الفهرس 
Aim of the WorkOnly 14 pages are availabe for public view
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Abstract
The results presented in this thesis indicated that 1209 isolates of pythia were isolated from the rhizosphere soils of 15 different plant species grown in different localities in
Egypt. Two hundred and thirty-eight isolates of P. oligandrum, representing 20% of the total Pythium isolates were obtained from these rhizosphere soils.
P. oligandrum populations fluctuated from 5 to 80 propagules per gram soil in the studied field over a 20-month periods. The population density during winter and spring was higher than those during summer and autumn. The total population of Pythium species increased
during the periods of cultivation of spinach and wheat. For mycelial growth, minimal, optimal and maximal temperatures of the 3 tested pythia were 10, 30 and 40°C, respectively. Mycelial growth occurred within the range of 6-9 with an optimum at pH 7. However, growth was inhibited at lower values. The effect of osmotic potential on mycelial growth at 25°C indicated similar responses for the tested isolates. No growth occurred at —1.00 MPa, whereas, at lower osmotic potentials (-0.13 — -0.27 MPa),
good growth was observed with an optimum of —0.13 MPa. zoospores production by selected 3 isolates of
P. oligandrum (El-U1122, El-U1123 and Ef-U1124) were generally high after 72 h incubation and then gradually decreased with time. The fungi tested started to produce zoospores after 24 h at 10-35°C but were inhibited at higher
or lower temperatures. At low temperatures (10, 15, 20°C) zoospores were produced only after 48 h. Low
temperatures required longer times for zoospore production than the higher ones. Production of zoospores by the studied pythia were inhibited below pH 5 but could occur within the pH range of
5-9 with an optimum at 7.0. In solutions prepared, using HCI or NaOH for pH adjustment, all the pythia tested
produced zoospores within a pH range of 5.5-9.b but their formation was inhibited at pH 10.5 (data not included). Distilled water (zero osmotic potential) was favourable
for good zoospore production by the tested fungi. In distilled water adjusted to different osmotic potential
between —0.13 MPa and —1.00 MPa with manitol, all isolates of P. oligandrum were able to produce zoospore in distilled water with zero osmotic potential up to —0.47 MPa osmotic potential but not above this range and production in distilled water was maximum. Oospores were produced over a temperature range of 15-35°C for the three isolates of P. oligandrum with an
optimum at 28°C. The pH range for oospore production ranged from 5.5-9. Oospores of P. oligandrum were produced at —0.13 to —1.65 MPa osmotic potential with optimum production rate at —0.27 to —0.47 MPa. These oospores were germinated between 15 and 35°C with an
optimum at 30°C and germinated over a pH range of 5 9.