Abstract
Internal necrosis in the upper taproot and in the stems is a common symptom associated with Phoma crown rot of alfalfa. Necrotic tissues were examined by using histological and histochemical techniques in permanent and semi-permanent preparations. Necrosis occurred in the woody tissue only, affecting the vascular elements, fibers, and ray parenchyma. In the upper taproot, tissues at the center of the stele were completely degraded with hyperplasia occurring in some parenchyma cells. Occlusion with wound gums and pectin occurred in elements of the secondary xylem. Wound periderm surrounded the infected areas, limiting radial but not longitudinal colonization by the pathogen. In the stems, different types of occlusions were observed, likely of composition similar to that found in the upper taproot. Hyphae were observed within tracheary elements.