The Solanum lycopersicum

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Tomato, Solanum lycopersicum belongs to the family Solanaceae and genus solanum, which has served the human being with many food plants. Almost 96 genra and 2800 species of Solanaceae have been reported within the subfamilies Solanoideae, Cestroideae and Solanineae (Nee et al., 1991). In the past the tomato was named as Lycopersicon esculentum while tree tomato was termed as Cyphomandra betacea but is now being replaced by the name S.betaceum. Some solanums in South America like cocona, pepino and naranjilla are also of importance (Heiser et al., 1999). According to FAO statistics, tomato (Solanum lycopersicum) has been cultivated over an area of 520 thousands hectares with an average production of 7420 thousand matric tons (FAO, 2002). This worldwide distribution of tomato reflects its importance and imperativeness. Tomato crop is perennial in nature and is being grown in moderate climatic regions as an annual. China is on the top position with an average production of 38 million tones, followed by US (25 million tons) and then Turkey, India and itlay (FAOSTAT, 2008). Suitable soil for tomato cultivation is deep loamy and well drained soils with PH of 6.2-6.8, but it can even grow in different types of soils. Due to warm season crop, tomatos do not tolerate the chill or frost.

Though tomato is native to South America, it was introduced in Europe by early explorers and was known as Apple of Love in France and Itlay. Thomas Jefferson, a former US president used to raise this crop for his guests during 1781. On the other hand tomatoes were not grown in US until 1835 because it was thought to be poisonous at that time. Tomato is very healthful due to its nutritive values. One medium size tomato provides with 57% of vitamin C, 25% of vitamin A and 8% of iron though it has only 35 calories. Besides this it has also been used in salads, pickles, juices, and bakery and so on. New cultivars appear in market each year expanding and improving disease resistance.

Tomato is the most popular in today's home garden but it is always not popular. This delicious crop is being faced by many threats in the form of pests, bacterial and viral diseases. There are certain geminiviruses affecting the tomato crops. One of the important virus affecting the tomato production is Tomato Yellow Leaf Curl Virus (TYLCV) was described in early sixties. TYCLV can infect 15 species in five different families.

In 1959, in Israel, the farmers were convinced by Government to replace the Marmande variety of tomato with money Maker for the purpose of export. After transplanting of this variety in the field, majority of the crop was affected by an unknown disease. Symptoms showed stunting, stiff shoots, smaller and deshaped leaves. Curling down of the leaves and upward curling of leaf margins was also observed. Young plants infected with this virus rarely produce any fruit (Cohen and Nitzany, 1960). Later on surveys were done and it was confirmed that this disease was transmitted by whitefly with the controlled transmission experiments in the laboratory. It was also observed that Marmande tomato was as susceptible as Money Maker to this disease which was found to be viral in nature (Cohen and Nitzany, 1960).This virus was named as Tomato Yellow leaf curl virus by professor I. Harpaz of the Hebrew university (Cohen and Nitzany, 1964). Similar disease symptoms were also observed in Jordan valley tomatoes as early as 1929 (Avidov, 1944). The attack or outbreak of this disease was always accompanied by a large population of whiteflies (Cohen and Berlinger, 1986). This virus was isolated in 1988 (Czosnek et al., 1988) while the geminate shape of the viral capsid was first observed in 1980 (Russo et al., 1980). It took another three years to make the replica and to chain this virus (Navot et al., 1991).

Being old world virus and its limited geographical distribution, it was not considered so important at that time until its outbreaks of B biotype of vector Bemisia tabaci in 1980 especially in European and Mediterranean regions (Czosnek et al., 1990) and also damaging in Middle East (Cohen and Harpaz, 1964, Cohen and Nitzany, 1966). Later on this virus established its position destroying all the crop of tomato during early seventies (Abak et al., 1991; Mazyad et al., 1979). This virus has been reported throughout the world in Israel, Itlay, Egypt (Gallitelli et al., 1991; Credi et al., 1989), in Asia including Taiwan, Thailand (Green et al., 1987; Thanapas et al., 1983) and African countries like, both eastern and western Africa including Nigeria, Mali and Senegal (Defrand et al., 1985), and Central America and Caribbean including Dominican Republic (Nakhla et al., 1994; Wernecke et al., 1995) have been reported of this virus. This virus contains small DNA virus with double particle shape due to which it is termed as geminivirus. TYLCV is only virus transmitted with whitefly having genome with only single circular DNA component, while all others have double component DNA genome. Due to its genome structure, this virus is given different names. Tomato Golden Mosaic Virus (TGMV) in South America. In Florida as Tomato Mottle Virus (TMoV) which is transmitted by silver leaf whitefly Bemisia argentifolii (Cohen et al., 1994) while in Australia and India it was referred as TLCV, Tomato Leaf Curl Virus. In US TYLCV has been established in Florida and also been found in Georgia, Louisiana, North Carolina, Texas and Arizona.

These all viruses have different symptoms on tomato (Green and Kallo, 1994) including different nucleotide structure and sequences. TYLCV symptoms do not appear immediately after the infection of virus on tomato. It can take two weeks to appear the symptoms after the virus inoculation. Plant become leathery with margins yellow and also curls upward (Fig. 1). The initial fruit loss and dropping of flowers are the general symptoms of the infected plants. Tomato plant becomes stunted and could easily be observed (Fig. 2). The earlier infection results in more loss of production.

Tomato Yellow Leaf curl virus (TYLCV) is one of the most important and sever disease which have a significant effect on tomato production (Abdel-saleem, 1999). This disease has a major economic impact, causing a decline in the income of the producers and growers and ultimately higher prices for consumers. Currently 125 million tons of tomatos are being produced worldwide of which china the principal producer is accounting almost quarter of the global production (FAS, 2009).

Biotype B of Bemisia tabaci serves as a vector for this disease (Mehta et al., 1994). The silvering of squashes caused by the Bemisia was observed as early as 1963 (Baery and Kapoller, 1963) and a very wide range of this insect indicate that Biotype B has been present in this region for a long time. The intensity of the transmission is very high (McGrath and Harrison, 1995). The acquisition time of this virus is between 20 to 60 minutes and inoculation feeding time ranges from 10 to 30 min. the persistence period of this period in the vector is generally 10 to 12 days. This virus develops with in plants in phloem for which symptoms appear after almost 15 days of inoculation (Ber et al., 1990). Bemisia whitefly can give up to 15 generations per year in Jordan valley due to favorable environmental conditions in that area and economic loss to vegetable crops due to Bemisia tabaci in Israel was confirmed in 1931 (Avidov, 1944).