Strong secondary fluorescence was induced in resting sporangia of Synchytrium endobioticum under long-wave ultraviolet radiation.  Sporangia were treated with 0.85% NaCl, phosphate-buffered at pH 7.4, and stained with 1 × 10^–4 M solutions of basic arylmethane, acridine, or quinone-imine dyes.  Secondary fluorescence was greatest in the walls of sporangia with organized contents, and in the internal regions of sporangia which had hyaline or disorganized contents.  The ratio of wall-fluorescence only to content-fluorescence decreased with increase in sporangial age.  It was hypothesized that infectivity would fall with age.  Pot experiments with sporangia of different ages at several concentrations supported this hypothesis; age was a more influential factor than was inoculum density.  Care was taken to develop a method for extracting sporangia from gall tissue to yield very low backgrounds of stainable debris.  It is suggested that the secondary fluorescence technique can be used as a novel basis for bioassay of the comparative activities of sporangial germinants; and, inter alia, for identifying viability in single sporangium samples.

Additional key words: acridines, sporangial viability, bioassay.