Genetical studies of resistance to Phytophthora porri in Allium porrum, using a new early screening method

A new screening method was developed to evaluate resistance of leek (Allium porrum) to Phytophthora porri, based on inoculation by 24 h-immersion of leek plantlets in the 3–6 leaf stage in a suspension of ca. 100 zoospores.ml-1. The immersion test was used for identifying new sources of resistance and to determine the genetic basis of resistance. Within winter leek, five resistance classes were defined on the basis of average field scores of 21 plants. Clones from these plants were tested with the immersion technique. The average scores per resistance class in immersion and field tests were significantly correlated (P<0.01). The correlation of single-date field scores with the immersion test scores was better in the second half of the epidemic season. A selection experiment yielded a strong response to selection for resistance (53–97%) but no response to selection for susceptibility. This may indicate that resistance is due to few recessive genes in the studied winter leek. Crosses between landraces and winter leek were analyzed by means of F2 (selfed F1) and BC1, progenies. This analysis indicated the presence of few loci with dominant genes for resistance in accession CGN 873243, and additive polygenes in accession Pl 368351.