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Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases The units of R are proportional to initial inoculum unit per unit time, if we integrate the above differential equation by: x = QRt (4), Furthermore, according to Thresh (1998), a virus disease tends to spread until a saturation level when there are no more plants to infect (100% infected). He went on to explain that disease spread tends to be arithmetic and not logarithmic for several reasons: (1) crop fields are finite; (2) there are spatial constraints; (3) host plants vary; (4) mature plants develop resistance; and (5) there are experimental effects imposed by applied treatments. Thresh (1998) indicated that spread is most effective from the local inoculum, which already exists on a few plants, outwards to the nearest hosts. He reported that experimental treatments could delay the build-up of initial inoculum and/or slow down disease spread. Thus, a susceptible variety would be infected at a higher rate than a resistant variety. Furthermore, disease occurrence and virus inoculum concentration are not directly proportional. The inoculum can occur as scattered foci, in which case virus inoculum is high or else occur as single foci, in which case virus inoculum concentration is considered to be low. Scattered foci are more damaging than single foci, because they are in contact with more healthy neighbours. Thresh (1998) further indicated that from the initial foci, spread can be either monocyclic or polycyclic. Monocyclic spread involves only one stage of disease spread, in that the disease develops on a plant, which eventually dies without the disease spreading out to other plants. A monocyclic situation arises when the vector is able to transmit a virus from weed foci to the crop, and not from plant foci to neighbouring plants within the crop. This then results into a localized high rate of infection at the early stages, which levels off and drops with time. Polycyclic disease spread involves successive cycles of disease spread, in that a disease that started with a few plants spreads out to other plants until the whole crop is infected. Polycyclic disease spread results into a sigmoid curve. Its upper limit is determined by host maturity, weather or lack of susceptible hosts. The latter is a more usual disease spread situation than the former. Thresh (1998) reported virus spread to be either from the crop itself 34
Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases (crop foci), or from an adjacent or distant field. Spread within the crop takes place whether virus transmission is persistent or non-persistent, while spread from distant fields is possible especially for persistent viruses. Thresh (1998) went further to report that if infection decreases from peripheral field rows to the centre, then the source of infection is outside the field. He further indicated that the smaller the field plot, the more the external influence of peripheral rows. Thus, an elongated field (rectangle) will be more affected than a square field having the same surface area. By considering the different biotic agents of disease spread, Thresh (1998) indicated that the more mobile the vector, the longer the risk distance along which it can transmit the disease. The risk distance is the path along which the vector that has acquired inoculum is able to move while still with potential to transmit the virus to other plants. The latter author also explained that planting in line and parallel to the prevailing wind direction could be expected to lead to less infection, because most vectors are blown away by wind. When planting is done in lines across the prevailing wind direction, vector movement is reduced. Sigiura and Bandaranayake (1975) working on viruses of chilli pepper found that two aphid genera are associated with virus transmission, i.e. genus Aphis and genus Myzus. Aphis fabae has an AAP of 60 min, and its optimum AAP is reported to be 12-16 hours (Roberts, 1940), while Myzus persicae has an AAP of 5 - 120 min with an optimum of 60 min (Sigiura and Bandaranayake, 1975). Sigiura and Bandaranayake (1975) further indicate that AAP varies according to the hosts involved. Aphids, whiteflies and thrips reported above also occur in Uganda (Baliddawa, 1990). Being that they transmit tomato viruses to crops in a range of other countries (Brunt et al., 1990), a similar situation is expected in Uganda. However, vector seasonality and ability to spread tomato virus diseases in Uganda, in space and time, is not known (Legg, 1996). This absence of data has affected development of appropriate management strategies for tomato viruses in Uganda (Varela, 1995). 35
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Incidence, Distribution and Charact
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Charles SSEKYEWA Incidence, Distrib
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CONTENTS 1 CHAPTER 1...............
Page 8 and 9: ACKNOWLEDGEMENT The following perso
Page 10 and 11: vii 7 mosaic virus (CMV), Alfalfa m
Page 12 and 13: ix 9 Consequently, our findings con
Page 14 and 15: SAMENVATTING Tomaat is een economis
Page 16 and 17: xiii 13 Beide genhomologie percenta
Page 18 and 19: xv 15 dit onderzoek geven ook aan d
Page 20 and 21: LIST OF TABLES xvii 17 Table Page 1
Page 22 and 23: ACRONYMS AND ABBREVIATIONS xix 19 A
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A A C D Incidence, distribution and
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8 ANNEXES Incidence, distribution a
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Table A.1.1 continued Incidence, di
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