Seed Series, Issue 6: Roots & Tubers

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Published by Kilimokwanza.org | Region: Kenya, Tanzania, Uganda, Rwanda, Burundi | Crops covered: Sweet potato, Cassava, Yam | Note: Irish potato planting materials were covered in detail in Issue 1.

Editor’s introduction

Roots and tubers feed East Africa’s most food-insecure households and increasingly its industrial value chains. Sweet potato carries the bio-fortification revolution in vitamin A; cassava carries the drought-coping calorie reserve and a rising starch-and-flour industry; yam holds a smaller but high-value urban niche concentrated in Western Uganda.

All three crops share a defining characteristic: they propagate vegetatively, not from true seed. Sweet potato runs on vine cuttings, cassava on stem stakes, yam on whole tubers or minisetts. This means the planting material itself is the principal disease pathway. A field of resistant variety planted from infected stakes is still an infected field. The seed system, the disease epidemiology and the agronomy are functionally one problem.

This Issue treats each of the three crops at greater depth than earlier instalments allowed, because the smaller crop count permits it and because the operational complexity of the vegetative-seed chain warrants it. Sweet potato in particular gets the OFSP story it has earned through two decades of public-health impact across the region.

The three crops at a glance

Read across: the yield gap is wider here than for any cereal or legume — a third of attainable in most cases. The reason is the same in each crop: degraded planting material carrying disease load that compounds across cycles. Closing the gap means changing the planting material, not just the agronomy.

Why this matters

Sweet potato is East Africa’s most under-counted staple. Uganda and Tanzania each rank in the global top-ten producers, with the Lake Zone (Mwanza, Mara, Kagera) and the Lake Victoria basin (Busoga, Teso, Bugisu) carrying production levels comparable to maize on a calorie-per-hectare basis. Kenya’s Western and Nyanza belts, Rwanda’s Eastern Province, and Burundi’s lower altitudes all rely on the crop for both food and income.

Sweet potato has done in nutrition policy what no other regional crop has done at scale. The Orange-Fleshed Sweet Potato (OFSP) bio-fortification programme — led by the International Potato Center (CIP) and HarvestPlus through the Sweetpotato Action for Security and Health in Africa (SASHA) and Building Nutritious Food Baskets (BNFB) projects — has reached more than 8 million households in sub-Saharan Africa, most of them in East Africa, with a vitamin-A solution delivered through a familiar food.

The planting-material landscape

• Vine Cuttings (the standard): 30 cm sections of stem cut from healthy vines, planted directly in mounds or ridges. The dominant smallholder method across the region.

• Apical / Top Cuttings: The top 20–30 cm of an actively growing vine, with 4–6 nodes. Roots faster than older mid-vine cuttings; preferred for nursery establishment.

• Tissue-Cultured Plantlets: Disease-free starter material produced in laboratory conditions. Used by NaCRRI Namulonge, KALRO Kakamega, TARI Ukiriguru, RAB Karama. The cleanest possible base for the multiplication chain.

• Triple-S (Storage in Sand and Sprouting): A drought-coping technique developed by CIP. Roots are stored in dry sand through the dry season, then sprouted three months before the rains to produce vines. Enables planting at the start of the rains, not weeks into them.

• Net-Tunnel Vine Multiplication: Insect-proof tunnels protect mother plants from sweet potato weevil and virus vectors. The standard for Decentralised Vine Multipliers (DVMs) and registered seed enterprises.

• Quality-Declared Vines (QDS): Inspected, certificate-backed vines from registered DVMs. The realistic supply route for clean planting material at smallholder scale.

Recommended varieties by country

• Kenya: OFSP — Vita, Kabode, Kakamega 12 (SPK 004), Kakamega 4, Tomulabula; white-fleshed traditional — Mtwapa, K-118, Naspot-1, KEMB-10; yellow-fleshed — SPK 031.

• Tanzania: OFSP — Kabode, Ejumula, Naruvai, Mataya, Vumbula, Polista; white/yellow — Sinia, Sukuma, Simama, Karoti dar.

• Uganda: OFSP — Naspot 9 O (Kakamega), Naspot 10 O, Naspot 11, Naspot 12 O (Kabode), Naspot 13 O, Ejumula; white-fleshed — Naspot 1, Naspot 2, Naspot 5, Naspot 7, New Kawogo, Tanzania.

• Rwanda: OFSP — Kabode, Ukerewe, Vita, Naspot 12, Cacearpedo, Mugande, Tomulabula, RW11-1860, RW11-2419; traditional — Gihingumukungu, Rusenya, Kabakara.

• Burundi: OFSP — Kabode, Ejumula, Naspot 12, Vita; ISABU local selections such as Ndamiraremeza.

Where to source

• Kenya: KALRO Kakamega (sweet potato research centre); CIP-Nairobi; Sweet Potato Action for Security & Health in Africa (SASHA) project partners; registered Decentralised Vine Multipliers across Western and Nyanza; Anglican Development Services; Farm Concern.

• Tanzania: TARI Ukiriguru (Lake Zone sweet potato centre); CIP-Tanzania; Lake Zone DVMs in Mwanza, Mara, Kagera; Tanzania Home Economics Association (TAHEA); Mtanga Foods.

• Uganda: National Crops Resources Research Institute (NaCRRI Namulonge); CIP-Uganda; HarvestPlus; Vision Fund Uganda; Operation Wealth Creation distribution channels; registered DVMs across Busoga, Teso and Bugisu.

• Rwanda: Rwanda Agriculture and Animal Resources Development Board (RAB Karama — the regional sweet potato centre); Imbaraga and Inkingi cooperatives; Urwibutso Enterprises (Akabanga — sweet-potato value-add).

• Burundi: ISABU; PAIVA-B / PRODEFI multipliers; CIP-supported DVMs in Bujumbura Rural and Cibitoke.

Field notes for multipliers

• Use vine tips (apical cuttings) where possible; they root 30–40% faster than mid-vine cuttings and carry less virus load.

• Roguing — removing visibly stunted, mottled or chlorotic plants from mother gardens — is the single highest-leverage practice for keeping virus pressure low.

• Multiply only inside insect-proof net tunnels for the foundation tier. Open-field multiplication is acceptable only for the final farmer-supply stage.

• Source new mother stock from a tissue-culture supplier (KALRO, NaCRRI, TARI, RAB) every 2–3 cycles to refresh the genetic and health status.

• For Triple-S, select small to medium roots without bruising, store in dry sand at 25°C in shade, sprout 8–12 weeks before the planting rains.

Bottom line

Sweet potato is the bio-fortification success story of African agriculture. Vitamin-A-rich OFSP varieties have moved from research stations into millions of household plots through a Decentralised Vine Multiplier model that other crops would do well to study.

The actionable change for an extension officer or a county agriculture department is to support a network of DVMs operating insect-proof net tunnels with TC-derived foundation vines. The multiplication ratios are dramatic: one tissue-culture plantlet can produce, through a 9-month nursery cycle, enough vines to plant several hectares of farmer fields.

Why this matters

Cassava is East Africa’s drought-and-poverty insurance. The crop survives where maize fails, stores in the ground for months as a living food bank, requires almost no purchased inputs, and supplies between 200 and 500 calories per person per day for tens of millions of households across the region.

It is also the region’s industrial frontier. Starch, ethanol, animal feed, gluten-free flour and high-quality cassava flour (HQCF) are all expanding markets. Tanzania’s Coast and Lake Zone, Uganda’s North and East, Kenya’s Coast and Western regions, and Rwanda’s Eastern Province all show rising commercial off-take.

But the seed system carries the heaviest disease load of any East African crop. Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) move through the region principally on infected cuttings, not through insects. Every recycled stake from a previous harvest is a potential disease vector. The work of the past two decades has been to build a clean-seed system that can outpace the viruses.

The planting-material landscape

• Stem Cuttings (Stakes): 20–30 cm sections of mature stem with at least 5–7 nodes. Planted angled or upright in well-prepared land. The standard propagation unit.

• Mini-Stem Cuttings: Shorter sections (10–15 cm) with 2–3 nodes, used to multiply scarce variety stock more efficiently. Common at multiplier stage.

• Tissue-Cultured Plantlets: The cleanest foundation material. NaCRRI Namulonge, TARI Kibaha, KALRO Mtwapa and RAB Rubona run TC programmes. Released into screenhouses, then multiplied through nursery and on to farmers.

• Semi-Autotrophic Hydroponics (SAH): A high-multiplication-rate technique developed by IITA and CIAT for rapid scale-up of clean cassava varieties. One mother plant can produce thousands of plantlets in a year. Deployed in Tanzania and Uganda foundation systems.

• True Cassava Seed: Botanical seed from cassava flowers — used only in breeding, not in farmer supply. Genetically variable, produces a long taproot rather than storage roots when grown directly.

• Quality-Declared Cuttings (QDS): Regulator-supervised but not fully certified. The dominant route for delivering clean material to smallholders through community-based cassava seed enterprises.

Recommended varieties by country

• Kenya: CMD/CBSD-tolerant — Karembo, Tajirika, Karibuni, MH95/0183, Kibandameno (coastal), Kaleso, Migyera; bitter types for industrial starch — Selele, Tajirika.

• Tanzania: CMD/CBSD-tolerant — Kiroba, Mkumba, Kizimbani, Mkuranga 1, Naliendele, Suma, Mahonda, Pwani, MM06/0083; Lake Zone selections — Mucuba, Muzege.

• Uganda: CMD/CBSD-tolerant — NASE 14, NASE 19, NAROCASS 1, NAROCASS 2, NASE 1, NASE 3, NASE 4, NASE 12, NASE 13; widely-planted older variety TME 14 with fading resistance.

• Rwanda: CBSD-tolerant — Mavoka, Nyaramirambo, Mbakungahaze, NDL 06/132, NAROCASS 1.

• Burundi: ISABU selections; introductions of NASE 14, NAROCASS 1, MM06/0083.

Where to source

• Kenya: KALRO Mtwapa (Coast cassava centre); KALRO Kibos and Alupe (Western); IITA-Kenya; registered Cassava Seed Entrepreneurs (CSEs); Anglican Development Services Western; Mwea Tebere ATC.

• Tanzania: Tanzania Agricultural Research Institute (TARI Kibaha — the regional cassava research centre); TARI Ukiriguru (Lake Zone); IITA-Tanzania; Cassava Seed Entrepreneurs (CSEs) under the Building an Economically Sustainable Integrated Cassava Seed System (BEST Cassava); Mtanga Foods; ASA.

• Uganda: National Crops Resources Research Institute (NaCRRI Namulonge — the regional cassava centre); IITA-Uganda; Operation Wealth Creation distribution channels; registered Community-Based Seed Producers (CBSPs); Equator Seeds; FICA Seeds.

• Rwanda: RAB Rubona; IITA-Rwanda; cooperative multipliers in Eastern Province (Bugesera, Kirehe, Ngoma).

• Burundi: ISABU; PRODEFI / PAIVA-B multipliers in Cibitoke and Muyinga.

Field notes for multipliers

• Cut stakes only from plants 8–12 months old; younger or older stems give weaker establishment.

• Discard the lowest woody portion and the very top soft tip; use the middle, well-lignified section.

• Inspect every stake — reject any with discoloured cut surface, sunken lesions, or visible scale or mealybug.

• Plant on the same day as cutting where possible; if delay is unavoidable, store stakes upright in shade for no more than 7 days.

• Refresh planting material from a registered multiplier or tissue-culture nursery every 3–4 cropping cycles to clear viral build-up.

• Position multiplication blocks at least 100 m from older cassava fields and away from prevailing whitefly migration paths.

Bottom line

Cassava planting material is the disease-control front line for the entire crop. Every other intervention — fertiliser, weeding, harvest timing — is built on the assumption that the cuttings going into the ground are clean. Most are not.

The actionable extension priority is the clean-seed chain: TC → SAH or screenhouse → registered multiplier → farmer. For governments, county agriculture departments and development partners, supporting CSE-style seed entrepreneurs is the single largest available cassava-yield intervention. For the farmer, the rule is simple: source cuttings only from a regulator-listed multiplier with a current inspection certificate, and refresh every three to four seasons.

Why this matters

Yam is the smallest of East Africa’s root and tuber crops by acreage and the most regionally concentrated. It is grown most prominently in Western Uganda — Ankole, Kabale, Toro, Kigezi — and across the Lake Victoria basin, with smaller scattered cultivation in coastal Kenya and Tanzania. The crop sits well outside the West African yam-belt powerhouse (Nigeria, Ghana, Côte d’Ivoire produce nearly all global yam) but holds important cultural and agronomic niches.

The crop is high-value where it is grown. Yam tubers fetch retail prices three to four times those of cassava in Kampala and Kigali markets, and the species mix is genuinely diverse: white yam (D. rotundata), water yam (D. alata), and the bitter / aerial yams (D. dumetorum, D. bulbifera) all have culinary and medicinal niches.

Like cassava and sweet potato, yam is propagated vegetatively. Unlike them, the propagation unit — the seed yam — represents a substantial fraction of the harvested crop, creating a structural tension between food and seed.

The planting-material landscape

• Whole Seed Yams: Small whole tubers (200–800g) selected from the previous harvest. The dominant smallholder method. Yields per hectare are good but seed-yam requirement is high.

• Yam Minisetts: 25–50g cut sections of mature tuber, dipped in a wood-ash and fungicide slurry, pre-sprouted in nursery beds, then transplanted. Developed by IITA in the 1980s. Reduces seed requirement by 70–80%; raises multiplication ratio from 1:5 to 1:30.

• Vine / Aerial Bulbils: Used in some Dioscorea species (notably D. bulbifera) to propagate from above-ground bulbils. Niche; mainly in food-security and agroforestry contexts.

• Tissue-Cultured Plantlets: Used by NaCRRI Namulonge and IITA-Uganda to clean foundation stock of yam mosaic virus and yam anthracnose. Limited but growing role.

• Aeroponics for Yam: Adapted from potato aeroponics; experimental but with strong multiplication-ratio promise. Trials at NaCRRI and IITA-Ibadan are showing 1:60 ratios per cycle.

Recommended varieties by country

• Kenya: Local landrace cultivation; KALRO collections of D. alata and D. rotundata at Mtwapa and Alupe.

• Tanzania: Local landraces in Kagera, Kigoma; minor research collections at TARI Kibaha and Ukiriguru.

• Uganda: Improved D. rotundata — NamYam-1, NamYam-2 (NaCRRI selections); local landraces — Karuziku, Mukozi, Kapalanga, Embirizi, Ng’oboole; D. alata local selections in Kabale and Kigezi.

• Rwanda: Limited cultivation; landrace material in Western Province; RAB collection at Rubona.

• Burundi: Limited cultivation; landrace selections.

Where to source

• Kenya: KALRO Mtwapa & Alupe; informal coastal multipliers; landrace seed-savers.

• Tanzania: TARI Kibaha and Ukiriguru (limited research material); informal Lake Zone multipliers.

• Uganda: NaCRRI Namulonge (the regional yam research focus); IITA-Uganda; Western Region informal multipliers; Operation Wealth Creation (limited).

• Rwanda: RAB Rubona (research only); informal multipliers in Western Province.

• Burundi: ISABU (limited).

Field notes for multipliers

• When using minisetts, treat cut pieces with a slurry of wood ash, water and a registered fungicide before nursery planting.

• Pre-sprout minisetts in shaded nursery beds for 4–6 weeks before transplanting to the main field.

• Stake yam vines vertically; supported plants out-yield ground-trailing crops by 30–60%.

• Source fresh seed yams from a known healthy field, not from grain markets where mixed-disease tubers circulate.

• Rotate yam blocks every 2–3 cycles; yam-on-yam build-up of nematodes and anthracnose is a major silent yield depressor.

Bottom line

Yam is East Africa’s most under-promoted root crop and its highest-value-per-tuber. The seed system is informal, the seed-cost ratio is high, and improved technology — minisetts, TC, aeroponics — reaches few farmers.

For NaCRRI, IITA, RAB and the regional development community, this is the unfinished agenda: scale the minisett technique, build a small but functional formal yam-seed channel, and capitalise on premium urban demand. For an Ankole or Kabale farmer asked where to start, the answer is the minisett — it pays back faster than any other yam intervention available.

Cross-cutting recommendations

• Treat cuttings, vines and setts as a disease-control issue first. Cassava brown streak, sweet potato virus disease and yam anthracnose are seed-borne. The single highest-leverage intervention in any of these crops is to source clean planting material from a TC-derived chain.

• Build the Decentralised Multiplier model. DVMs (sweet potato), CSEs / CBSPs (cassava) and yam-minisett nurseries are the workable last-mile bridges. Formal certified channels alone will not solve vegetative-seed supply at the smallholder scale.

• Refresh planting material on a discipline. Sweet potato vines every 2–3 multiplication cycles; cassava cuttings every 3–4 cropping cycles; yam every 2–3 cycles plus rotation away from yam-on-yam blocks. Build the schedule into the farm plan.

• Net-tunnel and screenhouse foundation tiers matter. The first multiplication step out of tissue culture should always be inside an insect-proof structure. Open-field foundation multiplication defeats the purpose of the TC investment.

• Promote the under-used techniques. Triple-S in sweet potato, semi-autotrophic hydroponics in cassava, and the minisett technique in yam are all proven scale-up tools that reach few farmers. Extension priority sits with bringing these into the village multiplier system.

Editor’s note

This Issue is the sixth instalment of Kilimokwanza’s Seed Series. The vegetatively propagated crops are the most operationally complex of the seed systems we have covered so far, and the institutions — NaCRRI, TARI, KALRO, RAB, ISABU, IITA, CIP — deserve recognition for the multiplication chains they have built. Corrections, updates and field reports from extension officers, multipliers, DVMs, CSEs and farmers are welcome at editor@kilimokwanza.org.

Coming next — Issue 7 • Vegetables: tomato, onion, cabbage, kale, spinach, carrot, eggplant, capsicum, hot pepper, okra. The single largest crop count in the Series.

Compiled by the Kilimokwanza Editorial Desk • Series Editor: Anthony Muchoki • kilimokwanza.org

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