Dispersal of cereal mildews across Europe

Following the evolution and spread of the barley and the wheat mildew patho gens, (Blumeria (Erysiphe) graminis fsp. hordei and tritici, respectively) over a long period and a large scale in Europe brought us new insight into their population biology which appears to be much more affected by aerial d ispersal than generally believed. Results on the local evolution of the num ber of colony forming units m(-3) of air (cfu) gave us an insight into the major features of population genetics and epidemiology of the barley mildew pathogen. Firstly, the evolution of cfu demonstrates and quantifies the ep idemiological significance of volunteer plants. Moreover, three or four per iods of primary infections become evident per year, forcing the wind-borne pathogen spores to move from one held (or region) to the next. This makes t he pathogen to be highly nomadic, and points out to be of major interest fo r studies across Europe. Pathogen samples were analyzed from within the geographical triangle Spain- Scotland-Byelorussia for virulence to host resistance. Most unexpectedly, v irulence complexity (i.e. number of virulence genes per spore) increased by approximately one virulence per 1000 km from west to east, from Scotland a nd France to central and eastern Europe. It is presumably caused by the int eraction of prevailing winds, pathogen spores migrating on them, and host s election. Prevailing westerly winds carry pathogen spores to the east. Alon g the way, they encounter barley cultivars with different genes for race-sp ecific resistance, and selection increases virulence complexity. The increa se is elucidating for basic concepts of population genetics of wind-borne s pecies, and important for gene deployment strategies. There was ample evidence for the dispersal of genes and genotypes from the regional to the continental level and beyond. Depending on year and locatio n, detailed analyses showed that major parts of the population in one regio n can be assumed to originate from neighboring or distant regions. Dispersa l is reduced by distance, barriers like mountain chains, and host-free spac es. Further, mildew 'waves' consisting of populations highly adapted throug h selection in one region appear to move across Europe on the prevailing we sterly winds, covering approximately 100 km/year. Another important unexpected finding relates to fundamental features of the evolution of new and dangerous pathotypes. The established way of thinking starts with the effect of (regional) mutation as the basis of variability. In contrast, our evidence stresses the importance of the extraordinary var iability already present in the pathogen population due to inoculum exchang e over the vast continuous areas of Europe, northern Africa and Asia where the pathogen is endemic. Though mutation will remain of importance in the e nd, variability through spore dispersal among regions appears to be of more immediate relevance for the rapid adaptation of cereal mildew populations. Implications of our findings for fundamental aspects of population biology and integrated pest management strategies, from the regional to the contin ental and intercontinental level, are outlined and discussed. (C) 1999 Else vier Science B.V. All rights reserved.