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 flavescence doreé‐like disease consisted of obliteration, necrosis and collapse of the sieve elements and associated companion cells, and excessive callose accumulation in lateral sieve areas and sieve plates of apparently normal mature sieve elements. Unusual structures, also found in degenerate sieve elements of diseased leaf vein specimens, were strongly electron‐dense and bounded by a unit membrane or an electron‐transparent border, and considered to be senescent forms of mycoplasma‐like organisms. The significance of these findings in relation to possible host responses to the yellows pathogen is discussed.