Diversity and management of common bunt of wheat in the Great Plains

Abstract

Common bunt is a significant disease of wheat caused by the fungal pathogens Tilletia tritici and Tilletia laevis. Dwarf bunt is a similar disease of wheat caused by Tilletia controversa Kühn, a genetically related pathogen that is regulated by many countries. Management of common bunt and dwarf bunt requires different strategies based on the unique timing of infection for each disease. Proper management thus requires diagnosis. The current diagnostic standards for differentiation of the causal agents of common bunt and dwarf bunt rely on differences in morphological characteristics of teliospores and dissimilarities in the environmental requirements for teliospore germination. Key morphological traits of teliospores include teliospore diameter, the presence or absence of reticulations on the spore surface, and the depth of these protrusions. The rate of teliospore germination at different temperatures is considered the most accurate method for the differentiation of T. tritici and T. controversa; however, this can take many weeks to complete. Here we evaluated the current standards for species differentiation across a large set of reference isolates and isolates collected from grain samples as well as the efficacy of current seed treatments for management. We characterized the isolates based on spore diameter, reticulation depth, and percent germination over time at 18 °C and 5 °C. We then evaluated conventional management recommendations for these isolates, including the efficacy of seed treatment fungicides in small plot trials. T. controversa reference isolates used in this study, as a group, had an average reticulation depth of 1.376 µm. This is smaller than previously suggested for this species, as 1.5-3.5 µm is the range reported by Durán et al. (1961). The average reticulation depth for all T. tritici reference cultures was 0.754 µm, which aligns with the published reticulation depth range of 0.5-1.5 µm (Durán, 1961). Variability within individual isolates makes these comparisons uncertain. For example, isolate UT0191 had reticulations with depths ranging from 0.498 µm to 3.435 µm. Isolate UU0215 had reticulation depths spanning from 0.313 µm to 2.245 µm. Isolate MI0080 had the greatest variability in reticulation depth with a range of 0.593 µm to 2.844 µm. This level of variability suggests that diagnoses based on reticulation depth would likely result in false positives. When unknown isolates were compared to reference cultures used in this study, twelve unknown isolates had average reticulation depths within the range observed for T. controversa. Out of these twelve, only two (MI0080 and KS0040) had depths similar to those reported in the literature for T. controversa. The mean teliospore diameter in this study (6.807-19.563 µm) was lower than that reported in the literature (T. tritici: 14-23.5 µm, T. laevis: 14-22 µm, T. controversa: 19-24 µm (Goates, 1996)) and appeared to provide no discernable distinction between all three reference species. All T. controversa reference cultures had no germination for the duration of the 15-day (18 °C) incubation period, followed by at least 5% germination after a minimum of 30 days incubated at 5°C. Nearly all T. tritici isolates surpassed 5% germination in only 6 DPI at 18 °C. Overall, T. laevis cultures had low viability. Most T. laevis isolates achieved 5% germination after 6 DPI. Most notably, reference isolate UU0238 did not achieve 5% germination until 45 DPI (30 days at 5 °C). Results of this study suggest that isolates of Tilletia may be present in the Great Plains that germinate like T. tritici but have spore morphology similar to T. controversa and these teliospores would likely be misdiagnosed based on the current diagnostic protocols. Our findings suggest that three currently marketed fungicide seed treatments are highly effective for controlling at least one of the morphologically ambiguous isolates characterized in this study. These results have important implications for the understanding, diagnosis, and management of these species in the Great Plains.