White plague is one of the most devastating coral diseases in the Caribbean, and yet important aspects of its epidemiology, including how the disease transmits, remain unknown. This study tested potential mechanisms and rates of transmission of white plague in a laboratory setting. Transmission mechanisms including the transport of water, contact with macroalgae, and predation via corallivorous worms and snails were tested on the host species Orbicella annularis. Two of the tested mechanisms were shown to transmit disease: water transport and the corallivorous snail Coralliophila abbreviata. Between these transmission mechanisms, transport of water between a diseased coral and a healthy coral resulted in disease incidence significantly more frequently in exposed healthy corals. Transmission via water transport also occurred more quickly and was associated with higher rates of tissue loss (up to 3.5 cm d -1 ) than with the corallivorous snail treatment. In addition, water that was in contact with diseased corals but was filtered with a 0.22-lm filter prior to being introduced to apparently healthy corals also resulted in the transmission of disease signs, but at a much lower rate than when water was not filtered. This study has provided important information on the transmission potential of Caribbean white plague disease and highlights the need for a greater understanding of how these processes operate in the natural environment.

Outbreaks of coral white plague (WP) disease have caused significant regional declines of reef‐building Caribbean corals. Due to a greater availability of epidemiological data, studies have primarily focused on shallow coral reefs (< 30 m). In the U.S. Virgin Islands, however, WP disease prevalence appears to be higher on upper mesophotic (30–40 m) coral reefs when compared with shallow reefs and may be inhibiting coral recovery after environmental disturbances. The aim of this study was to investigate the relationship of environmental drivers with spatio‐temporal patterns of WP prevalence across shallow and mesophotic coral reefs in the U.S. Virgin Islands. We recorded WP prevalence at 13 reef sites (five shallow, three mid‐depth and five upper‐mesophotic reefs) across the south shelf of St. Thomas approximately monthly between 2012 and 2015 using a drop camera method. The influence of environmental factors on disease prevalence was investigated using Bayesian inference with generalized linear mixed‐effect models. We found that WP tended to increase during the beginning of the rainy season (June), and when levels of water turbidity and temperature were higher, and levels of oxygen and salinity lower. The disease prevalence was higher on mesophotic than on shallow or mid‐depth reefs, probably due to higher availability of corals (host), and a possible temperature threshold for WP occurrence that allows long‐term persistence (year‐round) of the disease on upper‐mesophotic reefs. This is the first study to implement the drop camera method to survey a coral disease over several reef sites and depths. This method can be applied on surveys of other rapid tissue loss diseases, such as the newly emergent stony‐coral‐tissue‐loss‐disease (SCTLD).