Identification of New Pathogenic Races of Common Bunt and Dwarf Bunt Fungi, and Evaluation of Known Races Using an Expanded Set of Differential Wheat Lines

Abstract

Pathogenic races of Tilletia caries and T. foetida, which cause common bunt of wheat (Triticum aestivum), and Tilletia contraversa, which causes dwarf bunt of wheat, have been identified previously by their reaction to 10 differential wheat lines, each containing single bunt resistance genes Bt1 through Bt10. The reactions of races to the differential wheat lines follow the classic gene-for gene system for host–pathogen interactions. The pathogens are closely related and resistance to both diseases in wheat is controlled by the same genes. To better define pathogenic races, six additional wheat lines containing the genes Bt11 through Bt15 and a wheat line with a resistance factor designated as Btp were added to the set of 10 differentials and tested with all named U.S. races of common bunt and dwarf bunt. In addition, new isolates of dwarf bunt, and common bunt from hybrids and field collections, were tested with all 16 differentials for race identification. Six new races of T. caries, five new races of T. foetida, and two new races of T. contraversa were identified. Races of common bunt virulent to Bt8 or Bt12, and dwarf bunt races virulent to the combinations of Bt11 and Bt12, and Bt8, Bt9, Bt10, Bt11, and Bt12, were identified for the first time. Comparison of the reactions of the common bunt races with the Bt14 and Bt15 differentials grown in different environments after initial infection showed that these genes are temperature sensitive, indicating they should be excluded from the set of differential lines to avoid ambiguity in determining virulent or avirulent reactions. In the previous list of bunt races, there were races that had the same reaction to the set of 10 differentials but were designated as different races. These races were not differentiated further with the six additional differentials, indicating that the duplicate races should be dropped from the list of pathogenic races. The new races of common bunt and dwarf bunt identified have unique patterns of virulence that allow specific targeting and elucidation of bunt resistance genes in wheat and will aid the development of bunt-resistant wheat cultivars.

A novel QTL associated with dwarf bunt resistance in Idaho 444 winter wheat

“…The fungi are related to the extent that both dwarf and common bunt host plant resistances (HPR) are controlled in wheat by the same genes ( Bt ) in a classic gene-for-gene system of host–pathogen interaction (Metzger and Hoffmann 1978; Goates 1996, 2012). Thus, HPR genes effective against dwarf bunt also confer resistance to common bunt.…”

“…Thus, HPR genes effective against dwarf bunt also confer resistance to common bunt. Currently, 36 pathogenic races of T. caries , 15 races of T. foetida and 19 races of T. controversa are identified based on their reaction to 14 wheat differential lines that each putatively contains one of 14 recognized bunt resistance genes, Bt1 through Bt13 , and Btp (Goates 2012). Two additional HPR genes, Bt14 and Bt15 , were identified by R.J. Metzger in the tetraploid (durum) spring habit wheats Doubbi (CI 13711) and Carleton (CI 12064), respectively (Goates 1996).…”

“…They were excluded from recent pathogenic race tests primarily because they demonstrated temperature-sensitive responses in these wheat backgrounds. Thus, they may not reliably give a correct resistant or susceptible reaction to defined bunt races (Goates 2012). …”

“…Bt8 is likely responsible for the high level of resistance to dwarf bunt in some cultivars deriving resistance from PI 178383. It is the only known gene from that germplasm that is not compromised by known races of dwarf bunt in the U.S. (Goates 2012). The longevity of the resistance provided by PI 178383 demonstrates the durability of the resistance to bunt that can be provided by specific genes.…”

“…Race D-18 expresses weak virulence to the Bt8 differential line, and L-18 and D-19 express virulence to the Bt8 differential line. Race D-7 lacks virulence to the Bt8 differential, but it is virulent on CI 9342 and PI 636146, which putatively contain Bt8 due to their reaction to L-18, D-18, and D-19 (Goates 2012). One explanation for this is that there may be an additional minor HPR gene in the Bt8 differential.…”