Morphological and Pathological Characterization of Species ofElsinoeCausing Scab Diseases of Citrus

Abstract

Three scab diseases have been described on citrus: (i) citrus scab caused by Elsinoe fawcettii, which is cosmopolitan in humid citrusgrowing areas; (ii) Tryon's scab caused by Sphaceloma fawcettii var. scabiosa, which occurs in Australia and elsewhere; and (iii) sweet orange scab caused by E. australis, which occurs primarily in southern South America. In this study, we compared isolates from Australia, Argentina, and Florida (United States). Neither colony color nor conidial size or shape could be used to distinguish the three scab fungi on citrus. A detached-leaf assay was developed to compare pathogenicity of isolates on rough lemon, sour orange, and grapefruit. All isolates from Australia produced scab only on rough lemon, whereas all isolates from Florida produced scab on rough lemon and grapefruit and a portion of these also produced scab on sour orange. E. australis isolates from Argentina did not affect leaves of any species. One E. fawcettii isolate from Argentina produced the same reaction as Australian isolates, and two produced the same reaction as the Florida group that affected rough lemon, grapefruit, and sour orange. In greenhouse inoculations, all Australian isolates infected rough lemon, lemon, and Rangpur lime; all but a few also infected Cleopatra mandarin. In greenhouse inoculations in Florida, all isolates infected rough lemon and Cleopatra mandarin and some isolates infected sour orange. Although E. australis is not readily differentiated from E. fawcettii morphologically, we concluded that they are separate species on the basis of pathogenicity and molecular analysis. S. fawcettii var. scabiosa could be differentiated from E. fawcettii only by host range. We recognized four pathotypes of E. fawcettii: the Broad Host Range and Narrow Host Range pathotypes originally described in Florida, the Tryon's pathotype (formerly S. fawcettii var. scabiosa), and a new Lemon pathotype from northern Australia.