OCCURRENCE OF ANOMALOUS MYCOPLASMA-LIKE ORGANISMS IN GRAPEVINE YELLOWS DISEASED PHLOEM

Pathological changes in Vitis vinifera cv. Caveccia phloem from leaves showing symptoms of a flavescene doree-like disease consisted of obli teration, necrosis and collapse of the sieve elements and associated c ompanion cells, and excessive callose accumulation in lateral sieve ar eas and sieve plates of apparently normal mature sieve elements. Unusu al structures, also found in degenerate sieve elements of diseased lea f vein specimens, were strongly electron-dense and bounded by a unit m embrane or an electron-transparent border, and considered to be senesc ent forms of mycoplasma-like organisms. The significance of these find ings in relation to possible host responses to the yellows pathogen is discussed.