Cause Lettuce mosaic virus, which is transmitted by seed and by the green peach aphid.
Symptoms In young plants, first the seedling leaves roll up on the long axis and the first true leaf is irregularly shaped and slightly lobed. Vein clearing and bronzing follow. Bronzing is caused by necrotic flecking of the leaf blade, especially along the veinlets. Symptom expression varies with variety and climatic factors.
In older plants, the mottle symptom is absent or indistinct. In half-grown plants, the whole plant is severely stunted and dull green to slightly yellow. The outer leaf tip rolls down, so plants look wilted although leaves may be turgid. Midribs on stunted plants protrude abnormally on the leaf underside and give the lower portion of the heads a ribby appearance.
- Pathogen-free seed is the best means of control.
- Avoid planting varieties not tested for mosaic in the same field with tested material.
- Control weeds, particularly wild lettuce, near the planting.
- Remove diseased plants whenever apparent.