Fortune Favours the Brave: The story of BCMV in South Africa

By Antony Jarvie

Seed-borne diseases in common bean are arguably the most significant production constraints faced in bean production in Africa today. In the early eighties, South Africa opted to implement a seed certification scheme, the ultimate goal of which was to secure disease-free seed to the market. This move set us apart from the rest of Africa in terms of food security, and ultimately formed the foundation of a bean healthy industry. Within the seed certification scheme, we continually battle against seed-borne bacterial diseases (Common Blight, Halo Blight and Bacterial Brown Spot) and fungal diseases (Anthracnose) which we manage by producing parent seed in the winter rainfall region during the summer, and in the summer rainfall region during the winter. One of the seed-borne diseases that we do not have to worry about anymore is Bean Common Mosaic Virus (BCMV). At the time of the establishment of the seed certification scheme, BCMV was one of the major seed-borne diseases that plagued our industry. Tissue culture techniques were used to rid seed sources of the virus, and whilst this was successful, the huge reservoir of virus in uncertified fields meant that re-infection occurred despite the best efforts to maintain virus-free status.

A genetic solution to the problem was available, but it came with significant risks in deployment and required a unified breeding strategy from all cultivar developers. The ‘I gene’, as it is known, evokes a hypersensitive reaction to the necrotic strain of BCMV present in RSA. Plants infected with the virus die from what is commonly known as a ‘Black Root’ reaction, effectively systemic necrosis. The result is a total loss of seed yield from the infected plant, but in succumbing, the virus is prevented from becoming seed-borne. This mechanism eliminates the need for tissue culture and prevents a reservoir of virus from developing.

In 1988, under the guidance of the ARC, the players in the bean seed industry in RSA met to discuss and debate the merits of using the ‘I gene’ in local cultivars. At that time, the largest market share was held by speckled sugar bean called Bonus, which was susceptible to BCMV. Other susceptible types included Majuba (Natal Round Yellow), Heuningberg (Yellow Haricot), Nuweveld (Brown Haricot), Umvoti (Natal Speckled Sugar). Perhaps the only variety at the time that had any resistance to BCMV was Teebus, which carried the bc22 (race specific) resistance gene which did not induce the black root reaction. Given the backdrop of the predominance of susceptible varieties in the market and the large pool of virus inoculum present, it was indeed a brave decision to proceed with the unprotected ‘I gene’. At that meeting, bean breeders made the voluntary undertaking to introduce the ‘I gene’ into all new varieties in order to rid the industry of virus-related yield losses in the long term. The phasing in of ‘I gene’ and phasing out of susceptible cultivars occurred fairly quickly, restricting the number of vulnerable ‘mixed’ planting seasons.

Twenty-five years later, the bean industry is thriving and BCMV is practically extinct in South Africa. Two factors were crucial to the success of the strategy. A brave long-term vision and a unified approach from seed developers.