Scald of barley, caused by Rhynchosporium secalis (Oud) J.J. Davis, is prevalent in central Alberta as a result of environmental conditions conducive to disease development and intensive of barley production. Over the past several years it has been observed that the scald resistant cultivars CDC Earl, CDC Guardian and Duke have had high levels of scald infection at several locations in Alberta. The observed breakdown in cultivar resistance has resulted from changes in scald pathotypes due to the selection pressure exerted by the resistant cultivars that have been grown. Shifts in race composition may also be caused by the competitive or adaptive capacity of individual races on particular cultivars. A study was undertaken to investigate the relative competitive and adaptive capacity of two pathotypes on a set of differential hosts.
H97-2 and E97-2 of R. secalis were isolated from the leaves of cv. Harrington and cv. CDC Earl, respectively, and the two isolates were found to be among the most predominate pathotypes in Alberta. There were substantial differences in sporulation in vitro, DNA patterns and virulence between the two pathotypes. The level of sporulation of E97-2 was 5-10 times greater than H97-2. The two pathotypes displayed DNA polymorphisms when amplified by each of two random primers, OPA20 and OPX7. H97-2 was virulent on cvs. Harrington and Manley; E97-2 was virulent on Harrington and CDC Earl. To determine their competitive and adaptive capacity, the two pathotypes were mixed in equal proportions and inoculated on seedling of cvs. Harrington or CDC Earl. The fungus was then isolated from each of the cultivars and individual colonies were inoculated back to the same cultivar. The procedure of inoculation, isolation and reinoculation using each pathotype was repeated several times (generations). Disease assessments on Leaves 2 and 3 on cvs. Harrington, Manley and CDC Earl were made based on a 0-3 scale 14 days after inoculation. An average rating equal to or above 1.5 was considered to be virulent, whereas below 1.5 was avirulent. This rating was found to adequately differentiate the relative frequency of individual pathotypes.
In the first experiment, fungal colonies of a mixed population were isolated from CDC Earl and inoculated on the same host for four generations. The isolate E97-2 comprised 75% of the population after one generation on CDC Earl. This pathotype consisted of 90 and 100% of the population by the third and fourth generations, respectively. When the first experiment was repeated, it comprised nearly 100% of the population from Generations 1 to 4. Different results were found in the second experiment when the fungal colonies of a mixed population were isolated from Harrington and inoculated on the same host for four generations. H97-2 was found to decrease and E97-2 increased in the mixture. However, a repeated experiment showed that H97-2 consisted of 70% and E97-2 consisted of 10% of the population by the fourth generation. Undetermined pathotypes accounted for 10% of the populations, indicating that new pathotypes occurred and this was due probably to parasexualism or nuclear exchange. Further test showed that H97-2 and E97-2 comprised 66% and 24% of the populations, respectively, by the fourth generation. Undetermined pathotypes again accounted for 10% of the populations. Using Primer OPX7, DNA polymorphisms for individual colonies was found between the mixed populations that had been inoculated on cvs Harrington and CDC Earl for successive generations. Further determination of pathotype compositions based on DNA profiles is underway using this technique. Specific primers have been developed based on unique bands amplified by OPA20 and OPX7, and these specific primers are also being used for determination of pathotype composition.
It appears that the relative competitive capacity of a scald pathotype to multiply in a mixture is associated with sporulation capacity. This is evidenced by the fact that E97-2, with intensive sporulation in vitro, quickly predominated in the population after one or two generations. However, survival of a pathotype may primarily depend on its ability of infection, colonization and sporulation on particular cultivars, since E97-2 and H97-2 predominated on cvs. CDC Earl and Harrington, respectively, when the mixture was inoculated for successive generations. Selection pressure exerted by the resistant cultivars such as CDC Earl and CDC Guardian might explain the occurrence of the predominant pathotypes in Alberta barley fields