STRUCTURE OF POPULATIONS OF WHEAT POWDERY MILDEW (ELYSIPHE-GRAMINIS DC F-SP TRITICI MARCHAL) IN CENTRAL-EUROPE IN 1993-1996 - I - DYNAMICS OF VIRULENCE

In 1993-1996, the virulence of regional populations of the wheat powde ry mildew pathogen (Erysiphe graminis DC f. sp. tritici Marchal) from the Czech Republic, Austria, Hungary and Slovakia against 13 resistanc e genes was investigated. The populations differed mainly at the regio nal level. Populations from the Czech Republic, mainly from the wester n regions, showed higher values of virulence against the Pm4b gene. Lo wer frequency of virulence against Pm4b was found in Austria, and the lowest value was observed in Hungary. The differences in frequencies o f virulence against Pm4a and Pm4b showed a similar geographic pattern across the four countries: a continuous decline from west to east and from north to south. Virulence against Pm2 decreased in all countries considered; virulence to pm5, Pm6, Pm8 and Mli was high throughout. Ge nes and gene combinations that can ensure a relatively effective biolo gical protection against this pathogen across Central Europe at presen t are Pm3b, Pm2+Mld and Pm1+2+9. Czech and Slovak populations were the most complex: virulence complexity reached a maximum in Slovakia in 1 994. A similar evolution, though less significant, was observed in the Czech Republic. Data on complexity of isolates suggest that Central E uropean populations of wheat powdery mildew tend to reach an intermedi ate level representing the optimal number of virulence genes. This pro cess is probably a consequence of stabilizing selection.