Ring spot virus (PRSV) is the most destructive disease of papaya in India. PRSV is spread (transmitted) among papaya plants by mechanical activities like pruning, as it is transmitted through the sap.
Numerous aphid species like Myzus persicae, Aphis craccivora, and Aphis gossypii spread this infestation. Seed transmission has not been detected.
Initial symptoms appear as a light and dark green mosaic pattern on the leaves. Later on the leaves show a marked distortion and reduction in leaf area. Water soaked oily streaks get formed both on the petioles and on the upper parts of the trunk. Flower and fruit drop is noticed.
Pale green patches appear on the young fruits. Fruits from infected trees show bumps / raised projections similar to those observed on fruits from plants suffering from boron deficiency. Characteristic ‘ring spots’, which form the basis for the disease name start appearing on the fruit,
Number of rings on the fruit varies and the rings become less distinct as the fruit matures and yellows. Fruit yield from infected plants is lower when compared to healthy plants. Infection at early stages does not produce an economical crop.
Once the symptoms have become prominent in the field it would have become too late to spray for aphids as the disease would have already been transmitted to nearby plants.
In order to implement successful vector control, frequent preventive sprays are required. Regular monthly sprays of insecticide (75 ml / 100 L) of water and Multi Micronutrient sprays (0.1 per cent) or Soluble Boron (0.01 per cent) can be done at monthly intervals.
Barrier crops like Jowar / Maize should be sown densely along the perimeter one month before papaya transplanting. Plastic silver coloured mulch should be laid on the field. One year crop schedule to maximise production can be attempted in areas with severe PRSV infection.
Disease transmission can occur when infected seedlings are used. Hence nurseries should be raised under insect proof net cover.
(Dr. Duleep Kumar Samuel, Senior scientist and Dr .Krishna Reddy, Director, The Virology Lab, Division of Plant Pathology, Indian Institute of Horticultural Research, Bangalore , email: firstname.lastname@example.org)