Plants can be infected with either both RTBV and RTSV, or with each virus alone. Vector is the green paddy leafhopper. Doubly infected plants show typical so-called 'tungro symptoms', including plant stunting and reduced tillering. Their leaves become yellow or orange-yellow starting from the leaf tip and extending to the lower part. Discoloured leaves may also have irregular, small, dark-brown blotches. Younger plants may show interveinal chlorosis. Milder symptoms are found with RTBV or RTSV alone (for example, very mild stunting and no yellowing of the leaves).
Viruses are transmitted through a leafhopper called Nephotettix virescens. Tungro is rife in fields with high-yielding rice cultivars which have a shorter growth duration, allowing rice growers two crops of rice in a year. Once a rice plant is infected by tungro, it cannot be cured.Preventive measures are more effective than direct disease control. The double-cropping rice systems and the genetic uniformity of rice are major reasons why tungro disease appears in fields. Rice plants in irrigated areas are much more susceptible to develop the disease than rainfed or upland rice. Plant rests and stubbles are also a source of infection.
Light traps have successfully been used to attract and control the green leafhopper vectors as well as to monitor the population. In the early morning, the population of leafhopper alighting near the light trap should be caught and disposed of, alternatively killed by spraying/dusting insecticides. This should be practiced every day.
Always consider an integrated approach with preventive measures together with biological treatments if available. Using insecticides to control leafhoppers is often ineffective, because green leafhoppers continuously move to surrounding fields and spread tungro rapidly in very short times. Thiamethoxam or imidacloprid at 15 and 30 days after transplanting have been used with some success. The vegetation around the field should also be sprayed with the insecticides.
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