Nonlinear microscopy, which in combination with fluorescence spectroscopy is called nonlinear spectral imaging (NLSI), affords access to specimen morphology and (auto)fluorescence spectra at the same time. Thus, it measures the biochemical composition, while also distinguishing different parts of the tissue. Data interpretation is simplified by a new analysis method, the spectral phasor [1], which results in a robust, quick, and semi-blind spectral unmixing of fluorescent species.NLSI and the spectral phasor are a very user-friendly technique and have the potential to address a broad range of microbiological questions. We introduce them as a novel minimum-invasive technique to monitor the state of "fungal cells" (hyphae). Fungi, both used as consumables and organisms to produce industrial and pharmaceutical compounds, require stringent quality control during their growth. To this end, and as one possible application of NLSI and the spectral phasor, we present their use in monitoring the quality and freshness of white button mushrooms.[1] F. Fereidouni, A.N. Bader, and H.C. Gerritsen, Optics Express, 20,12729-12741 (2012).RGB-representation of a nonlinear spectral image of the white button mushroom (A. bisporus, left) and the corresponding phasor.View Large Image | View Hi-Res Image | Download PowerPoint Slide