Sunflower Necrosis Disease: A New Threat for Sunflower (Helainthus annuus L.) Production in Southern India
Digvijay Kumar Bhargav1* and H. P. Meena 1Ph.D. Scholar, Division of Genetics, Indian Agricultural Research Institute, New Delhi
*Scientist (Plant Breeding), Directorate of Oilseeds Research, Rajendranagar, Hyderabad *Email of corresponding author: [email protected]
Introduction Many problems of modern agriculture, such as the overuse of fertilizers and pesticides are a result of the trend in crop production toward homogeneous crop genotypes for certain agricultural areas (Browning and Frey, 1969). In fact, monoculture is presently the dominant form of crop management worldwide, which plays the major role in disease progression (Zhu et al., 2000). Sunflower is a major oil seed crop in India. More than 30 diseases have been reported on sunflowers worldwide (Gulya et al., 1994). However, under Indian conditions, only few of them are of common occurrence. Among the viral diseases affecting the crop, sunflower necrosis virus disease caused by Tobacco streak virus belongs to the genus Ilarvirus (Prasada Rao et al., 2000, Bhat et al., 2002) and spread by thrips was reported for the first time in Karnataka during 1997 in seed production plots causing panic among farmers and seed growers (Anon., 1998; Aravind, 2002; Nagaraju and Hanumantha Rao, 1999; Harvir Singh, 2005; Singh et al., 1997). In Andhra Pradesh, sunflower necrosis disease was observed first time in kharif 2000 as a new virus disease in groundnut in Anantpur district (Jain et al., 2000). Later SND was observed in the districts of Jalna, Aurangabad, Latur and Akola of Maharashtra and because of its fast spreading nature, this necrosis virus was considered as one of the deadly virus diseases on crop in India (Nagaraju et al., 1998). Since then, the disease has become increasingly important in sunflower growing areas of southern states of peninsular India.
The sunflower necrosis disease (SND) of sunflower is one of the important diseases causing heavy losses in edible oil seed in the country. Sunflower necrosis virus disease caused by a strain of Tobacco Streak Virus (TSV) genus Ilarvirus group belongs to Bromoviridae family. It creates epiphytotics in many sunflower plants. It is an emerging threat for sunflower production system in southern India, reported for the first time at Bagepally region of Koalr district and around Bangalore in Karnataka during 1997. Later, its occurrence was reported from almost all sunflower growing states of India, posing threat to sunflower cultivation. On an average, yield losses ranging from 30 to 100 percent have been reported. In southern India especially Maharashtra, Karnataka, Andhra Pradesh and Tamil Nadu states are highly exposing to Sunflower Necrosis Disease (SND).
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The disease has been noticed in an epidemic form consecutively for the last eight years and the incidence is ranging from 5-90 per cent both in open pollinated varieties and hybrids. Among the virus diseases of sunflower, necrosis disease has gained importance because of its heavy devastating nature in most of the sunflower growing states in India. It is a major threat to the economic and social well being of farmers in the Asian region. The disease has the potential to attain menacing proportions threatening the sunflower production in southern states. In subsequent years, outbreaks of this disease in major sunflower-growing states of India, especially Andhra Pradesh, Karnataka and Maharashtra, have virtually threatened the sunflower cultivation and yield losses ranging from 30 to 100 percent have been reported (Chander Rao et al., 2000). All hybrids in cultivation show disease in varying proportions in different seasons. The SND is turning out to be a complex disease and necessitates concerted breeding efforts. All open-pollinated varieties in cultivation now are susceptible to SND indicating the vulnerability of sunflower production in the country. The cultivated sunflower has a narrow genetic base and is deficient in resistance genes. Most of the sunflower hybrids currently under cultivation in India have shown various degrees of susceptibility to the disease. This has to be the major thrust area in disease resistance breeding. To overcome this problem there is an urgent need to identified resistance genes in cultivated and wild species. Presently no reliable resistant sources are available in cultivated species. Directorate of Oilseeds Research initiated the work on development of transgenic lines resistant or tolerant to SND.
Symptoms Symptoms of sunflower necrosis disease comprised chlorotic and necrotic ring spots and leaf distortion (Fig. 1). A general leaf and stem necrosis extending to mid-veins resulting in twisting of the leaf, petioles and flower bracts eventually results in stunting and dieback especially, when plants become infected in early stages of development. The tip of the growing plants becomes necrotic giving typical necrosis symptoms. The plants fail to produce flowers if infected early and finally die. Moreover, the occurrence of necrosis at bud formation stage leads to the partial twisting of the capitulum. Necrosis on bracts and back of the capitulum is also common. The disease was found to infect the sunflower on all the stages starting from seedling to maturity (Nagaraju and Hanumantha Rao, 1999). The infected plant and flower buds were partially twisted which later became sterile. Plants infected prior to flowering remain sterile. Apart from necrosis symptoms, mosaic symptoms are also seen on the leaves. Sometimes the stem below the head bends into `S' shape. Plants infected at grain filling stage exhibit necrosis below the head (Raghavendra, 2005). Systemic symptoms of mosaic, mottling and yellowing accompanied by small ear heads and necrosis of calyx, corolla and other floral parts were also observed (Nagaraju et al., 1998). The necrosis continued to extend throughout the plant growth, on one side of the plant and finally terminated at the apical bud causing tip necrosis and twisting and sterility of the capitulum. Plants at all stages of growth were found to be susceptible.
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Figure 1: Sunflower plants stunting and distortion in early and late infections by SND
Incidence of SND The incidence of the disease ranged from 5-34 per cent, being maximum in May-June sown crop but declined with the onset of rains during Kharif (Anon., 1997). Nagaraju et al., (1998) reported incidence up to 80 per cent on breeding lines of CMS-234 during 1999. In Raichur district, 7 to 36
At seedling stage At star bud stage
At flowering stage No seed setting due to SND
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per cent incidence of the necrosis disease was recorded around Raichur, Manvi and Deodurg taluks during kharif-1998 (Anon., 1999). Fifteen to twenty per cent incidence of necrosis disease was also recorded around north Arcot in Coimbatore district of Tamil Nadu during the survey undertaken in August-September, 1998 (Anon., 2000). The intensity of the disease varied from 30 to 100 per cent in Andhra Pradesh (Chander Rao et al., 2000). Similarly, survey conducted in Maharashtra revealed that the necrosis disease appeared with a high intensity from 20 to 80 per cent (Anon., 2000).
Host Range Anjula (2000) reported that the causal agent infected groundnut, soybean, sunflower, cowpea, cucumber, horsegra:m, watermelon and ridge gourd by mechanical sap inoculation and also through the vector. Host range studies carried out by Ramaiah et al. (2001) revealed that an isometric virus causing sunflower necrosis disease could infect members of plants belonging to fa:milies Amaranthaceae, Chenopodiaceae and Fabaceae. The virus infected the plant species viz., Acanthospermum hispidum, Euphorbia geniculata, Ipomoea aquatica, Stachytarpeta jamaikensis, Galinsoga parviflora, Amaranthus spinosa and Commelina benghalensis (Aravind, 2002). Linga Reddy, (2003) reported that the virus infected soybean, groundnut, watermelon, cucumber, cowpea, Acanthospermum hispidum, Euphorbia geniculata and Galinsoga paruiflora. Lavanya et al. (2005) reported that sunflower necrosis virus has wide host range, which included many cultivated crops and weed hosts. Fifteen plants species belonging to Fabaceae; three plant species belonging to Malvaceae, six plant species belonging to Cucurbitaceae, three plant species belonging to Solanaceae and one plant species each belonging to Cruciferaceae and Moringonacae were found to be additional hosts for sunflower necrosis virus.
Alternate Host The disease is transmitted through thrips. The obnoxious weed Parthenium is suspected as alternate host, as its pollen is known to possess the virus. The symptoms of necrosis virus disease are also observed in weed plant Xanthium. As the disease is caused by virus, control measures are not possible after the infection. As prevention is the strategy to manage the disease, the stress should be on controlling the thrips which are carriers of the virus. Farmers may adopt the following measures to control the pest: Planting 4-6 lines of tall growing border crops like bajra, sorghum or maize all around the sunflower.
Protection from Thrips Treating the seeds with insecticide Imidacloprid 70 WS at 5 g/kg of seeds before sowing which will give protection against thrips for 30 days. Spraying of insecticide Imidacloprid 17.8 SL at 0.25 ml/litre or oxydematon methyle at 1.5 ml/litre after 30 days of sowing. Planting in the month of June and August may be avoided. Instead a short duration pulse crop may be planted during this season. Sunflower sown during September records less disease and yields more. The fields should be kept free from weeds.
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Management Presently no reliable resistant sources are available. The disease being viral in nature is very much difficult to combat by single approach. Most of the sunflower hybrids currently under cultivation in India have shown various degrees of susceptibility to the disease. Recently, adjusting sowing period has been proposed for management of the disease. Also, sunflower crop sowing in post rainy season (September onwards) was found to be beneficial for minimizing necrosis incidence (Shirshikar, 2003). Shirshikar (2008) concluded that for the management of sunflower necrosis disease, the sunflower crop should be bordered with 6 rows of sorghum (border crop should be sown 15 day prior to main crop) and the sunflower seeds should be treated with imidacloprid (Gaucho 70 W.S., 5 g/kg) along with three sprays with imidacloprid (Confidor 200 S.L., 0.05 percent) applied 15, 30 and 45 days after sowing. Bordering of sorghum crop recorded lowest mean necrosis incidence of 10.2 percent and maximum yield of 1222 kg/ha along with highest BC ratio of 3.4 over non-border treatment Shirshikar (2008). The sunflower necrosis disease can be managed by treating seeds with thiomethoxam at 4 g/kg along with two sprays of the chemical at 0.05% 30 & 45 DAS (Shirshikar, 2010). Some practices which help in reducing the incidence and further spread of the virus diseases in the field and thereby reducing the yield losses, have been suggested. Like, following clean cultivation by removing the weeds both inside and neighbouring plots which helps to destroy the virus source and avoid the primary inoculum to the crop. Prophylactic sprays with suitable insecticides to control the insect vectors which come from outside the field and also harbouring inside the field, helps to avoid the primary sources and also further spread in the field. Careful destruction of the infected plants as soon as they are noticed in the field will also help to avoid further spread of the viruses in the field by destroying the source of inoculum within the field. Conclusion Sunflower necrosis disease is an emerging threat for sunflower production system in southern India. Among the virus diseases of sunflower, necrosis disease has gained importance because of its heavy devastating nature in most of the sunflower growing states in India. The cultivated sunflower has a narrow genetic base and is deficient in resistance genes. This has to be the major thrust area in disease resistance breeding. To overcome this problem there is an urgent need to identified resistance genes in cultivated and wild species. But the inheritance pattern or genetics of this disease is unknown. So, it will create major problem to transfer into cultivated crosses and other problem related to the use of wild species in sunflower improvement programme is frequently obstructed by incompatibility, structural heterozygosity, ploidy differences and limited gene exchange in hybrids involving resistant wild species.
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