Brown spots on sawtooth oak (Q. acutissima) leaves were investigated for elemental composition using analytical electron and X-ray microscopy. The spots were approximately 1 mm in diameter and had a yellow halo. Leaf fragments with spots were directly mounted to a loader and observed using X-ray microscopy. Three-dimensional reconstructions showed that grain-like structures were mostly concentrated in the spots as well as along the leaf veins. Field emission scanning electron microscopy revealed no distinct surface differences between the spots and asymptomatic leaf regions. In situ X-ray microanalysis showed a homogeneous distribution of calcium and potassium on the asymptomatic leaf regions. However, a notable increase in calcium concentration was detected in the spots. Based on the elemental composition and distribution in leaves, the grain-like structures were assumed to be calcium oxalate. These results suggest that the necrotic leaf tissues may have undergone the elemental relocation in response to infection. Noninvasive X-ray microscopy can provide insight into the pathological changes in plants.