ABSTRACT: Colorimetric techniques, such as tetrazolium salts and resazurin, are indicators of viability (respiratory activity) of fungi, bacteria, and seeds widely used in different areas of research. The objective of this study was to develop a protocol with two screening methods, qualitative and quantitative, to detect the viability of Fusarium semitectum in soybean seeds using 2,3,5-triphenyl tetrazolium chloride (TZ). The experimental design was completely randomized in a triple factorial arrangement (incubation times, TZ concentrations, and inoculum serial dilutions) with levels of 3, 5, and 5 of each factor, respectively, and 3 replications in both methods. For the qualitative method, sensitivity was more satisfactory at 72 hours, at the minimum TZ concentration of 0.1 mg mL-1, and with the inoculum concentration of 10-¹. In the quantitative method (spectrophotometry), it was not possible to quantify the colony forming units (CFU) on plates, with the Optical Density (OD) being interfered with by structure of the mecelium in the microtube. Although it is a reproducible methodology, it is conditioned by fungal morphology. The present study allowed easy visualization of the structures of F. semitectum in soybean seeds, based on the TZ. However, it was not possible to quantify the inoculum density, due to low spore production, requiring greater adjustments in the methodology.
