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Abstract SUMMARY Macrop.10mina phaseolina, Rhizoctonia solani and Sclerotium Iolfsii are considered the most common destructive patho1rens that cause root-rot, charcoal-rot and damping-off diseases on sunflower. Fusarium Oxysporum is considered also one of the most pathogenic fungi that infect roots of sunflower plants causing damping-off, wilt, and sometimes rClot-rot. The present investigation was planned to study the causal orga~isms of sunflower root diseases, the effect of some physiological and environmental factors on the isolated fungi and their control. The attained results could he summarized as follows: 1. Macrol’homina phaseol ina, Rhizoctonia solani, Sci eroti um rolfsii and Fusarium Oxysporum were isolated fom diseased sunflower plants collected from different Goverlorates in A.R.E. Fusarium Oxysporium were more frequ~ncy isolated than the other fungi. 2. Patho~enicity tests proved that S. rolfsii was the most destluctive fungus causing high percentage of pre-or post-emergence and root-rot. M.pbaseolina was less effective in the pre-emergence phase hut becomes more serious in the post-emergence phase and later stages of -g-rol’<t:h c au s’Ln c charcoal-rot and root-rot after - 116 - relatively long time. 3. Pathoger.icity tests of the other fungi showed generally that t.heae fungi could be arranged descendingly according to their virulence in the pre- and post-emergence phases is follows: S.rolfsii, F.oxysporum, R.solani and M.pbaseolina. However. 4. The conbinations of fungal inoculations revealed that all cOllbinations included S.rolfsii showed very high percen1.ages of pre- and post-emergence phase, which may indica’:e its synergistic effect to other tested fungi. Other .louble combinations including M.phaseo1 ina showed low pe~centage of pre- and post-emergence phases, which may ir.dicate its antagonistic effect to other tested fungi. However, when all the isolated fungi were used, the p,~rcentages of pre-, post-emergence and diseased plant~ were high. 5. Under greenhouse condi tions, the bacterial, actinomrcetes and fungal counts, including the appearance of T1 i choderma sp. and the hi gh frequency of Peni ci 1- lium spp., Aspergillus spp. and the low frequency of Fusali um spp., in the rhizosphere region of Giza 2 cv . were more than that in the rhizosphere of Miak cv. duriIlq the seedling and vegetative stages and thus the perc~ntaqes of pre- and post-emergence phases were lowe~ than that of Miak cv. In the second and third .- --- - - _.--’-- _.- - -”- - .- -- - 6. - 117 - stages of plant growth (flowering, seed formation and maturity I the former count was reversed and hence the percenta;re of infection with wilt and/or root-rot was lower in Miak cv. than Giza 2 cv. Generally, the rhizosphere effect increase by increase of plant age. In v i t:r o, three out of the four bacterial isolates isolatec from the rhizosphere region of sunflower cultivars a.nd identified as Bacillus sp . were antagonistic to S. ro : fs i i and M.phaseol ina and reduced the growth of the twc fungi. On the other hand, these bacterial i so Lat.e.s were not antaogonisti c to R. solani and did not cause a~y growth reduction. The other 4 Bacillus isolates w er e antagonistic to the wilt fungus F.oxysorum and caused growth reduction. 7. Additic,n of Trichodermia harzianum isolated from the rhizQsphere region, to soil infested with the 4 tested fungi :’educed the pre- and post-emergence phases and the pe:~centaqe of diseased plants compared with soil i.nfes t e.d only with the fungi. This trend was found in both cllitivars under study. 8. Labora.:ory screening of various fungicides for the control of the 4 sunflower pathogenic fungi, showed that the 4 fungicides tested gave partial or complete inhibition to the fungal growth. The inhibition power varied among the fungicides and the concentration used, - 118 - as thi s :)ower increased with the increase in fung icide concentration. The fungicides Rhizolex and Homai 80 were t.he most effective fungicides against the 4 pathogers, on the other hand, Topsin M and Monceren were less effective against the pathogens. 9. In vi t.s:o experiments were confirmed by in vivo experiment. Soed dressing with the fungicide Rhizolex (3g/kg seeds) Jave the best results in controlling the preemergence damping-off phase and reducing the percentage of diseised plants, followed by Homai 80, Monceren and Topsin M. These results were true for both cultivars under study. 10. Giza 2. Miak, Hybrid ”All., Hybrid ”B” and Hybrid ”en sunflo~er cultivars were tested under field conditions for thl~ir suscept ib i1ity to pre- and post- emergence dampin~’-off, root-rot and wilt diseases and the results showed that Hybrid ”B” and Hybrid ”crt were highly susceptib ie to the pre- and post-emergence phases, whi Le Miak end Giza 2 cv s . were tolerant to both phases. Hybrid ”A” was susceptible to the pre-emergence phase and irrmune to the post-emergence phase, Hybrid ”B” and Giza 2 cvs. were highly susceptible to wilt disease and resis~ant to root-rot disease. On the other hand, Hybric. ”Cit and Miak cvs. were resistant to wilt infection Lod hiqhly susceptible to root-rot infection. -- --- -- --- - - - - ---- ----------------~--- - -,- _.-’-’ - ._. - - - 120 - 14. Generally, under field conditions it could be recommended to treat Giza 2, Miak and HYbrid ”A” seeds cUlti’rars with 3g/Kg seeds of either Rhizolex or Homai 80 or Topsin M or Monceren for obtaining best yield comporents (Head diameter, seed yield/plant, weight of 100 s~ed and number of seeds/laO g seeds). These fungicides were also to some extend good in controlling the di3eases under field conditions. 15. Plants infected with F.oxysporum have less seed oil conten1 and lower iodine value than the healthy ones. On the other hand, infection with the wilt fungus caused an increase in the acid number of diseased seeds. Diseased seeds showed also a change in the normal col’ur of oil except in case of Miak cultivar . |