Department of Plant Pathology, Kasetsart University, Chatuchack, Bangkok 10900, Thailand

Background and objectives
Bacterial pustules, caused by Xanthomonas campestris pv. glycines, is one of the most prevalent soybean (Glycine max) diseases in the central area of Thailand [1]. Pathogenic variation among population of this bacterium has been observed [2] where the distribution of strains within the host crop is largely unknown. We identify the strains of X. ;campestris pv. glycines detected on field-grown soybeans and determine strain aggressiveness occurring during the past 7 ;years.

Materials and methods
The study included selection of collection sites, sampling of infected leaves, isolation procedures, bacteriological tests, physiological strain determination and grouping of the strain.

Results and conclusions
One-hundred strains of X. ;campestris pv. glycines (KU-P-34001 to KU-P-34100) collected from various soybean-growing regions in Thailand were characterized for differences in biochemical and physiological characteristics and aggressiveness on three soybean varieties (Doikam, Nakornsawan 1 and SJ4). After numerical phenotypic features were analysed, those strains were determined into 10 type strains (TS) of which the most and least prevalent types were in TS 9 (24 strains) and TS 1 (1 strain), respectively. Ten representative strains of each type strain were selected using nonparametric methods to determine their virulent variability. When a ranking of strains for aggressiveness on all soybean varieties were considered, strain KU-P-34070 was the most aggressive (57.61% disease severity), strain KU-P-34035 and KU-P-34088 were moderately aggressive (19.03 and 17.19% disease severity, respectively) and the least aggressive strain was KU-P-34041 (1.15% disease severity). Variability in leaf lesion size and colony forming unit (cfu) of causal bacteria/lesion were also observed within 10 type strains tested. The most aggressive strains, KU-P- 34070 produced largest leaf lesion sizes and highest of cfu/lesion with 0.4X1.8 cm and 8.17X10 cfu respectively; where the least aggressive strain, KU-P-34041 produced smallest leaf lesion size and lowest of cfu/lesion with 0.27X0.11 cm and 3.33X10 cfu/lesion, respectively. The result also revealed that the number of leaf lesion or infected leaf area was the most conveniente to be evaluated for severity of soybean bacterial pustule regardless site and cfu of lesion. This is the first report of pathotypic strains of X. ;campestris pv. glycines which could be compared as a type strain for all soybean bacterial pustule occurring in the region.

1. Prathuangwong S, 1983. Tropical legume improvement. ACIAR Proceedings Series 8, 40-41.
2. Prathuangwong S, Amnuaykit K, 1987. Kasetsart Journal (Natural Science) 20, 353-359.