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Nonpathogenic Isolates of the Citrus Black Spot Fungus, Guignardia citricarpa, Identified as a Cosmopolitan Endophyte of Woody Plants, G. mangiferae (Phyllosticta capitalensis)

May 2002 , Volume 92 , Number  5
Pages  464 - 477

R. P. Baayen , P. J. M. Bonants , G. Verkley , G. C. Carroll , H. A. van der Aa , M. de Weerdt , I. R. van Brouwershaven , G. C. Schutte , W. Maccheroni Jr. , C. Glienke de Blanco , and J. L. Azevedo

First and seventh authors: Plant Protection Service, P.O. Box 9102, 6700 HC Wageningen, the Netherlands; second and sixth authors: BioInteractions and Plant Health Unit, Plant Research International, P.O. Box 26, 6700 AA Wageningen, the Netherlands; third and fifth authors: Centraalbureau voor Schimmelcultures, P.O. Box 85167, 3508 AD Utrecht, the Netherlands; fourth author: Department of Biology, 1210 University of Oregon, Eugene; eighth author: Outspan Citrus Centre, P.O. Box 28, Nelspruit 1200, South Africa; tenth author: Department of Genetics, Universidade Federal do Paraná, Curitiba 81531-990, PR, Brazil; and ninth and eleventh authors: Department of Genetics, Escola Superior de Agricultura “Luiz de Queiroz,” Universidade de São Paulo, Piricicaba 13400-970, SP, Brazil

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Accepted for publication 5 February 2001.

The population structure of Guignardia citricarpa sensu lato (anamorph: Phyllosticta citricarpa), a fungus of which strains pathogenic to citrus are subject to phytosanitary legislation in the European Union and the United States, was investigated. Internal transcribed spacer sequences revealed two phylogenetically distinct groups in G. citricarpa. This distinction was supported by amplified fragment length polymorphism analysis that also supported the exclusion of two isolates that had apparently been misclassified as G. citricarpa. On cherry decoction agar, but not on other media, growth rates of group I isolates were lower than those of group II isolates. Conidial dimensions were similar, but group I isolates formed conidia with barely visible mucoid sheaths, whereas those of group II formed conidia with thick sheaths. Cultures of isolates belonging to group I produced rare infertile perithecia, whereas fertile perithecia were formed by most isolates of group II. Colonies of isolates belonging to group I were less dark than those of group II, with a wider translucent outer zone and a lobate rather than entire margin. On oatmeal agar, exclusively group I isolates formed a yellow pigment. Group I harbored strains from citrus fruits with classical black spot lesions (1 to 10 mm in diameter) usually containing pycnidia. Group II harbored endophytic strains from a wide range of host species, as well as strains from symptomless citrus fruits or fruits with minute spots (<2-mm diameter) without pycnidia. These observations support the historic distinction between slowly growing pathogenic isolates and morphologically similar fast-growing, nonpathogenic isolates of G. citricarpa. The latter proved to belong to G. mangiferae (P. capitalensis), a ubiquitous endophyte of woody plants with numerous probable synonyms including G. endophyllicola, G. psidii, P. anacardiacearum, and P. theacearum. G. mangiferae occurs in the European Union and the United States on many host species including citrus, and does not cause symptoms of citrus black spot, justifying its exclusion from quarantine measures.

The American Phytopathological Society, 2002