1. Contribution from the Dipartimento di Chimica Organica e Biochimica Complesso, Universitario Monte Sant'Angelo, Via Cintia 4, 80126, Napoli, Italy, Section for Plant Pathology, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg, Denmark, BIOMERIT Research Centre, Department of Microbiology, BioSciences Institute, National University of Ireland, Cork, Ireland, and Bioorganische Chemie, Gebäude NWI, Universität Bayreuth, D-95440 Bayreuth, Germany