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CROP BREEDING, GENETICS & CYTOLOGY

Agronomic Traits Affecting Resistance to White Mold in Common Bean

^a Dep. of Crop and Soil Sciences, Oregon State Univ., Corvallis, OR, 97331
^b Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI, 48824

* Corresponding author (kellyj{at}msu.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Resistance to white mold [caused by Sclerotinia sclerotiorum (Lib.) de Bary] in common bean (Phaseolus vulgaris L.) is complexly inherited, and highly influenced by environmental factors. The identification of agronomically desirable avoidance mechanisms in advanced or elite lines and segregating populations is essential. The objectives of this study were to determine the relationship between nine agronomic traits (growth habit, days to flower, canopy height and width, branching pattern, lodging, days to maturity, seed size, and yield) and resistance to white mold in the field, corresponding heritabilities of resistance and agronomic traits, and the effect of the agronomic traits on disease and seed yield. A group of elite lines, and two recombinant populations derived from crosses between two resistant navy bean cultivars, Bunsi and Huron, to a susceptible cultivar, Newport, were evaluated for disease severity index (DSI) and agronomic traits in multiple field seasons. Heritability estimates for DSI were moderate (0.47) in the Bunsi/Newport population and high (0.82) in the Huron/Newport population. All agronomic traits displayed moderate to high heritability estimates. The agronomic traits that associated significantly with DSI in the three populations differed greatly. The most important agronomic trait that reduced DSI and contributed to yield was indeterminate growth habit. Increased canopy height, width, and lodging were generally associated with an increase in DSI, whereas traits such as days to flower and maturity varied in relation to DSI and yield across environments and populations. The complexity of the relationship of agronomic traits to DSI and yield highlights the difficulties bean breeders face in selecting among elite lines and within segregating populations for resistance to white mold in the field.

Abbreviations: BN, Bunsi/Newport • BP, branching pattern • DI, disease incidence • DSI, disease severity index • DTF, days to flower • DTM, days to maturity • GH, growth habit • HN, Huron/Newport • HT, canopy height • LDG, lodging • MRF, Montcalm Research Farm • RIL, recombinant inbred line • SCF, Sanilac Cooperator Farm • WD, canopy width

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
WHITE MOLD IS A destructive yield-limiting fungal disease that seriously affects common bean production in temperate regions (Steadman, 1983; Purdy, 1979). White mold infections in bean are typically initiated during flowering, coinciding with the closure of the plant canopy and the development of apothecia from soil-borne sclerotial bodies (Boland and Hall, 1987). Ascospores disperse from the apothecia into the plant canopy and require a nutrient source, such as senescent flowers, for germination and subsequent mycelial infection (Abawi and Grogan, 1975; Hunter et al., 1978). Symptoms of white mold on bean plants include wilting, lesions, bleached stems, the presence of sclerotial bodies in infected tissue, and plant death. Resistance to white mold in bean is quantitatively inherited (Fuller et al., 1984a; Miklas and Grafton, 1992) consisting of physiological resistance and avoidance (Miklas et al., 2001). Previous studies have indicated that heritability estimates were low to moderate for partial physiological resistance, and moderate to high for field resistance (Miklas and Grafton, 1992). Resistance to white mold has been described in several navy bean cultivars, such as Bunsi (also known as Ex Rico 23) and C-20, (Schwartz et al., 1987; Miklas and Grafton, 1992; Tu and Beversdorf, 1982; Kelly et al., 1984).

Plant avoidance of white mold can be due to plant architectural traits or agronomic management practices. A decrease in plant row width, resulting in higher plant density was shown to increase the level of white mold in bean (Park, 1993; Steadman et al., 1973). Elevating the canopy of a prostrate highly susceptible indeterminate great northern bean in a semiarid climate reduced white mold infection and increased yield (Fuller et al., 1984b). In comparison to determinate navy bean types, upright indeterminate navy bean cultivars escape white mold infection or spread because of a narrower canopy (Park, 1993). Reduced levels of white mold in the field have been associated with the open porous canopy of the indeterminate navy bean cultivar Bunsi (Tu and Beversdorf, 1982; Park, 1993). An open, porous canopy of larger seeded determinate beans was also identified as an architectural avoidance mechanism (Coyne et al., 1974; Weiss et al., 1980; Schwartz et al., 1987). Dense canopies in bean generally resulted in higher white mold severity than porous canopies because of the development of a favorable microclimate within the canopy (Blad et al., 1978; Coyne, 1980). Fewer apothecia were produced underneath the open canopy of the determinate dark red kidney bean cultivar Charlevoix and the upright canopy of the indeterminate small white bean cultivar Aurora compared with the dense canopy of several prostrate indeterminate great northern bean cultivars (Schwartz and Steadman, 1978). Plant height was found to be associated with lower field white mold levels in an A55/G122 recombinant inbred line (RIL) population (Miklas et al., 2001) and a PC-50/XAN159 RIL population (Park et al., 2001). In soybean [Glycine max (L.) Merr], however, increased plant height was associated with increased white mold levels in the field. Other agronomic traits in soybean, such as an increase in days to R1, lodging, and maturity date, were also found to be associated positively with white mold levels in the field (Kim and Diers, 2000).

Progress in breeding for white mold resistance in bean is hindered by environmental conditions and morphological factors that confound the expression and detection of physiological resistance mechanisms. Combining physiological resistance with agronomically desirable avoidance mechanisms may offer a stable approach to the long-term improvement of resistance to white mold in bean. The identification of agronomically desirable avoidance mechanisms in advanced or elite lines and segregating populations is essential. The objectives of this study were to identify agronomic avoidance traits that contribute to resistance to white mold in the field, determine the heritabilities of agronomic traits and resistance to white mold in the field, and investigate the resulting effect of the agronomic avoidance traits on resistance to white mold and yield levels.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Plant Materials
A group of elite lines and two segregating navy bean populations were used to study relationships among agronomic traits and field resistance to white mold. The elite lines were tested in three field trials and consisted of 30 (Test 1) and 36 (Tests 2 and 3) genotypes, including cultivars and breeding lines from seven commercial bean classes, new sources of improved resistance, and genotypes entered for testing in the National Sclerotinia Bean Nursery. Twenty-seven genotypes were common across all three tests. The elite lines were typically high-yielding cultivars or advanced breeding lines selected for useful agronomic attributes and previously evaluated in three corresponding greenhouse tests for resistance to oxalate (Kolkman and Kelly, 2000).

The Bunsi/Newport (BN) population consisted of 98 F₃-derived lines developed from a biparental cross between Bunsi, an indeterminate (Type II; Singh, 1982) resistant cultivar with an open porous canopy, and Newport, a determinate (Type I) susceptible cultivar (Kelly et al., 1995; Kolkman and Kelly, 2000). Bunsi has physiological resistance to white mold, resistance to oxalate, and exhibits plant avoidance through an open, porous canopy (Tu and Beversdorf, 1982; Schwartz et al., 1978; Miklas and Grafton, 1992; Tu, 1985; Kolkman and Kelly, 2000). The 98 F₂ individuals were advanced in the greenhouse to the F₃ generation by single seed descent. Seed from individual F₃ plants was bulked and advanced in the greenhouse. Seed harvested from three F_3:4 plants was bulked and F_3:5 plants were increased in a winter nursery in Puerto Rico.

The Huron/Newport (HN) population consisted of 28 F_5:6 RILs, developed by single seed descent from a biparental cross between the resistant indeterminate (Type II) cultivar, Huron (Kelly et al., 1994), and the susceptible determinate (Type I) cultivar, Newport. Huron has both physiological resistance to white mold, as determined through resistance to oxalate and resistance to white mold in the field (Kolkman and Kelly, 2000). The HN population was advanced by single seed descent and bulked in the F₅ generation for seed increase. No intentional selection was made for agronomic traits during the development of either population.

The elite lines and two segregating populations were evaluated for resistance to white mold in the field in three individual experiments. The elite lines were grown in field trials at the Montcalm Research Farm (MRF), Entrican, MI, in 1996 (MRF96; Test 1), 1997 (MRF97; Test 2), and 1998 (MRF98; Test 3). The BN F_3:6 population was grown in MRF97, and at the Sanilac Cooperator Farm (SCF97) location, Palms, MI, in 1997, and the F_3:7 lines were grown in MRF98. Only 88 of the 98 F_3:6 lines in the BN population were grown in SCF97, due to limited seed availability. The HN population was grown in MRF96 (F_5:6), MRF97(F_5:7), SCF97 (F_5:7), and MRF98 (F_5:8). Planting was delayed to the second week in June in all field experiments to favor disease development. Plot rows were 6 m in length, with a 0.5-m-row spacing in the MRF experiments, whereas all plots at the SCF97 location had a 3-m-row length and 0.76-m-row width. The two center rows of each four row plot were planted with the experimental line, while the two outer border rows were planted with a highly white mold susceptible cultivar, Midland. Standard agronomic practices for tillage, fertilization, and herbicide were applied to ensure good crop growth and development at both field sites. To promote uniform disease pressure across the field at MRF, plots were overhead irrigated five times in 1996, three times in 1997, and six times in 1998 during the flowering period with approximately 13 mm of water each irrigation. The field site at SCF97 was selected on the basis of past history of heavy white mold infection in previous years.

Traits
Plots were rated for DSI and disease incidence (DI; Kolkman and Kelly, 2000; Steadman, 1997; Steadman et al., 1998) by means of a "quarter scale" (Hall and Phillips, 1996) shortly before harvest, when the plants had reached physiological maturity. Thirty plants per plot were rated from 0 to 4, where 0 = no disease present, 1 = 1 to 25% of the plant with white mold symptoms, 2 = 26 to 50% of the plant with white mold symptoms, 3 = 51 to 75% of the plant with white mold symptoms, and 4 = 76 to 100% of the plant with white mold symptoms. DSI was calculated for each plot on a percentage basis by the following formula:

DI was calculated as the number of plants out of the 30 individuals with white mold infection on a percentage basis.

The elite lines and segregating populations were evaluated for the following agronomic traits: growth habit, days to flower, mid-season canopy height, mid-season canopy width, branching pattern, days to maturity, lodging, seed size, and yield. Growth habit was determined during the growing season as either indeterminate (Type II) or determinate (Type I). Days to flower were recorded as the number of days following planting, when 50% of the plants in a plot had at least one open flower. At midseason (post-main flower flush) canopy height and width measurements were averaged on each individual plot, from six measurements per plot (three measurements per row) in all experiments except for the MRF96 trials, where 10 measurements per plot were taken. Plots were evaluated for branching pattern on the basis of a 1 to 5 scale, where 1 = acute upright branching, and 5 = obtuse prostrate branching. Days to maturity were recorded as the number of days following planting, until 90% of the pods were physiologically mature and drying down. Lodging was determined at maturity, on the basis of a 1 to 5 scale, where 1 = no lodging, and 5 = excessive lodging. All plots were harvested at maturity after the disease ratings were taken. Seed yield and seed size (the weight of 100 seeds) were adjusted to 18% moisture content by weight.

Statistical Analysis
In the elite lines, the MRF96 experiment was analyzed as a rectangular lattice and the MRF97 and MRF98 field experiments were each analyzed as a partially balanced triple lattice by means of PROC LATTICE (SAS, 1995). The 27 common elite genotypes were analyzed across all three field seasons as a RCBD, by means of PROC GLM, with environments considered as a random effect and genotypes as a fixed effect. The BN and HN populations were evaluated in individual field environments as RCBDs by means of PROC GLM. Both segregating populations were analyzed across the three years as a RCBD, by means of PROC GLM, with both genotypes and environments considered as random effects. Estimates of heritability for all traits were calculated on an entry-mean basis, where h² = (Hallauer and Miranda, 1988). The 90% confidence intervals for the heritability estimates were determined on the basis of Knapp et al. (1985). A chi-square goodness-of-fit test was used to test growth habit for a normal determinate to indeterminate segregation ratio in the segregating populations. Pearson correlation coefficients (r) were calculated by PROC CORR, and multiple stepwise regression of significant agronomic traits (P < 0.15) on DSI and yield (excluding seed size) and R² values were determined by PROC REG (SAS, 1995).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Significant genotypic variance among the elite lines and within the BN and HN populations was observed for DSI, DI, and all agronomic traits measured (Table 1). Genotypic expression for all traits was influenced by environment, as indicated by significant genotype x environment interactions, except for DSI, DI and days to maturity in the HN population. Mean DSI and DI values indicated that adequate disease pressure was attained in each environment (Table 2). In both the BN and HN populations, parental genotypes varied for disease reaction and for all agronomic traits, except lodging (Table 3). The DSI and DI means for the elite lines were reported previously (Kolkman and Kelly, 2000) and the agronomic trait means for the elite lines are available upon request. Lodging scores between parents were identical, yet the progeny segregated for lodging in both populations. Determinate and indeterminate growth habit segregated as expected in the BN (² = 1.98; P = 0.16) and HN (² = 0.57; P = 0.47) populations.

Heritability estimates for resistance to white mold ranged from 0.47 for DSI in the BN population to 0.82 in the HN population (Table 3). Previous estimates of heritability for resistance to white mold in the field have been moderate to high in a Bunsi-derived population (h² = 0.70), and in two snap pod bean populations (h² = 0.77; h² = 0.58), suggesting that progress in breeding for resistance is possible (Miklas and Grafton, 1992). Heritability estimates of agronomic traits in the BN population were generally high, ranging from h² = 0.65 for canopy width, to h² = 0.90 for days to flower. In the HN population, heritability estimates for agronomic traits were generally moderate to high, ranging from h² = 0.58 for canopy width to h² = 0.95 for days to maturity. The estimates of heritability for yield under white mold pressure were moderate in both populations. The strong influence of DSI on yield (Table 4) under white mold pressure most likely accounted for a clear separation in yield among segregating progeny, resulting in the atypically moderate to high heritability values for yield (Table 3).

Multiple regression analysis of agronomic traits on DSI and yield also depicted the complexity of resistance to white mold in bean (Table 5). Desirable avoidance mechanisms are recognized as those traits that reduce DSI and either increase yield or have no negative effect on yield under white mold pressure. In contrast to the single regression analysis (Table 4) growth habit was one of the most important desirable avoidance mechanisms identified in the elite lines and segregating populations, contributing to both DSI and yield (Tables 5 and 6). The indeterminate growth habit was associated with a decrease in disease pressure in the BN and HN populations, except for the SCF97 location. Indeterminate growth habit was also generally associated with an increase in yield, accounting for up to 34.4% of the yield in the BN population in MRF97, and up to 51.6% of the yield in the HN population in MRF96. Yield stability associated with indeterminate navy bean genotypes has been previously demonstrated in the absence of white mold (Kelly et al., 1987). Indeterminate growth habit as a desirable avoidance phenotype in navy bean is reversed in large-seeded bush bean, where the determinate growth habit is an avoidance phenotype (Miklas et al., 2001; Park et al., 2001; Schwartz and Steadman, 1978).

Increased canopy height was associated with an increase in DSI and yield in the elite lines and the BN and HN populations, except for the elite lines tested at MRF98, where an increase in height was associated with a decrease in DSI (Tables 5 and 6). An increase in plant height has been associated previously with reduced disease levels (Miklas et al., 2001; Park et al., 2001). The contrasting results may be due to the extreme differences in resistance phenotypes evaluated and environments tested. Most significantly, in the HN population, an increase in canopy height resulted in a 19% increase in DSI at MRF96, and a 26.2% increase in yield at SCF97. Interestingly, the susceptible parent had a greater canopy height than either resistant parent. For the two segregating populations, reduced plant height would be considered an undesirable avoidance mechanism since it also reduced yield potential.

An increase in canopy width was a desirable avoidance mechanism in the elite lines, since it reduced DSI levels while increasing yield. Larger-seeded genotypes that may have had wider but more porous canopies that those of the segregating populations may have contributed to this result (Tables 5 and 6). In general, greater canopy width allows for more rapid canopy closure, which could result in higher white mold pressure and less avoidance. Although decreased canopy width was associated with a decrease in DSI in the segregating populations, it is still an undesirable trait because of the negative impact on yield.

Branching pattern, lodging, and days to maturity also had variable impacts on DSI and yield in each environment and between the elite lines and segregating populations (Tables 5 and 6). The parental genotype Bunsi has a more open, wider branching pattern than the susceptible parent and the wider branching pattern resulted in a decrease in DSI for the BN population. In contrast, Huron has a narrower upright branching pattern than does Newport and branching pattern was not a factor for DSI in this population. For the elite lines, an increase in branching pattern was attributable to an increase in yield at MRF97 and a decrease in yield at MRF98. An increase in lodging in the BN population at MRF97 and MRF98 was associated with an increase in DSI and a decrease in yield. The microclimate within a lodged canopy is more conducive to heavy white mold infection and spread of disease. For the elite lines, an increase in lodging also contributed to an increase in DSI (13.4%) and a decrease in yield (6.1%) in the MRF97 environment. Generally, more days to maturity resulted in less disease and higher yield in the multifactor analysis except at the MRF98 location, where more days to maturity reduced yield. The function of maturity on DSI as a desirable or undesirable avoidance mechanism is highly variable and dependent upon specific environmental conditions, genotypes evaluated, and the desired yield potential.

The DSI score in each trial had a major impact on seed size and yield. Heavy white mold pressure typically reduces seed size and subsequently yield (Kerr et al., 1978). In this study, as DSI increased, seed size decreased for BN at all three locations and in the HN population at MRF97 and MRF98 (Table 4). Similarly, as DSI increased, yield decreased in most environments for the elite lines and BN and HN populations. Although genotypes and environments that produce high yield potential are also conducive to heavy white mold levels, realized yield was severely curtailed since high DSI negatively affected yield across most environments in the elite lines and segregating populations.

The complexity of the relationship between agronomic traits and white mold in the field in contrasting environments and differing tests or populations underlies the difficulties bean breeders face when trying to select for resistance to white mold. Agronomic avoidance mechanisms played a major role in the final disease development in the segregating populations. Of the traits measured, growth habit was the most important avoidance mechanism affecting resistance to white mold. The indeterminate growth habit of the Bunsi and Huron cultivars was associated with reduced DSI and increased yield. Many of the other agronomic traits varied in their avoidance characteristics to white mold between the elite lines and segregating populations, as well as between environments. Specifically, such traits as days to flower and canopy height must be considered individually in relation to the environment and population tested, and the potential direct effect on yield.

The choice of the resistant parent is an important factor in dictating how much variability in DSI is attributable to agronomic avoidance traits versus physiological resistance. The resistant parents, Bunsi and Huron, possess a similar growth habit, yet differ in branching pattern. The variation in correlations between agronomic and disease-related traits indicates that the overall architecture of the resistant parent is very important in determining potential avoidance mechanisms in a segregating population. Early generation selection against highly heritable, undesirable agronomic avoidance traits such as the determinate growth habit in navy bean, may be a useful approach in minimizing the selection of less desirable genotypes.

Given the complexity of interactions between agronomic traits on white mold development and yield in bean, we would propose that the small-seeded resistant ideotype for the Midwest should possess physiological resistance to white mold and be indeterminate, with a Type II growth habit, resistant to lodging, medium canopy width and branching pattern, and medium height, days to flower and maturity, to ensure adequate vegetative growth and yield potential without creating a favorable microclimate within the plant canopy for disease development. Choosing among the types of variability present for different agronomic traits in populations segregating for resistance needs to be conducted in different environments if critical progress is to be made in breeding for resistance to white mold in common bean.

Received for publication February 22, 2001.

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