lPalackg University, Faculty of Sciences, Department of Botany, 783 71 Olomouc-Holice, Czech Republic; 2TU Munich, Faculty of Agriculture and Horticulture, Department of Plant Pathology, 85350 Freising-Weihenstephan, Germany

Background and objectives Lettuce downy mildew, caused by Bremia lactucae Regel, is a worldwide distributed and serious disease of glasshouse and field crops. Interaction between Lactuca sativaand L. serriola conforms a gene-for-gene relationship, each resistance gene (Dm gene) is matched in the pathogen by a specific aviruience gene (A), which is dominant to virulence [1]. B. lactucae belongs to the enormously variable oomycete pathogens, existence of many physiological races was recognized in various countries around the world [1]. Incorporation of different combinations of race-specific resistance genes to the lettuce cultivars has very often been followed by the appearance of new races with the matching virulence genes and thus,,overcome" the resistance [2].

Materials and methods
The data on physiological specialization of B. lactucae from Germany collected in the years 1974-1997 (totaly five periods) were used as a model system for host-pathogen coevolutionary study. Elucidation of the genetic basis for cultivar specificity in B. lactucae provided the opportunity to describe specific pathogen virulence factors (v-factors) and interpret the changes in pathogen population structure in terms of v-phenotypes, v-factors frequencies and v-complexity [3,4]. Data on a total of 176 B. lactucae isolates collected from L. sativa cultivars in 11 German federal states and 48 sites were analysed. The genetic structure of resistance in the host population was quantified by the frequencies of Dm genes (R-factors) [3] in different subsequent periods. To the frequency calculation were included all host cultivars from which the pathogen was isolated, with known or expected resistance genotype (Dm gene, R-factor).

Results and conclusions
In the period 1974-1997 totaly twelve well specified race-specific resistance genes (factors) occurred in the studied sample of German lettuce population. However, only some of these R-genes played an important role in host population resistance management (e.g. Dm3, Dm7, Dmll, Dml6, R18). Substantial changes in the frequency of these R-genes were recorded during twenty years period. Generally, the durability of resistance was very unstable and short. Host resistance based on recently most efficient and frequently used R-genes (Dmll, Dml6, R18 ans RLSal) derived from wild Lactuca spp. (L. serriola, L. saligna) was overcomed in the period 1996/1997. Distinct regional patterns were observed for the frequencies of different v-factors in the pathogen population. The obtained data on changes of virulence structure well exemplified the process of host-pathogen coevolution. During the evaluated period there was a considerable increase of the frequency of some v-factors (v2, v3, v518, v6, v7, vl 1, vl 6) and a complexity of v-phenotypes. In the period 1996/1997 12 of 14 evaluated v-factors reached an absolute or nearly absolute values (frequency higher then 0.90) in the pathogen population. In Germany, from recently available lettuce cultivars as the most efficient one could be considered cvs. with R-genes Rl 8 and RLSal (e.g. cv. Titan), however, their efficiency may be only local and very unstable. Searching for sources of durable resistance [5] in wild Lactuca species by using of a new highly virulent isolates of B. lactucae is considered to be the most important topic of recent research.

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