Apple Bitter Rot and Glomerella Leaf Spot: A Comprehensive Review of Causal Species and Their Biology, Fungicide Sensitivities, and Management Strategies.

Bitter rot and Glomerella leaf spot (GLS) are two distinct diseases of apple fruit and foliage caused by members of the ascomycete fungal genus . Although GLS is restricted to subtropical and, in some areas, to temperate climates, bitter rot is responsible for significant yield loss worldwide, particularly during the postharvest period. Initially thought to be caused by just two species of , . , and . , advances in molecular biology and sequencing techniques enabled the identification of 25 different species capable of causing bitter rot and/or GLS of apple belonging to the . species complex (CGSC), . species complex (CASC), and . species complex (CBSC). Three species (. , . , and . ) of CGSC cause both bitter rot and GLS, 18 species (6 of CGSC and 12 of CASC) only cause bitter rot, and 4 species (. and . of CGSC, . of CASC, and . of CBSC) only cause GLS. These species were found to differ in their geographical distribution, environmental and host tissue preference, pathogenicity, and fungicide sensitivities. In this review, we summarize the distribution, life cycle, and pathogenicity mechanisms of all currently known species responsible for bitter rot and GLS of apple. Furthermore, we describe known apple defense mechanisms and management strategies for the control of these economically significant pathogens and identify gaps in our present understanding for future research.