Dr Fengyi Wang*
Because of climate change, the reduction of arable land, increasing population, and frequent occurrence of natural disasters, food security has become a crucial issue. To face this situation, increased food supply has become a priority in the world’s development agenda. In terms of nutritional value, adaptability to diverse environments and yield potential, the potato is a preferred crop, especially in developing countries. According to FAO statistics, potato production in developing countries has increased by 94.6 percent over the last 15 years (Table 1). Out of the four major food crops (rice, wheat, potato and maize), the potato has the best potential for yield increases.
Table 1. World potato
production in 1991-2007
(million ton)
|
Countries |
1991 |
1993 |
1995 |
1997 |
1999 |
2001 |
2003 |
2005 |
2007 |
|
Developed |
183.13 |
199.31 |
177.47 |
174.63 |
165.93 |
166.94 |
160.97 |
159.99 |
155.56 |
|
Developing |
84.86 |
101.95 |
108.72 |
128.72 |
135.15 |
145.92 |
152.11 |
160.12 |
165.15 |
|
WORLD |
257.25 |
301.27 |
285.97 |
303.36 |
301.08 |
312.86 |
313.09 |
320.11 |
320.71 |
|
Source: FAOSTAT |
|||||||||
The potato is grown in more than 150 countries with an average yield of about 16 tons/ha. However, yields in North America and some European countries are over 40 tons/ha; even 70 to 80 tons/ha can be realized in experimental plots. The yield in developing countries is less than 20 tons/ha, even less than 10 tons/ha in some countries. There is a big gap among the various countries between high and low yields, even with the same variety of potato. If constraints could be overcome to some extent, it would be possible to increase the yields in the developing world significantly.
Potato yield is determined both by the crop per se and the environment. The former can be defined as internal causes including genetic identity, health and physiology. The latter are external factors that consist mainly of temperature, light, nutrition and water. The genotype determines tuber number, tuber size and yield potential for any given cultivar. Then, the performance of yield is largely influenced by the health status of seed tubers and plants. The physiological age of seed tubers is also a factor that can affect final yield. Since some external factors (such as light and temperature) cannot be controlled, we can only adjust the inputs of nutrition and water by appropriate fertilizer application and irrigation. Then, appropriate crop management practices should be applied to harmonize the relationship of crop and the environment. It is the only way to get good yields. Unfortunately, we cannot control all external factors, but efforts can be made to optimize yields by using high quality seed.
Genetic identity is modified by breeders through developing new cultivars. Healthy seed is the responsibility of seed producers, and only the physiological age of seed tubers can be adjusted by growers. After reviewing the history of potato breeding, it seems that all registered cultivars have similar yield potential; even some cultivars that have been cultivated for over
100 years can still yield 50 or 60 tons/ha if seed quality is good and, of course, under appropriate crop management. We can say that genotype at present is not a major constraint for getting higher yields. The physiological age of seed tubers can influence yield to a certain extent through adjusting growth speed. Old seed develops fast at an early stage and senesces earlier, which leads to relatively lower final yields. In contrast, young seed develops slowly at an early stage, but it can keep vigour longer and get higher final yields. The range of yield fluctuation adjusted by physiological age is not clear, but yield effects of physiological age are evident. Therefore, it is clear now that seed quality in terms of health and physiological status is a determinant factor of the yield potential of the potato crop.
Crop management practices are related to local conditions, production purposes and utilization, and growers’ experience. In many countries irrigation and fertilizer application are restricted by economic factors. Under these conditions what we can do for yield improvement is just to have healthy seed and good crop management. In fact, high productivity is based on good quality seed, combined with the use of other critical inputs and application of appropriate crop management practices.
Potato yields are affected by several factors. Quality seed is a very important factor. The average yield increase from the use of good quality seed is 30 to 50 percent compared to farmers’ seeds. Two cases from China and the Democratic People’s Republic of Korea can illustrate the differences of yield. The results of investigations carried out in 2005 in Shandong, China are shown in Table 2. In the whole province, the use of good quality seed accounted for 24 percent. Medium quality seed accounted for 43.3 percent; and 32.7 percent of production was based on poor quality seed. The yield difference between good quality and poor quality was 28.4 percent.
Table 2. Comparison of yields from different quality categories of seed
|
Seed quality |
Yield (tons/ha) |
Yield comparison (%) |
|
Good |
37.6 |
100.0 |
|
Medium |
30.7 |
81.5 |
|
Poor |
22.0 |
71.6 |
Data from Academy of Agriculture Sciences, DPR Korea, 2007.
The second case, in the Democratic People’s Republic of Korea, shows the yield differences among various classes of seed (Table 3). The basic seed was microtubers. In Class 1 the tubers were harvested in normal size net-houses. In Class 2 the tubers were multiplied in the field.
Table 3. Yield comparison of different classes of seed
|
Variety |
Seed
class |
Yield(T/ha) |
Ratio
(%) |
|
Jangjin-6 |
Basic |
22.3 |
120.2 |
|
Class 1 |
30.7 |
162.0 |
|
|
Class 2 |
18.9 |
100.0 |
|
|
Yolmaejo-saeng |
Basic |
25.2 |
142.0 |
|
Class 1 |
28.5 |
160.0 |
|
|
Class 2 |
17.7 |
100.0 |
Data
from Academy of Agriculture Sciences, DPR Korea, 2007.
Quality indicators of potato seed have two dimensions: the biological attributes (biological quality) and the appearance attributes (commercial quality). Biological quality is crucial for productivity, whereas commercial quality mainly affects seed price.
The biological quality includes two aspects: a) the level of disease infection and b) the physiological age of seed tubers. The former is quite complicated and important. It is well known that seed tubers planted continuously for several years will show degeneration. The degeneration is aroused by several kinds of viruses and virus-like organisms. Because of asexual propagation, viruses and viroids can be accumulated in tubers, and lead to degeneration of the potato.
The biological quality includes two aspects: a) the level of disease infection and b) the physiological age of seed tubers. The former is quite complicated and important. It is well known that seed tubers planted continuously for several years will show degeneration. The degeneration is aroused by several kinds of viruses and virus-like organisms. Because of asexual propagation, viruses and viroids can be accumulated in tubers, and lead to degeneration of the potato.
Major viruses affecting the potato are potato virus Y (PVY), potato virus X (PVX), potato virus M (PVM), potato virus A (PVA), potato leaf roll virus (PLRV) and potato spindle tuber viroid (PSTV). Infection of any one alone or some of them jointly would retard plant growth and reduce tuber yield. Apart from viruses, fungal and bacterial pathogens borne by tubers lead to late blight, ring rot, black-leg and others, and are also limiting factors for seed quality.
Commercial quality is defined by uniformity and size of tubers, as well as external appearance. For normal production, a reasonable size of seed tuber or tuber pieces should be about 40 to 50 grams. Big size seed will increase cost and seed that are too small can rot before emergence.
Good biological quality seed is free from any pathogens, including viruses, viroids, fungi and bacteria, that may lead to degeneration of seed. Seed multiplication started with clean stocks should be the key step. After that, appropriate multiplication technology should be applied according to classes of seed multiplied. Figure 1 shows the pattern of the general flow of seed multiplication in most seed production programmes.
Figure 1. General flow of potato seed production
Under given conditions, this flow could be modified. No matter what improvements are made, the quality of seed should be guaranteed. Of course, more generations of multiplication always increase the risk of degeneration, but seed costs can be reduced.
Seed supply systems are quite diverse. Many seed supply schemes have been adopted by local seed producers especially in tropical and subtropical regions. Some examples used in Asia and the Pacific region are shown below.
China is the world’s foremost potato producer in terms of harvest area and amount. In 2007, the harvested area was five million hectares and total production was 72 million tons. Nevertheless, national average yield was only 14.4 tons/ha, even lower than the world average (16.64 tons/ha). The adverse natural environment such as infertile soil in southwest mountain zones and shortage of water supply in the north-central zones are negative factors, but poor quality of seed is more significant. It is estimated that only 20 percent of the total cultivable area is planted with quality seed (Figures 2 and 3).
Figure 2. Potato seed production scheme in Northern China (single cropping)
Figure 3. Potato seed production scheme in Central China (double cropping)
During the last ten years, rapid potato development has taken place in Democratic People’s Republic of Korea. Up to 2006, the harvested area was 188 388 hectares and total production was 470 451 tons with average yields of 9.3 tons/ha for spring potatoes and 10.7 tons/ha for summer potatoes. Poor quality of seed is a major constraint on yield increase. For this reason, supply of quality seed became a key issue for potato production. The first step was to install hydroponic facilities to produce mini-tubers of virus free stocks. There are already six tissue culture laboratories equipped with hydroponic facilities around the country. Mini-tubers produced in the laboratories will be distributed to provincial seed farms, and then followed by county seed farms and sub-work teams of cooperative farms. In the northern regions, a four-year multiplication scheme was adopted. In the lowland zones, a scheme of four generations in two years was adopted. As an alternative, true potato seed (TPS) was tried as well for seed production.
Although several approaches to seed multiplication have been tried in lowland zones, the supply of mini-tubers and aphid-proof facilities is still not satisfactory. Sexual reproduction can eliminate almost all pathogens; thus TPS as basic material can be used for seed production. Hybrid or open-pollinated TPS were sowed in July between rows of maize, and harvested in late October. After four to five months of storing, those tubers from TPS can be used as seed for commercial production in lowland zones. All potato clones are heterozygous, and tubers harvested from TPS do not represent a pure variety. However, they are available for family consumption after selection of parental clones (Figures 4 and 5).
Figure 4. Multiplication system in northern region
Figure 5. Multiplication scheme in lowland zones
In 2004, the average potato yield was only 8.5 tons/ha in Mongolia. Results of interviews with 300 growers revealed that seed quality was the major cause of low yields. Most growers said that they did not know the origin of the seed they planted, as a seed supply system does not exist to which small-scale growers can have access.
From 2005, revitalization of Mongolia’s potato sector (funded by donors) started. Improvement of seed quality was a major component of the project. Based on natural and financial conditions, a decentralized seed system was adopted (Figure 6).
Figure 6. Proposed seed production scheme
Although the system is not perfect, and still needs to be improved in many aspects, it is already showing results. Up to 2006, 103 informal seed producers were operating in 21 aimags (top-level administrative divisions). The average yield reached 10.2 tons/ha. However, technologies for virus detection and quality control are still under development.
The use of quality seed involves two aspects: whether seed of good quality and in large enough quantities can be supplied and whether growers are willing to use quality seed considering the cost—benefit ratio. Both aspects are related to market conditions. The only factor motivating growers to grow potatoes is competitive returns compared to other crops. If higher profits can be achieved, growers would be eager to get quality seed, and then seed producers could have a market for their product. For example, in Shandong, a special area for potato production in China, potatoes can be harvested from late April to June when fresh potato supply is short in most other regions of China. Fresh potatoes harvested in Shandong are easily sold at a good price. Potato growing has become a major source of income for local farmers. Since the profits from increasing yield are higher by using quality seed, growers are willing to buy good quality seed. Shandong has the highest potato yield in China now. Provincial average yield is over 30 tons/ha, with a total harvest area of 1.4 million hectares. Furthermore, Inner Mongolia, which is located on a plateau and has sufficient sunshine, has become a major area for potato production with irrigation facilities. Higher profits from potato growing are encouraging growers to use quality seed.
With the growth of the potato seed market, some large scale seed producers have emerged in recent years. The biggest one is the Shandong Xisen Group, with a capacity of 250 million mini-tubers per year. After multiplication in fields, 75 000 tons of certified seed can be produced yearly. Annual seed production of Inner Mongolia’s Hesheng Potato Industry is already stable at 40 000 tons of certified seed. There are also many small-scale seed producers in China. With the improvement of seed quality, potato yields in China will be significantly increased in the near future.
Potato yields are affected by several factors, but the basic factor is seed quality, especially its biological quality. Application of fertilizers and irrigation, as well as appropriate crop management, could be more effective when good quality seed is used.
Good returns from potato production are the driving force for using quality seed. As long as potato growers can achieve higher profits, they are willing to use quality seed. The key is that the profit from using quality seed must offsets its higher cost.
The only way to increase potato production in the region is by increasing productivity per unit area of land. Increasing productivity rather than area expansion will be the key factor to meet the increasing potato demand in the future.
* Coordinator for CFC/CIP DPR Korea Project, CIP-ESEAP, Training Building in the Campus of Northeast Agricultural University, 59-Mucai Street, Xiangfang District, Harbin, Heilongjiang Province, China 150030.
