Mapping QTL for bacterial brown spot resistance under natural infection in field and seedling stem inoculation in growth chamber in common bean

Bacterial brown spot (BBS), caused by Pseudomonas syringae pv. syringae van Hall (Pss), is an important bacterial disease of common bean (Phaseolus vulgaris L.). The objective of this study was to identify random amplified polymorphic DNA (RAPD) molecular markers linked to quantitative trait loci (QTLs) for BBS resistance. Resistance was assessed by three methods: (i) stem inoculations of plants grown in a greenhouse evaluated by a 1-to-5 rating scale for stem symptoms, (ii) percentage of BBS infected leaves per plot in noninoculated field trials, and (iii) Pss population sizes, on noninoculated field grown plants determined by a leaflet freezing assay (LFA). F(8:9) recombinant inbred lines derived from the Mesoamerican cross of Belneb RR-1 (susceptible to BBS) x A 55 (resistant to BBS) were grown in replicated experiments in two years (1996, 1998) in Wisconsin. In addition, two separate experiments were performed in the greenhouse in a randomized complete block design with two replicates for seedling stem inoculation with Pss strain Bs191. One genomic region on linkage group (LG) 2, from a previously published genetic linkage map, was significantly associated with QTLs for BBS resistance measured by three assays over two years. Phenotypic reactions were significantly correlated with the measurement of freezing temperatures as determined by LFA under a favorable environment for the growth of epiphytic bacterial populations. Marker assisted selection for resistance to BBS using molecular markers found during this study may improve selection efficiency for resistance because of the low heritability of the reaction to BBS, and the independent QTLs for resistance to different bacterial diseases. Furthermore, disease screening solely dependent upon natural field inoculations is not reliable because of genotype x environment interactions. The leaflet freezing assay method could be utilized to screen progenies or breeding germplasms, even when no distinct phenotypic disease symptoms are present on the plants.