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Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases 2.1.3.1.1 Virus Transmission by Whitefly Vectors Whiteflies are vectors of viruses causing many diseases in the tropics and subtropics (Duffus, 1992). Bohmfalk et al. (2006) described the whitefly as a snowwhite insect measuring about 1mm in length. The adult whitefly starts laying eggs immediately after emerging from the nymph. Eggs are laid underneath leaves to protect them from adverse weather conditions such as rainfall and direct solar radiation (Marks, 2006). The adult is able to lay eggs for 3 weeks. Eggs hatch into nymphs, which measure about 0.5mm in length. Nymphs start by crawling about for short distances, but come immobile when they start feeding on the host leaf. Nymphs feed for 5-6 days, and after another 5-6 days, they go into pseudo-pupal stage for 2 days, after which they become adult flying whiteflies (Figures 2.2). Whitefly species Bemisia tabaci and B. argentifolii (Cohen et al., 1992; Bellows and Perring, 1994; Riley et al., 1995; Legg, 1996), as well as Trialeurodes vaporariorum and T. abutilonea (Larsen and Kim, 1985) transmit geminiviruses. Among these there many now known biotypes (Demichelis et al., 2000). There is variation between whitefly biotypes in the ability to colonize plants and transmit geminiviruses (Rataul and Brar, 1989; Legg, 1996). Whiteflies colonize plants by laying their eggs underneath leaves, where they hatch into nymphs. Ugandan cotton and sweet potato whitefly biotype strains are said not to reproduce on cassava (Legg, 1996). Neither their adults nor their nymphs survive on cassava unlike the Ivory Coast strain, but both their adults and nymphs survive on cotton (Legg, 1996). B. tabaci also infests other hosts such as Commelina benghalensis L. and Euphorbia heterophylla L. (researcher’s field observation). These plants are weeds found growing along with tomato, tobacco, cucurbits, and many other tropical crops. In spite of these efforts, B. tabaci ability to colonize tomato, tobacco and cucurbits in Uganda is not yet well known (Legg, 1996). 36
Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases A B C Adult Pseudo-pupae Whitefly life cycle 2 nd &3 rd Instars Figure 2.2: Whitefly vector (Bemisia tabaci): A- Winged adults and nymphs, B- Sketch drawings to give an impression of actual size of the adult whitefly and nymph (www.bijlmakers.com/guide), C- Whitefly Life cycle (www. 2.dpi.qld.gov.au/image) Whitefly vectors (B. tabaci) are phloem feeders and transmit tomato yellow curl viruses from chilli to tomato and vice versa (Rataul and Brar 1989; Jiang et al., 2000). However, according to Caciagli et al. (1995) transmission is only possible from tomato to tomato. Caciagli et al. (1995) and Rataul and Brar (1989) found that virus acquisition, referred to as AAP (access and acquisition period, or access to plant cells and sucking sap from it to acquire virus inoculum), takes 31 min to 24 hours, while virus inoculation, IAP Eggs 1 st Instar nymph 37
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Incidence, Distribution and Charact
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Charles SSEKYEWA Incidence, Distrib
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CONTENTS 1 CHAPTER 1...............
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ACKNOWLEDGEMENT The following perso
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8 ANNEXES Incidence, distribution a
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Table A.1.1 continued Incidence, di
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