1307 KHON KAEN AGR. J. 45 SUPPL. 1 : (2017). KHON KAEN AGR. J. 45 SUPPL. 1 : (2017). แก่นเกษตร 45 ฉบับพิเศษ 1 : (2560). 1 Land Resources and Environment Section, Department of Plant Science and Agricultural Resources, Faculty of Agriculture, Khon Kaen University, Khon Kaen, 40002, Thailand * Corresponding author: phrueksa@gmail.com The appearance of allelopathic substances affecting the presence of Burkholderia pseudomallei Saowalak Somboon 1 , Chanikan Klinpratum 1 , Tippawan Phungpoo 1 , Kanogkorn Sawatwor 1 and Phrueksa Lawongsa 1* ABSTRACT: Besides highest incidence of melioidosis, an infectious disease in humans caused by Burkholderia pseudomallei, Northeastern part of Thailand is as well facing major problem with soil organic matter loss and fertility degradation under large sandy soil contents. In order to reduce the presence of B. pseudomallei in parallel with restoration of soil degradation, thus, this study investigated the population of B. pseudomallei in relation to the appearance of natural allelopathic substances from tamarind leaves residue and commercial allelopathic substances (gallic acid, tannic acid, and caffeic acid). The fndings showed that long-term continuous application of tamarind leaf residue has potential to improve soil fertility by increase soil organic matter in parallel with decrease the population of B. pseudomallei. In addition, crude water-soluble extract of tamarind leaf and commercial allelopathic substances have potential to reduce the population of B. pseudomallei under in vitro condition. Keywords: Burkholderia pseudomallei, melioidosis, organic residues, tamarind, allelopathic substances Introduction Burkholderia pseudomallei (earlier known as Pseudomonas pseudomallei), a gram-negative motile bacillus, is commonly found in soil and water in southeast Asia and northern Australia (White, 2003; Leelarasamee, 2000) and is the cause of melioidosis, a notoriously diseases in humans and animals (White, 2003). The infection is acquired by percutaneous inoculation or inha- lation (Yeager et al., 2012; White, 2003). In Thai- land, most of the cases occur in the northeastern part of the country. Thai farmers are high exposed to a risk of infection with B. pseudomallei by cu- taneous inoculation due to they rarely wear pro- tective footwear (Withatanung et al., 2016). The observation of 20% of community-acquired sep- ticemias caused by B. pseudomallei was found (Chaowagul et al., 1989) and the mortality is around 50% (35% in children). Besides highest incidence of melioidosis recorded in the world (Limmathurotsakul et al., 2010), Northeast Thailand is as well facing serious low soil productivity issue due to tropical sandy soils under agricultural land uses in Northeast of Thailand have undergone continuous fertility degradation as seen in low content of soil or- ganic matter. Nevertheless, restoration of soil degradation and increase in soil organic matter accumulation in degraded tropical sandy soil can be achieved by application of organic residues with appropriate qualities (biochemical composi- tion) (Puttaso et al., 2011; Vityakon, 2007). How- ever, different biochemical quality including al- lelopathic potential of some organic residue may affect microbial decomposition, nutrient supply