A study of progressive stages of discoloration and decay in Malus using scanning electron microscopy

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Abstract
Scanning electron microscopy has elucidated the roles of microorganisms during various stages of discoloration and decay in living apple trees Malussylvestris Mill. Disks were removed sequentially at 30-cm intervals up the tree from the bases of decayed trunks into clear sapwood from ten 20- to 30-year-old (Red Delicious) apple trees variously affected by internal decay. Evidence for a microbial succession and morphological changes in cell constituents was observed. Bacteria, yeasts, and other nonhymenomycetes (e.g., Cytospora spp.) appeared to be primary colonists of discolored tissues, which altered cell wall components, and degraded wound-initiated vessel plugs. Basidiomycetous hyphae were observed only in tissues where amorphous vessel deposits had been degraded by pioneer microorganisms. Associations of secondary bacteria and yeasts with wood-destroying hymenomycetes, indicating a mutualistic existence, were frequent. Decay was of the white rot type with numerous crystalline structures observed in regions of advanced decay.