Life Science Journal 2012;9(4) http://www.lifesciencesite.com 3147 Evaluation of some Plant Growth Promoting Rhizobacteria (PGPR) to Control Pythium aphanidermatum in Cucumber Plants Ahmed M. Elazzazy 1* , Omar A. Almaghrabi 1 , Tarek A. A. Moussa 1 , Tamer S. Abdelmoneim 1,2 1 Biological Sciences Department, Faculty of Science (North Jeddah), King Abdulaziz University, P.O. Box 15758, Jeddah 21454, Saudi Arabia, 2 Suez Canal University, Faculty of Agriculture, Department of Agricultural Botany, P.O. Box 41522, Ismailia, Egypt. ahmedazazy8@hotmail.com Abstract: Twenty isolates of bacteria were successfully isolated from rhizosphere soil samples collected from different plant species growing at various locations in North Jeddah, Saudi Arabia to investigate their effects on Cucumis sativus L. cv. Marketmore growth and damping-off disease caused by Pythium aphanidermatum. The results of dual culture method showed that only three isolates out of twenty, which have a great antagonistic effect on the growth of P. aphanidermatum referring to inhibition index. The selected isolates were identified in two genera with three species as Bacillus subtilis, B. amyloliquefaciens and Pseudomonas aeruginosa, that causing inhibition in fungal growth by rate 38.1±3.8, 66±5.0 and 78.1±6.8%, respectively. Under greenhouse condition, the efficacy of different treatments with bacterial isolates and their possible combination were recorded a varied significant effect to suppress damping off symptoms caused by P. aphanidermatum. All treatments with the isolates of PGPR were recorded growth promoting effect in the absence of pathogenic fungus comparing with untreated plant. The best result in plant height, stem length, plant fresh and dry weight were recorded in treatment with B. subtilis (23.7±1.04 cm, 13.5±0.87 cm 0.62±0.02 g and 0.033±0.007 g), flowed by P. aeruginosa (19.3±1.15 cm, 9.00±1.0 cm, 0.61±0.07 g, 0.029±0.002 g) then B. amyloliquefaciens (17.0±1.30 cm, 8.97±0.90 cm, 0.53±0.01 g, 0.031±0.004 g). On contrast, all seeds were completely dead, when treated with P. aphanidermatum individually or in combination with PGPR isolates except two treatments, which used B. amyloliquefaciens and P. aeruginosa for coating seeds. Consequently, these two isolates have the potential not only to protect cucumber seedling against P. aphanidermatum infection but also to improve the plant growth parameters. [Ahmed M. Elazzazy, Omar A. Almaghrabi, Tarek A. A. Moussa, Tamer S. Abdelmoneim. Evaluation of some Plant Growth Promoting Rhizobacteria (PGPR) to Control Pythium aphanidermatum in Cucumber Plants. Life Sci J 2012;9(4):3147- 3153] (ISSN:1097-8135). http://www.lifesciencesite.com . 461 Keywords: Damping off, PGPR, Cucumber plant, Biological control. 1. Introduction Cucumber plants (Cucumis sativus L.) are one of the main greenhouse crops widely grown in Saudi Arabia. The total planting area for cucumber production decreased from 3636 hectares in 2006 to 3149 hectares in 2010, and production decreased from 242004 ton in 2006 to 220978 ton in 2010. Among all vegetable produced in greenhouse, cucumber production area decreased from 3.2% to 2.8% and yield decreased from 9.2% to 8.7%, respectively (Ministry of Agriculture, 2011). One of the major factors inhibits growth and yield of cucumber is damping-off caused by Pythium species. In most cases Pre- and post-emergence damping-off in cucumber is caused by Pythium aphanidermatum and other Pythium spp. (Abbasi and Lazarovits, 2006). In major Pythium can result economic losses in both greenhouse and field production systems under favorable conditions for disease development (Georgakopoulos et al., 2002; Abbasi and Lazarovits, 2006). The damage effects causing by Pythium spp. can increase in the association with some plant-parasitic nematode (Meloidogyne incognita), which known as complex diseases that can cause highly significant losses to field and greenhouse grown cucumbers and other cucurbits (Zitter et al., 1996; Koening et al., 1999). They are ubiquitous in soil and in water, distributed worldwide, and with very diverse host ranges. Pythium species include some of the most important and destructive plant pathogens, causing losses of seeds, pre-emergence and post-emergence damping- off. In addition to causing rots of seedlings, roots, or basal stalks, decays of fruits and vegetables during cultivation, storage, transit or at the market and serious damages of a wide variety of crops in KSA (Yu and Ma, 1989; Al-Sheikh, 2010). The part of problem can effectively be solved by the use of chemical fungicides. However the indiscriminate use of chemical fungicides is not recommended for the management of plant diseases because of their collateral adverse effects on the environment, along with negative effects on animal and human health. Moreover, their efficacy has been reduced by the appearance of microbial resistance (Sanders, 1984; Cook and Zhang, 1985) and their