Association of a Bacterium-Like Organism With Rugose Leaf Curl Disease of Clovers

Abstract

White and crimson clover plants affected by rugose leaf curl disease showed temporary remission of symptoms when treated with weekly doses of penicillin applied as a soil drench. A small bacterium or bacterium-like organism was consistently associated with diseased plants of red, white and crimson clover and Trifolium semipilosum when sections of diseased tissues were examined by thin-section electron microscopy. No similar bacterial bodies were seen in sections cut from healthy plants. The bacterial bodies were restricted to phloem sieve tubes and phloem parenchyma cells and were irregularly distributed along vascular bundles within infected tissue. Infected phloem cells also contained electron-dense droplet material not seen in adjacent cells. The bacterial bodies associated with rugose leaf curl measured approximately 0�25.um in diameter and were 1-2.um in length, although more elongated bodies also occurred. The organism was bounded by a cell wall and plasma membrane, or a double membrane, both trilaminar in structure and separated by a lightly stained intermediate layer. The cell wall or outer membrane was darker-stained than the inner membrane and was undulating in outline. Although this organism has not yet been isolated in pure culture, the sensitivity of the pathogen to penicillin suggests that the bacterium-like organism seen by electron microscopy is the causal agent of rugose leaf curl disease.