August2012. Spotting on lettuce from an NGO in Cambodia and thought to be caused by lettuce mosaic virus. Is this so? The organisation is facing a substantial problem for a few months. It has been removing affected plants as soon as theyare seen. However, the phenomenon has got worse. In some fields where this disease occurs, it seems that the disease remains in the soil and the plants get re-infected after harvest and replanting.

We have been putting a thin net around some plants to prevent insect infestations, and sprayed insecticide inside them, but disease is still spreading. We are very surprised since we thought aphids were the only vectors.

We are also starting planting cucumber in fields where the salads were previously and they too have yellow spots on the leaves.

Members pointed out that lettuce mosaic virus was spread by aphids and by seed.

There were also suggestions that the problem was caused by other pathogens. The slow spread may indicate the presence of nematodes although mosaic symptoms are not typical of Meloidogyne sp. Reduced vigour and yellowing of margins would be the expected symptoms. Rotylenchus, another nematode species, is known to produce more yellowing and stunting. It would be well worth checking the roots for galls and the soil for nematodes using the Whitehead tray method. If the field is found to be nematode infested, a multi-pronged medium to long-term strategy would need to be put in place, using:

• Crop rotation e.g. Sun hemp (Crotalaria),French Marigold (Tagetes)
• Bio-control agents like Paecilomycessp., Pochonia chlamydosporia, etc
• Resistant varieties.

In the short term it would help to look for locally available chemical nematicides (?Carbofuran).

Soil fungi like Phytophthora would spread much faster and cause necrosis rather than yellowing, and hence could be ruled out but never under-estimated!

There were other suggestions:

• Soil nutrient deficiency
• pH of the soil, for lettuce it should be pH7-7.5; they are sensitive to low pH
• Doing an ELISA test for lettuce mosaic, eg http://www.hydros.cc/planttest.htm
• The observation that some of the beds “keep” the disease suggests a soil borne virus may be present;at least three are transmitted by Olpidium brassicae.

Later, it was reported that a soil test showed that the pH was 5-6. An application of lime was recommended, working it thoroughly into the soil to a depth of at least 20 cm. The amount applied will depend on whether the type of lime available and the type of soil.