Developing clean seed systems for cassava

James Legg, j.legg@cgiar.org

Cassava stems for future crop. Photo by L. Kumar, IITA.
Cassava stems for future crop. Photo by L. Kumar, IITA.

Cassava is one of those crops that uses part of the plant for propagation. It is very convenient to use vegetative material from a previous crop to plant a new one. This is one of the beauties of vegetatively propagated crops. However, this convenience comes at a price. The use of planting material from a previous generation to establish the next provides an easy way for disease-causing pathogens, particularly viruses, to pass directly from one plant generation to another. So, while they offer convenience, vegetatively-propagated crops are often more widely affected by pathogens than those planted in the form of true seeds.

In Africa, cassava is the most widely cultivated of the vegetatively-propagated crops, being grown on more than 12 million ha across the continent. The exotic pest introductions, cassava mealybug and cassava green mite, caused great damage to Africa’s cassava crop in the 1980s and 1990s, but both have been effectively managed through the implementation of a classical biological control program.

The fungal diseases, cassava bacterial blight (Xanthomonas axonopodis pv. manihotis) and cassava anthracnose (Colletotrichum gloeosporioides f. sp. manihotis) are locally important. The greatest current constraints to cassava production, however, are the virus diseases, cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses (CMGs) and cassava brown streak disease (CBSD) caused by cassava brown streak viruses (CBSVs), which together cause crop losses worth more than US$1 billion annually.

One of the most important approaches to controlling these virus diseases, as well as other pathogens of cassava, is through the avoidance of infection. This can be achieved by starting out with pathogen-tested plants, and then bulking the planting material through a series of quality controlled multiplication steps. Although it sounds very simple, this can be difficult to achieve in practice.

Pathogen testing requires well-equipped laboratories run by adequately trained staff. Quality management in the field requires extensive grassroots knowledge of disease symptoms and the involvement of an appropriately trained and resourced national plant protection organization. In many parts of sub-Saharan Africa, capacity for these functions remains insufficient to meet the demands.

IITA and its partners have made significant progress in developing and implementing new systems to maintain the health of cassava through seed systems. For instance, through the Great Lakes Cassava Initiative (GLCI), a multi-partnered project implemented from 2007 to the present in Burundi, Democratic Republic of Congo, Kenya, Rwanda, Tanzania, and Uganda, a rigorous system has been put in place to assure the health of cassava planting material. This has been particularly important in view of the rapid recent spread of a devastating pandemic of CBSD in East Africa.

Healthy cassava plant. Photo by IITA.
Healthy cassava plant. Photo by IITA.

The key components of the quality and health management system are as follows: Primary (centralized seed production sites) managed by researchers or qualified seed producers, secondary, and tertiary multiplication sites (usually in farmers’ fields) are all assessed, at least once in a year, using the Quality Management Protocol (QMP). This sets out quality levels, primarily in terms of disease and pest incidence and material quality that must be met if the field is to “pass”.

The QMP standards for CMD and CBSD incidences ascertained by diagnostic tests are <10% for primary and secondary sites and <20% for tertiary sites in endemic areas. Planting materials from fields that fail to meet QMP standards are not distributed or used for further multiplication, although the tuberous roots can be used by the growers for consumption. Fields that meet the QMP standard and test negative for CBSVs are approved for more widespread dissemination.

This is the first time that this level of rigor has been applied to maintaining the health of cassava through multiplication programs in sub-Saharan Africa. It has been invaluable in assuring the health of the planting material provided to more than half a million beneficiaries in six countries, and provides an important model for other current and future cassava development programs.

Much remains to be done before such an approach can be used in a more sustainable way. Most importantly, basic capacity needs to be strengthened in most countries. Key elements of this include the laboratory and human capacity for virus indexing, as well as the knowledge of QMP and the capacity of the national plant quarantine organization to monitor cassava seed systems.

In addition, the management of cassava diseases could be greatly enhanced by the establishment of isolated nuclear multiplication sites planted with virus-tested cassava plantlets derived from tissue culture, as well as by raising awareness among growers about the importance of establishing the next crop with healthy planting material.

A long-term goal, as the commercial value of cassava increases, will be to provide a mechanism through which planting material certified through the QMP attracts a price premium. Creating added value is certain to be the key to the future development of clean seed systems for cassava in Africa. IITA and its partners are strongly committed to reaching this goal.

Clean yam tubers from vine cuttings

Hidehiko Kikuno, h.kikuno@cgiar.org

Production of yam seed tubers using vine cuttings and in vitro micropropagation is quick, cost-effective, and results in clean planting material. This new propagation system for yam developed by IITA uses vine cuttings grown on carbonized rice husks combined with in vitro micropropagation (tissue culture).

The traditional system uses tubers as seeds, is inefficient and costly. High production costs are attributed to the use of seed yam tubers, which account for about 30% of the total yield and as much as 63% of the total variable cost incurred per season of cultivation. The multiplication rate in the field using the traditional system is also very low (1:5 to 1:10) compared, for instance, with some cereals (1:300). Low quality seed yam containing pests (nematodes) and pathogens (viruses) also result in a poor yield of ware yam tubers.

Clean seed tuber production system using vine propagation in combination with tissue culture techniques. Source: H. Kikuno, IITA.
Clean seed tuber production system using vine propagation in combination with tissue culture techniques. Source: H. Kikuno, IITA.

The use of vine cuttings as a planting material gives a higher multiplication rate that is about 20−50 times more than the traditional system. It also significantly lowers the risk of nematode infestation and promotes faster multiplication and better and more uniform crop quality. Although viruses are difficult to eliminate, planting materials (seedlings or tubers) produced by this approach are relatively clean compared with those from other propagation methods used in the open field.

An experiment conducted from 2009 to 2010 using seed tubers produced by vine propagation and planted at 25 cm × 1 m spacing resulted in the production of tubers both large (200−400 g) and small (<10−30 g). Large tubers are suitable for use as seed yam for planting in the field, whereas small tubers are resown to obtain appropriately sized seed yam (about 200−400 g) (Table 1).

Tubers from vine cuttings. Photo by. H. Kikuno, IITA
Tubers from vine cuttings. Photo by. H. Kikuno, IITA

Attempts are also being made to standardize the procedure for the direct use of vine cuttings as planting material using cv. TDr 95/18544. The success of this approach could change the way in which yam is propagated in the future and eliminate the dependence on seed yam for planting needs. It would also boost the availability of yam by ~30% (Table 1).

Another trial conducted to understand the appropriate time to excise vine cuttings established from tissue culture materials revealed that the best time for vine cutting is before the rapid tuberization stage. Vine cuttings taken after tuberization were poorly established (see figure below; Kikuno et al. 2010).

Correlation between rooting of vine cuttings and dry weight of tubers formed on mother plants. Time course of rooting of wild vine cuttings and growth of tubers of mother plants on yam (D. alata cv. 95/00361). Bars in each figure indicate % of vine cuttings with rooting. Source: H. Kikuno.
Correlation between rooting of vine cuttings and dry weight of tubers formed on mother plants. Time course of rooting of wild vine cuttings and growth of tubers of mother plants on yam (D. alata cv. 95/00361). Bars in each figure indicate % of vine cuttings with rooting. Source: H. Kikuno.

This new technology offers a rapid solution for a high-output production of seed yam or yam planting material. At the same time, it addresses the need for large numbers and the quick distribution of improved varieties to farmers. This knowledge would be useful for NARES, CSOs, and farmers involved in producing and distributing seed yam, and in maintaining and multiplying breeder and foundation seeds. The technologies can also be used as a research tool by scientists.

The project was funded by the Japanese Government (Ministry of Foreign Affairs), Sasakawa Africa Association, Tokyo University of Agriculture, and International Cooperation Center for Agricultural Education at Nagoya University in Japan under the Ministry of Agriculture, Forestry and Fisheries funded the project. Partners include the Tokyo University of Agriculture; National Root Crops Research Institute at Umudike, Nigeria; Crop Research Institute, Ghana; and Institute of Agricultural Research for Development, Cameroon.

Reference
Kikuno H, Matsumoto R, Shiwachi H, Toyohara H, and Asiedu R. 2007. Comparative effects of explants sources and age of plant on rooting, shooting and tuber formation of vine cuttings from yams (Dioscorea spp.). Japanese Journal of Tropical Agriculture 51, Extra issue 2.