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المستخلص SUMMARY a) Effect of the fungicides Vitavax Captan a~d Topsin M 70 on total bacterial count: The application of fungicides gradually inhibited the rhizosphere microflora of cotton plants, at the early stages of plant groWth. This was found with the normal recommended dose and this trend lasted till 15 days from sowing, then microorganisms gradually increased till the end of the experiment where the total bacterial count approximated that of control with Vitavax Captan and surpassed that of control with Tpsin M 70- • However, with both fungicides the harmfUl effect of fungicides on the total count was more obvious at the higher dose (10 fold) than the lower dose (normal dose) and in the uncultivated than cultivated soil. b) Effect of fungicides on spore-former bacteria: The applioation of fungicides reduced the density of $pore-former bacteria. M±nimal spore-former counts were obtained after 15 days from sowing. At the normal dose of Vitavax Capt an , spore-former count in the rhizo138 sphere of cotton plants surpassed that of control after 90 days from sowing. However, the effect of Vitavax Captan was more drastic on the spore-former count in the uncultivated soil. Topsin M 70 greatly decreased sporeformers counts at both ooncentrations used in the cultivated and uncultivated soils allover the experimental period. c) Effect of fungicides on actinomycetes count: Fungicides application reduced the,actinomycetes count. ~he extent of inhibition was in line with the rate of application. By the end of the experiment; at the normal dose of fungicides, actinomycetes count mostly surpassed that of control. This was found with both fungicides in the cultivated and uncultivated 80ils. With the higher dose of fungicides, actinomycetes count at the end of the experiment approximately levelled that of control. d) Effect of fungicides on fungal count: The application of fungicides highly decreased the density of fungi in the rhi.zosphere of cotton plants and in the unoultivated 8011 till the end of the experiment (90 days from sowing). This was found with both f’ungicides. The reduction in fungal count was in line with the rate of’ application. e) Effect of fungicides on anaerobic nitrogen fixers count: The application of Vitavax Captan at the normal dose deoreased the anaerobic nitrogen fixers count in the rhizosphere of cotton plants till 7 days from so~ then anaerobic nitrogen fixers started to increase and ,counts surpassed that of control after 30 days from sowing and thereafter. In case of the 10 fold dose of Vitavax Captan and both rates of Topsin M70% application, anaerobic nitrogen fixers counts were lower than their respective control in the majority of the investigated samples till the end of the experiment. f) Effect of fungicides on aerobic nitrogen fixers oount: This group was also affected by the application of fungicides. At the normal dose of fungicides application, Azotobacter count decreased in the early stages after fUngicides application, then counts levelled or even surpassed their respective control. !he higher dose of the fungicides greatly dec~ased. Azotobacter count in all treatments allover the experimental period. g) Effect of fUngicides on nitrifying bacteria: Both fungicides showed drastic effect on nitrifiers count in the cultivated and uncultivated soils. A- Effect of the fungicides Vitavax Captan and Topsin M70% on pathogenic fungi, in vitro. The minimal concentrations of the fungicides Vitavax Captan which completely inhibited the growth of the pathogenic fungi,in vitro, were: _ 500 ppm of Vitavax Captan were necessary for the complete inhibition of Fusarium 0xYsporum. _ 100 ppm of Vitavax Captan completly inhibited R.solani. _ 5 ppm or lower of Vitavax Captan completely inhibited the growth of ~.rolfsii. The minimal concentrations of the fungicide Topsin It70%, which completely inhibited the growth of the pathogenic tungi, in vitro I were _ 1000 ppm of Topsin II70% were required for the complete inhibition of R-.solani. _ 100 ppm of Top.in JI70% completely inhibited ~.oxysporum _ 10000 ppm of Tops1n • 70% were not sufficient for the complete inhibition of -Scl.rolfsii. B- Effect of fungicides Vitavax Captan and Topsin M70% on f’ungal count in soil and rhizosphere of cotton plants, sown in sterile soil infested with the pathogens R.solani and ~.rolfsii. The application of fungicides, decreased the fungal count in sterilized soil infested with the pathogens. The effect was more dras~ic in the uncultivated soil. The toxic effect was obvious after 7 days from fungicide application and thereafter. V1tavax Captan seemed to be more toxic than Topsin K 70’ • c- Effect of fungicides on reducing the dampLng-off in cotton plants sown in sterilized soil infested with the root-rot pathogens. Fungicides application increased the percentages of germination and survival plants and a1most decreased the the pre- and post-emergence damping-off percentages as compared to their respective oontrol (sterilized Boil infested with the pathogen, without fungicide application). This was found with both fungicides and for all investigated pathoge:cs. 14.3 on fungal counts in cultivated and uncultivated soils during 7-15 days from application. At the later stages, fungal counts increased, but were always lower than oontrol in the rhizosphere of’ the cultivated and in the unoultivated soil. Effect of soil infestation with ~.rolfsii and fungicides applioation on microbial counts: Soil infestation with Scl.rolfeii increased the total baoterial, actinomycetes and fungal counts. Fungicides application decreased the bacterial, actinomycetes and fungal counts in the cultivated and uncultivated soils. The extent of inhibition was in line with the rate of application. IV.A- Antagonistic fungi: The study showed that out of 675 fungal isolates investigated, 170 isolates antagonized R.so1ani. Out of these antagonists, 82 isolates were potent antagonists against R.solani. All the light brown Aspergillus isolates (30 isolates), the Trichoderma isolates (16 isolates) and 80% of the 25 isolates of the olive green Penicillium were potent antagonists against R.so1aniThe study showed that out of 675 fungal isolates investigated, 150 isolates antagonized Scl.rolfsii, out of these, 83 isolates were potent antagonists. All the investigated isolates of the light brown Aspergillus (30 isolates), olive green Penicillium (25 isolates) and Trichoderma (16 isolates) proved to be potent antagonists against Scl.rolfsii. B- Antagonistic bacteria: 1- Bacterial antagODi.sts against !.aola.ni: The study indicated that out of 722 bacterial isolates investigated. 160 islates antagonized !.solani, out of these antagonists -. 34 isolates highly antagonized !.solani. Most of the potent antagonists were found to belong to genera Pseudomonas and Bacillu8. 2- Bacterial antagonists against ~.ro1fsii: The study showed that out of 722 bacterial isolates, 171 isolates antagonized Scl.rolfeii, out of these antagonists, 54 isolates proved to be efficient antagonists against Scl.rolfsii. The preliminary identification showed that most of the pGtent antagonists belonged to genera Pseudomonas and Bacillus. c- Antagonistic actinomycetes: 1- Actinomycetes antagonists against ~.solan1: The investigation showed that out of 627 actinomycetes isolates investigated, 281 isolates antagonized !.solani, out of which, 122 isolates proved to be potent antaggonists. ~he maJor! ty fit the coloured aetinomycetes showed higher percentages e£ antagonists than non coloured actinO]Q”cetes • Among the coloured antagonists, 86.~ of the violet antagonists, 80% of the yellow antagonists and 66.7 of the orange pis-ented antagonists were potent antagonists against R.solani as compared to their respective groups. 146 2- Antagonistic actinomycetes isolates against Scl.rolfsii: The study showedthat out of 627 actinomycetes isolates investigated, 279 isolates antagonized Scl~rolfsii. Out of these antagonists, 99 isolates were found to be potent antagonists. Many of the coloured actinomyceteBshowed hi higher percentages of antagonists against Sel. rolfs!1 than uncoloured actinomycetes ’ • Amongthe coloured antagonists, 57.3%of the grey antagonists and 52%of the violet antagonists proved to be potent antagonists against ~.rolfsi1. Effect of fungicides on the potent antagonists, in vitro. The potent antagonist, Aspergillus ap. isolate was very sensitive to Vitavax Captan, but could tolerate high concentrations of Topsin 1170%. !he potent antagonist Penicillium. ap, isolate was sensitive to Topsin ]I 70 than Vitavax Captan. The efficient antagonist, Trichodermaap, isolate was 8ensitiTe to Topsin • 70, but could tolerate moderate ooncentrations of Vitavax Captan. 147 BVitavax Captan was more toxic, than Topsin M 70, to the potent bacterial antagonists. Vitavax Captan inhibited 4 bacterial antagonists at very low conoentrations, while the fifth isolate tolerated 10,000 ppm. It was found that potent antagonists belonging to the same genus differed in their tolerenoe to different fungicides. Antagonistio actinomyoetes showed more sensitivity to Vitavax Captan than Topsin JC70. Potent antagonistio aotinomyoetes isolates having the same pigment (colour) differed in their tolerence to different trmgicides. Biological oontrol &8 compared to chemical control of damping-off in cotton: Seed inoculation with the potent antagoniat,Aspergillus ap. isolate ~incr.a8ed the percentages of germination and survival plants ana decreased the damping-off percentages as oompared to oontrol. J.48 Seed inoculation with the potent antagonist, Penicillium sp. isolate was not efficient 8S the Aspergillus sp. isolate inoculation. However, seed inoculation with a mixture of the two potent antagonists (Aspergillus sp. isolate + Penicillium sp. isolate) showed the greatest effect than when any of the antagonists was inoculated solely. The study showed that biological control nearly levelled chemical control of damping-off in cotton. |