Sweden

A number of rusts affect grain crops in Sweden, but stem rust on oats and stripe (yellow) rust on wheat appear to create the greatest problems in production. The epidemiology of these diseases is intimately connected to the overall cropping patterns of these two crops. In Sweden, oats are only sown in the spring, thus forcing any overwintering pathogen to survive a Swedish winter. This is easiest for Puccinia graminis f. sp. avenae, which apparently completes its full, sexual life cycle on the abundant barberry plants. The presence of barberry and clear indications of sexual reproduction by P. graminis suggests that Pgt could be a problem on wheat, but there are only sporadic reports of stem rust on wheat. Wheat cultivars grown in Sweden possess few effective genes for resistance to stem rust, and the lack of rust is probably due to a lack of Pgt in the region. Given the resurgence of barberry in the landscape this implies that stem rust on wheat could be a major problem if (or when) the pathogen returns. P. striiformis, in contrast, can survive the Swedish winters on fall sown cereal crops, and thus it is the fittest clones that survive and dominate in the population. A large number of factors can affect this fitness, most markedly resistance genes in the cultivated wheat, but it is also possible that extended asexual reproduction can reduce the fitness of these persistent clones (Muller's ratchet) so that they can be displaced by fitter clones. Despite the widespread occurrence of barberry plants, we have not found any aecia of P. striiformis, although there does seem to be some genetic variation in the alternate host. Simple models that simulate the appearance and competition between different clonal lineages of the pathogen indicate that fitter individuals will eventually dominate the population, but their initial appearance will be difficult, since they are only detectable after enough generations have passed to increase the population size above a detectable level.