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Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases In the case of Uganda poor tomato yield is attributed to three factors, (a) lack of improved varieties, which are well suited for high yield and resistance to diseases, (b) occurrence of pests and diseases, and (c) lack of knowledge on sustainable agronomic practices (Defrancq, 1989; Hansen, 1990; Mwaule, 1995). Tomato varieties Caribe, Magrobe, Pakmor and Tropic are reported to be acceptable to tomato farmers and consumers in Uganda, while tomato varieties VF 6203 and Peto-C-8100 159 with resistance to Verticillium spp. and Fusarium spp. are recommended for processing (Mwaule, 1995). Mwaule further confirmed the Asian Vegetable Research and Development Centre (AVRDC) tomato lines MT 40, 41, 55, 56 and 57 to be resistant to bacterial wilt (Ralstonia solanacearum). It is also reported that sustainable agronomic practices, such as plant spacing of 45 cm x 90 cm, mulching, staking and pruning to two leader vines, help to achieve better fruit quality and higher yields (Mwaule, 1995; Rice et al., 1987). Pests and diseases such as blight (Phytophthora infestans and Alternaria solani), bacterial wilt (R. solanacearum), root nematodes (Meloidogyne spp.), African bollworm (Helicoverpa armigera), thrips (Thrips tabaci), whitefly (Bemisia tabaci) and aphids (Myzus persicae) are reported to infect tomato (Defrancq, 1989; Mwaule, 1995). There are also a number of viral diseases whose symptoms have been encountered in many tomato fields in Uganda (Defrancq, 1989; Hansen, 1990). Virus-infected plants are typically stunted, crinkled and have small fruits, or no fruits at all. In some cases, infected plants show dieback symptoms before flowering (Hansen, 1990). Worldwide, of about 146 viruses belonging to 33 genera are reported to infect tomato, a majority 27 viruses belong to genus Potyvirus; 17 are Begomoviruses, 15 Nepoviruses, 9 Potexviruses, 8 Tobamoviruses, 6 Luteoviruses, and 6 Tymoviruses, whereas the remaining 67 viruses are distributed among genera Alfamovirus, Carlavirus, Carmovirus, Closterovirus, Comovirus, Cucumovirus, Cytorhabdovirus, Dianthovirus, Fabavirus, Furovirus, Ilarvirus, Ipomovirus, Luteovirus, Necrovirus, Nucleorhabdovirus, Ourmiavirus, Phytoreovirus, Sequivirus, Tobravirus, Tombusvirus, Tospovirus, Tymovirus, Umbravirus and Varicosavirus (Annex 4) 1 . 1 http://image.fs.uidaho.edu/vide/descr002.htm, 2003 4
Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases However, not much tomato virus research work has been done in the East African region, partly because of the costs involved in virus identifications. As such, the few available reports from neighbouring countries list only six tomato viruses: Tomato spotted wilt virus (TSWV) reported in Tanzania, South Africa, Zambia, Zimbabwe and Madagascar as early as 1969 (CMI, 1969); Tomato leaf curl virus (ToLCV) first reported in Sudan in the 1980s (Yassin, 1989); and Tomato yellow leaf curl virus-Israel (TYLCV-Is), Tomato mosaic virus (ToMV), Cucumber mosaic virus (CMV) and Potato virus Y (PVY) first reported in Tanzania, Zambia and Malawi in the mid-1990s (Nono-Womdim et al., 1996). It is not clear which of these viruses could be infecting tomato in Uganda. Defrancq (1989) observed that probably a complex of tomato viruses occurs in Uganda, and suggested that proper identification should be done as a basis for developing appropriate control technologies. Unfortunately, to the best of our knowledge there was no immediate follow-up on her findings, apart from the general one by Hansen (1990), until the present study was initiated in 1998. According to Bock (1982) and Green and Kim (1991), it is important that viruses occurring in a specific geographical area are identified and characterized prior to developing sustainable, environment-friendly disease management programmes. Consequently, this study was designed to determine which viruses infect tomato in Uganda. 1.1.2 The Research Problem Prior to this one, and as reported in 1.2 above, there have been only two general studies on tomato virus diseases in Uganda, i.e. by Defrancq in 1989 and Hansen in 1990. Reports from neighbouring countries indicate the presence of only six viruses, as mentioned in section 1.2 (CMI, 1969; Yassin, 1989; Nono-Womdim et al., 1996). It is, however, not clear whether only these viruses, which are found in neighbouring countries or many more of the 146 viruses reported to infect tomato worldwide (http://image.fs.uidaho.edu/vide/descr002.htm) occur in Uganda. Likewise, information 5
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Page 8 and 9: ACKNOWLEDGEMENT The following perso
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