Chrysanthemum phloem necrosis: symptomatology and histopathology

The symptomatology and histopathology of chrysanthemum phloem necrosis, a new disease of florists' chrysanthemum was investigated. The disease results in flecking, yellowing, and premature deterioration of lower leaves and flower distortion in the Dendranthema grandiflora Tzveler. (syns. D. morifolia Ramat., Chrysanthemum morifolium Ramat.) Marble cultivar group. Cleared leaves of 'Pink Marble' revealed vascular browning in midveins and secondary veins that was enhanced at 32 °C. The sieve elements and associated cells in affected leaves were often necrotic and collapsed, a condition that was aggravated by elevated temperature. Spherical-to-elongate mycoplasmalike organisms, 0.2–1.0 μm diameter, were observed only in infected plants within collapsed sieve elements. Mycoplasmalike organisms were not detected in uninfected plants; sieve-plate elements contained merely fibrillar p-protein, a normal cell component. In comparison to uninfected plants abnormal callose accumulation and lignin formation were obse...