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Cytology and Histology

Graft Union Histology and Distribution of Tomato Ringspot Virus in Infected McIntosh/Malling Merton 106 Apple Trees. Margaret A. Tuttle, Former graduate research assistant, Botany Department, Plant and Soil Science Department, University of Vermont, Burlington 05405; Alan R. Gotlieb, associate professor, Plant and Soil Science Department, University of Vermont, Burlington 05405. Phytopathology 75:347-351. Accepted for publication 14 September 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-347.

Tissues from the graft union of Malus domestica 'McIntosh'/Malling Merton 106 (MM 106) trees infected with tomato ringspot virus (TmRSV) were sectioned, stained, and examined through a light microscope. After at least 8 yr of compatible growth, the orientation of vessels and fibers just above the union became inclined rightward with respect to the main stem axis. The angle of inclination increased gradually, reaching almost 90 degrees after about 5 yr. Three to 5 yr after the initiation of counterclockwise spiral grain, a zone of solid parenchyma and a xylem indentation developed at the union. Most trees also had a scion overgrowth or inverted shoulder. These trees did not produce pegs of dark tissue, which characterizes apple union necrosis and decline in other cultivars. Abnormal tissue at the union interfered with mobilization or translocation of carbohydrates. TmRSV was readily detected with enzyme-linked immunosorbent assay in MM 106 tissue. Rootstock bark tissue was more reliable than rootstock sprout leaf tissue for indexing trees. TmRSV was detected in three of 21 McIntosh scions. Tobacco ringspot virus was not detected in rootstock tissue.