Virulence phenotypes in powdery mildew (Blumeria graminis) populations and resistance genes in triticale (x Triticosecale)

Triticale (xTriticosecale) is a new host for powdery mildew (Blumeria graminis) being not infected by this pathogen in Germany before 2001. To evaluate population structure of the pathogen and race-specific resistances in the host, 694 isolates were collected in 12 states of Germany in the years 2007 to 2009 on triticale. They were tested by a newly developed initial differential set of 20 triticale cultivars. Corresponding virulences were found for all differentials except for cultivar Grenado. In total, 272 different virulence phenotypes (=pathotypes) were detected. The virulence complexity of the isolates ranged from 6 to 19 of 20 possible virulences with a mean of 15. In all years, a high level of diversity of the powdery mildew populations was observed with Simpson indices in the range 0.95 to 0.97. The distribution of the pathotypes was even across Germany with an Evenness index in the range 0.82 to 0.88. A set of 19 isolates with different virulence pathotypes and 10 cultivars were selected to be used to identify race-specific resistances of triticale cultivars and breeding lines. Some cultivars susceptible to most of the isolates in seedling stage were moderately resistant in adult-plant stage. The high diversity and complexity of the pathotypes found in German powdery mildew populations as well as an increasing acreage of only a few dominating triticale cultivars accelerate the adaptation of the pathogen to race-specific host resistances suggesting restricted durability only. More durable resistance might be achieved by combining new effective race-specific (qualitative) resistance genes with race-nonspecific (quantitative) resistances effective in the adult-plant stage that are already available.