Chapter 9 Microbial Suppressiveness of Pythium Damping-Off Diseases Mona Kilany, Essam H. Ibrahim, Saad Al Amry, Sulaiman Al Roman, and Sazada Siddiqi 9.1 Introduction Soilborne plant pathogens causing wilts, root and crown rots, and damping-off are major yield-limiting factors in the production of fiber, food, and ornamental crops. Most soilborne pathogens are difficult to control by conventional strategies such as the use of synthetic fungicides. The lack of reliable chemical controls, the occur- rence of fungicide resistance in pathogens, and the breakdown or circumvention of host resistance by pathogen populations are among the key factors underlying potentials to develop other control measures. The search for alternative strategies has also been stimulated by public concerns about the adverse effects of soil fumigants such as methyl bromide on the environment and human health. Cook and Long (1995) postulated that many plant species have developed a defense strategy against soilborne pathogens that involves the selective stimulation and support of populations of antagonistic rhizosphere microorganism. Over the past century, evidence has accumulated that such plant-associated microorganisms M. Kilany (*) Biology Department, King Khalid University, Abha, 9004, Saudi Arabia Microbiology Department, National Organization for Drug Control and Research (NODCAR), Giza, Egypt e-mail: mona.kilany@yahoo.com E.H. Ibrahim Biology Department, King Khalid University, Abha, 9004, Saudi Arabia Blood Products Quality Control and Research Department, National Organization for Research and Control of Biologicals, Cairo, Egypt e-mail: essamebrahim@hotmail.com S. Al Amry • S. Al Roman • S. Siddiqi Biology Department, King Khalid University, Abha, 9004, Saudi Arabia e-mail: amri555@yahoo.com; sar200@hotmail.com; kalasaz@yahoo.co.in © Springer International Publishing Switzerland 2015 M.K. Meghvansi, A. Varma (eds.), Organic Amendments and Soil Suppressiveness in Plant Disease Management, Soil Biology 46, DOI 10.1007/978-3-319-23075-7_9 187