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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases 119
whereby N is the number of infected plants, t is the unit of time, dN is the difference
between that number of plants infected, and dt is the difference in time, from when the
previously infected plants were observed to the currently recorded ones. Using
Vanderplank formula (7), the rate at which tomato yellow leaf curl virus (sensu lato)
spreads in the field was established.
5.2.4 Yield Records
Mature fruits were harvested weekly and using a clock weighing scale range of (1g-
10kg), yield was determined per plot. Note was taken of number of fruits harvested per
treatment, total yield, marketable fruit weight and unmarketable fruit weight. Fruits were
considered marketable when they had grown to maturity and had neither sign of damage
nor rot.
5.2.5 Data Analysis
Data were analysed for ANOVA. Linear regression and Pearson correlations were
calculated using SAS and SPSS 11.0 statistical programmes. Duncan’s Multiple Range
Test was also used. Sometimes General Linear Models, Least Square Means Procedure,
or square root transformation was used to separate means, in case significant differences
were not easy to determine. Means generated were used in Excel to develop related
graphs. Standard error values (SE) were used as basis to determine variability within
treatment and date effects, while F-distribution values were used to determine
significance of differences between those effects (Mead and Curnow, 1990).