Anatomic effects of curly top and aster yellows viruses on tomato

Abstract

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. The effects of the two viruses are fundamentally similar, but differ in detail. The proliferated phloem shows a more orderly cell arrangement in aster yellows plants than in those affected by curly top. This phloem consists almost entirely of sieve elements in curly top plants, whereas it also contains parenchyma and companion cells in aster yellows plants. The diseased phloem collapses in large, continuous masses in curly top plants, and in small, scattered masses in aster yellows plants. Rod-like crystals, pointed at one end and diamond-shaped in cross section, were found in phloem cells of aster yellows plants, but none was seen in curly top plants.