Septoria epambrosiae sp. nov. on Ambrosia artemisiifolia (common ragweed) David F. Farr & Lisa A. Castlebury Systematic Botany and Mycology Laboratory, USDA, Agricultural Research Service, Beltsville, Maryland 20705-2350 Farr, D. F. & L. A. Castelbury (2001). Septoria epambrosiae sp. nov. on Ambrosia artemisiifolia (common ragweed). - Sydowia 53(1): 81-92. Recently an unidentified Septoria was reported to be pathogenic on Ambrosia artemisiifolia L. (common ragweed) in Hungary and may have potential as a biocontrol agent for this noxious weed. After reviewing the two previously known species of Septoria on Ambrosia (Asteraceae), the Septoria from Hungary is described as a new species, Septoria epambrosiae. A synopsis of the three species of Septoria on Ambrosia is provided. Based on a molecular analyses of the nSSU and complete ITS region of the rDNA, the taxonomic relationships of S. epambrosiae are determined. Septoria epambrosiae clusters with members of the Dothideales with the teleomorph most likely a species of Mycosphaerella. Keywords: Ascomycetes, molecular taxonomy, biocontrol, taxonomy. Ragweed {Ambrosia artemisiifolia L., Asteraceae) is a noxious weed common throughout temperate regions that produces quantities of pollen to which many humans have an allergic reaction, often referred to as hay fever. A species of Septoria Sacc. in Hungary was discovered and tested as a potential biocontrol agent of ragweed (Bohär & Schwarczinger, 1999). Ragweed plants grown in the greenhouse were inoculated with a conidial suspension of this fungus. After 3-4 weeks the infected leaves became completely necrotic and occasionally the entire plants died (Bohär & Schwarczinger, 1999). The possibility of developing this fungus as a biocontrol agent necessitated an accurate identification. The coelomycetous genus Septoria includes about 1,000 described species for which no monographic account exists (Rossman & al., 1987), thus identification is difficult. Host- specificity in Septoria has been assumed and the few recent studies suggest that this assumption is justified, at least, at the level of host genus (Farr, 1991, 1992; Goodwin & Zismann, 1999). Two species of Septoria have been described from Ambrosia, differing primarily in conidial characteristics; none of these previously known species, however, agree with the potential biocontrol fungus of ragweed. The fungus studied by Bohär & Schwarczinger (1999) and three 81 ©Verlag Ferdinand Berger & Söhne Ges.m.b.H., Horn, Austria, download unter www.biologiezentrum.at