# On the Trail of Stubborn Bacterial Yellowing Diseases

## TL;DR

This paper reviews the history and challenges of bacterial plant yellowing diseases, highlighting the shift from viral to bacterial causes and the need for better management strategies.

## Contribution

The paper provides a retrospective analysis of key plant yellowing diseases and their bacterial causes, emphasizing the need for improved control strategies.

## Key findings

- Citrus little leaf disease and papaya dieback share unpredictable outbreak patterns, complicating management.
- Huanglongbing's global spread highlights the urgent need for better strategies against phloem-limited bacterial pathogens.
- Mollicutes, including Spiroplasma and Phytoplasma, are now recognized as key causes of plant 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 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.