Yellow-blotch disease (YBD) is most commonly observed on boulder star corals including Montastraea annularis M. faveolata and M. franksi, and occasionally on large cup star coral (M. cavernosa) and brain coral (Colpophyllia natans). In corals with YBD, there is no mat of microorganisms as observed in BBD, and very little recently exposed (white) skeleton is apparent.

The disease may first appear as a small (1-2 cm diameter) circular area of pale to lemon yellow tissue, completely surrounded by live green to brown tissue. Over time the blotch increases in size, and eventually the tissue that was first affected dies.

Colony affected by YBD in1999

YBD also often starts somewhere along the outer perimeter of the coral, or within the center of a colony, forming a narrow band or yellowish lightened tissue that slowly spreads outward. As the infection progresses, the discolored band radiates outward slowly killing tissue at a rate of 7-10 cm/year. Unlike most other diseases, recently denuded white skeleton is rarely observed - typically the area behind the band is covered in filamentous and coralline algae.

Colony of M. faveolata with a rapidly advancing YBD. The affected areas include tissue that has recently died (A), diseased tissue that has become a dark yellow color (B) and a rapidly advancing disease front that is very light yellow in color (C).
 

In some situations, the leading edge of the band adjacent to healthy tissue often becomes very light yellow while tissue behind this gradually darkens and then dies.

Although tissue affected by YBD has fewer symbiotic algae (zooxanthellae) than normal tissue, this condition should not be confused with Bleaching.

Colony of M. faveolata is affected by YBD in multiple locations

Although the rate of tissue loss is fairly slow colonies can lose substantial amounts of tissue because colonies can be infected in multiple locations and the slow tissue loss can continue for many years.

 
Close-up of a single yellow blotch. The center of the blotch is dark yellow and is beginning to die; sediment is starting to accumulate on affected tissue. The leading edge of the diseased tissue is pale yellow.