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Histology of Delicious/Malling Merton 106 Trees Affected by Apple Union Necrosis and Decline. Margaret A. Tuttle, Former graduate research assistant, Botany Department, Plant and Soil Science Department, University of Vermont, Burlington 05405, Present address: Department of Plant Pathology, Pennsylvania State University, University Park 16802; Alan R. Gotlieb, associate professor, Plant and Soil Science Department, University of Vermont, Burlington 05405. Phytopathology 75:342-347. Accepted for publication 14 September 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-342.

Tissue sections from the graft union of apple union necrosis and decline-affected Malus domestica trees were examined by light microscopy to determine the anatomical basis of this delayed incompatibility, which has been associated with tomato ringspot virus (TmRSV) infection of rootstock tissue. Histopathological changes in Delicious/Malling Merton 106 trees began abruptly after at least 8 yr of compatible growth. The relative number of ray and axial parenchyma cells increased greatly at the union and above it. Local areas of unlignified parenchyma developed at the union in place of xylem. This tissue was removed when bark was peeled off to expose the union, thereby leaving indentations in the xylem. Pegs of peridermlike tissue were differentiated from cells within these indentations. These three changes occurred within 1- 2 yr after symptom induction. Abnormal tissue at the union interfered with mobilization or translocation of carbohydrates. These histological changes did not appear to be the result of a hypersensitive reaction of cultivar Delicious to TmRSV in rootstock tissue.

Additional keywords: incompatibility.