Including seeds of local crop varieties in the relief seed packages distributed to small-scale farmers after natural calamities could help indigenous crop diversity to recover faster. In addition, existing social networks which act as vital channels for seed distribution hasten the recovery of diversity in disaster-affected communities. These are among the findings of a recent study by IITA that looked into the loss and subsequent recovery of cowpea diversity in Mozambique when widespread flooding, followed by severe drought, hit most of the country about 11 years ago.
Farmers in Mozambique usually receive relief seed packages as a stop-gap measure to alleviate the effects of natural disasters that often wipe out their crops. However, most of the seeds are generally of introduced and genetically uniform varieties purchased from markets or provided by seed companies or by well-meaning relief agencies, which slow the recovery of crop diversity.
The study noted that the speedy recovery of Mozambican cowpea diversity after the back-to-back disasters of 2000 was largely due to the exchange of seeds among farmers through making gifts and other social interactions involving friends, family members, and relatives within the same community or those adjacent to it. Morag Ferguson, a molecular biologist with IITA and one of the studyâ€™s lead researchers, says that farmers in Africa traditionally grow many crops and several varieties of each crop on the same plot of land to cope with unforeseen economic or environmental instabilities. They usually set aside part of their harvest to serve as seeds for the next cropping season. They also share or trade some of these seeds with friends and relatives. When natural disasters strike, many farmers often lose the seeds that they have set aside and are forced to rely on relief seeds, buy seeds from the market, or receive seeds as gifts from friends and relatives.
â€œWe found that the substantial recovery of cowpea genetic diversity two years after the calamities was mainly due to the informal exchange of seeds among farmers that served as a socially based backup for the safety of crop diversity. It is therefore important that seed relief strategies recognize and capitalize on this existing traditional network, based on social relations, to help restore diversity especially after natural upheavals,â€ Ferguson said.
The study was initiated in 2002, two years after the floods-then-drought disaster, in Chokwe and Xai Xai districts of the Limpopo River Valleyâ€”areas that were among those severely affected. The findings of the research have been published in the current issue of Disasters, a publication of the Overseas Development Institute.
The research established that nearly 90% of the farmers in the affected areas received cowpea relief seeds immediately after the back-to-back calamities. Two years afterwards, only one in every five of the recipient farmers were still growing the seeds, whereas more than half sourced their seeds from markets. However, this did little in restoring cowpea diversity in the affected communities as the seeds bought by farmers from the market were mostly uniform, since they came from other districts that grew just one variety or a few select varieties.
On the other hand, about one-third of the affected farmers obtained seeds from friends and relatives from nearby districts not affected by the disaster and with excess seed to restock their farmsâ€”the same people with whom they had been exchanging seeds before the disasters. This practice was the main reason why cowpea diversity was restored in these areas, the study showed.
Ferguson says that such a seed distribution system based on social relations is already in use in an approach developed and implemented by the Catholic Relief Services in partnership with other relief agencies in which seed vouchers are exchanged for seeds at â€Seed Fairsâ€. In this approach, farmers from nearby districts not affected by disaster and with surplus seeds come to the Seed Fair to sell seeds to disaster-affected farmers in exchange for vouchers, which they then cash-in with the relief agency.
â€œThis approach recognizes that farmersâ€™ seed systems are robust and resilient, and can provide seeds even in emergencies. This study shows that such an approach will be more effective in restoring diversity faster and more efficiently than a system based on direct distribution only.â€
The study was the first of its kind to investigate in detail the effects of disasters on crop diversity and its recovery. It combined agronomic observations (e.g., looking at the seedsâ€™ color, size, pattern, and shape) with biotechnology tools to determine the seedsâ€™ genetic makeup.
Bananas are an important crop for global trade and nutrition where they are intensively cultivated, but few efforts exist to breed superior bananas. One of the reasons for this is that humans have intensively â€œselectedâ€ against seeded bananas and it is difficult or impossible to pollinate many banana varieties and successfully produce seeds.
Many of the most important banana varieties are triploid, which means that they carry an extra copy of each chromosome compared to the normal diploid. Being a triploid means that it is difficult for normal chromosome pairing and segregation to make fertile eggs or pollen, which results in most triploids being nearly sterile. Sterile bananas are great for people who donâ€™t like to crack their teeth on banana seeds, but mean that bananas have to be multiplied via vegetative propagation, similar to propagation of potatoes, sweet potatoes, cassava, and selected varieties of other fruit trees or ornamental species.
Gardeners are familiar with â€œseed potatoes,â€ small potato tubers that are planted to produce a potato crop. Bananas do not form tubers; new plants derive from â€œsuckersâ€ that emerge from the lower banana stem (corm). These suckers can be uprooted and used to plant new banana plants. Similar to potato tubers, these suckers were a part of the original mother plant, which means that they potentially carry whatever disease pathogens or pests had infested the mother plant. Therefore, banana suckers are one of the main means of transport and spread of certain disease-causing agents, including important fungi, bacteria, and viruses.
Nematodes and pests can also hitchhike on banana suckers to infest the new crop. Not only does such hitchhiking result in early infection/infestation of new banana plants in a farmerâ€™s field, but transporting long distances may help introduce a new disease or pest problem in a new location. This dual hazard of reduced yield potential of already infected planting material that may introduce new pests and diseases emphasizes the need for superior disease-free planting material produced through a â€œseed systemâ€ designed to minimize the risks of spreading pathogens and pests.
The traditional means of obtaining banana planting material (â€œseedâ€) is to acquire suckers from oneâ€™s own banana garden, from a neighbor, or from a more distant source. This method served to spread common varieties around the world and to multiply them in their new locations. This system can be modified to produce more banana suckers or shoots by manipulating banana corms to allow more buds to sprout. One such method that is described here is called macropropagation. A higher tech procedure to rapidly produce many plants in just a few generations of propagation is called tissue culture. In tissue culture, plants are first surface sterilized and then grown in aseptic culture in test tubes using an artificial growth medium based on a gelling agent like agar. The tender tissue-cultured plants can then be planted in the field after rooting and hardening under protected conditions.
Seed systems for producing clean planting material can be operated at various levels of technology and efficiency. In some cases, plant health could be improved by merely raising the awareness of the negative impact of planting â€œsickâ€ suckers. Where infected plants look visibly different from healthy plants, either because of reduced vigor or visual disease symptoms in infected plants, the propagator could practice negative selection against â€œsickâ€ plants or positive selection for â€œhealthyâ€ plants (or both). Such plants could be multiplied faster by applying a rapid propagation method such as macropropagation. However, while low-tech and affordable for farmers, such a system does not eliminate problems that cannot be detected by visual observation. Unfortunately, many diseases and pests fall into this category for at least part of their infection cycle.
For crops such as cereals, seed certification systems were developed to assure varietal purity, and later expanded to include freedom from weed seeds and seed-transmitted pathogens. Since most pathogens are seed-transmissible for vegetatively-propagated crops like potato or banana, disease management is the major focus of most seed potato certification programs and banana multiplication programs. Modern technology has provided diagnostic tests to detect significant pathogens. These tests are similar to those used in modern laboratories to diagnose human diseases, and can be expensive. For this reason, it is more efficient to test a small number of plants and multiply those that were negative for all pathogens tested in the battery of diagnostic tests.
It is possible to use tissue culture to efficiently and rapidly multiply plants that tested â€œcleanâ€ in the pathogen testing. Most potatoes eaten in the Western world are just a few field generations removed from tissue-cultured plants used to produce â€œseed potatoesâ€ in screened glasshouses to start the seed production cycle. Similarly, most dessert bananas in the global export trade are from plants originally propagated in tissue culture from plants that tested clean for known banana diseases. A modified form of tissue culture can also be used to eliminate pathogens from plants that did not test clean, after which they can be propagated to produce â€œseedâ€ planting material. There is great potential to improve the health of banana plantations in the developing world through increased use of this technology.
Tissue culture is the process of growing plants that have been surface sterilized and planted in test tubes or similar containers in sterile medium that contains all the nutrients they need to grow. This is almost always done in indoor laboratory facilities and the medium also contains the sugars needed to grow, since there isnâ€™t enough light for photosynthesis.
Sanitation is extremely important, since a single mold spore is enough to contaminate a test tube. Tissue-cultured plants are generally tested for pathogens before commencing the multiplication cycle so that infected plants are not multiplied. The small banana plantlets produce small suckers that can be detached and planted as new plants, or an experienced technician can cut sections that contain buds that will grow. Extra shoots can sometimes be induced by cutting through the growing points so that multiple plants develop from single buds. This process can be repeated every 5-8 weeks so that a single plant can produce many new plantlets in a relatively short period of time.Bananas are sometimes unstable in tissue culture and mutant versions can develop. For this reason, most multiplication labs try to limit the number of multiplication cycles before renewing their cultures from field plants observed to have all the correct traits for that variety.
When tissue-cultured plants are rooted in soil, hardened, and then planted back in the field, they can be more susceptible to some pests and diseases than the original plant was. To restore natural levels of resistance, these plants can be reinfected with the endophyte microorganisms that normally coexist with bananas, similar to the gut bacteria that are important for human intestinal health (see related article on endophytes).
Macropropagation falls somewhere between tissue culture and traditional systems of distributing suckers. In macropropagation, large suckers from healthy banana plants are removed and the roots and soft stem portion (pseudostem) of the sucker are cut away to expose the buds of the corm (the hard stem portion at the base of the sucker). The bare corms are briefly dipped in boiling water to kill any nematodes (micro-worms) that were not removed when cutting off roots. Small cuts are made through the buds to encourage development of multiple sprouts from each bud. The apical (top) bud is often removed because it can suppress development of lower buds. The corm is then covered with moist wood shavings and incubated in a small plastic-covered chamber for a few weeks to encourage shoot development.
Primary shoots can be rooted and used as planting material, or cut off and the growing point again cut to promote additional shooting. Shoots that develop are broken off with a bit of hard stem and roots attached, placed in small nursery bags in a similar high humidity chamber for a few days to allow root development, and finally moved to a nursery for hardening. Hardened plants can be planted in the field, similar to suckers or hardened plants from tissue culture.
A major drawback of macropropagation is that rustic or low-tech methods of detecting pathogens have not been developed, so this method can propagate infected plants if they were chosen as mother plants. Both macropropagated plants and tissue-cultured plants have less food reserves than suckers and require more care (compost/manure, watering) after planting than suckers. Careful siting of â€œmother gardensâ€ established from tissue-cultured plants in clean areas may be the best way to produce suckers for macropropagation.
Traditional seed systems have produced most of the nearly 6 billion banana and plantain plants in Africa currently spread over nearly 4 million hectares of farm and gardens. Many of these are in excellent condition; others have become infected with one or more banana diseases and need to be replaced. Since new banana diseases have been introduced to Africa in the last century, and many diseases have increased in distribution and prevalence, greater care needs to be practiced to multiply â€œhealthy seedâ€.
Breeding programs are nearly ready to release new varieties with resistance to some of the disease problems.
A combination of new and old seed systems can improve the overall health of new plantings by providing healthy plants of both preferred older varieties and resistant new varieties.
IITA’s research on macropropagation is supported by the Directorate General for Development Cooperation (Belgium) and Agricultural Productivity Enhancement Program (APEP-USAID) Uganda Agricultural Productivity Enhancement Project.