Xylella fastidiosa is the causative agent in Pierce's disease (PD) in Vitis vinifera L. (grape) vines. Xylella fastidiosa colonizes and disseminates itself from one xylem element to another by dissolution and breach of pit membranes. These studies on naturally infected V. vinifera grown under vineyard conditions document by transmission electron microscopy that there is dissolution and breach of pit membranes by X. fastidiosa in vertical and lateral colonization in PD. These processes were documented in two cultivars of V. vinifera: 'Syrah' and 'Cabernet Sauvignon.'