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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
(inoculum access period, or the period during which a vector pierces cells and infects
plant with virus inoculum), takes 32 min to 24 hours. In contrast, Ber et al. (1990),
reported an AAP of 48 hours, and that infected plants take a maximum of 15 - 29 days to
show symptoms. However, it was found that there is a need for a pre-and postacquisition
fasting period of at least 1 - 2 hours to enhance transmission efficiency.
Cohen and Nitzany (1966) reported a latent period of 21 hours before the vector transmits
the virus. Caciagli et al. (1995) found that the latent period is 17 - 20 days. They also
established that the virus persists in the vector for 20 days. These scientists seem to agree
that whitefly transmission is persistent, i.e. able to infect plants with virus inoculum all
the time, whereas inoculum is not carried over to their offsprings.
Furthermore, Caciagli et al. (1995) found that virus acquisition is more efficient than
inoculation, and females are more efficient in transmission than males, while nymphs are
as efficient as adults in acquiring the virus, but of little epidemiological importance
because of being immobile.
Rataul and Brar (1989) determined transmission efficiency by use of χ² analysis of data
from number of plants infected and expected number of plants to be infected. Other
methods of analysing transmission are through calculating the probability of disease
transmission (Rataul and Brar, 1989) by:
p = 1-Q ¹/k (5),
whereby p is probability of disease transmission by single vector, Q is observed fraction
of non-infected plants; and k is number of insects used per plant.
Furthermore, the effect of IAP on transmission by a single vector is determined by
expressing the growth rate of increase in infection, which is expressed as (dp/dt), and is
said to increase linearly with the proportion of uninfective insects.
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