Photinia Black Spot is a widespread and destructive disease caused by the fungus Entomosporium. Most damage occurs during periods of cool, wet weather and when active growth is occurring. Initial signs of infection appear as small spots on both the upper and lower surfaces of young leaves. As the infection progresses, these small spots may coalesce into large maroon blotches.
A low level of leaf spot infection usually causes little more than cosmetic damage, however these spots remain as a source of spores for future infections. In years with severe infections, the leaves may drop prematurely. Heavy leaf drop severely reduces the landscape value of Photinias and can cause plant death. If the infestation is widespread, it may be necessary to remove severely diseased plants and replace them with another plant species that is not susceptible to leaf spot.
This disease is very difficult to control after plants are severely infected. However, during extended cool, wet periods, protective sprays may be applied. The recommended treatment is with Bordeaux Mix, which is made of Hydrated Lime and Copper Sulphate.