Myrmaeciella caraganae was recollected in and around Vienna, Austria and found to be morphologically different generically from the type species of Myrmaeciella, M. endoleuca. We redescribe M. caraganae in the new genus Stromatonectria. Phylogenetic analyses of LSU sequences place the genus in the Bionectriaceae, Hypocreales. S. caraganae occurs on branches of Caragana spp., Colutea arborescens and Laburnum anagyroides of the Fabaceae. It is characterized by spheroid perithecia partly or entirely immersed in a Hypocrea-like stroma, a Nectria-like centrum and bicellular hyaline ascospores. Conidia of S. caraganae are produced in compound pycnidia that are formed prior to or in association with perithecia. Sporodochia but no pycnidia are formed in culture. We discuss the genus Myrmaeciella and compare S. caraganae with species of the Nectriaceae, including Nectria balansae, N. eustromatica and N. paraguayensis.