Morphological and chemical studies of Penicillium oxalicum, newly identified as a pathogen on greenhouse cucumbers

Abstract

Identification of the fungus designated as Penicillium crustosum Thom causing stem and fruit rot of greenhouse cucumbers (Cucumis sativa) as Penicillium oxalicum Currie and Thom has been substantiated by morphological and chemical studies. Scanning electron microscopy and light microscopy revealed elliptical conidia produced on smooth-walled conidiophores characteristic of P. oxalicum. Colonies were velvety in texture and blue-green in color. Secalonic acid D and oxaline were isolated from Czapek – yeast extract agar cultures and identified through their physical constants and mass, infrared, ultraviolet, and proton nuclear magnetic resonance spectra. Secalonic acid D was obtained also from Czapek–Dox broth cultures together with a previously undescribed compound, which we have named penoxin. In addition, oxalic acid and calcium oxalate were detected in these cultures by gas chromatography and light microscopy of crystals. Morphological and chemical observations are consistent with published descriptions of P. oxalicum. This is the first report of P. oxalicum causing a storage rot of cucumber and tomato fruit. Key words: stem and storage rot, identification, penoxin.