Inheritance and QTL Analysis of Field Resistance to Ashy Stem Blight in Common Bean

Ashy stem blight [caused by Macrophomina phaseolina (Tassi) Goid.] can be a serious disease of common bean (Phaseolus vulgaris L.) under drought and high temperature conditions in some regions. The mode of inheritance of valuable sources of resistance is lacking. We studied inheritance of field resistance to ashy stem blight in a recombinant inbred population (’Dorado’ × XAN 176) consisting of 119 Fs:7 recombinant inbred lines (RILs) tested in replicated experiments across 2 yr. A score from 1 to 9 (no disease to severe disease) was used to measure disease reaction. Moderate HNs (0.53 and 0.57) and near-normal frequency distribution of RILs for mean disease score each year indicated a lack of discrete segregation classes. The phenotypic variation across a subgroup composed of 79 RILs was further investigated with 165 randomly amplified polymorphic DNA (RAPD) markers by one-way analyses of variance and interval mapping. Five quantitative trait loci (QTL), explaining 19, 15, 15, 13, and 13% of the phenotypic variation for disease score, were detected in 1993. Three of these QTL, explaining 15,12, and 12% of the variation in disease reaction, were detected in 1994. Multiple QTL regression models (P < 0.01) explained up to 47% (four loci) of the phenotypic variation for disease score in 1993 and 28% (three loci) in 1994. The five QTL, all derived from XAN 176, generally showed additive effects. These QTL-linked RAPD markers may prove useful for indirect selection of field resistance to ashy stem blight derived from XAN 176. A HY STEM BLIGHT or charcoal rot, incited by Macrophomina phaseolina, is a serious disease of common bean in tropical and subtropical areas and some temperate regions characterized by high temperatures and drought conditions. Zaumeyer and Thomas (1957) reported 65% yield reduction in common bean from ashy stem blight in the USA, and Miklas et al. (1998b) observed disease severity was correlated with yield loss in tepary bean (Phaseolus acutifolius A. Gray latifolius G. Freeman). Periodic ashy stem blight epidemics of common bean occur in Puerto Rico (Echavez-Badel and Beaver, 1987). In addition to common bean, this fungus can inflict substantial yield loss on other crops (Schwartz, 1989) such as soybean [Glycine max (L.) Merr.], maize (Zea mays L.), and sorghum [Sorghum bicolor (L.) Moench]. Pastor-Corrales and Abawi (1988) report that losses in common bean result from either preemergence and postemergence death of seedlings (seedling rot disease) or reduced vigor and yield of adult plants (ashy stem blight disease). The pathogen is seed transmissible (Abawi and Pastor-Corrales, 1990b), but primary inoculum causing epidemics are P.N. Miklas, USDA-ARS-IAREC, 24106 N. Bunn Rd., Prosser, WA 99350-9687; V. Stone, Whitehead Institute, 9 Cambridge Center, Cambridge, MA 02142; C.A. Urrea, Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105; E. Johnson, Crop Science Dep., North Carolina State Univ., Raleigh, NC 27695; J.S. Beaver, Dep. of Agronomy and Soils, Univ. of Puerto Rico, Mayag0ez, PR 00681. Received 3 March 1997. *Corresponding author ([email protected]). Published in Crop Sci. 38:916-921 (1998). sclerotia embedded in organic debris or free in the soil. Crop rotation is recommended as an integrated control practice to reduce inoculum in the soil. Genetic resistance to M. phaseolina is also advocated for integrated control practices, and high levels of resistance have been observed in beans (Echavez-Badel and Beaver, 1987; Pastor-Corrales and Abawi, 1988). The combining of resistances from different sources will likely provide more durable resistance. Olaya et al. (1996) determined in greenhouse inoculations that resistance of BAT 477 was conditioned by two dominant complementary genes, Mp-1 and Mp-2. However, a direct relationship between Mp-I and Mp-2 and field resistance exhibited by BAT 477 (Pastor-Corrales and Abawi, 1988) was not shown. Some researchers (Beebe and Pastor-Corrales, 1991; Pastor-Corrales and Abawi, 1.988) allude to, or report, strong correlations between field and greenhouse reactions to ashy stem blight, whereas, Miklas et al. (1998b) working with tepary bean observed none. Perhaps the greenhouse method used by Miklas et al. (1998b) selectively measured resistance to seedling rot (Abawi and Pastor-Corrales, 1989) but not adult-plant resistance to ashy stem blight in the field. Additional information concerning inheritance of field resistance is needed to enhance breeding efforts. Herein, field resistance to ashy stem blight in a recombinant inbred population (Dorado/XAN-176) is investigated by means of multiple environments and a linkage map for identifying putative QTL. MATERIALS AND METHODS A recombinant inbred population of 119 randomly derived F5:7 lines (RILs) generated from a Dorado (formerly 364) × XAN 176 hybridization was evaluated for field reaction to ashy stem blight at the University of Puerto Rico, Fortuna Substation at Juana Dfaz, PR, (fine-loamy, mixed, isohyperthcrmic Cumulic Haplustoll) during the summers of 1993 and 1994. Ashy stem blight epidemics frequently occur in Fortuna because of prevailing high temperatures and low rainfall (Abawl and Pastor-Corrales, 1990a). In addition, the plantings of dry bean on the same experimental area for many years preceding this research likely resulted in elew~ted soil inoculure levels. The F5:7 lines were randomly assigned to three separate groups, two of 40 and one of 39 lines. Each year, each group was sown as a separate randomized complete block design with three replications. The groups were planted side by side, and each group contained the parents. Plots consisted of single 1-m rows spaced 80 cm apart in 1993 and 160 cm apart in 1994. Plots were rated for ashy stem blight infection at the end of the R-8 growth stage with a 1-to-9 scale similar to CIAT’s (Schoonhoven and Pastor-Corrales, 1987). The scale combined incidence (number of plants infected) and severity (plant area affected by the pathogen) of infection on a whole Abbreviations: cM, centimorgans; DAP, days after planting; HNs, narrow-sense heritability; QTL, quantitative trait loci (locus); RAPD, random amplified polymorphic DNA; RIL, recombinant inbred line. 916 Published July, 1998