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Evidence that Clover Club Leaf is Caused by a Rickettsia-like Organism. I. M. Windsor, Research Assistant, Department of Botany, University of Illinois, Urbana 61801; L. M. Black, Professor, Department of Botany, University of Illinois, Urbana 61801. Phytopathology 63:1139-1148. Accepted for publication 17 March 1973. DOI: 10.1094/Phyto-63-1139.

Electron micrographs of thin sections of phloem tissues from periwinkle, Vinca rosea alba, and crimson clover, Trifolium incarnatum, showing symptoms of clover club leaf disease demonstrated the presence of organisms bounded by a double membrane, or membrane and wall system, each structure measuring approximately 8 nm in thickness. In cross sections of phloem, some bodies appeared roughly spherical, with a diameter of approximately 200 nm, whereas others were elongated, measuring about 2 µm in length and 200 nm in diameter. No such organisms were ever found in the tissues of healthy plants. A probability study showed that the chance that these organisms were associated with the diseased plants purely fortuitously was not greater than 1/924. This probability indicates that this organism is the causal agent of clover club leaf disease. The organism appears to be more closely related to the rickettsias than to the mycoplasmas or chlamydiae.