Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/2/2011
Publication Date: 10/13/2011
Citation: Goates, B. 2011. Identification of new pathogenic races of common bunt and dwarf bunt fungi, and evaluation of known races using an expanded set of differential cultivars. Plant Disease. DOI:10.1094/PDIS-04-11-0339. Interpretive Summary: Common bunt and dwarf bunt are fungal diseases of wheat that are under excellent control in the U.S. utilizing resistant cultivars and fungicide seed treatments. A high level of disease resistance has been developed by identifying resistant wheats and incorporating the resistance into cultivars through breeding. Incorporating resistance into cultivars is important in many wheat breeding programs world-wide, and has had an increased interest with the increase in organic farming. Knowledge of the virulence characteristics of the pathogens, and their reaction to specific resistance genes in wheat is essential to this effort. Different isolates of the fungal pathogens that cause the diseases are characterized by their reaction to specific wheat resistance genes which defines the pathogenic race of the fungi. Previous reports have described pathogenic races based on a set of 10 wheats (called differential cultivars), each carrying a single resistance gene. Six additional wheats that have new resistance genes have been identified and were added to the set of differentials which were tested to all named pathogenic races to better define the races of these fungi. The differentials were also used to describe unique virulence patterns of additional fungal isolates that demonstrated new pathogenic races. A summary of the reaction of the differentials to all these fungal isolates is presented. Two of the differentials were found to have resistance that was sensitive to temperature suggesting they should be excluded from the differentials. Previously named common bunt races that have the same reaction to the set of 10 differentials, but are designated as different races, were not differentiated further with the additional differentials indicating the duplicate races should be excluded from the list of races. Thirteen new races of bunt fungi were identified that have previously unreported virulence to individual resistance genes, or combinations of resistance genes in wheat. These new races can be used in tests to target specific resistance genes that will aid the identification of known and unknown resistance genes in wheat, and the development of wheat cultivars that are resistant to the diseases.
Technical Abstract: Pathogenic races of Tilletia caries and T. foetida, which cause common bunt of wheat (Triticum aestivum), and T. contraversa, which causes dwarf bunt of wheat, have been identified previously by their reaction to ten monogenic differential wheat lines, each containing single bunt resistance genes Bt1 through Bt10. The reactions of races to the monogenic wheat lines follow the classic gene-for gene system for plant/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 monogenic wheats containing the genes Bt11 through Bt15, and Btp, were added to the set of ten differentials and tested to all named U.S. races of common bunt and dwarf bunt. In addition, new isolates of dwarf bunt from field collections, and common bunt from hybrids and field collections, were tested to all the 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 the combination of Bt8, Bt9, Bt10, Bt11, and Bt12 were identified for the first time. Comparison of the reactions of the common bunt races to the Bt14 and Bt15 differentials grown in different environments after initial infection indicated these genes are temperature sensitive suggesting they should be excluded from the set of differential lines. Previously named common bunt races that have the same reaction to the set of 10 differentials, but were designated as different races, were not differentiated further with the six additional differentials indicating the duplicate races should be excluded from the list of pathogenic races. The new races of common bunt and dwarf bunt identified have unique virulence patterns that allows specific targeting and elucidation of bunt resistance genes in wheat and will aid the development of bunt resistant wheat cultivars.