Differentiation of species of Elsinoe causing citrus scab
So far, three scab diseases have been described on citrus: 1) Elsinoe fawcettii (citrus scab) which is cosmopolitan in humid regions, has a rather broad host range and attacks leaves and fruits; 2) Sphaeceloma fawcettii var. scabiosa (Tryon's scab), present in Australia but also in some other countries, mainly on lemon and rough lemon; 3) Elsinoe australis (sweet orange scab), occurs primarily in South America, on sweet orange and mandarin and attacks fruits only. Studies have been carried out in Florida (US) and Australia on different citrus scab isolates from Florida, Australia and Argentina, representing the three species. Elsinoe species are listed by the European Union as quarantine pests (EU Annex II/A1). Although the three forms can be distinguished by host range, and are considered to be morphologically different (colony colour, conidial shape), they cause very similar diseases and some doubts have been previously expressed on the validity of the distinct taxa. Also, the teleomorphs are only known from South America.
Morphological characteristics of the three scab fungi were studied, and it was found that neither the colony colour or conidial shape could differentiate them. Detached-leaf assays were performed to compare pathogenicity on various citrus species. It was found that E. fawcettii could readily be differentiated from E. australis on the basis of the host range. Molecular analysis of the different isolates was also conducted. Restriction analysis of the internal transcribed spacer (ITS) of rDNA, and nucleotide sequence of the ITS showed that E. australis is different from both E. fawcettii and S. fawcettii var. scabiosa. PCR amplification of segments of the ITS region and endonuclease cleavage produce different profiles for the three scabs but correlate with their geographical origin and the results of host-range studies. Australian isolates of S. fawcettii var. scabiosa and the Florida isolates of E. fawcettii appear more closely related to each other than to E. australis isolates. The authors concluded that E. fawcettii and E. australis could be considered as two valid and separate species, and that S. fawcettii var. scabiosa could be a pathotype of E. fawcettii. However, further studies on more isolates from other hosts or origins, are still necessary as other pathotypes may exist within E. fawcettii. Finally, the authors felt that molecular analysis could provide a rapid and useful tool in identifying citrus scab types on shipments of fruit and reduce the risk of introduction of exotic types into new areas.
Sources
Timmer, L.W.; Priest, M.; Broadbent, P.; Tan, M.K. (1996) Morphological and pathological characterization of species of Elsinoe causing scab diseases of citrus.
Phytopathology, 86(10), 1032-1038.
Tan, M.K.; Timmer, M.; Broadbent, P.; L.W.; Priest; Cain, P. (1996) Differentiation by molecular analysis of Elsinoe spp. causing scab diseases of citrus.
Phytopathology, 86(10), 1039-1044.