ISSN 0367-6315 Korean J. Soil Sci. Fert. 46(2), 112-116 (2013) http://dx.doi.org/10.7745/KJSSF.2013.46.2.112 Effect of Fungicides on Phosphate Solubilization by Klebsiella oxytoca and Enterobacter ludwigii Buddhi Charana Walpola, Mi-Jung Keum, and Min-Ho Yoon* Department of Bio-Environmental Chemistry, College of Agriculture and Life Sciences, Chungnam National University, Daejeon, 305-764, Korea. The aim of the present study was to isolate phosphate solubilizing bacteria (PSB) and to assess their potential tolerance to fungicides. Out of thirty PSB, two strains Klebsiella oxytoca and Enterobacter ludwigii were selected on the basis of their tolerance to fungicides. Both strains were assessed for their phosphate solubilizing ability using three different fungicides (difenoconazole, fluazinam and streptomycin) each with the concentrations of 0, 1, 2 or 3 times of the recommended rate. Both strains showed increased phosphate solubilization with difenoconazole at 1, 2 and 3 times of the recommended rate as compared to the phosphate solubilization of the control. The phosphate solubilization in Klebsiella oxytoca was recorded as 326, 538, 518 and 481 µg mL -1 at 0, 1, 2 and 3 times of the recommended rate respectively, whereas in Enterobacter ludwigii it was recorded as 395, 499, 529 and 533 µg mL -1 respectively at various doses. Based on the present findings, it may be concluded that both strains have the potential to be used as bio-inoculants which can solubilize phosphate even at the higher doses as compared to the recommended rate of fungicides. Key words: Phosphate solubilization, Fungicides, Klebsiella oxytoca, Enterobacter ludwigii, Difenoconazole, Fluazinam, Streptomycin Received : 2013. 1. 10 Accepted : 2013. 2. 25 *Corresponding author : Phone: +82428217888 E-mail: mhyoon@cnu.ac.kr Article Introduction Use of agrochemicals (such as fertilizers, pesticides, soil conditioners and phytohormones) aiming at optimizing crop production has become very common in agriculture (Ahemad and Khan, 2011). Pesticides, the major plant protection agents include a wide range of compounds including insecticides, fungicides, bactericides, herbicides, rodenticides, molluscicides and nematicides (Aktar et al., 2009). Use of pesticides increased by 3-fold over the past several decades due to intensified agricultural systems (Fox et al., 2007). Many pesticides showed no detectable effects on soil microorganisms at the recommended application rates. However, application at increased rates is often reported as most of the farmers decide based on their own experience of the effective pest control. Repeated and overuse of pesticides in agriculture is a matter of concern because these chemicals are recognized as a source of potential adverse impacts on the metabolic activities of soil microorganisms as well as their plant growth promoting characteristics (Wani et al., 2005; Ahemad et al., 2009). Some microorganisms (called as phosphate solubilizing microorganisms-PSMs) perform phosphate solubilization. Their growth and phosphate solubilizing activity may also be affected by the pesticides leading to the imbalance phosphorous nutrition for the crop plants. However, the effect of pesticides on microbial growth and their activity especially phosphate solubilization can only be assessed using microorganisms which are tolerant to the pesticide of concern (Oves et al., 2009). Under this background, in the present investigation, an attempt has been made to examine the effects of different fungicides applied at various rates on Klebsiella oxytoca and Enterobacter ludwigii, the two phosphate solubilizing bacteria. Materials and Methods Isolation of phosphate solubilizing microorganisms Phosphate solubilizing microorganisms used in this study were isolated from the soils collected from green houses in Chungcheongnam-do, Gongju-gun area and abounded mines of Boryeong area in South Korea. In order to isolate, a serial dilution assay was carried out in 0.85% NaCl solution and 10 µl of