Graduate School, Experimental Plant Sciences, Department of Plant Breeding, Wageningen Agricultural University, PO Box 386, 6700 AJ Wageningen, The Netherlands

Background and objectives
Partial resistance is defined as a quantitative resistance that reduces epidemic development despite a fully compatible infection type. In barley, the partial resistance to the leaf rust fungus Puccinia hordei is largely race-non-specific, but very small cultivar x isolate interactions have been reported. By using a high-density AFLP marker map [1], six QTLs for partial resistance to barley leaf rust isolate 1.2.1 were identified in barley cultivar Vada [2]. Three QTLs, Rphq1, Rphq2 and Rphq3, were effective at the seedling stage, and five QTLs, Rphq2 to Rphq6, were effective at the adult plant stage. We set out to investigate whether the same or different QTLs would be effective in another isolate of P. hordei, isolate 24. The results would contribute to the long-standing scientific dispute about the possible 'minor-gene-for-minor-gene' interaction proposed by Parlevliet and Zadoks [3].

Materials and methods
A set of 103 F9 recombinant inbred lines (RILs) was obtained from a cross of L94 x Vada. L94 is extremely susceptible and Vada has a high level of partial resistance. This set of RILs had been used to construct a map of 566 AFLP markers covering the barley genome, and to map QTLs for partial resistance to barley leaf rust (P. hordei) isolate 1.2.1 [2]. A skeletal map with a uniform distribution of markers at 5 cM intervals was extracted and used for mapping QTLs effective against rust isolate 24. Disease was evaluated by measuring the latency period (relative to L94) at the seedling stage and at the adult plant stage. A multiple-QTL mapping approach was used to detect regions of putative QTLs. A LOD value of 3.0 was chosen as a significant threshold value for declaring a QTL.

Results and conclusions
In total, 10 QTLs for partial resistance were found to operate at the seedling and/or adult plant stage. Three QTLs, Rphq2, Rphq3 and Rphq4, were effective in both isolates. The other eight QTLs were isolate-specific. These results support the hypothesis of Parlevliet and Zadoks [3] that a 'minor-gene-for-minor-gene' interaction can also occur in partial resistance.

[1] Qi X, Stam P, Lindhout P, 1998. Theoretical and Applied Genetics, in press.
[2] Qi X, Niks RE, Stam P, Lindhout P, 1998. Theoretical and Applied Genetics, in press. [3] Parlevliet JE, Zadoks JC, 1977. Euphytica 26, 5-21.