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

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
2.1.1.1.1.1.1 Genus Begomovirus
Viruses in this genus were recognized in 1978 by ICTV as forming a distinct class with a
specific size, a geminate appearance, and with one or two species of single-stranded
DNA. Russo et al. (1980) confirmed the presence of geminate particles in tomato tissues.
Later, Czosnek et al. (1988) isolated and purified TYLCV-Is particles of 20 nm x 30 nm.
Navot et al. (1991) reported that TYLCV had a monopartite, circular, single-stranded
DNA genome of 2.8 kb, which is encapsulated by a coat protein. It has 2,787 nucleotides
with six open reading frames (ORFs), two viral-sense and four ORFs on the
complementary strand (Novot et al., 1991). Other scientists in Sardinia, and Italy
(Kheyr-Pour et al., 1991) confirmed these findings.
Tomato Yellow Leaf Curl Viruses
The major tomato virus in this group of viruses having monopartite single-stranded DNA
is Tomato yellow leaf curl virus (TYLCV) (sensu stricto). Symptoms caused by this
virus are chlorotic and leathery leaves, leaf curling, blistering, reduced leaf size,
shortened internodes, chlorosis of leaf margins, rounding of leaflets, flower abscission
and poor bearing (Cohen and Nitzany, 1966: Yassin, 1982; Makkouk et al., 1983;
Thomas, 1984).
According to Padidam et al. (1995), there are three distinct TYLCVs based on nucleotide
sequence comparisons. It is also considered that viruses of the genus Begomovirus,
which have nucleotide sequence similarity levels below 90 % are distinct from each other
(Padidam et al., 1995), although later on ICTV reported that this can only be concluded
when complete genome sequences have been compared (Fauquet et al., 2003), and not on
the basis of the intergenic region (IR) or coat protein gene alone. Similarity comparisons
have previously been done on the basis of the intergenic region and partial sequences for
other TYLCVs including isolates from Egypt and Israel, which are similar but different
from isolates from Spain (GenBank No. L 277081) and Sicily (GenBank No. Z28390)
(Noris et al. 1993; Antignus and Cohen, 1994), so there is need to characterize and
15