Sclerotinia sclerotiorum is a necrotrophic fungal pathogen with a very wide host range. Isolates of this pathogen are normally described as having fluffy and white mycelium; however, isolates of S. sclerotiorum with darkly-pigmented mycelium on potato dextrose agar medium have been identified in eastern New Mexico and western Texas from Valencia peanut fields. Mutant non-pigmented S. sclerotiorum isolates (SW) were created in an earlier study from wild-type pigmented isolates (SD) using melanin inhibitors. The SD isolates were pathogenic on Valencia peanut, whereas the SW isolates were not. The current study was conducted to further characterize the differences between SD and SW isolates in regards to metabolite production and utilization, including the effects of carbon sources and oxalic acid precursors on oxalic acid production and pathogenicity on Valencia peanut. Gas chromatography–mass spectrometry (GC/MS) metabolomics analysis revealed a down-regulation of several sugars and compounds within the citric acid cycle as well as oxalic acid for SW isolates of S. sclerotiorum. The addition of glucose to potato dextrose agar medium allowed for the production of oxalic acid and restored pathogenicity in SW isolates that were previously non-pathogenic on Valencia peanut. This study indicates that glucose alone plays a major role in oxalic acid production and pathogenicity of S. sclerotiorum.