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.

The genus Tilletia is a group of smut fungi that infects grasses either systemically or locally. Basic differences exist between the systemically infecting species, such as the common and dwarf bunt fungi, and locally infecting species. Tilletia indica, which causes Karnal bunt of wheat, and Tilletia horrida, which causes rice kernel smut, are two examples of locally infecting species on economically important crops. However, even species on noncultivated hosts can become important when occurring as contaminants in export grain and seed shipments. In this review, we focus on T. indica and the morphologically similar but distantly related T. horrida, considering history, systematics, and biology. In addition, the controversial generic placement and evolutionary relationships of these two species are discussed in light of recent molecular studies.

Key message A novel QTL, Q.DB.ui-7DS, and the PCR-based markers identified in the current study will accelerate variety development for resistance to dwarf and common bunt of wheat. AbstractDwarf bunt [Tilletia controversa J.G. Kühn [as ‘contraversa’], in Rabenhorst, Hedwigia 13: 188 (1874)] is a destructive disease of wheat (Triticum aestivum L.) that reduces grain yield and quality. A number of distinct genes conferring resistance to dwarf bunt have been used by breeding programs for nearly 100 years. However, few markers were identified that can be used in selection of dwarf bunt resistance. A recombinant inbred line (RIL) population derived from the bunt-resistant germplasm, Idaho 444 (IDO444), and the susceptible cultivar, Rio Blanco, was evaluated for phenotypic reaction to dwarf bunt inoculation in four trials in two locations (USU and USDA) over 3 years. The population was genotyped with the Diversity Arrays Technology (DArT) and the Illumina Infinium 9K iSelect marker platforms. A total of three QTL were detected, and resistant alleles were from IDO444. QTL Q.DB.ui-7DS on 7DS was determined based on the location of a DArT marker wPt-2565 (X116197), which was consistently detected and explained 32 to 56 % of phenotypic variation among the four trials. QTL Q.DB.ui-1A on 1A was detected in three Utah State University (USU) trials and explained 11–15 % of phenotypic variation. QTL Q.DB.ui-2B on 2B was detected in two USU and one United States Department of Agriculture (USDA) trials and explained up to 6 % of phenotypic variation. Two PCR-based markers were developed based on the sequence of wPt-2565 and validated in the RIL population and used in genotyping of dwarf bunt differential lines, known resistance sources, and resistant cultivars.

Landrace accessions of wheat (Triticum aestivum L. subsp. aestivum) from the USDA‐ARS National Small Grains Collection (NSGC) have been tested systematically for the past 25 yr for disease resistance. We analyzed the resistance of 10 759 common wheat accessions to common bunt (CB) caused by Tilletia tritici (Bjerk.) Wint. and T. laevis Kühn, and 8167 to dwarf bunt (DB) caused by T. controversa Kühn with respect to geographic origin, relationship to color of awn, glume, and kernel of accessions, and phenotypic variation within areas of high frequency of resistance. A clear center of concentration was evident for CB resistance extending from Serbia and Montenegro through Macedonia, Turkey, and Iran with the highest frequency of resistance occurring in Kosovo province in Serbia and Montenegro (36%) and Bakhtaran province in Iran (40.8%). Compared to CB resistance (5.5% of total tested), DB resistance was more rare (1.3% of total tested). DB resistance was concentrated in accessions from Iran, Turkey, and Serbia and Montenegro with the highest frequency (58%) occurring in Hakkari province in southeastern Turkey. CB resistance was positively associated with lightly pigmented kernels and negatively associated with lightly pigmented awns and glumes. Analysis of accessions from areas with unusually high frequency of resistance suggested that DB resistant accessions from Hakkari are genetically diverse, whereas CB resistant accessions from Bakhtaran may be much less so.