Sesame Phyllody: Potential threat to sesame cultivation in India

Sesame and its importance

Sesame or gingelly (Sesamum indicum L.), is an important oilseed crop in India. India is the largest sesame producer in world with cultivated area of 19.47 lakh ha and average production of 8.66 lakh tonnes. India is the second largest exporter of sesame next to China. In addition to edible purpose, sesame oil is used in margarine, soaps, pharmaceuticals, paints and lubricants etc. The leftover residue after oil extraction is oil seed cake which is being used as cattle feed. In India, sesame is being majorly cultivated in the states of Madhya Pradesh, Gujarat, West Bengal, Rajasthan, Tamil Nadu, Orissa, Andhra Pradesh, Maharashtra, Uttar Pradesh, Punjab and Karnataka. Sesame is suitable to grow in kharif, rabi and summer seasons and on wide range of soils. 

Although sesame is widely cultivated, sesame cultivation is hindered by many biotic and abiotic factors. Among biotic factors, in recent days, sesame phyllody is the threatening disease all over India which results in severe yield reduction in almost all sesame growing regions. This occurrence of sesame phyllody disease was first noticed in Burma (Myanmar) in 1920s and subsequently in all sesame growing areas of the world.

Sesame Phyllody - Causal organism and symptomatology

Sesame phyllody is caused by phloem-limiting Phytoplasmas which are intracellular pathogens that belong to Mollicutes class and spread primarily by various species of leafhopper. This cell wall-less bacteria are scientifically named as Candidatus Phytoplasma which inhabits the phloem and known to cause phyllody disease in many crops. So far two major groups, viz. 16Sr I (B subgroup) and II (C and D subgroups) of Candidatus Phytoplasma were reported to incite sesame phyllody in India. The symptom of disease could be observed mostly during flowering stage, where the floral parts are transformed into green leafy structures. Various types of symptoms are reported to be associated with phyllody infected sesame plants are flowery phyllody, virescence, witches’ broom, shoot tip fasciation, flattening of the shoot apex, intense leaf and flower bud proliferation; twisting and bending of main stem; stunted plants; and cracking of seed capsules, etc. however the main symptom is conversion of partial or entire floral parts in to leaf like structures and thereby no yield or low yield.

Primary and secondary spread

Weed hosts and vectors play a significant role in multiplication and spread this disease. Sesame phytoplasma has been effectively transmitted to the healthy plants from infected plants through leafhopper species, especially Orosius albicinctus and Hishimonus phycitis has been reported for transmission of sesame phyllody. The weed hosts like Parthenium hysterophorus, Crotalaria juncea, Croton sp., etc. found in and around sesame fields acts as reservoir and acts as primary inoculum for phyllody multiplication throughout the year for infection.

Effectiveness of Integrated Disease Management

• Complete host plant resistance/ resistant varieties are not so far reported/identified. The tolerant/ moderately resistant varieties reported were also not showing consistent phyllody tolerance throughout the season or varied with respect to different agro ecological conditions.
• The adjustment of date of sowing also plays significant role in reduction of this disease. Phyllody incidence was more in July sown crop followed by November sown crop than August and January sown crops. Also the phyllody incidence was reported to be lesser in rabi/summer sown crop comparative to kharif crop.
• Field sanitation, removal of phyllody infected weed and crop hosts also helps in reduction of disease incidence.
• Vector management in initial cropping season plays crucial role in containing the spread of disease.
• The seed treatment with imidacloprid and foliar application of karanj oil/ neem oil were found to be more efficient in reducing the phyllody incidence.
• Antibiotic sprays such as tetracycline and oxytetracycline in combination with insecticide seed treatment and foliar application have also been found to be highly efficient in the prevention of phyllody incidence.

Conclusion

Phyllody is a serious threat to sesame cultivations in recent years throughout India. The disease incidence ranged from 30% to even 100% in some locations. Hence, it is very much necessary to create awareness among farming community on available disease control measures for successful avoidance of phyllody disease incidence. Also research effects should focus on identification of host plant resistance and its utilization in breeding program for disease resistance and in-depth study of pathogen biology for effective containment of sesame phyllody either by biological or chemical means. 

Contributors

K. Sakthivel, T. Boopathi*, P. Duraimurugan and M. Santha Lakshmi Prasad, ICAR-Indian Institute of Oilseeds Research, Rajendranagar, Hyderabad-500 030, Telangana, India.

*Corresponding author email: boopathiars@gmail.com