The international Institute of Tropical Agriculture (IITA) is the leading institution conducting research on CMD in Africa and has developed improved cassava germplasm for Africa for several decades. IITA scientists identified in the early 1990s an outstanding source of resistance that was named CMD2. Most of cassava varieties developed thereafter in Africa carry CMD2 which is dominant and very stable. There has been no evidence of braking of the resistance based on this gene which has been located to be in chromosome 12. There is another region of the genome in chromosome 14 that seem to have an epistatic boosting effect on the expression of CMD2. IITA has developed and released a series of clones which are highly resistant to CMD and offer other agronomic characteristics suitable for the market in Asia:
- CR36-5 – erect, high
- IITA-TMS-IBA961632 – erect, high starch, high yield
- TMEB419 – erect, high farmer preference in Nigeria, high starch
- IITA-TMS-IBA9722005 – high flowering, high breeding value
- IITA-TMS-IBA980505 – best CMD resistance
- IITA-TMS-IBA980581 – drought tolerance, high yield
- IITA-TMS-IBA920057 – erect, high yield, high starch
CTCRI in India
CTCRI is based in Kerala state in southern India. The breeding program has released commercial varieties resistant to CMD. Some of the newest resistant varieties were developed taking advantage of the introduction of CMD2 into India in the early 2000s. The strain of CMD present in SE Asia is the Sri Lankan version, which is the prevailing version of the pathogen currently present in India. CTCRI has developed a series of clones that are grown by farmers and support the starch industries in Andra Pradesh and Tamil Nadu. They are all resistant to CMD. This is the list of clones from CTCRI that could benefit SE Asia farmers:
- CR24-4. Sree Reksha – meaning escapes CMD. Erect-nonbranching PPD tolerant. Accepted by farmers in Tamil Nadu
- CR43-7. Released in Andra Pradesh for drought tolerance. Semi spreading plant habit
- CR43-2. To be released this year. preferred in Tamil Nadu
- Clone 8501
- CR35-8 Early bulking
- CR43-8 Nutrient efficient
- CR54-A3. High flowering, it is used a breeding parent
- CR43-11. Accepted in Tamil Nadu, high yielding and nonbranching
Kasetsart University in Thailand
It was stated above that CMD2 was introduced into Thailand several years ago before the arrival of CMD into the region. Crosses were made with the elite variety HB80 and five out of many produced were found to carry the molecular markers related to resistance to CMD in chromosomes 12 and 14. There are few plants of these genotypes currently in the field. Kasetsart University is a leading institution in cassava breeding which has released successful varieties such as KU50, HB60 and HB80
CIAT in Colombia
A productive collaboration took place between IITA and CIAT through efforts to introduce genetic variability from South America into African breeding populations. Sources of CMD2 resistance were introduced from IITA into CIAT (Colombia) and crosses between these sources and elite local germplasm made. Botanical seeds were germinated in vitro (e.g. embryo rescue) and further multiplied and maintained in tissue culture. Plantlets of these crosses were then shipped to Africa and grown to asses their reaction to CMD. This was part of the work that led to the publication of the first molecular SSR markers for the CMD2 locus. The technology has been now further refined and is now based on SNPs. The germplasm was also evaluated for other characteristics. Two genotypes (CR41 and CR36) were officially released in Nigeria. They are not only resistant to CMD but also are high starch lines. The advantage of this material is that they have been maintained in vitro ever since the germination (also in vitro of the seeds – actually embryo rescued). They are, therefore, considered very safe from the sanitary standpoint.
The NextGen project has arranged for a crossing nursery in Hawaii. Germplasm from IITA and CIAT has been planted so that useful genetic variability from South America can be combined with sources of resistance to CMD can be combined. The first batch of germplasm planted in Hawaii came from botanical seed that had been generated from elite germplasm. The plants in the nursery, therefore, were not elite germplasm but the bulk of gametes they produced provide, on average, a representative sample of the allelic frequencies expected from the original elite germplasm.
Botanical seed is available from directed crosses or open pollinations that generate full- and half-sib families, respectively. It is expected that 50% of full-sib families between South American and IITA germplasm, on average will carry CMD2. They would also offer a wide genetic variability for other relevant traits.
The seed is immediately available for shipment as soon as import permits are issued by authorities.
The seed will be germinated and the seedlings grown for six months under high disease pressure. It is expected that phenotypic reaction to the disease would facilitate the selection process of resistant plants. Selection may be further supported by available molecular markers. Starting four months after planting a process of rapid multiplication will began from promising materials. Only 30 plants per genotype are necessary to be able to plant single row trials in three different locations (if possible the ten plants in each location should be planted in two replications with 5 plants each). Strong selection pressure can be exerted because of the availability of data from three different locations. The following stage will be planting preliminary yield trials in up to 10 locations. The planting material will be produced through the production of plantlets from green micro-cuttings collected even before the single row trials are harvested.
Preliminary yield trials (three replications with ten plants each) planted in as many as 10 locations should provide information reliable enough to initiate rapid multiplication of promising clones.