Status of viruses infecting sunflower and strategies for their management

Abstract

Among different diseases infecting sunflower (Helianthus annuus) worldwide, sunflower necrosis, sunflower mosaic, and sunflower leaf curl are important diseases in India. After thorough investigation, the causal organism was identified as tobacco streak virus (TSV) for sunflower necrosis disease (SND) in India. It belongs to the genus Ilarvirus and family Bromoviridae. It appeared as epidemic form from 1997 to 1999 in Southern India and caused yield loss up to 90%. It is occurring in all the sunflower-growing states in India such as Andhra Pradesh, Karnataka, Tamil Nadu, and Maharashtra. It poses potential threat to the cultivation of sunflower in India. The early infection kills entire plants and produces characteristic symptoms, namely, necrosis of leaf, petiole, stem, and bracts and malformation of head with chaffy grains. TSV has wide host range infecting both crop and weed species growing in sunflower cropping system. TSV is pollen borne, and its seed transmission is not yet proved from any crop and weed plants. Infection is occurring in the presence of both infective pollen and thrips, and relationship of vector with virus is nonpersistent as there is no specificity between virus and vector. In India, effective screening was carried out against TSV in hot spots. However, complete resistant sources are not available in both germplasm and varieties/hybrids. Hence, effective breeding approaches should be developed to augment the resistance using resistant A line/CMS lines/R lines. Marker-assisted selection (MAS) has also not been exploited in this crop against TSV due to lack of resistant sources. Effective management practices such as late sowing (October sowing), border cropping (7-11 rows of sorghum), removal of weeds before flowering, intercropping with castor and red gram, treating the seed with imidacloprid (5 g/kg seed) or thiamethoxam (4 g/kg), and vector control using systemic insecticide at 15, 30, and 45 days after sowing were very effective to contain the disease. Systemic epidemiological studies were conducted in India and established the positive correlation of SND with thrips population and low temperature. So, effective disease forewarning system to be planned as the primary inoculum of the virus is provided by weed hosts prevalent in and round the sunflower fields. So far, coat protein (CP) gene of sunflower TSV isolate was very well characterized in India. Hence, CP gene-mediated resistance should be incorporated in the cultivated high yielders by genetic transformation using biotechnological tools. Combination of conventional breeding methods, different management practices, and transgenics should be adopted to tackle the SND menace in India.