Curly top and aster yellows viruses affect the food-conducting tissue, that is the phloem, of the tomato plant. This tissue undergoes an abnormal increase in number of cells, most of which mature to resemble the food-conducting cells, or sieve elements. The abnormal phloem dies precociously and is crushed. Since the first signs of phloem degeneration occur near normal sieve elements that are first to mature in a given part of leaf, stem, or root, it appears that the viruses move through these elements in their spread through the plant.
