Scanning Electron Microscope Examination of Sugarbeet Flowers and Fruits Infected with Phoma Betae

Abstract

There are three natural openings in a mature sugarbeet fruit which serve as avenues of entry for microorganisms: 1) the basal pore which contains dried parenchyma and vascular tissue and is the point where the flower was connected to the stalk; 2) the apical pore where the style was inserted; and 3) the peripheral zone of dehiscence where the operculum separates from the fruit cavity wall during germination. The apical pore was first described in this study. Scanning electron microscopy of the naturally infected fruits showed, for the first time, hyphal penetration through both the basal pore and the peripheral zone. Examination of sugarbeet flowers artificially infected with Phoma betae also showed fungal penetration through the apical pore. Dense hyphal growth was associated with stigmal lobes and ungerminated pollen grains. Fungal growth apparently was stimulated by excretions from the stigma. Penetration of the fruit cavity wall and the operculum would render the fungus inaccessible to protectant fungicides and explains why the most successful seed treatments for P. betae have included volatile mercury fungicides or seed soak in thiram. Such treatment allows direct contact between the toxin and the pathogen.