Production of disease-free seed tubers
Potato diseases can dramatically reduce both tuber yields and quality.
Tissue culture of plantlets in vitro for production of disease-free seed tuber requires expensive technology and highly trained staff.
A low cost alternative is the use of cuttings - a single-node, leaf-bud or other type of very small plant cutting (above) - for propagation of plantlets under non-sterile conditions.
Potatoes are susceptible to a variety of diseases that lower yields and tuber quality. What's more, pathogens accumulate in successive clonings of tubers and in the soil used to grow them. That is why sustainable potato production depends on a constantly renewed supply of disease-free planting material.
A major innovation for the potato industry in developed countries was the widespread adoption in the 1970s of tissue culture – or micropropagation - as a means of multiplying disease-free plants that can then be used to produce healthy seed tubers for farmers. First, viruses and other pathogens are eliminated by growing potato plants in a controlled environment at high temperature. The disease-free shoot tips of the plants are then placed on a standard nutrient medium in glass containers (in vitro) in a completely sterile laboratory environment. The tips develop into plantlets that are then transferred to either a greenhouse or a field protected from insect pests, where they grow at the same rate as normal potato plants but produce smaller tubers (called "mini-tubers").
After harvesting, mini-tubers need to be stored at low temperature. After about 45 days – and for a period of up to seven months thereafter - they can be moved to a warmer environment to induce sprouting. Once planted, they go on to produce normal-size, disease-free seed tubers ready for delivery to farmers. (While growing, the plants need to be protected from insect pests to avoid new disease infections.)
A low-cost alternative: plant cuttings
While the above process does deliver healthy seed tubers, micropropagation of plantlets is costly, requiring sophisticated technology and well-trained staff. In many developing countries, simpler and less expensive ways of propagation are needed. FAO is promoting a promising, low-cost alternative: the use of very small cuttings, i.e. a single-node, leaf-bud or other type of plant cutting of about 1.5 cm, which can be grown to produce plantlets on a commercial scale.
Tissue culture and micropropagation
Elementary methods of tissue culture were developed in the 1950s, and micropropagation has been used commercially for multiplying stock plant material since the late 1960s. The annual volume of plants micropropagated from tissue culture is estimated at hundreds of millions of plants, representing tens of thousands of varieties. Commonly micropropagated plants include flowers, strawberry, ornamental shrubs and forestry trees.
The starter plant material remains a small number of disease-free micropropagated plantlets which, in regions such as sub-Saharan Africa, are often imported from developed countries. However, they are multiplied not in vitro but in vivo (i.e. in non-sterile, natural conditions). Cuttings are propagated in a growing room or a shaded greenhouse in a mixture of peat and sand (or other rooting media) in plastic trays placed on metal stands.
The cutting technique takes advantage of etiolation – i.e. growing the plantlets under low light intensity. Etiolated plants retain their juvenile characteristics, producing new shoots for further cuttings that root easily. In addition, the plants remain small, so many can be grown in a limited space - each tray can hold up to 500 cuttings per square metre. The cuttings grow into new plantlets within three weeks, providing a source for further cuttings. Within six months, a single cutting can yield up to 100 000 progeny.
Once the plant material is multiplied to the quantity needed, plantlets can be transferred to an environment free of insect pests (in a greenhouse or an open field under shade). Planted in deep soil, the plantlets root easily within a week, grow into perfectly normal potato plants and produce mini-tubers.
The technique produces plantlets at the same rate as in vitro propagation at a fraction of the cost. However, it is essential that the disease-free starting plant material is kept in vitro and all standard phytosanitary measures are followed throughout the propagation process.This fact sheet was written by Tapani Haapala, of FAO's Plant Production and Protection Division. Roger Cortbaoui and Enrique Chujoy, of the International Potato Center, also contributed.