Halo blight of beans in South Africa

Halo blight caused by the bacterium Pseudomonas syringae pv. phaseolicola is a destructive disease of dry beans in South Africa. Chemical control is not effective and the only long term control is with resistant cultivars. Breeding for resistance is complicated by the presence of nine different races of the pathogen that were identifed across the world. Race identification is a prerequisite for resistance breeding. During the past three seasons pathogenic strains, representative of various hosts and geographical regions, were submitted to race identification by means of host differential lines. From 1992 to 1994 race identification was done on 101 isolates. Sixty percent of all the isolates were identified to race one. A low percentage of isolates of races 2.6 and 9 were found. During the 1994/95 season race identification was done with 62 isolates. Thirty eight percent of the isolates were identified as race 1; 32,3 percent as race 6; 16.2 percent as race 8 and 6.5 percent as race 2. Two new races were identified namely race 7 (4.8 percent) and race 4 (1,6 percent). Race 7 was isolated from cv. Drakensberg collected from Cedara in Natal. Race 4 was an isolate received from PPRl at Rietondale which was isolated from bean seed from Malawi. In the past, race 1 was isolated only from large white kidney beans which are mainly cultivated in the Eastern Transvaal Highveld. This year the majority of race 1 isolates were collected from a wide variety of cultivars in this region. Kidney beans are not included in the disease free seed scheme and it would appear that this bean type was a source of inoculum in spreading race 1. The reaction of thirty commercial cultivars to the prevailing races (races 1, 2, 6, 8, 9) was also determined Susceptible and resistant cultivars as checks were included Bean seedlings were inoculated with each race and maintained under high humidity conditions. All cultivars tested were moderately. Plants rated 3) to highly (plants rated 4-5) susceptible to all local races. These results stress the importance of breeding for resistance to this disease in South Africa. Cultivar Edmund which was included as one of the checks showed resistance to all the races tested. A backcross program was initiated to incorporate the race non-specific recessive gene present in Edmund into 11 local cultivars and promising breeding lines.