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On the Trail of Stubborn Bacterial Yellowing Diseases
1The S. Tolkowsky Laboratory, Department of Plant Pathology and Weed Research, Agricultural Research Organization-The Volcani Center, P.O. Box 15159, Rishon Lezion 7528809, Israel.
Abstract:
This retrospective review traces personal encounters along the complex path of plant yellowing diseases-graft-transmissible disorders historically attributed to elusive viruses, but later linked to phloem-invading, wall-less bacteria known as Mollicutes. These include two plant-infecting genera: the cultivable Spiroplasma and the non-cultivable 'Candidatus Phytoplasma'. A third group-the walled, psyllid-transmitted Candidatus Liberibacter-was later implicated in closely similar syndromes. This shift in understanding marked a major turning point in plant pathology, offering new insights into yellowing diseases characterized by stunting, decline, and poor or deformed growth. The review focuses on key syndromes: citrus little leaf disease (LLD), or citrus stubborn disease (CSD), caused by Spiroplasma citri; and several Mollicute -related disorders, including safflower phyllody, Bermuda grass yellowing, and papaya dieback (PDD) (Nivun Haamir), the latter linked to 'Candidatus Phytoplasma australiense'. Despite differing causes and vectors, citrus LLD-CSD and PPD share an erratic, unpredictable pattern of natural outbreaks-sometimes a decade apart-hindering grower engagement and sustained control efforts. While scientific understanding has deepened, practical management remains limited. The recent global spread of Huanglongbing (HLB), caused by Candidatus Liberibacter species, underscores the urgent need for improved strategies to manage this resilient group of phloem-limited bacterial pathogens.
Insights
Plant yellowing diseases, once blamed on viruses, are now understood to be caused by wall-less bacteria called Mollicutes and related pathogens. Effective management strategies are urgently needed due to their unpredictable outbreaks and spread.
Area of Science:
- Plant Pathology
- Bacteriology
- Phytopathology
Background:
- Historically, plant yellowing diseases were attributed to viruses.
- Recent discoveries link these disorders to phloem-invading bacteria: Mollicutes (Spiroplasma, Ca. Phytoplasma) and Ca. Liberibacter.
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