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Factors Affecting Staining of Sclerospora graminicola Oospores with Triphenyl Tetrazolium Chloride. R. J. Williams, Principal pathologist, Millet Improvement Program, International Crops Research Institute for the Semi-Arid Tropics, ICRISAT Patancheru Post Office, Andhra Pradesh 502 324, India; M. N. Pawar(2), and I. Huibers-Govaert(3). (2)(3)Research technician, and research scholar, respectively, Millet Improvement Program, International Crops Research Institute for the Semi-Arid Tropics, ICRISAT Patancheru Post Office, Andhra Pradesh 502 324, India. Phytopathology 70:1092-1096. Accepted for publication 16 May 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-1092.

Oospores of the pearl millet downy mildew pathogen, Sclerospora graminicola, at various concentrations, were treated with 2,3,5-triphenyl-tetrazolium chloride (TTC) at various temperatures, TTC concentrations, and for various time periods following presoaking in distilled water at two temperatures for several time periods. TTC-treated oospores remained unstained or developed colored cytoplasm varying from light pink to deep red. The proportion of oospores stained varied greatly with presoaking time, incubation time, incubation temperature, TTC concentration, and oospore concentration. In some instances, oospores stained at one examination had lost the stain by the next examination (destaining). The most staining occurred when an aqueous oospore suspension containing ~48 × 10⁵ oospores per milliliter that had been presoaked at 40 C for 48 hr was incubated in 1% TTC solution at 40 C for up to 9 wk. The results indicate that the technique previously reported for routine screening of S. graminicola oospores for viability using TTC is unreliable and emphasize that a technique useful for one fungal species may not be directly applicable to another. More work is needed to determine whether TTC can be used to give a reliable quantitative measure of viability of S. graminicola oospores, and the problems of differential response to environmental factors, oospore wall permeability, mycoparasitism, concentration effects, and destaining all must be recognized and dealt with.