The Developmental Anatomy of Pierce's Disease Symptoms in Grapevines: Green Islands and Matchsticks.

Symptoms of Pierce's disease were studied in an anatomical context from infected grapevines (Vitis species) collected from field sites within Napa Valley, CA. Two symptoms, green islands and matchsticks, are reported in this study. Green islands formed as a result of incomplete initiation of the phellogen. In regions of the stem where a phellogen and subsequent periderm arose, immediately exterior tissue was cut off, causing it to brown. In regions of the stem where no periderm is formed, the exterior tissues remained green. Consequently, the stem is mottled with both green living epidermis and brown dying epidermis as determined by the presence or absence of an underlying periderm. Matchsticks formed when the leaf lamina separated from the petiole, and the petiole remained attached to the stem. Lamina broke off from the petioles consistently in a fracture zone where xylem from the petiole anastomoses into the five major veins of the leaf. No separation layer was found to explain this pseudoabscission.