CORAL DISEASES

Many thanks to CORDAP for their excellent Roadmap Report on Coral Diseases (PDF), published September 2025.

Coral reefs worldwide are facing an unprecedented crisis. Large-scale bleaching events, caused primarily by marine heatwaves and exacerbated by declining water quality have drastically reduced coral cover. Infectious diseases and other stressors have further contributed to coral loss, disrupting reproductive connectivity and natural recruitment, which are essential for reef resilience.

Understanding and management of disease outbreaks on both natural and restored reefs are an urgent priority. Without proactive disease mitigation, restoration efforts risk being undermined by recurring outbreaks. To effectively address coral disease we need to integrate ecology, microbiology, medicine, predictive modelling, and investment in research.

In recent decades, coral diseases have emerged as one of the primary global threats to the health of coral reefs. The increasing frequency and intensity of marine heatwaves have triggered mass mortality events and drastically reduced live coral cover, resulting in significant biodiversity loss and widespread degradation. Generally, coral diseases are characterised by changes in coral colouration, rapid tissue loss, and disruption of the symbiotic relationship with Symbiodiniaceae, often culminating in coral mortality. Various pathogens, including bacteria, fungi, protozoa, and viruses, have been associated with coral diseases. Based on their pathology, coral diseases can be classified into three categories: growth anomalies, pigmentation disorders, and tissue-loss diseases.

Conceptual model: holobiont components interact with external factors and biological agents in the development of coral diseases.

The coral holobiont comprises three main components –

– Coral: The fundamental structure forming the reef, produced by scleractinian corals, providing a physical habitat and energy through carbon fixation carried out by its symbionts.
– Symbiotic microalgae (Symbiodiniaceae): perform photosynthesis, supplying essential energy products to the coral. Environmental disturbances, such as elevated sea surface temperature (SST), lead to coral bleaching due to the disruption of this symbiotic association.
– Microbiome: A diverse microbial community including bacteria, archaea, fungi, viruses, and protozoa and their interactions within the holobiont. It plays a crucial role in regulating coral health but can also include pathogens such as Vibrio spp. and Roseofilum reptotaenium.

These 3 main components interact dynamically with external factors and biological agents that influence coral health and the prevalence of diseases –

– Environmental stress: Elevated SST and eutrophication are key triggers that exacerbate coral susceptibility to diseases. These factors disrupt the holobiont’s homeostasis, weakening coral immune defences and favouring proliferation of pathogens.
– Biological factors: Associated biotic pathogenic agents, including bacteria, fungi, and viruses, interact with environmental conditions to promote disease emergence and transmission.
– Coral disease dynamics: Changes in coral colouration, rapid tissue loss, and disruption of the symbiosis, often culminating in coral mortality.

Conclusions

To address the knowledge gaps in coral pathology, it’s crucial to gain a comprehensive understanding of the dynamics of coral diseases across multiple spatial and temporal scales. Future studies must extend beyond localised assessments and establish long-term multiregional monitoring programs that capture the variability in disease prevalence and the environmental stressors triggering different diseases. Attention should focus on the functional interactions between corals, their microbiomes, and environmental stress factors. This includes targeted investigations into microbial manipulation strategies, such as probiotics, phage therapy and microbial transplants, to enhance coral immunity and mitigate pathogen infections.