1Agricultural Research Institute, CZ-767 41 Kromeriz, Czech Republic; 2USDA-ARS, National Small Grains Collection, Aberdeen, ID-83210, USA

Background and objectives
Powdery mildew, Erysiphe (Blumeria) graminis f. sp. hordei ranks among important pathogens on barley in most European countries and some regions of North America, as well as in other regions in the world. In the Czech Republic, powdery mildew is the most common disease on this crop [1]. Its harmful effects may be limited by growing resistant varieties. The powdery mildew population, however, is very adaptable, and after some time it is able to overcome a resistant variety. Therefore new varieties should possess new effective resistances. They are very rare within cultivated barley, thus attention is given to seeking new resistances from other sources, particularly in wild barley Hordeum spontaneum ssp. spontaneum. This was also the aim of this study.

Materials and methods
In 1997, 575 of about 1400 accessions from the USDA-National Small Grains Collection (NSGC) wild barley collection, Aberdeen, USA were studied. Firstly, glasshouse tests were carried out using chosen powdery mildew pathotypes exhibiting high complexity of known virulences. The number of grains of the examined samples was strongly limited, therefore young plants were inoculated at the full primary leaf stage with two pathotypes at an inoculum ratio 1:1 at the same time. Seven days later, infection types (ITs) were scored. The higher IT was recorded when various ITs occurred on the same leaf. In the samples which appeared resistant after the first testing, the procedure was repeated twice using two different pairs of pathotypes. The samples resistant to all pathotypes used (ITs 2-3 and lower) were sown in the field. During the growing season, the highest natural powdery mildew occurrence was assessed using an estimation of the percentage of pathogen colonies on the most infected leaf. Seed of samples which had shown susceptible responses in the glasshouse (IT 3 and higher) and had not been sown in the field were used in the other two tests with either pathotype of the lowest complexity of known virulences. This enabled us to detect other resistances.

Results and conclusions
The glasshouse tests showed susceptible responses in 176 samples. Of this number, 91 samples were susceptible to all powdery mildew pathotypes; in 85 samples, resistance to at least one pair of pathotypes or one of two individual pathotypes was assessed in addition to susceptibility. One sample did not germinate. 398 samples (69%) exhibited resistance to all three pairs of pathotypes, with 84 samples characterized by IT 1 and lower (three ITs of higher resistance) and 314 samples with ITs 1-2 to 2-3 (three ITs with lower resistance). All samples which had been resistant to all three pairs of pathotypes in the glasshouse were sown in the field. Twenty-five of them did not show any symptoms of powdery mildew infection. In another 93 samples, only traces of powdery mildew up to 1% of leaf area were found. In 249 samples, powdery mildew occurrence ranged from 1 to 10%, and 31 samples were infected on more than 10% of leaf area.

The USDA-NSG Collection showed its richness and high diversity in powdery mildew resistance. This is likely to be a result of long-term co-evolution of wild barley and the pathogen in the Mediterranean region, and particularly in Israel [2], where 94% of all studied samples and 99% of resistant ones originated. It can be assumed that many of the resistant samples in this collection will be used for breeding barley varieties resistant to powdery mildew.

This study was conducted within the grant No. 521/97/0389 of the Grant Agency of the Czech Republic.

1. Dreiseitl A, Jurecka D, 1997. Ochrana Rostlin (Plant Protection) 33, 177-186.
2. Anikster Y, Wahl I, 1979. Annual Review of Phytopathology 17, 367-403.