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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
Based on NARO and MAAIF zones, a survey was conducted using a random stratified
design (Elkinton, 1993). Zones and districts formed the first stratum of sampling, while
farmer fields were randomly selected using calculator random numbers to form the
second level of sampling. Five tomato fields (of about 0.25 – 1 ha each) were randomly
selected in each district.
3.2.1.1 Collection of Virus Symptoms-bearing Tomato Samples
For each of the five fields in every selected district, twenty leafy shoot samples were
picked from individual plants showing virus-like symptoms by the smart-sampling
approach (Bragard, 2006 personal communication), and were placed in paper bags. At
least five leaf curl samples were collected from each district, except for Kasese, where
neither leaf curl nor mottling symptoms were observed during the visit made in the dry
season (January-February 1998) until later at the beginning of the rain season (March-
July 1998), probably because inoculum-bearing plants had died and vector population
decreased to very low levels at the end of the previous growing season, a situation
referred to as random extinction ² by Garcia-Arenal et al. (2000). Ohnesonge et al. (1981)
report about death of whiteflies during the winter season, which is another good example.
In addition to the 800 leafy tomato shoot samples, suspected weed samples were also
collected from within and around tomato fields to determine tomato viruses host range
(Bock, 1982). Detailed field notes were made on virus symptoms, presence of vectors
such as aphids by simple observation, and varieties grown as well as general cropping
practices such as intercropping by interviewing farmers (Annex 3). Vector infestations of
tomato fields were recorded as low, medium, or high, if (≤25%), (25% < to 50%), and
(50% < to 100%) of tomato plants in the field were infested, respectively (Cho et al.,
1989). Virus symptom severity was scored on a scale of 1-5, based on extent of leaf
damage and percentage number of leaves showing symptoms, whereby 1=1-20% (very
mild); 2=21-40% (mild); 3=41-60% (severe); 4=61-80% (very severe); and 5=81-100%
(almost dead). Virus disease incidence, defined as the extent of infection in the field, and
calculated according to Allen et al. (1983) formula:
² In this report by Garcia et al., 2000, the world random seems to be used to mean sudden
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