Differences Between Compatible Parasite/Host Genotypes Involving the Pm4 Locus of Wheat and the Corresponding Genes in Erysiphe graminis f. sp. tritici

Abstract

Infection efficiency, defined as percent of E. graminis f. sp. tritici conidia applied to wheat leaves that produced secondary hyphae over 10 .mu.m long, was determined for each of 8 parasite/host genotypes. Near isogenic lines of wheat with either Pm4 or pm4 were inoculated with cultures MS-1(P4), MS-2(p4), Kh .times. Cc(p4), and MS-3(p4). All cultures gave infection efficiencies of 76-78% and infection type 4 on wheat containing pm4. Isolate MS-1(P4) inoculated on wheat containing Pm4 gave an infection efficiency of 6% and an infection type 0. MS-2(p4) inoculated on wheat containing Pm4 gave 78% infection efficiency and an infection type 4, but primary infection proceeded more slowly and required 28 vs. 26 h for completion. Kh .times. Cc7(p4) and MS-3(p4) inoculated on wheat with Pm4 gave an infection efficiency of 19 and 33%. respectively, but the successful primary infections developed into type 4 infections in 7 days. Although all compatible parasite/host combinations produced type 4 infections, none of the cultures with the p4 gene conferred complete compatibility to the P4/pm4 parasite/host genotype in terms of infection efficiency or rate of development of primary infection. These results show how gene-for-gene relationships can contribute to the phenomenon known as horizontal resistance.