Histo-pathologic changes in the phloem of American elm affected with the virus causing phloem necrosis

Mclean, D. M.

Phloem necrosis is a virus disease of elm. The virus is readily transmitted by grafting diseased bark tissue to healthy suscepts. A natural vector has not been discovered. Following infection, the fibrous roots die first. Necrosis progresses from the fibrous roots into the larger roote and finally, after death of the roots, the inner phloem in the lower portion of the stem may be killed. Miscroscopic pathological changes in the primary phloem tissue of the root include hypertrophied cells and nuclei in the vicinity of mature sieve tubes, followed by hyper-plasia and finally crushing of the sieve-tube cells and companion cells in the older tissues. Hyperplasia and hypertrophy of parenchyma are the most striking microscopic symptoms in the phloem of older roots and stem. A yellow to brown discoloration of the phloem tissue accompanies the death of the roots and usually progresses into the stem and lower branches before the tree dies. Associated with the discoloration is a faint odor of wintergreen in freshly cut tissues, which is not detectable in comparable healthy tissues.