Aster yellows is a disease caused by a mycoplasma-like organism which attacks a wide range of plants. Plants may be stunted or with numerous secondary shoots. Foliage is yellow and seeds are usually sterile. Plants have an upright habit of growth. In many plants the veins of immature leaves are clear.
Affected leaves are somewhat narrower than healthy leaves.
Old leaves may develop a slightly reddish, brownish, or purplish tinge in the late stages. The main branches will be shortened. Flower parts may develop into leafy structures. In lettuce, the head leaves fail to fully develop and they have pink to tan spots. There is a curling and twisting of inner leaves.