Endopolygalacturonase is essential for citrus black rot caused by Alternaria citri but not brown spot caused by Alternaria alternata

Alternaria citri, the cause of Alternaria black rot, and Alternaria alterna ta rough lemon pathotype, the cause of Alternaria brown spot, are morpholog ically indistinguishable pathogens of citrus: one causes rot by macerating tissues and the other causes necrotic spots by producing a host-selective t oxin. To evaluate the role of endopolygalacturonase (endoPG) in pathogenici ty of these two Alternaria spp, pathogens, their genes for endoPG were muta ted by gene targeting, The endoPGs produced by these fungi have similar bio chemical properties, and the genes are highly similar (99.6% nucleotide ide ntity). The phenotypes of the mutants, however, are completely different. A n endoPG mutant of A. citri was significantly reduced in its ability to cau se black rot symptoms on citrus as well as in the maceration of potato tiss ue and could not colonize citrus peel segments. In contrast, an endoPG muta nt of A. alternata was unchanged in pathogenicity. The results indicate tha t a cell wail-degrading enzyme can play different roles in the pathogenicit y of fungal pathogens, The role of a cell wall-degrading enzyme depends upo n the type of disease but not the taxonomy of the fungus.