Recebido em 20/11/12; aceito em 15/2/13; publicado na web em 24/5/13A bioassay-guided fractionation of culture filtrates of the fungus Alternaria euphorbiicola, a pathogen of the weed Euphorbia heterophylla, led to the isolation of anhydromevalonolactone (1), tyrosol (2), (R)-(-)-mevalonolactone (3), and cycloglycylproline (4). When tested on the punctured leaves of the host plant, these compounds produced bleached lesions with dark brown margins at concentrations as low as 80 µM. When tested on the leaves of other relevant weeds, only cycloglycylproline showed selective activity against E. heterophylla. This is the first report on the isolation of phytotoxins from A. euphorbiicola and on the phytotoxicity of anhydromevalonolactone, (R)-(-)-mevalonolactone, and cycloglycylproline.Keywords: fungi; phytotoxin; wild poinsettia. INTRODUCTIONSurveys for fungal pathogens of wild poinsettia in Brazil and their evaluation as potential mycoherbicides for the management of this weed were initiated in the late 1980s. Among the fungi collected and studied, preliminary results indicate that Alternaria euphorbiicola Simmons & Engelhard holds great promise for herbicidal control. This fungus was found to cause severe inflorescence necrosis, foliage blight, and stem canker on Euphorbia heterophylla L. 1 Euphorbia heterophylla (wild poinsettia) is regarded as a major weed in many tropical and subtropical countries, being responsible for substantial agricultural losses in important crops, particularly soybeans and corn. 2 During early investigation, when A. euphorbiicola was evaluated as a putative mycoherbicide, it was observed that the application of fungal spore suspensions on the host plant resulted in the production of widespread necrosis at time intervals ranging from few hours to 1-2 days after application. However, this was regarded as too short a period to allow for the occurrence of the whole process of spore germination, fungal penetration of tissues, colonization of plant organs, and development of symptoms due to plant infection. These observations led to the conjecture that A. euphorbiicola might produce phytotoxic metabolites capable of causing the first wave of damage to the host plant.It is well recognized that phytopathogenic fungi commonly produce toxins that may play a role in plant disease development, adversely affecting their infected hosts. 3,4 In this paper, we describe the isolation of anhydromevalonolactone (1), tyrosol (2), (R)-(-)-mevalonolactone (3), and cycloglycylproline (4) from A. euphorbiicola and their phytotoxic activity against the host plant wild poinsettia. EXPERIMENTAL Fungal strain, culture media, and growth conditionsAlternaria euphorbiicola was isolated from naturally infected E. heterophylla plants collected in Viçosa, Minas Gerais State, Brazil. The fungal species was identified by Prof. Robert Weingart Barreto, from the Department of Phytopathology, Universidade Federal de Viçosa (UFV), and stored at 5 °C in the culture collection Octávio de Almeida Drumond of UFV. The fungus was c...