Tomato Yellow Leaf Curl Virus Tylcv Infection Biology Essay

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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 now it is 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 with a PH of 6.2-6.8, but it can even grow in different types of soils. Due to warm season crop, tomatoes do not tolerate the chill or frost.

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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 (TYLCV) 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 the only virus which is transmitted through 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.

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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.

Fig.1.jpgFig. 2.jpg

Fig. 1 Fig. 2

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 tomatoes are being produced worldwide of which china the principal producer is accounting almost quarter of the global production (FAS, 2009). TYLCV has a significant number of host range but Tomato served as a major host plant for this disease. Some other important hosts for this disease include spider vine (Boehavia erecta), beans (Phaseolus vulgaris), tick weed (Cleome viscose) and knot weed ( Polygonum spp.)

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 but this disease can be transferred in seed or soil (McGrath and Harrison, 1995). Due to piercing and sucking mouth parts, the acquisition time of this virus is between 20 to 60 minutes and inoculation feeding time ranges from 10 to 30 minutes with a persistence period of generally 10 to 12 days. High temperatures with less or no rain, whiteflies, weeds and infected transplants are the factors that favor the development of TYLCV. 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). Though the nymphal stages of silver leaf whitefly have the capability to acquire the TYLCV from the infected plants but only the adults are involved in the transmission of virus disease in crops.

This disease, TYLCV is silent killer and this is one of the factors for affecting the production and quality of tomato in the world. This very important crop though has very low yield in tropical and subtropical areas in comparison with temperate areas. The average production of tomato during the year 1980 was estimated to be 55 tons/ha for USA, 52 tons for Japan, 9 tons and 10 tons/ha for Philippines and Thailand consecutively (Jones et al., 1993). TYLCV is a limiting factor in tomato production due to loss of tomato plant in early stage of infection by this virus.

TYLCV causes severe stunting in plants. Besides the downward cup shaped form of leaves immediate after infection, leaves shape becomes abnormal afterward. Upward movement of the leaf margins is also observed. Vector resistance to insecticide may occur, when insecticide are used to reduce the attack of Bemisia tabaci, a vector for TYLCV. Irrespective of the insecticide treatment, there are certain other methods to suppress the population of Bemisia tabaci and ultimately the attack of TYLCV. This method includes cultural practices; seedling planting dates can be adjusted when the population of whitefly is high both in green house and open fields (FAO, 1998). Due to fact that the control and management of TYLCV is expensive and have limited options, tradional methods of controlling vector is being used (Cohen and Anderson, 1994) by insecticides treatments. But these methods have certain limitations due to the development of resistance in vector against the pesticide which ultimately results in the hazardous environmental effects (Palumbo et al, 2001). Furthermore insecticide are only the solution when there is a chance of too high vector population. Other methods which are used to control TYLCV are physical barriers and screens, which could result in additional production cost and problems of shading, too much heat and high humidity respectively. So it is best suggested breeding crops which are resistant or tolerant to TYLCV so that the yield losses due to this virus can be reduced.

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Zhenxing in 1999 conducted an experiment selecting 13 TYLCV resistant sources and AVRDC breeding lines (Table. 1). In his experiment tomato were sown in November and transplanting was done after one month of sowing with normal cultural practices.

Rating Scale: Zhenxing rated the TYLCV with following severity scales.

Light leaf yellowing

Moderate plant stunting and leaf culture and yellowing

Severe plant stunting and leaf curling and yellowing

Disease index (DI) was calculated by following

DI= (n1+2.n2+3.n3).100/3.N

ni=is the diseased plant in each grade

N= Total number of plants

The disease was rated at vegetative growth, 50 % of flowering and fruit maturation.

The data was collected on mean fruit weight, thickness and firmness, its shape and soluble solid contents. In his experiment during vegetative growth T1, TYLCV was observed after 25 days of planting. Out of thirteen varieties, four varieties started developing symptoms while remaining varieties did not show any symptom. In T2 of 50 % flowering stage one more variety showed symptom. Similarly in T3 of fruit maturation stage after 65 days of transplanting disease index ranged from 0% to 60% with the check cultivar with 10.7 % index. Out of thirteen varieties, nine varieties showed symptoms of TYLCV from T1 to T3 (Table. 2)

Table. 1 Screening for tomato yellow leaf curl virus resistance sources.

AVRDC accession

Origin

Reported resistance

BL 937

H24(India)

R

BL938

H36(India)

R

BL932

TY52 (Israel from Dani Zamir)

R

Hirseptyle

France(Laterrot)

R

Avinash#2

Novartis hybrid

R

FLA 438-17

Florida (J. Scott)

R

FLA 456-17

Florida (J. Scott)

R

FLA 582-17

Florida (J. Scott)

R

TY-king

Royal sluis hybrid

R

TY-8487

Hezora seed comp.

R

CLN 2117dcl-26-19-15

AVRDC breeding line

R

TLCV (271/1 x 26)-1

Breeding line from Dr.Deshpande, IIDR, India

R

CL 5915-93D4-1-03

Susceptible check

S

Table. 2 Tomato TYLV disease index and fruit characteristics

Entry

Disease Index

Fruit characteristics

T1

T2

T3

Thickness(cm)

Firmness(g/cm2

Soluble solids(%)

Mean fruit wt(g)

Shape index(length/width)

BL 937

0.00a

0.00b

0.00c

0.32f

1173d

4.67f

57

0.82

BL938

0.00a

0.00b

0.00c

0.33f

1460cd

4.47g

89

0.83

BL932

0.00a

2.22b

22.22b

0.67bc

2225a

4.87ef

72

1.38

Hirseptyle

2.92a

18.89aa

60.00a

-

-

-

-

-

Avinash#2

0.00a

0.00a

5.56bc

0.70ab

2230a

4.87ef

130

1.03

FLA 438-17

1.11a

3.45b

4.67bc

0.70ab

2133ab

5.40c

121

1.00

FLA 456-17

0.00a

0.00b

3.33c

0.33f

1490cd

6.13a

52

0.84

FLA 582-17

0.00a

0.00b

0.00c

0.59de

1300cd

5.00de

66

0.92

TY-king

0.00a

0.00b

0.00c

0.58e

1403cd

4.93e

120

0.79

TY-8487

0.00a

0.00b

11.33bc

0.72a

1667cd

4.40g

191

0.84

CLN 2117dcl-26-19-15

2.22a

3.33b

3.33c

0.70ab

2510a

5.00de

71

1.19

TLCV (271/1 x 26)-1

0.00a

0.00b

2.22c

0.63cd

1723bc

5.60b

123

0.95

CL 5915-93D4-1-03

2.22a

3.33b

10.67bc

0.60de

1467cd

5.20d

50

1.05

Asian Regional Centre-AVRDC, Report 1999.

Scale development and screening to evaluate the TYLCV

The removal or control of vector is not the solution of TYLCV, as vector resistance against these insecticides develops in Bemisia tabaci. So in spite of this, it is best to develop/or breed resistant tomato cultivars to avoid TYLCV (Morrales, 2001). In this method genes from the wild resistant tomato are transferred to field cultivated tomatoes because at earlier, these wild species were screened for resistance against TYLCV.

As we compare the controlled green house inoculation with spontaneous field inoculation, it's not reliable to concentrate only on spontaneous inoculation because many of the field plants may escape infection even under heavy inoculation (Vidavsky et al., 1998). It could be misleading if we select the susceptible plants during screening as resistant which were actually escaped while inoculation and thus could be used for further crosses as resistant parents. So relying only on the symptoms while selecting the resistant tomato plants could be wrong (Vidavsky et al., 1998).

While inoculation in green house also have some limiting factors. White fly may not inoculate a particular cultivar and may prefer to feed on another plant just creating a problem of non preference. Waxy or thick cuticle and certain other physical barriers may contribute in non preference of white fly. (Bellotti and Arias, 2001). So individual inoculation in cages may prevent the non preference in white fly.

v.jpg

Mass inoculation of young tomato seedlings. (A) Whiteflies immediately after landing on

the tomato plants, prior to moving to the abaxial side of the leaf. (B) Close-up on plants from A.

(C) Inoculation access feeding of whiteflies on tomato seedlings.

Clip cages could be used while mass inoculation with large number of population of white flies to maintain the accuracy of white flies on leaves.

f.jpg

(A) Side view of the clip cage attached to a

Tomato leaf. (B) Bottom view of the clip cage, showing the caged whiteflies feeding from the tomato

Leaf abaxial side.

Lepidot et al in 2006 developed a scale for the evaluation of TYLCV resistance level in tomato.

In green house: They used an insect proof green house to maintain the cultures of tomato yellow leaf curl virus isolates in tomato. Biotype B of the whiteflies were also reared and maintained in an insect proof green house. These adult whiteflies were then given 48 hours acquisition access to infected TYLCV plants following with another 48 hours of inoculation to the scale tomatoes. Almost 50 whiteflies were released on each plant to ensure the 100% infection at first leaf stage. Similar method was also repeated for control and non inoculated plants with virus free whiteflies. White flies were then removed by applying imidacloprid and after that plants were maintained in insect proof green house until symptoms appear. The plants were selected by repeated selection and screening methods up to 4 to 5 generations so that resistance for segregation ceased.

symptoms.jpg

TYLC symptoms in (A) a tomato plant infected with TYLCV, (B) a tomato field. TYLC, (C) a banana pepper plant infected with TYLCV, (D) a banana pepper field. TYLC, (E) Datura stromonium (F)common bean(Phaseolus vulgaris cv. Topcrop) infected with TYLCV.

Symptoms severity Scale: Friedmann et al in 1998 described following symptom severity scale (Fig. 3)

0= no visible symptoms, same symptoms for inoculated and non inoculated plants

1= very slight yellowing of leaflet margins on apical leaf

2=some yellowing and minor curling of leaflet ends

3= a wide range of leaf yellowing, curling and cupping with some reductions in size

4= very severe plant stunting and yellowing, pronounced leaf cupping and curling and plant growth stops.

Plant height is given as the percentage of control plants height that the inoculated treatment achieved e.g. height of the inoculated plant/height of the un-inoculated plant * 100

dsi.jpg

Fig. 3 TYLCV disease severity index

In field: In their experiment after the controlled green house inoculation, they transplanted the plants in field. They treated the plants with imidacloprid before transplantation in the field. Ten pairs of rows of randomly distributed were planted. Each pair of rows was planted with one inoculated and one control comprising of five plants in a single row. The within rows distance was 0.5 m while between rows it was 1.2 m. imidacloprid was used 4-8 weeks after transplantation before collecting the data. During first and 2nd harvest all mature and red fruits were harvested while in the 3rd harvest all mature red and immature green were collected to calculate the average yield, fruit weight and number of fruits. Statistical analysis was done by one way ANOVA test (SAS Institute, Cary, NC).

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