ABSTRACT
The major objective of our cucumber breeding program involves breeding for CMV disease resistance to reduce losses in quality and yield. Seven half diallel cross hybrids resistant to CMV were developed at the Vegetables Breeding Dept., Hort. Res. Inst., Egypt during March, 2006. Sixteen genetically different pure inbred lines of Cucumis sativus were used to develop the hybrids obtained from different sources and selected for their CMV disease-resistance trait. The inbred lines and one commercial [Beit alpha] cultivar as a susceptible control were examined to CMV resistance using biological, serological and molecular methods. The CMV isolate identified by Plant Path. Res. Inst. Virus and Phytoplasma Res. Dept. [ARC] was used in mechanical inoculation of all cucumber genotypes used during this study. The seeds of the genotypes were incubated and the seedlings were cultivated in foam trays with peat soil and kept under greenhouse conditions. At the cotyledon stage, i.e. before the development of the first true leaf, the seedlings were mechanically inoculated by rubbing with virus inoculum. Disease severity was assessed visually 7- 10 days [on cotyledons] and 14-25 days [on true leaves] after inoculation with CMV. The results revealed that six out of sixteen cucumber inbred lines [Cus 260/1980, 6-5-23-2 Kaha, 1-180-309- 18-105 Dokky, 5-57-22-17 Kaha, Cus 38/1991, and 25-2-1-90 Kaha] were found to be without systemic symptoms of CMV infection and proved to be resistant to CMV when tested by DAS-ELISA and RT-PCR. The promising accessions as sources of resistance have been intercrossed with leading commercial type [Beit-alpha] in half diallel system. In order to determine the genetic polymorphism and discriminate between cucumber inbred lines, RAPD-PCR analyses were conducted on the DNA isolated from each line. Dendrograms representing genetic distances were performed on the studied genotypes using the UPGMA [Unweighted Pair Group Method with Arithmetic Average]. Twenty one cucumber hybrids obtained from the half diallel crossing between the six resistant genotypes and the local commercial cultivar [Beit-alpha] were subjected to CMV artificial inoculation in a separate greenhouse and symptoms were visually monitored for two months. Only seven cucumber hybrids showed high a level of resistance to CMV were screened in the greenhouse and evaluated for CMV resistance. The resistant hybrids obtained did not develop visual symptoms of CMV infection on cotyledons and true leaves. These resistant lines could serve as potential sources of resistance in breeding programs