الفهرس 
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Abstract
In this study, investigation was conducted at various locations br within the citrus growing orchads in Qalubia governorate (Qaha - br Kanater). ١٩٢ budwood samples thought to be infected with citrus br tristeza virus (CTV)were collected -#102;-#114;-#111;-#109; sweet orange orchards. br Two positive trees -#102;-#114;-#111;-#109; the total numbers were CTV isolates and br classified into mild to moderate severe via the symptoms on br Mexican lime plants. CTV isolates in Egypt were studied, through modes of br transmissions, biologically by host range plants indexing on br indicator plants and by aphids, serological detection using br polyclonal and monoclonal antibodies, electron microscopy, br polymerase chain reaction for cp٢٧ gene, tissue culture and cross br protection for virus control. The results showed that none of the br CTV-infected trees, even though grafted on sour orange, showed br decline or obvious tristeza symptoms. All CTV isolates induce br strong vein clearing, leaf cupping, seedling yellow and tolerant br when indexed on specific woody indicators seedlings: Cleopatra br Mandarin, Mexican lime, sweet orange, sour orange and Duncan br grapefruit. ELISA tests for all indexed plants confirmed the presence of CTV. The vector represent worldwide for CTV and exist in Egypt are both aphids (A.gossaypii, M. persicae), in semipersistent maner. The higher transmission rates after ٢٤ hr acquisition period for aphids on infected Mexican lime plants by severe CTV isolate are ٤٠, ١٠% by A.gossaypii, M. persicae respectively. None of CTVcharacteristics symptoms on Mexican Lime was appeared with br mild isolate after different acquisition periods.Coat protein gene detection in severe and mild isolates by br PCR amplification of the cp٢٧ gene with specific primers produced br a fragment with the same expected size (٥٨٠ bp)for both isolates br as revealed by electrophoresis in a ١% agarose gel. The effect of ultrastructure of sweet orange (Citrus sinensis br (L.)infected with moderate severe isolate of Citrus tristeza virus, br as well as the difference between moderate severe and mild isolates were illustrated upon examination of ultrathin sections of infected Mexican lime (Citrus aurantifolia (L.)by transmission electron microscopy. Attempt to provide resistance to the disease, a protocol for shoot tip grafting used to recovery infection of Mexican lime plants by moderate severe isolate of CTV, volkamariana seeds used as a br rootstock, has shown the best effect on virus elimination through tissue culture technique. Success of cross protection by mild isolate CTV preimmunization to prevent infection with moderate br severe isolate to Mexican lime plants.