Incidence, Distribution and Characteristics of Major Tomato Leaf ...

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases 118
after turning over infested leaves is also not feasible, because many fly off before they are
counted.
To get an estimate of whitefly population density per plant per plot, the Kubwa trap was
inverted on one plant at a time for at least 3 minutes. Ten plants were sampled per plot.
The plant stem was shaken below to induce whiteflies to fly. Whiteflies escaping to the
translucent part of Kubwa trap would be trapped on the sticky surface of the bucket.
Trapped whiteflies were counted and removed from the trap. Data were taken on a
weekly basis starting from transplanting date.
5.2.3.1 TYLCV Incidence and Spread
Firstly, the number of plants expressing tomato yellow leaf curl symptoms was recorded
at different tomato growth stages, i.e. vegetative stage, flowering stage and fruiting stage,
per treatment plot. Spread of viral infection was determined in space and time by
measuring the distance between old and new infections in relation to the time interval
between detection of first symptoms and subsequent symptoms, in the field (Raccah,
1986). At each stage of disease spread, the number of diseased plants was recorded to
develop temporal and spatial patterns of disease-spread curves. Disease spread from the
first plant infected (single foci) to other plants in space was calculated using Allen’s
equation (Allen et al., 1983; Plumb and Thresh, 1983), in which distance x from the first
infection source to the new infection is calculated by:
Px = 1-exp (-x/x) (7),
whereby x is the mean distance between all new infections and their sources.
Spread in time was determined using the formula of Vanderplank, i.e. by:
dN/dt (8),