ARC-Grain Crops Institute, Private Bag X1251, Potchefstroom, 2520, South Africa

Background and objectives

Production of dry beans (Phaseolus vulgaris L.) in South Africa is seriously affected by halo blight, a bacterial disease incited by Pseudomonas syringae pv. phaseolicola (Burkh.) Dows. The disease is widespread throughout the production area and is favoured by moderate to cool temperatures. Nine races of the pathogen have been identified world-wide according to their reaction on a set of eight differential cultivars and lines [1]. Race differentiation is a prerequisite for resistance breeding which is considered the most effective method of disease control. The aim of the study was to characterize local races, determine their distribution and to evaluate resistance of commercially grown cultivars to each of these races.

Material and methods
Isolates were collected from the major bean producing areas in South Africa from 1991 to 1996. Samples were taken from various cultivars of P. ;vulgaris, P. ;coccineus L. (large white kidney beans) and P. ;lunatus L. (lima beans) and were collected from 255 disease occurrences. Races were identified by spray-inoculating the set of differential cultivars with a DeVilbiss atomizer. Inoculated plants were kept in a humidity chamber (191C, RH=100%) for 48 ;h before being transferred to a greenhouse equipped with a humidifier (18C night/25C day, RH=70%). Plants were rated for infection 10 ;days after inoculation on a 1 to 5 scale, with 1 being highly resistant and 5 being highly susceptible. The same inoculation and rating methods were used to evaluate 30 locally grown commercial cultivars in the greenhouse for resistance against all the locally occurring races.

Results and conclusions
Seven races of P. ;syringae pv. phaseolicola were identified in South Africa (1,2,4,6,7,8 and 9). Race 8 was widely distributed and occurred in most of the localities. Race 1 was initially (during survey of 1991 to 1993) found only in the Mpumalanga Highveld where large white kidney beans are cultivated, but has since spread to new areas. Race 7 was confined to KwaZulu-Natal where it had been isolated from two localities (Cedara and Greytown) and race 9 was only found in KwaZulu-Natal and Mpumalanga. Although the occurrence of races 2 and 6 was low, these races were widespread throughout the production areas. The evaluation of commercial cultivars showed that all the locally grown cultivars are moderately to highly susceptible to most of the local races. In order to breed for resistance against halo blight in South Africa, known sources of resistance, i.e. the cultivar Edmund (race nonspecific resistance), should be used in a breeding programme.

1. Taylor JD, Teverson DM, Allen, MA, Pastor-Corrales MA, 1996. Plant Pathology 45, 69-78.