الفهرس 
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Abstract
The search included two sections, on the first section the samples were obtained from paddy fields at different stations of Al-Qalyoubia province. Ten sites were chosen in different areas of the paddy fields at five cities. The mean values of physico-chemical parameters of studied sites showed some differences in water samples of fields irrigated from Nile compared to those irrigated from water wells. There were marked monthly differences in the quantitative and qualitative composition of the algal communities at each site. A total of 114 genera were recorded with 32 genera of Chlorophyceae, 25 of Bacillariophyceae, 51 of Cyanophyceae and 6 of Euglenophyceae. The maximum number of genera (99) appeared in site (10) and the minimum (47) in site (5). In the second section the extracts of three algal species (Nostoc muscorum , Phormidium fragile and Chlorella vulgaris) had an anti-Pythium activity, whereas Phormidium orientale did not shown any obvious anti-Pythium activity. Pre-emergence damping-off of rice grains was greatly reduced when grains were soaked in algal metabolites before planting in soil infested with Pythium aphandermatum. Nostoc muscorum extracts improved grains emergence when subjected to the pytopathogenic Pythium aphandermatum isolate from 0 % to 100 %.