6.5 Powdery mildew isolates and their use in breeding of mildew resistant hops

6.5 Powdery mildew isolates and their use in breeding of mildew resistant hops

Sponsor: Bayerische Landesanstalt für Landwirtschaft, Institut für Pflanzenbau und Pflanzenzüchtung, AG Hopfenbau, Produktionstechnik (IPZ 5a) [Bavarian State Research Center for Agriculture, Institute for Plant Production and Plant Breeding, AG Hop Production, Production Technology (IPZ 5a)] AG Züchtungsforschung Hopfen (WG Hop Breeding Research) Financing: Gesellschaft für Hopfenforschung e.V. (GfH) (Society of Hop Research) (2013 -2014; 2017 – 2021) Erzeugergemeinschaft HVG e. G. (HVG Hop Producer Group) (2015 - 2016) Project Management: Dr. E. Seigner, A. Lutz Team: AG Züchtungforschung Hopfen (WG Hop Breeding Research): A. Lutz, J. Kneidl EpiLogic: S. Hasyn Collaboration: Dr. F. Felsenstein, EpiLogic GmbH, Agarbiologische Forshung u. Beratung, (Agri-biological Research and Consulting), Freising Duration: January 1, 2013 to December 31, 2021

Objectives

Improved resistance to diseases, especially to powdery mildew, is a top priority in the development of new hop varieties. This is why seedlings from all breeding programs are tested for powdery mildew resistance each year. Starting in 2000, powdery mildew isolates with virulence properties have been used for powdery mildew resistance tests in the greenhouse and the laboratory (Seigner et al., 2002). In conjunction with constantly improving test systems in the greenhouse and laboratory, these resistance tests form the backbone of effective resistance breeding (Seigner et al., 2005; Seigner et al., 2006; Seigner et al., 2009). For this purpose, 10 powdery mildew isolates with all currently known virulence genes are obtained from EpiLogic, Agrarbiologische Forschung und Beratung (Agri-biological Research and Consulting), in Freising. These isolates are made available for the various efforts related to mildew resistance breeding.

Description of work Powdery mildew isolates – preservation and characterization of their virulence properties

• Before the start of testing, the virulence properties of all mildew isolates are checked once a year in February. To this end, a range of eleven hop varieties that carry all the resistance genes known to date are used to differentiate the virulence levels. This ensures that available isolates have not lost their virulence genes through mutation, even years after the culture was first created. In addition, as new mildew populations appear in the growing regions or in the greenhouse, these are also examined for their virulence properties.

Testing for powdery mildew resistance in the greenhouse in Hüll

• In the greenhouse, under standardized infection conditions, all seedlings (approx. 100,000) that were produced from the crosses of the previous year are artificially inoculated with three mildewisolates. In this process, only powdery mildew strains are used that are known to have virulences that are widespread in the Hallertau. This allows for a large number of seedlings to be tested, which, in turn, clarifies the extent to which they show the type of resistances that are urgently required for cultivation in the Hallertau. Only seedlings that are classified as resistant move on for further selection.

Figure 6.1: Resistance test in the greenhouse with seedling trays amidst inoculator plants

Testing for powdery mildew resistance in the laboratory using the leaf test system

• In addition, breeding strains, varieties and wild hops that have proven their resistance in the greenhouse in previous years, are examined in the laboratory at EpiLogic using the leaf test system. An English powdery mildew isolate (“R2 resistance breaker”) and a Hallertau isolate (“RWH18 breaker”), which has regional significance, are used for inoculation.

Only breeding lines and varieties that show broad resistances to powdery mildew in both tests (greenhouse and leaf test) are used for further breeding.

Assessment of the state of virulence in growing areas and assessment of resistance sources using the leaf test system

▪ The virulence genes of current powdery mildew populations in German hop gardens are identified every year. The reactions of 11 cultivated and several wild varieties that carry all the resistance genes known to date worldwide (= so-called hop differential assortment), are tested against all currently available mildew isolates. This makes it possible to assess whether existing resistances are still fully effective in current varieties

(such as the fully mildew-resistant Hallertauer Blanc aroma variety). Last year, the powdery mildew strain, which is starting to affect Herkules in more and more regions of the Hallertau, was examined by EpiLogic for its virulence properties. The presence of well-known virulences that break the R1 and R3 resistance gene could be confirmed.

The virulence was also identified on mildew-infected leaves of Callista in commercial plots. According to our current understanding, the R18 resistance in Callista in these commercial plots appears to have been broken by regionally specific mildew strains with complementary v18 virulence. • These virulence tests provide crucial insights into the mildew strains that are found in commercial plots or greenhouses. These insights are necessary for assessing if the resistance of our varieties is still effective or has since ceased.

Phenotypical data about powdery mildew resistance of the assortment of hop reference varieties for the GHop project as a contribution to the establishment of precision breeding

Starting in mid-2015, work has been underway to develop the foundations for precision breeding in hops. The project is a collaboration between the Universität Hohenheim (University of Hohenheim), and, since 2017, the Gesellschaft für Hopfenforschung (Society for Hop Research) and Hopfenverwertungsgenossenschaft (Hop Sales Collaborative) HVG. The resistance tests were conducted with defined mildew strains in the greenhouse, as well as in subsequent leaf tests at EpiLogic. These provided crucial insights into the mildew resistance or susceptibility of all individuals in the hop reference assortment, which consists of 192 cultivated varieties, both female and male breeding lines, as well as wild hops. These powdery mildew tests on a range of reference plants in the greenhouse and in the laboratory (leaf test) allowed for controlled, reliable assessments about mildew resistance under controlled conditions, which could never have been possible in field tests. In addition, resistance reactions could be associated with actions of special resistance genes (complementary to the virulence of the mildew strains used).

Table 6.3: Overview of mildew resistance tests in 2020 with mildew isolates of defined virulence

2020 Tests in greenhouse Leaf tests in laboratory

Plants Assay data Plants Assay data

Seedlings from 89 crossings approx. 100,000 mass-selected - Breeding lines* 133 136 189 1,455 Varieties* 22 39 23 81 Wild hops* 1 2 1 0 Virulence powdery mildew - - 10 485 isolates Total (Individual tests) 156 177 223 2,029

Mass selection in plant trays; Single tests = selection of individual plants in pots * partial data for the GHop project (precision breeding)