen 1986). Potato production in the developing world has increased faster in recent years than that of any other major food crop (Hesen 1986). The increase in area planted with potato in the developing world has been facilitated by the development of new cultivars and the funding of seed production schemes utilizing modern multiplication techniques. Besides being the largest vegetatively propaged crop in the world, the potato has become an important staple food in parts of the world where there is a limited (but increasing) purchasing power, an increasing demand for food and an increasing pressure on scarce land (Struik and Wiersema 1999). In most African countries with low income farmers and erratic rainfall, there is a need to produce the highest tonnage of food in the shortest time. Potato is known to produce the highest protein per hectare per day. Nutritional studies also show how healthy potatoes can be in terms of vitamins, minerals, proteins, essential amino acids and carbohydrates (Buckenhüskes 2005, Struik et al. 2006).
Seed potato production is mostly vegetative and based on the use of in vitro plantlets or microtubers (Ranalli 1997). The multiplication rate of potatoes is very low compared to other crops, from between four to six times under optimal conditions. For this reason, a large portion of crop area is devoted to the production of seed tubers and it takes a considerable time to build up a sufficient amount of commercial tubers. With every field multiplication the build-up and transfer of pathogens can increase, leading to seed degeneration. Therefore it is essential to investigate methods of increasing the number of minitubers (G0) produced from disease free in-vitro plantlets. Stellenbosch University http://scholar.sun.ac.za