Ultrastructural aspects of AY group phytoplasmas in sieve tube elements of diseased plants have been studied by several researchers using transmission and scanning electron microscope observations (Hirumi and Maramorosch, 1973;Haggis and Sinha, 1978;Marcone et al., 1995;Marcone and Ragozzino, 1996;Fránová and Šimková, 2009;Fránová et al., 2009). The phytoplasma bodies varied in size and shape. They showed a very high polymorphism, appearing in round, ovoid, encurved and elongated forms. Octopus-like structures, as well as budding, dimpled- and dumbbell-shaped forms were also observed. The size of spherical forms ranged from 100 to 800 nm and filamentous bodies were up to 2600 nm in length. However, the morphological variations observed most probably represented various developmental stages of phytoplasmas and they cannot be considered as distinctive characteristics.
For reliable diagnosis, the identity of phytoplasmas occurring in plants characterized by the symptoms described (see Symptoms), should be determined by molecular techniques.
Related invasive species
- Candidatus Phytoplasma asteris
Related Farm Practice
- Transmission
- Host plants
- Development
AY group phytoplasmas affect plants by causing extensive abnormalities in plant growth and development, suggestive of profound disturbance in plant hormone balance. Symptoms typical on herbaceous plant hosts include yellowing of the leaves, stunting, proliferation of auxiliary shoots resulting in a witches'-broom appearance, bunchy appearance of growth at the ends of stems, virescence of flowers and sterility, phyllody, shortening of internodes, elongation and etiolation of internodes, small and deformed leaves. Yellowing, decline, sparse foliage and dieback are predominant in woody plant hosts. However, it is well-known that distantly related phytoplasmas can cause identical symptoms in a given host plant, whereas closely related phytoplasmas can cause distinctly different symptoms. Lee et al. (1992) determined that different symptoms could be induced in Catharanthus roseus (periwinkle) by closely related strains of the AY phytoplasma group.
AY group phytoplasmas appear to have a wide host range. The vast majority of strains in the AY group infect herbaceous dicotyledonous plant hosts. However, a number of strains that belong to subgroups 16SrI-A, 16SrI-B and 16SrI-C are capable of infecting monocotyledonous plants (e.g., maize, onion, gladiolus, oat, wheat and grass). Some strains in subgroups 16SrI-A, 16SrI-B, 16SrI-D, 16SrI-E, 16SrI-F and 16SrI-Q can induce disease in woody plants (e.g., grey dogwood, sandalwood, blueberry, mulberry, peach, cherry, olive, grapevine and paulownia). For many of the plant hosts which have previously been reported to be affected by AY diseases on the basis of symptomatology and/or microscopic examinations (see McCoy et al., 1989), the identity of the infecting phytoplasmas has never been determined with molecular techniques, or proved to be different from that of other established AY phytoplasma strains (Schneider et al., 1997;Marcone et al., 2000).