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 resistance 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 plants, which can have devastating consequences to a crop if there is a proximal virus source. In succumbing however, plant death prevents the virus from becoming seed-borne and eliminates the need for tissue culture.
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 dominantly inherited ‘I gene’ in local cultivars. At that time, the largest market share was held by a speckled sugar bean called Bonus, which was susceptible to BCMV. Other susceptible types included Majuba (Natal Round Yellow), Heuningberg (Yellow Haricot), Nuweveld (Brown Haricot) and Umvoti (Natal Speckled Sugar). Perhaps the only variety at the time that had any resistance to BCMV was Teebus, which carried the recessive bc22 (race specific) resistance gene which did not induce the necrotic 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’ strategy. At that meeting, bean breeders made the voluntary undertaking to introduce the ‘I gene’ into all future new varieties 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 reasonably rapidly once the breeding programmes came on-line, restricting the number of vulnerable planting seasons (where ‘I gene’ and potentially infected susceptible varieties co-existed commercially) to a minimum.