Detection of linked QTL for soybean brown stem rot resistance in 'BSR 101'as expressed in a growth chamber environment

The objective of this study was to map the gene(s) conferring resistance to brown stem rot in the soybean cultivar BSR 101. A population of 320 recomb inant inbred lines (RIL) was derived from a cross of BSR 101 and PI 437.654 . Seedlings of each RIL and parent were inoculated by injecting stems with a suspension of spores and mycelia of Phialophora gregata, incubated in a g rowth chamber at 17 degrees C, and assessed for resistance by monitoring th e development of foliar and stem symptoms. The population also was evaluate d with 146 RFLPs, 760 AFLPs, and 4 probes for resistance gene analogs (RGAs ). Regression analysis identified a significant association between resista nce and several markers on Linkage Group J of the USDA-ARS molecular marker linkage map. Interval analysis with Mapmaker QTL identified a major peak b etween marker RGA2V-1 and AFLP marker AAGATG 152M on Linkage Group J. A sec ond peak, associated only with stem symptoms, was identified between the RF LP B122I-1 and RGA2V-1, also on Linkage Group J. When composite interval ma pping with QTL Cartographer was used, two linked QTL were identified with b oth foliar and stem disease assessment methods: a major QTL between AFLP ma rkers AAGATG152E and ACAAGT260, and a minor QTL between RGA3I-3 and RGA3I-2 , These results demonstrate that composite interval mapping gives increased precision over interval mapping and is capable of distinguishing two linke d QTL. The minor QTL associated with the cluster of RGA3I loci is of specia l interest because it is the first example of a disease resistance QTL asso ciated with a resistance gene analog.