A DISSERTATION SUBMITTED IN PARRTIAL FULLFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF SCIENCE IN CROP SCIENCE OF SOKOINE UNIVERSITY OF AGRICULTURE.
MOROGORO, TANZANIA.
A study was done to enhance the understanding of the diversity of begomoviruses and to identify tomato varieties resistant to Tomato yellow leaf curl disease (TYLCD) that can be used by farmers in the coastal belt of Tanzania. Twenty four out of 225 diseased leaf samples collected tested positive for DNA-A component for primer pairs VD360/CD1266 and VD360/AC1048. No DNA-B or satellites were detected. Their phylogenetic tree constructed with other closely related sequence from the GeneBank showed four begomovirus clusters each with a distinct species indicating a great diversity of begomoviruses along the coast regions of Tanzania and Zanzibar Islands. The entire cluster IV formed a new species of begomovirus. Four of the begomovirus sequences namely ToLCVTz-Ch-4, ToLCVTz-Ch-7, ToLCVTz-Ch-9 and ToLCVTz-Ch-26 tested positive for DNA-A PCR amplification for primers VD360/CD1266, VC 2305 – F1/VC 442 – R1, C1 – 1700 – F2/C1 – 2422 – R2 and Int 1232 – F3/C1 – 1763 – R3. Their full length genomes were 2760, 2760, 2774 and 2767 base pairs, respectively. ToLCVTz-Ch-9 and ToLCVTz-Ch-26 were found to be variants of a completely new species. Phylogenetic analysis confirmed the viruses to display recombination. Betasatellites were not found to be associated with these begomoviruses. On the other hand, a total of 20 tomato lines were screened for resistance to TYLCD. Fifteen of the lines contained Ty genes which confer resistance to TYLCD. In the seventh week after transplanting, tomato lines AVTO1122, AVTO1130, AVTO1141 and AVTO1219 were considered resistant as they had a severity of below 2.4. Tomato Lines AVTO1080, AVTO0301, AVTO0922, AVTO1226, AVTO1229, AVTO1260, AVTO1010, AVTO1132, AVTO1008, AVTO1005, AVTO1143, Assila F1 and Mwanga were considered as moderate resistant with severity scores ranging from 2.4 to 3.4. The susceptible lines were line VI045743 and Tanya with severity scores greater than 3.5. Ty-2 was found to be the strongest gene to show resistance to begomoviruses while Ty-1 was the weakest gene. The study concludes that there is a great genetic diversity of begomovirus on the coastal areas of Tanzania which has been caused by recombination occurring in the begomovirus populations. Breeding resistant tomato lines is the most reliable and sustainable approach to withstand the begomovirus infections. Therefore more breeding for effective resistant tomato varieties is recommended.