The effect of pH on staining of oospores of Peronospora viciae with tetrazolium bromide

Tetrazolium bromide was tested as a vital stain for oospores of Peronospora viciae f. sp. fabae and P. viciae f. sp. pisi in phosphate buffers with a pH ranging from 6.0 to 8.0. Oospores stained mostly pink to red, seldom black. The number of black oospores increased with incubation time but the total number of pink, red and black oospores did not increase. The results suggest overstaining as the cause of black oospores. The number of stained, nonautoclaved oospores showed a small increase at higher pH. The pH had a large effect on staining of oospores killed by autoclaving. Similar numbers of autoclaved and nonautoclaved oospores stained at pH 7.5 and 8.0, but few autoclaved oospores stained at pH lower than 7.0. When using tetrazolium bromide (0.1% solution) as a vital stain for oospores of Peronospora viciae, good staining results are obtained with a 48-h incubation at 35 C in solutions at pH 6.0 to 6.5.