Elite disease resistant germplasm successfully exchanged and evaluated in five African countries

Edward Kanju IITA-Uganda, Silver Tumwegamire IITA-Rwanda, James Legg IITA-Tanzania

A technician multiplies hardened tissue culture plantlets of elite clones using node stem cutting technique at Kibaha, Tanzania. Photo by S. Tumwegamire, IITA




They say two is better than one, but for this one-of-a-kind, four-year initiative, five was better than one. The project, that came to an end in June 2017, saw five countries (Kenya, Malawi, Mozambique, Tanzania, and Uganda) join forces to tackle one of the major challenges to cassava production— cassava brown streak disease (CBSD) and cassava mosaic disease (CMD). The two viral diseases are fast spreading across the continent affecting food security and incomes of smallholder farmers.

The project, New Cassava Varieties and Clean Seed to Combat CBSD and CMD, shortened to 5CP supported the countries to successfully share their five best cassava varieties in terms of yield and resistance to the two diseases. This was to speed up efforts to develop and supply farmers with improved varieties of cassava that are resistant to the two viral diseases, the greatest constraint to the crop’s production.

CBSD damages tuberous roots, rendering them unfit for human consumption as well as depressing overall yield. The effects are insidious, since the plants often show no symptoms with farmers only seeing the damage at the time of harvest. CMD causes more obvious damage, as affected plants have yellow-green, twisted leaves, and plants can be severely stunted, producing either very small roots or none at all. Cassava varieties resistant to these two diseases are therefore urgently needed by rural farmers in eastern and southern Africa (ESA), to protect their food and income security.

5CP borrowed lessons from joint action against CMD in the 1990s, and brought together the national agriculture research systems (NARS) of the five countries most affected by the two deadly cassava diseases to share and test their elite germplasm to speed up the breeding process and develop varieties with strong resistance to CBSD and CMD. The other objective of the project was to pilot a commercially viable seed system in Tanzania to ensure farmers have access to a new pipeline of improved cassava varieties from research.

How did we tackle the problem?

Virus testing and indexing: The project started with each country selecting and submitting its five best cassava varieties in terms of yield and resistance to the two viral diseases, as well as four local checks. These were then cleaned of viruses at both the Kenya Plant Health Inspectorate Service (KEPHIS) and the Natural Resources Institute (NRI).

The 25 varieties were Sangoja, Sauti, Yizaso, Kalawe, and CH 05/203 (Malawi); Colicanana, N’ziva, Okhumelela, Orera, and Eyope (Mozambique); LM 08/363, F19-NL, Tajirika, Shibe, and F10-30-R2 (Kenya); Kipusa, Pwani, Mkumba, Mkuranga 1, and Kizimbani (Tanzania); NAROCASS 1, NASE 14, NASE 1, NASE 3, and NASE 18 (Uganda). Two regional checks—Kibandameno (from Kenya, a susceptible check for both CMD and CBSD) and Albert (from Tanzania, a susceptible check for CBSD) were also included. All clean plantlets were dispatched to Genetic Technologies International Limited (GTIL), a private tissue culture (TC) laboratory in Nairobi, Kenya, for micropropagation.

Micropropagation: At GTIL, at least 300 TC plantlets of each variety were raised and sent back to the five countries, allowing the exchange of germplasm between them. Each country therefore received 25 resistant varieties including the best five, two local checks, and four own local checks. Genotype purity verification was done in collaboration with Biosciences eastern and central Africa (BecA) (Nairobi, Kenya) while both Mikocheni Agricultural Research Institute (MARI, Dar es Salaam, Tanzania) and IITA confirmed that plantlets were virus free.

To ensure the TC plantlets were well handled, two partners in each country were trained on post-flask management in collaboration with Uganda’s cassava research program at the National Crops Resources Research Institute (NaCRRI). Screen houses in Malawi and Mozambique were repaired and new ones constructed in Kenya and Tanzania. The project also conducted a mock shipment to test the partners’ readiness to receive the materials and identify possible challenges during the actual shipment.

Management of the exchanged elite varieties in target countries: Upon arrival in the countries, the TC plantlets were hardened, macropropagated using two-node stem cuttings, and field multiplied in sites with very low
CBSD and CMD pressure that were isolated by at least 200 m from any cassava crop to minimize any virus spread. They were then distributed and planted in field trials at over 30 sites
across the five countries. The sites had varying levels of CBSD and CMD pressure, and climatic and soil conditions.

CBSD root necrotic damage among the susceptible clones Albert, Sagonja and CH05/203 compared to no necrotic damage among the tolerant clones F10-30R2, TZ-130, Orera and Mkuranga1 as observed at Bunda Tanzania in November 2016.Photo by S. Tumwegamire.

Sagonja

CH05/203

Albert

F10-30-R2

Tz-130

Orera

Mukuranga1

What was achieved?

Three major achievements were accomplished Firstly, up to 31 varieties (25 elite, 2 standard checks and 4 national checks) were successfully virus-cleaned and indexed, and 27 varieties were exchanged among the target countries. The other four were returned to their respective mother countries as national checks. This germplasm presents a unique opportunity to identify varieties with high levels of resistance to both CBSD and CMD under the diverse range of virus/virus vector/environmental conditions in these five ESA countries.

Additionally, these varieties have great potential for use as parents to generate superior
progeny. The clean stocks are also a great asset for initiating extension programs for multiplying and disseminating high-quality, pre- basic “seeds”. Hitherto, most of the target countries have had no access to field-based stocks of high-quality, virus-tested planting material.

Secondly, a strong partnership was built between breeders and virologists in national and international institutions as they worked together to combat these diseases through elite resistant germplasm and clean seeds. The partnership also allowed cross-learning between partners at all levels of the process. It is envisaged that these partnerships will continue after the project.

Thirdly, the elite varieties were successfully tested in 33 different sites across the partner
countries and comprehensive data collected, some of which has been uploaded to www. cassavabase.org. The remaining sets will soon be uploaded. Upon analysis, the collected
data should elucidate the magnitude of genotype by environment interactions for CBSD/CMD, yield, and other performance characteristics.

Although combined data analysis is yet to be done, preliminary selected data analyses by graduate students on the project are already helping to guide stakeholders on which varieties are likely to be most appropriate in which agroecologies and CBSD/CMD disease pressure conditions within the eastern and southern African region.

Moreover, the preliminary data and the germplasm exchange described here have provided a foundation for the development and implementation of new CBSD mitigation programs in Tanzania (BEST Cassava Project), Burundi, Rwanda (CBSD Control Project), and eastern Democratic Republic of Congo (Action to control CBSD in DRC).

Acknowledgement

The initiative was implemented as part of a wider project “New Varieties and Clean Seed to Combat CBSD and CMD” led by IITA and funded by the Bill & Melinda Gates Foundation. The process and lessons of this success story have since been published as a journal article (http://link.springer. com/article/10.1007/s12571-018-0779-2) by the Food Security Journal in March 2018.

Posted on October 30, 2018 in Making Crops Healthy

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