الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Citrus is one of the most important commercial fruit crops all over the world, it is grown in more than one hundred countries. Citrus is affected by variable diseases. Huanglongbing (HLB) or citrus greening is considered one of the most dangerous and destructive diseases of citrus trees all over the world. This disease is caused by Candidatus Liberibacter asiaticus (CLas), a phloem-limited bacterium. HLB is transmitted through different means; infected propagation sources, the parasitic plant dodder (Cuscuta sp.), and insect vectors in nature. The Asian citrus psyllids, Diaphorina citri, had been identified as the most potent insect vector for the transmission of the disease. HLB has very bad symptoms on citrus trees, those symptoms can be summarized as follows: asymmetric blotchy mottling, yellow leaves and shoots, corky veins, twig-dieback, also has bad effects on the fruits like small size, lopsided, bad taste (acidic and bitter), dry fruits have aborted seeds, fruit from HLB infected tree tend to fall prematurely and those that remain on the tree fail to mature correctly and retain their green color, hence the name greening for this disease, reduction of the fruit production, this disease has serious effects on the roots too, finally it causes tree decline Nearly all cultivated and wild citrus species are likewise sensitive to HLB in varying degrees. However, within individual species and among some citrus hybrid families, number of accessions can be found that display substantial tolerance. Some citrus cultivars released by the UF-CREC breeding program have exhibited tolerance to HLB. Here in, we aimed to evaluate field tolerance to HLB among citrus breeding populations at CREC-UF, as well identifying the most tolerant cultivars for that disease. On the other hand, identifying the transcriptomic, anatomical and metabolomic differences between some tolerant and susceptible cultivars in the field, also detecting the reasonable genes or chemical compounds that may be related to tolerance in some tolerant or healthy trees in the field, which may lead to the development of efficient breeding methods utilizing HLB tolerant germplasm in a citrus cultivar improvement program. In the first study, we examined HLB responses of 6609 citrus trees from 1248 crosses made or mutations induced by the UF-CREC breeding program. The evaluated trees had been divided to five grades (tolerant, good looking, medium, bad and dead) based on disease symptoms. Additionally, we focused on tolerant trees only to assess some characters that can be affected by HLB such as amount of production, also, fruit taste and seediness. 86 Summary On the other hand, real time PCR analysis had been used for detection of CLas titer in each tolerant tree, whereas some times those tolerant trees can be infected with the bacterium without any symptoms for that disease.