On the Trail of Stubborn Bacterial Yellowing Diseases.

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 <i>Spiroplasma</i> and the non-cultivable 'Candidatus Phytoplasma'. A third group-the walled, psyllid-transmitted <i>Candidatus Liberibacter</i>-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 <i>Spiroplasma citri</i>; 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 <i>Candidatus Liberibacter</i> species, underscores the urgent need for improved strategies to manage this resilient group of phloem-limited bacterial pathogens.