ABSTRACT
Introduction and significance Aster yellows is periodically important on lettuce crops throughout the world. Disease incidence is usually low, so this problem is considered a minor disease.
Symptoms and diagnostic features Symptoms and disease severity of aster yellows on lettuce can be highly variable, depending on the strain
of the phytoplasma, age of the plant when infected, and other factors. Plants are generally stunted and yellowed (388). Leaves can be malformed in various ways, and often remain small and thickened (389). When the plant is sliced lengthwise, the arrangement of the stems sometimes shows a twisted, spiral configuration (390). Characteristic, diagnostic pink to orange-tan latex oozes to leaf and petiole surfaces and results in oblong to circular deposits in the interior parts of lettuce heads (391). Plants infected early in their development will not form heads.
