Alberta, Canada

 

Competition and survival of Rhynchosporium secalis pathotypes

 in relation to barley cultivar  resistance  to scald

 

K. Xi¹, T.K. Turkington², J. Meadus², J.H. Helm³ and J.P. Tewari⁴

 

¹Alberta Agriculture, Field Crop Development Centre, 6000 C & E Trail, Lacombe, AB, T4L 1W1, ²Agriculture & Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail, Lacombe, AB, T4L 1W1, ³Alberta Agriculture, Field Crop Development Centre, 5030 - 50 Street, Lacombe, AB, T4L 1W8, ⁴Dept. of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, T6G 2P5  

 

Changes in the frequency of virulence in the barley scald pathogen (Rhynchosporium secalis Oud. J.J. Davis) are often directly related to changes in the genetic composition of the host. Such changes have led to the increasing prevalence of pathotypes virulent on widely used and formerly resistant varieties.  Over the past several years it was observed that the scald resistant cultivars have high levels of scald infection in experimental plots and commercial fields across Alberta.  However, it is unclear how these barley cultivars affected the shifts of major R. secalis pathotypes causing breakdownofresistance.  The prevalence of virulent races may also be the consequence of the survival and competitive abilities of one pathogen race over others on particular host cultivars. 

 

The objective of this study was to test the hypothesis that shifts in R. secalis pathotype composition could result from the selection pressure exerted by the resistant genotypes in the host population.  Pathotypes E97-2 and H97-2 differed widely in virulence and represented two of the major pathotypes in R. secalis populations of Alberta.  From naturally infected leaves, the pathotype E97-2 was isolated from cv. CDC Earl and pathotype H97-2 from cv. Harrington..  The two pathotypes were mixed in equal proportions and inoculated on seedlings of cvs. Harrington, CDC Earl and Manley.  Subsequently, the fungus was isolated from Harrington and CDC Earl, and single spore colonies were inoculated back to the three cultivars.  The experiment consisting of four cycles was repeated four times.  The random amplified polymorphic DNA (RAPD) assay was used to fingerprint the parent and progeny pathotypes from the inoculation mixtures.  The resulting molecular phenotypes were used for comparison with the pathotype virulence identified by the host cultivars. 

 

            Following greenhouse coinoculation with the two pathotypes, E97-2 predominant over H97-2 from the first to the last infection cycles on both cvs. CDC Earl and Harrington, indicating that the host can instantly influence pathotype composition and that E97-2 may have a competitive advantage over H97-2.  DNA polymorphisms were found between single or mixed pathotypes that were inoculated on cvs. CDC Earl and Harrington for successive cycles.  Coinoculation with the two pathotypes resulted in a shift to a molecular phenotype closer to E97-2 compared with that of H97-2.  The competitive advantage of E97-2 over H97-2 combined with the selective pressure exerted by the host would explain an increase in susceptibility of cv. CDC Earl and cultivars with a similar genetic basis of scald resistance in fields across Alberta.  H97-2 will probably remain the major pathotype in R. secalis populations as long as the susceptible Harrington is grown over a large acreage.