The minor root and tuber crops are a rich but neglected resource for food, nutrition, and income. The Andean root and tuber crops (ARTC) (achira, ahipa, arracacha, maca, mashua, mauka, oca, ulluco, yacón) play a major role in Andean potato-based farming systems, where they are of great economic and nutritional importance to subsistence farmers.
They yield well with low inputs and withstand most biotic and abiotic stresses. They have a wide range and mix of desirable characteristics: high-vitamin, micronutrient, and starch content; high yields; and medicinal properties. Diploid Ahipa yam beans (Pachyrhizus spp.) are drought-tolerant legumes with considerably more protein and trace minerals (Fe, Zn) than better-known root crops.
The aroids, Colocasia spp. and Xanthosoma spp., are important food crops in the tropics (12.2 MT), especially in Africa (76%). Taro (Colocasia spp.) and cocoyam/tannia (Xanthosoma spp.) were domesticated independently in southeastern Asia and tropical America, respectively. Cultivation is concentrated in the coastal forest regions of West and Central Africa, but they are often intercropped with bananas in highland Eastern Africa. Cocoyam and taro are important traditional crops, playing a vital role for women in family food supply and income generation. Similar to yam, these crops are preferred staples, but they have not been sufficiently researched.
Bananas, including dessert banana, plantain, and cooking banana, are the fourth most important food crop in the least developed countries ranked by total production and food consumption. Total global production in 2010 was 136 million metric tons.
Banana production in many areas rests upon a narrow genetic base. Many major production zones are dominated by cultivar sets that are clonal variants of a base cultivar. Bioversity International’s International Transit Center is the world’s largest collection of bananas with more than 1,400 accessions maintained as in vitro plantlets. The genebank includes mostly cultivars, plus some improved material and wild Musa, which make up less than 15% of the accessions. Increasing the representation of wild species is one of the genebank’s priorities. Like roots and tubers, bananas are vegetatively propagated polyploids, traits that affect their breeding, propagation, and distribution. Key traits for breeding include yield, crop cycle, resistance/tolerance to pests, diseases, and abiotic stress, nutritional qualities, postharvest attributes, and preferred enduser qualities.
Yams are a multispecies crop with a wide range of ploidies and rank eleventh among global food crops in both total production (49 MT) and food consumption in developing countries. Production is concentrated in tropical Africa (96%), mostly by smallholders.
Guinea yams, D. rotundata (white yam) and D. cayenensis (yellow yam), originated in Africa and account for most of the yam production; they are preferred for the organoleptic properties of the tubers.The water yam, D. alata, originated in Asia and has superior agronomic flexibility and wide geographic distribution.Yams are adapted to the tropics and subtropics and can be propagated by seed tubers or vine cuttings. Breeding objectives include high and stable tuber yields, resistance to pests and diseases (nematodes, viruses, anthracnose, and scale insects), tuber quality, ease of harvest and long storage, suitability to cropping systems and markets, tolerance to abiotic stresses, and textural and nutritional attributes. In addition to common research needs with other RTB, agronomic research on soil fertility management investigates the role of mycorrhizal fungi in yam mineral nutrition, while product development looks at functional properties required for products for household and industrial purposes.
Sweetpotato is the eighth most important food crop in developing countries. Total production (110 MT) has historically been centered in Asia with Africa taking an increasing share.
Like the potato, sweetpotato (Ipomea batatas) is a major energy crop, and some of its genotypes such as orange fleshed sweetpotato (OFSP) have very high levels of pro-vitamin A, which provides a means to reduce VAD in the tropics and subtropics. Whilst sweetpotato originated in the Americas, no wild form of I. batatas has been found, but domesticated I. batatas was probably selected from an inter-specific cross between diploid and tetraploid species in the I. trifida complex. Major pest and diseases are sweetpotato chlorotic stunt virus and the sweetpotato weevils (Cyclas spp.). The crop has large genetic variation for crop duration, as adaptation, partitioning, and nutrient composition.Breeding has been limited to date, and this needs to be increased to produce optimum combinations of broadly adapted productivity of highly nutritious genotypes and high foliage production for animal feed. Transgenic approaches mainly serve for providing weevil and virus resistance. Marketing and processing of sweetpotato are still limited and require further research. The economic value of products varies with dry matter content and color. Hexaploid hybrids are propagated and distributed as tender cuttings called vines, or storage roots.
Cassava is the second most important food crop in the least-developed countries, and the fourth most important in developing countries, with total production (218 MT), of which over half is in Africa and another third in Asia.
It is mostly grown by smallholders. Commonly considered to provide only carbohydrates, cassava also contains significant minerals including micronutrients. High pro-vitamin A (PVA) cultivars exist and leaves are consumed as a nutritious vegetable in some countries.
There is a significant global market for dehydrated cassava that is boosting production with new market opportunities. Originating in the Americas and adapted to the tropics and subtropics, wild Manihot exhibit significant variability for adaptation, nutritive content, and toxins (cyanogenic glucosides), and resistance to pests, diseases, and postharvest deterioration.
This perennial species is handled as an annual crop that is tolerant to many abiotic and biotic stresses, including low-fertility soils, and can be left unharvested until needed. The short shelf-life requires efficient marketing/fresh consumption or processing. Cassava cultivars are heterozygous hybrids propagated by woody cuttings called stakes. Key breeding objectives are yield, high dry matter, resistance to viruses such as cassava mosaic disease and cassava brown streak disease, compatibility with integrated pest management (IPM), tolerance to drought and low fertility, and low toxins, high-carotene, and fodder.
Potato is the world’s fifth most important food crop in the least developed countries ranked by total production (159 MT) and the third most important ranked by food consumption (118 MT). Potato is grown in diverse environments in Asia, Africa, and LAC. Originating in the Americas, wild potatoes are a tremendous resource for breeding, with a wide range of ecological adaptation from the high Andes through cool temperate rain forests to coastal plains, and carry resistances to many important pests and diseases.
They provide food, employment, and income as a cash crop for table, processing, and industrial use. Potatoes are a major source of energy and protein with significant amounts of vitamins C, B6, and B1, folate, potassium, phosphorus, calcium, Fe, and Zn. They are high in dietary fiber and rich in antioxidants, including polyphenols and tocopherols. As with other RTB, their reproductive biology allows variability to be rapidly generated and fixed, with sexual reproduction followed by vegetative propagation via tubers.
Pollinated potato flowers set true seed in berries, with obligatory out‐crossing for many species. Most cultivated potatoes are tetraploids (2n = 4x = 48). Key breeding objectives are stable tuber yield, durable resistance to diseases such as late blight (LB) and bacterial wilt, resistance to multiple viruses (potato virus Y, potato leafroll virus) and pests, adaptation to heat and drought, short vegetative cycle, tuber quality, and nutritional attributes.