Viral Diseases of Crops in Nepal

Viral diseases are the important diseases next to the fungal and bacterial in Nepal. The increase in incidence and severity of viral diseases and emergence of new viral diseases causes the significant yield losses of different crops in Nepal. But the research and studies on plant viral diseases are limited. Most of the studies were focused in viral diseases of rice (Rice tungro virus and Rice dwarf virus), tomato (Yellow leaf curl virus) and potato (PVX and PVY). Maize leaf fleck virus and mosaic caused by Maize mosaic virus were recorded as minor disease of maize. Citrus Tristeza Virus is an important virus of citrus fruit in Nepal while Papaya ringspot potyvirus, Ageratum yellow vein virus (AYVV), Tomato leaf curlJava betasatellite and Sida yellow vein Chinaalphasatellite were recorded from the papaya fruit. The Cucumber mosaic virus (CMV) and Zucchini yellow mosaic potyvirus (ZYMV) are the viral diseases of cucurbitaceous crop reported in Nepal. Mungbean yellow mosaic India virus (MYMIV) found to infect the many crops Limabean, Kidney bean, blackgram and Mungbean. Bean common mosaic necrosis virus in sweet bean, Pea leaf distortion virus (PLDV), Cowpea aphid‐borne mosaic potyvirus (CABMV), Peanut bud necrosis virus (PBNV) in groundnut, Cucumber mosaic virus (CMV). Chili veinal mottle potyvirus (CVMV) and Tomatoyellow leaf curl gemini virus (TYLCV) were only reported and no any further works have been carried out. The 3 virus diseases Soyabean mosaic (SMV), Soybean yellow mosaic virus and Bud blight tobacco ring spot virus (TRSV) were found in soybean.


Introduction
Nepal is an Agricultural country where about 65% of total people are involved in agriculture.Most of them are dependent in the agriculture for sustaining the life.At present context there are various constraints in Nepalese agriculture.Among them plant diseases one of the major constraints in crop production in Nepal.Plant diseases cause both qualitative as well as quantitative losses in Agriculture with negative impact on the economy of Nepal.Past and the present viral disease scenario indicated that the occurrence of several viruses on cereals, cucurbits, legumes and Cite this article as: N.S.Poudel and K. Khanal (2018) Int.J. Appl.Sci.Biotechnol.Vol 6(2): 75-80.DOI: 10.3126/ijasbt.v6i2.19702

Rice tungro badnavirus/machlovirus
Rice tungro disease had been first reported from Parwanipur research station based on field symptoms and transmission studies (John et al., 1979a).Later from the plant sample collected from Hardinath Agriculture Farm, Janakpur; virus particle of tungro was identified in electron microscope examination (Omura et al., 1981;Dahal et al., 1996a).This disease was also reported in Kathmandu Valley later (Amaty and Manandhar, 1986).Based on particle morphology and serology Omura et al. (1981Omura et al. ( , 1982) ) identified the presence of rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV) from tungro-like rice plants.Dahal et al. (1996a) surveyed from 21 districts of terai region of Nepal and found that rice tungro was primarily restricted to the Hardinath (Janakpur) and Parwanipur (Bara) regions.The tungro incidence in Hardinath ranged from 17% to 51% and in Parwanipur from 6% to 61% causing about 89% grain yield loss in Hardinath.In the same year the rice tungro virus was found in some pockets of Janakpur and Bara districts and of rice dwarf in Kathamndu Valley (Dahal et al., 1996b).On mass inoculation of 15 popular rice cultivars most became more than 50% infected and only cultivar Radha 9 had low (22.2%)infection (Dahal et al., 1996a).

Rice dwarf virus
Rice dwarf virus was reported in cultivars Taichung 176 and KT 32-2 (John et al., 1979b).The finding was later confirmed by Omura et al. (1982) by electron microscopic examination of infected material collected in Nepal.Based on SDS-immunodiffusion test, the Nepalese isolate Of RDV was shown to be similar to the Japanese RDV isolate.In Nepal the virus is transmitted by both male and female leafhopper Nephotettix nigropictus in a persistant manner (John et al., 1979a;Omura et al., 1982).Nymphs are more efficient in transmitting the virus than adult (Pradhan and Khattri, 1980).According to Upadhyay and Lapis (1982) among the 44 screened germplasm 10 are observed to be tolerant while other are moderately to highly susceptible.Out of them, seven were designated as IR20, IR 2071-627-1, IR C3707-117 2, IR2797-125, Tetep, IR 1416-128-5-8 and IR 1905-81-3-1showed the 0.5% seedling infection.Other three entries named IET2938, IR 1544-340-6-1 and IR 1905P 11-29-4-1showed 6-25% seedling infection.Remaining showed susceptible reaction to rice dwarf raging from moderate to high.

Viral Diseases of Maize
Maize disease investigation was initiated in 1964-65 when there was not one improved variety of maize in the country (Manadhar, 1983).According to Manandhar, (1983) the incidences of virus diseases have been reported in maize in Nepal, but none of the virus diseases have been unquestionably identified.The initial reports of Leaf fleck caused by Maize leaf fleck virus and Mosaic caused by Maize mosaic virus were recorded both in hill and Terai (Subedi, 2015).

Citrus
Citrus virus and virus-like diseaseswere unknown in Nepal priorto the importation of grafted citrus from Saharanpur (Uttar Pradesh) inIndia in the 1960's (Lama, T. K. 1996).The first record of Citrus tristeza virus (CTV) Nepal was in 1971 (Knorr and Moin-shah, 1971).According to Regmi et al. (2001) among two strain of CTV virus found in Nepal first strain is endemic and causes stem pitting and vein clearing symptoms only on lime and does not show symptoms in sweet orange and manderian while second strain shows symptoms on sweet orange and mandarin.Regmi and Shrestha (1993) introduced the practice of shoot-tip grafting to eliminate viruses from budwood sources.Regmi et al. (2001), found the incidence of CTV (non-stem pitting strain on mandarin) was about 7.7 percent with several colonies of aphidswhich are vectors of Citrus Tristeza Virus (CTV) in different orchards of Baitadi.Dahal et al. (1997) observed the symptomsof papaya ringspot potyvirus-watermelon strain (PRSV-W) on papaya in18 terai and inner terai districts of Nepal during1989 and 1992 to 1994.The virus isolated from papaya was confirmed as papaya ringspot potyviruswatermelon strain (PRSV-W).Fromleaf samples of Carica papaya plants with leaf curling, vein darkening, vein thickening, and a reduction in leaf size collected from Rampur, Nepal in 2010 an isolates of Ageratum yellow vein virus (AYVV), Tomato leaf curl Java betasatellite and Sida yellow vein China alphasatellite were isolated (Shahid et al., 2013).This was the first identification in papaya in Nepal.

Potato
PLRV, PVS, PVX, PVY, PVA and PVM are known to be the major potato viruses of economic importance in Nepal ( Akius and Kloos 1990;Ranjit et al., 1994;Sakha et al.,2007).Khatri and Shrestha (2004) tested 21 local cultivars and the tuber of almost all tested stocks were found infected by virus diseases and cultivars Farse White, Rosita, Khumbule and Lumle Red were superior to Kufri Jyoti in tuber yields.Jumli Local, Kathmandu Local, Sarkari Seto, Syang Dorje and Tharu Local were cleaned against six potato viruses (A, X, M,Y, S an and leaf roll) at Khumaltar (Khatri and Rai, 2000).

Tomato
Tomato leaf curl virus (TLCV) is becoming a serious problem in the tomato crop of terai and foothills (Shrestha, etal., 1997).Ghimire, et al. (2001) reported incidence of the disease in most tomato growing pockets and yield losses of 40% or even higher in some areas like Risingpatan, Tanahun and Kudule, of western hills of Nepal.Plant disease monitoring by Lumle Agricultural Research Centre (LARC) through its network of off-station research sites (OSR), outreach research sites (OR) found severe infestation of the Tomato Yellow Leaf Curl Virus (TYLCV virus) disease occurred in tomato growing areas of Dhanubase, Yampaphant and Rishing Patan in fiscal year 1994/95 (Sharma et al, 1996).The causal agent of leaf curl disease is known as geminivirus that is transmitted by white fly (Bemisia tabici Gen.) in a semi-persistent manner (Shrestha, et al.1997).All exotic tomato cultivars are susceptible to leaf curl virus however, some local indeterminate cultivars bave been observed as tolerant to Tomato Leaf Curl Virus.Malla et al. (2000;2002 ) reported varieties ATY 3, ATY 5, and ATY 6 were resistant to TYLCV where as NCL1, BL410 and Pusa ruby showed moderate level of TYLCV incidence.The seedling raised under musling cloth+without mulching showed the highest fruit yield and lowest Tomato Yellow Leaf Curl Virus (TYLCV) incidence and the use of straw mulching on tomato field had significant effect on reducing disease (Malla et al., 2000(Malla et al., , 2001)).

Cucurbitaceous crops
The virus isolated from zucchini, ash-gourd, bitter-gourd, snake gourd, spongegourd, watermelon, bottlegourd and cucumber virus like symptoms and mosaic or yellow mosaic, blisters, and leaf distortion was confirmed as papaya ring spot potyvirus watermelon strain and Leaf extract of ash gourd cucumber and pumpkin reacted to antibody against cucurbit aphid borne yellow luteovirus (CABW) also From leaf samples of Cucumber, chayote, pumpkin, zucchini and snakegourd isolated cucumber mosaic virus(CMV) and zucchini yellow mosaic potyvirus (ZYMV) (Dahal et al., 1997).ZYMV and CABW was first report in Nepal.The zucchini yellow mosaic virus was epidemic in most of farmers' fields at low elevations (about 250 m) and much less at higher altitudes (>1500 m) and the spread of disease was more than 8.5% per day during both pre-and post-monsoon seasons (Dahal,1992).A research on management of viral diseases of cucurbits crops conducted by (Joshi et al., 2012) concluded that there is no chemical which control virus but fresh cow milk, Anti-VS, Liposome and Victovirus helped to lower the virus severity.Shahid et al. (2012) first reported MYMIV (Mungbean yellow mosaic India virus) from Phaseolus vulgaris (Kidney bean) and lima bean (Phaseolus lunatus) plants showing severe mosaic, yellowing and leaf curling symptoms from vicinity of Chitwan, Nepal and the incidence of disease was 70-80%.

Sweet Bean
The first report of Bean common mosaic necrosis virus (BCMNV) was from sweet bean with mottle and leaf deformation, severe mosaic, necrosis, malformation of leaves in Nepal in December 2010 (Pudashini et al., 2013).

Pea
Electron microscopic examination of leaf extracts from leaves showing symptoms narrow and/or curled, and stunting of pea plants at Rampur revealed pea seed-borne mosaic potyvirus (PSbMV) (Dahal and Albrechtsen, 1996).Pea leaf distortion virus (PLDV) was also reported from Rampur, Nepal (Shahid et al., 2017).Dahal and Albrechtsen, (1996) recorded symptoms of cowpea aphid-borne mosaic potyvirus (CABMV) during 1989-1990 at Rampur which was transmitted by both aphids and sap inoculations.The incidence was higher in the late planted pea crop (January) than those planted in November and December.Different virus-like symptoms have been reported from the experimental plots of IAAS farm (Timilsina, 1988;Neupane and Pant, 1988) and farmer's field of Chitwan district.(Dahal, 1990) reported the virus like symptoms on cultivar Pusa sawani with reduced leaf lamina and a typical interwoven network of yellow veins and complete chlorosis of younger leaves at the advance stage.

Chili
The viral disease problem of pepper in Nepal is complex more than one viruses attack simultaneously (Joshi and Shrestha, 1999).Cucumber mosaic virus (CMV), Chili veinal mottle potyvirus (CVMV) and Tomato yellow leaf curl gemini virus (TYLCV) were the most common viruscs identified in Nepal (Joshi and Shrestha, 2001).During March and September 1989, few dwarf plants of chilies, both sweet pepper and hot chilies (cv.Pusa jwala) were observed with Initial symptoms of dwarf, light green foliage, leaf curl, cupping upward/downward, and leaf distortion (Dahal, 1990).Timila et al. (1994) also reported the occurrence and distribution of TYLCV in tomato and chilli.

Brinjal
Brinjal plants with symptoms like mosaic, reduced and distorted leaves, and short internodes exhibiting a bushy appearance were observed during first week of September, 1989 (Dahal, 1990).

Broad leaf mustard
The occurrence of the mosaic disease of broad leaf mustard was reported, diagnose& and identified (Shrestha 1983.The possible occurrence of mosaic virus on radish and turnip has also been suspected (Shrestha, 1984).

Viral Diseases of Oilseed crops (Soyabean and Groundnut)
The most common virus is soyabean yellow mosaic virus among the 3 virus diseases of soyabean that were found in Nepal (Manandhar and Sinclair, 1982).Soyabean mosaic (SMV) and bud blight (tobacco ring spot virus, TRSV) in soyabean in Kathmandu valley and terai region was reported by (Manandhar, 1979).Gharti et al, (2010) reported 6 genotypes PI 94159,G-8754, Dashratpur, SB 0095, CM 9125 and TAMPOMAS were found highly resistance to Mungbean yellow mosaic virus (Gharti et al. 2010;Shrestha et al., 2011).Sharma (1996) conducted yield loss assessment study between 1992 and 1995 at Nawalpur, revealed that cumulative disease incidence of Peanut bud necrosis virus (PBNV) was 20%; pod yield loss reached 27.7% and infects during the early stage of crop growth.

Viral diseases of Legumes (Mungbean and Blackgram)
Mungbean yellow mosaic virus is the most important disease of summer legumes in warm humid subtropical regions of Nepal.Srivastava (2010) reported Pratikshya and kalyan have been recently released by National Seed Board were resistance to mungbean yellow mosaic virus.In screening nursery of Mungbean Yellow Mosaic Virus of mungbean and blackgram , IPM-16 genotype of mungbean were found highly resistant whereas Bari Mash-1, Bari Mash-2, Bari Mash-3 of blackgram and Bari Mung-2, Pratikshya, Hum 12genotypes of mungbean were resistant and BLG 0003-2-1 genotype of blackgram were found moderately tolerant (Gharti, 2013).According to Gharti (2013) spray of Cow's milk and leaf extract of mixture of botanicals were effective to reduced severity, increased hundred seed weight and improved grain yield.

Spices crop (Cardamom)
The 'Foorkey' (dwarf) and 'Chhirke' (mosaic or streak) disease of large cardamom have been reported as serious problem in cardamom cultivation (Dahal et al., 1988).The author has indicated that the dwarf disease of cardamom was more serious than the mosaic or streak diseases.The experiments carried by Subedi and Paudyal (2008) found that invitro plant regeneration by multiple shoot formation from meristemic tissue of large cardamom helps for the rapid multiplication of disease free planting materials.

Conclusion
The viral diseases are as important as the fungal and bacterial diseases in Nepal causing subsequent losses in yield of different crops but the studies and research works are limited in Nepal.The works should now be focused on the molecular level as well as the best management practices of the various diseases as the farmers want the best results and the high output.Only the researches are being focused in some of the major crops like rice, potato and tomato but not in the minor and indigenous crops which are the main crop for the farmers of rural Nepal.Therefore, the Agriculture research body of Nepal (NARC) should focus in the viral disease of crops also should coordinate with NAST, Department of Agriculture, AFU and IAAS.