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Presenting commercially available wheat varieties with resistance to common bunt.

Breeding for Crop Mixtures and Agroforestry in Organic and Low Input

455 wheat varieties and breeding lines were grown in the field,contaminated with 7 to 11 different races of common bunt. Based on the reaction of the lines to the different virulence races, it was possible to group the lines by differential varieties with known resistance genes, indicating that they may have one or two of the resistance genes Bt1, Bt2, Bt5, Bt7, Bt13, BtZ or Quebon-resistance. Based hereof, genetic markers will be developed using a genome-wide association study (GWAS).

When assesssing races of common bunt for virulens pattern within a region, it is important to take into account that collected spores may represent a diverse population of different virulence races. When screening spores on a differential set of wheat lines with known resistance genes, a low infection rate on a resistant wheat variety does not necessarily demonstrate that virulence is absent in the spore collection, but could be a sign that virulence is present, but only present in a low frequency among the spores. If just a few spores within a spore sample are indeed virulent, they may infect some plants and from there multiply the virulence quite rapidly next years. Previous studies have shown that virulence against most resistance genes were present in Denmark after purifying races of common bunt (Tilletia caries) on resistant varieties. So far, only wheat differential varieties with Bt4, Bt6, Bt9, Bt11 and Bt12 cannot be infected with bunt races purified from Danish collections [1, and later own unpublished data]. Virulence against Bt4, Bt6 and Bt9 has been found in other European studies [2], and Bt11 may not be only one gene but a combination of at least two genes [3]. Therefore, Bt12 seems to be the only gene for which virulence have not been found in European population of common bunt. This leads to the conclusion that if resistance breeding shall safely control common bunt in wheat, we need not only one effective gene, but a combination of pyramided genes. Since it is very difficult to test if a resistant line has only one gene or more genes, the most effective tool to achieve this at present are genetic markers. Using Genome Wide Association Studies (GWAS) to find QTLs and markers for the major resistance genes in wheat have so far led to only few commercial useful markers. Till now, only markers for Bt9 [6] and Bt10 are used in practice, but a marker for Bt12 [4] and Blizzard [7] have also been found. One of the problems in developing markers for bunt resistance have been that spores used in GWAS trials have been divers or unknown in virulence, and that phenotypic results not distinguishes between different resistance genes. Therefore, the most successful studies have used segregating populations of single crosses where the resistance gene is known on before hand [5]. In the LIVESEED project, we have the ambition to develop genetic markers on several different resistance genes at the same time. We will do so by testing segregating populations of several different crosses between varieties with 7 different resistance genes, and infect them with 7-11 different virulence races of common bunt able to distinguish between the resistance genes. A total of 300 varieties will be pheno- and genotyped. Using this experimental design, we attempt during 2018 and ‘19 to develop markers for Bt1, Bt2, Bt5, Bt7, Bt13, BtZ and Quebon-resistance, and hopefully also a couple of minor QTLs.

Bunt is one of the most devastating diseases of wheat, in Europe and Northern America mainly caused by Tilletia caries and T.controversa, in the warmer climates of the Near East by T.foetida. The generally obligate biotrophic pathogen is trans-mitted by contaminated seeds or can ersist in the soil. Infection occurs at the very first moments after germination of the grain. Once penetrated into the plant, the fungus grows endophytically, remaining undetected until early maturity stages when grains are replaced by spores. Besides yield losses, the malodorant spores contaminate the grains impairing their use as seeds or as food, thereby leading to serious economic losses. Usually the disease is controlled by the use of seed dressings and the use of certified seeds. For organic farming, however, re-sistance of wheat varieties to bunt s crucial as seed treatments with chemical pesticides are not allowed and several proposed alternatives, e.g. treatments with bacteria, hot water, vaccum-steam, steam-ultrasound or electrons, are expensive and lack efficiency. Major and some minor genes conferring resistance to Tilletia infections are described, however, the performance and stability and, therefore, usefulness of these resistances are under debate. Common bunt resistance is based on Flors’ gene for gene principle with an effector of the pathogen and a resistance gene in the host plants, able to detect the effector and to unleash the appropriate resistance mechanisms. To officiently use this type of resistance, it is important to characterize the effectors in the pathogen population as well as to monitor the presence and the efficacy of the resistance genes. While many of this information is available at local and regional level, only little is known at an interregional or even continental dimension. In order to obtain a better overview on the efficacy of resistance genes and the presence and distribution of pathogen races, the European Tilletia ringtest (ETR) was established including also the USA and Iraqi Kurdistan. The ringtest consists of a set of 65 wheat accessions including differentials to characterize the pathogen strains and 40 modern varieties and landraces with specific resistance features. The ringtest took place in 2015 and 2016. First results display a wide diversity of pathogen strains, allowing to recommend the deployment of the most appropriate resistance genes in the different cropping areas. Important differences in disease severity were observed among sites. Resistance was not always linked to the postulated resistance genes. Across all test sites no susceptible plant was observed in PI636170, a breeding line selected from a Turkish landrace. Low infection levels (≤25%) were observed in some breeding lines/genetic resources and also in released cultivars, e.g. Stava and SW Magnifik.