Prof. Dr. Paul Struik

Potato is becoming increasingly important in Sub-Saharan Africa (SSA), as a strategic crop for food security and rural development. Yields are still low: on average about 9 t/ha. Potential yields in SSA are much higher, 30 t/ha and more, despite the short growing seasons.

In SSA, often more than one crop can be grown per year, as there are two rainy seasons with suitable temperatures in the mid- and higher altitudes. Having two potato crops per year is advantageous, but increases late blight pressure, while the knowledge of farmers on integrated disease management is sub-optimal, resulting in over-use of fungicides and unnecessary crop losses. Moreover, having two seasons requires cultivars with relatively short dormancy. Long-term storage of ware potatoes is uncommon, and the time from harvest to consumption is short. Even though prices fluctuate with potato availability over the year, this apparently offers insufficient economic incentive to invest in professional storage facilities. For seed storage, diffused-light stores are wide-spread, and the storage period is shorter than in countries with only one growing season per year. A vibrant processing industry would lift the entire value chain to a higher level, while creating more added value.

But for that a viable and reliable seed system based on an integrated seed health strategy with emphasis on host resistance against seed-borne pests and diseases and on-farm disease management is vital. Concerted action is often required to combat diseases such as bacterial wilt. Seed growers need to make major efforts to improve seed quality, but also need to be rewarded for those efforts. This does not go without saying among smallholder farmers who sometimes hardly discriminate between seed and ware. Alternative quality-declared seed systems without formal certification have been promoted but partly failed because the control of indigenous pathogens that are both soil borne and seed borne was inadequate. Imported certified seed is too expensive for smallholder farmers and has the risk of introducing harmful seed-borne organisms, such as new mating types of Phytophthora infestans or nematodes.

In addition to late blight, the major threats are viruses and bacteria. Farmer-saved seed is usually loaded with many different viruses. The most harmful and prevalent viruses are Potato leaf roll virus and Potato virus Y. Positive selection can be achieved by marking healthy looking, vigorous plants whose tubers finally will be harvested tubers for seed. This simple and low-cost technique has been advocated and in places successfully introduced. Positive selection might not only slow down seed degeneration but could even reverse it.

In SSA, new models of seed potato systems are being developed, tested and implemented. Systems with decentralized seed growers, who have been trained to produce healthy starting material and multiply that as fast as possible with as little infestation as possible are gaining momentum. Programmes to advise such decentralized growers when investing in innovative systems (such as seed production through rooted apical cuttings, hydroponics or sand cultures) are under way. But continuously demonstrating to smallholder farmers how important good seed is while enabling them to get access to and purchase such seed is paramount.

A promising development is hybrid true potato seed, which is easier to store and transport, hardly contains seed-borne pests and diseases, and can be multiplied very rapidly. Growing a crop from true seed is challenging under SSA conditions but possible. Moreover, hybrid true seed might be the starting point of an innovative way of creating alternative seed systems, supply chains and knowledge systems of potato. ●