Strawberry green petal and lethal yellows diseases are associated with Phytoplasma australiense
Strawberry lethal yellows phytoplasma (EPPO Alert List) is associated with a severe decline of strawberry (see EPPO RS 98/171). This disease is reported in New Zealand and Australia. In New Zealand, it has been found that the strawberry lethal yellows phytoplasma was related to, or identical with, phytoplasmas associated with phormium yellow leaf disease, Australian grapevine yellows (the phytoplasma associated with this disease has been designated Candidatus Phytoplasma australiense) and papaya dieback. Strawberries can be affected by a number of phytoplasma diseases, including green petal. Green petal disease occurs sporadically in Europe, North America and Australia, and symptoms (green petals) are usually less severe than those of strawberry lethal yellows. A recent Australian study was carried out on strawberry plants showing symptoms of green petal (3 plants) and lethal yellows diseases (8 plants) using molecular techniques (RFLP analysis of 16S rRNA gene and adjacent spacer region, and sequence comparison). Results showed that the phytoplasmas detected in strawberry plants with green petal and lethal yellows diseases were indistinguishable and were most closely related to P. australiense. The authors noted that this also supports the view that strawberry lethal yellows diseases are caused by the same phytoplasma in New Zealand and in Australia.
Many questions are raised by this study. As green petal disease occurs in other parts of the world, the geographical distribution of P. australiense which was previously considered as endemic to New Zealand and Australia, may need to be revised. The fact that the same phytoplasma was associated with two distinct diseases in strawberry cannot yet be explained (cultivar susceptibility, environmental conditions, disease progression?). The authors thought that mixed infections with other phytoplasmas could perhaps explain differences in symptomatology, but this hypothesis could not be verified. In particular, another lethal yellows in Australia has been associated with rickettsia-like bacteria [EPPO note: this disease was described in 1979 and nothing has been published on it since; its status is distinctly doubtful]. But in the present study, no rickettsia-like bacteria were found in diseased strawberry plants testing positive for P. australiense. More studies on a larger number of plants are needed to try to better understand the etiology and the epidemiology of these strawberry diseases.
Sources
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Padovan, A.; Gibb, K.; Persley, D. (2000) Association of ‘Candidatus Phytoplasma australiense’ with green petal and lethal yellows diseases in strawberry.
Plant Pathology, 49(3), 362-369.
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