K. Xi¹, T. K. Turkington² and M. Cortez¹ and J. Helm¹

¹Alberta Agriculture, Food and Rural Development, 50th Street, Lacombe, AB. T4L 1W8
²Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail, Lacombe, AB T4L 1W1

Introduction

    Barley production in Alberta is estimated to be around 13 million metric tonnes from 2 million hectares of land annually from 1995 to 1999, and accounted for close to 50% of the total barley production in western Canada. Scald caused by Rhynchosporium secalis J.J. Davis is one of the major barley leaf diseases in Alberta as a result of intensive crop production, and cool, wet environmental conditions that favor disease development. Yield losses due to scald are estimated to be 1-19% in field trials and severe losses over 25% have been reported. Average barley yield reductions can result in annual monetary losses of approximately $7 million in Alberta.

    Over the past several years it has been observed that resistance to scald has frequently been overcome in barley fields in Alberta. This has occurred as a consequence of changing scald pathotype composition and frequency. It was observed that cultivars like CDC Earl, CDC Guardian and Duke, that rated as resistant when first registered for production, are now susceptible to scald at several sites in Alberta. Despite being a major disease in Alberta, where intensive barley production over a large geographic area is practiced, scald variability and distribution have not been well studied, and the current status of scald resistance in commercial barley cultivars remains to be clarified. The objectives of this study were (1) to evaluate the scald reaction of commercially available cultivars in relation to spatial and temporal patterns of scald distribution across the major barley production areas in Alberta and (2) to provide information on potential sources of resistance that are effective against the most prevalent pathotypes in this province.

Materials and methods

    Barley accessions and commercial cultivars were evaluated for scald reaction in hill plots from 1997 to 2001 at various sites in Alberta. Hill plots were set up at Edmonton, Beaverlodge (500 km of NW of Edmonton), Calmar (30 km S of Edmonton), Carstairs (200 km S of Edmonton), Lacombe (130 km S of Edmonton), Stettler (160 km SW of Edmonton), Trochu (200 km SE of Edmonton), Vegreville (80 km E of Edmonton) and Westlock (80 km N of Edmonton). In each of these trials there were 44 - 54 entries including accessions and commercial cultivars with different levels of scald resistance. The major scald resistance genes were determined in the accessions that were used for pathotype differentiation in numerous studies. The majority of commercial cultivars used in the present study were from western Canadian breeding programs and the genetic basis of scald reaction in these cultivars has not been determined.

    During 1997 to 1999, the sites for most trials were set up where barley had been grown and scald had occurred in previous seasons. During 2000 to 2001, barley stubble from previous seasons at the test sites was spread to the plots to provide a natural source of inoculum. Assessment of disease severity was made using a 0-9 scale at growth stage GS 40-50 (boot) and GS 80-90 (grain filling). At each site and season, disease severity data on accessions and commercial cultivars were converted to percentage severity relative to the average of the three susceptible cultivars, Argyle, Harrington and Jackson. Relative severity for the accessions and cultivars was used for analysis of variance to determine interactions between accession or cultivar, location and season. The commercial cultivas were grouped into scald reactions using cluster analyses. More entries were tested in 2000 - 2001 than for 1997 - 1999, thus, analyses were carried out separately for the two study periods.

Results and discussion

    Accessions Abyssinian, Hudson, Atlas, Atlas 46, Atlas 68, Kitchin and Turk with major genes representing the majority of the known resistance sources determined in barley cultivars were found to be resistant at all sites and seasons. This indicated the potential of these genes as genetic sources for breeding scald resistance. The scald reaction of commercial cultivars was significantly different among sites and seasons. Resistance in a few commercial cultivars including Kasota, Mahigan, Seebe and CDC Dolly held up at most sites with the majority of cultivars being intermediate to moderately susceptible. Cultivars previously considered as resistant were found in the present study to be intermediate in reaction and became increasingly susceptible. Commercial cultivars were classified in scald reactions into five groups, those in the intermediate group for the 1997 to 1999 test period were classified into the susceptible group for the 2000 to 2001 period, suggesting a continuous erosion of scald resistance as a result of development of virulent pathotypes at the test sites.

    The sites where cultivars became increasingly susceptible were more diverse in pathogen virulence compared with those where the same cultivars were found to be resistant. Scald reactions for commercial cultivars were location dependent suggesting an uneven distribution of scald pathotypes in Alberta. Consequently, scald management via the choice of cultivar will be dependent on location. Pathogenic variability associated with location in Alberta is important in the context of breeding for resistance and use of the most resistant cultivars by producers.