1. Heritability of Resistance to White Mold in Snap Bean (Phaseolus vulgaris L.) Based on Field Evaluation, Oxalic Acid, and Straw Test
Yong Suk Chung , Michell E. Sass, Scott C. Schaefer, James Nienhuis
Department of Horticulture, University of Wisconsin – Madison
Introduction
Sclerotinia sclerotiorum (Lib.) de Bary (phylum Ascomycota) is a ubiquitous necrotrophic fungi that is a common pathogen in cool and wet environments worldwide (Purdy, 1979; Bolton et al., 2006). The disease has many common names depending upon host and symptoms, the most common include stem rot and white mold. White mold of dry bean (Phaseolus vulgaris L.) was first reported in 1944 in Colorado and Wyoming, but due to sporadic environmental conditions favorable to disease development and lack of inoculum, the disease was limited to localized outbreaks until early 1990’s (Dickson et al., 1982). Currently, white mold is an important production constraint to processing snap bean in the cool-wet northern states including Wisconsin.
Objective
The objective of this research is to estimate the heritability of white mold resistance based on both field and greenhouse evaluations in two snap bean populations derived using a common resistance parent.
Materials and Methods
The germplasm accession, ‘G122’, has over years and locations ranked among the most tolerant in the White Mold Cooperative nursery coordinated by Jim Steadman, University of Nebraska. G122 was used as the donor parent in the development of a recombinant inbred line population (Astrel x G122) in a cross to the small sieve snap bean cultivar ‘Astrel’. An independent inbred backcross population’TR781’ was developed by crossing and backcrossing to a large sieve snap bean, ‘PLS8088’. The two populations, A x G and TR781, represent theoretical contributions of 50 and 25 percent of the G122 genome, respectively.
The two populations were evaluated over two years, 2004 and 2005, at the Arlington Agric. Res. Station, Arlington, WI, in a field inoculated with white mold sclerotia and mist irrigated. The populations were evaluated for disease incidence (percentage of plants in a row in which white mold symptoms were observed) and disease severity (the mean number or nodes affected in a sample of 10 plants). The populations were evaluated in the greenhouse for reaction to the oxalic acid toxin produced naturally by the white mold fungus. The populations were also evaluated using the straw test for the number of nodes that mycelial inoculum progressed over a three week period.
Field Evaluation
Greenhouse Evaluation
Disease Incidence
Disease Severity
Oxalic Acid Test
Straw Test
Resistant and susceptible reactions to 20 mM oxalate solution (pH 4.0)
Susceptible
Resistant
Scale 5: Stem/branch progression of fungus passed the first node
Scale 7: Stem/branch progression of fungus to the second node
Scale 9: Stem/branch progression to the third internode > 1 inch
Modified scale of Petzoldt and Dickson (1996) for white mold rating in common bean
Scale 3: Stem/branch infection of the first internode > 1 inch
Results and Discussion
All greenhouse and field evaluation methods distinguished among genotypes; however, the correlation among methods was low.
Low correlation may be due to the fact that most greenhouse tests are based on the growth and development of mycelial infections, whereas the primary inoculum source in the
field is ascospores (Kull et al., 2003) as well as due to avoidance mechanism.
Marker assisted selection may be useful in selecting for architectural and physiological components of white mold resistance in snap bean.
Figure 1. Distribution of two populations (Astrel x G122 and TR781) evaluated for white mold
resistance using two greenhouse and two field based evaluation procedures. 1
0.00ns 67
0.27* 66
0.26*
Straw Test
0.37** 60
0.10ns
0.06ns
Oxalic Acid Test
0.31* 62
0.20ns 64
0.80***
Disease Severity
0.18ns
0.19ns
0.87***
Disease Incidence
Table 2. Spearman correlation coefficients among two greenhouse tests, and two methods of field evaluation for resistance to white mold over years (A x G)
is above diagonal, and TR781 is below diagonal).
*, **, *** Significant at the 0.05, 0.01, and probability level, respectively.
ns = Nonsignificant at P < 0.05.
References
Bolton, M.D., B. Thomma, and B.D. Nelson Sclerotinia sclerotiorum (Lib.) de Bary: biology and molecular traits of a cosmopolitan pathogen. Molecular Plant Pathology 7:1-16.
Dickson, M.H., J.E. Hunter, M.A. Boettger, and J.A. Cigna Selection for resistance in Phaseolus vulgaris L. to white mold disease caused by Sclerotinia sclerotiorum (Lib.) de Bary. J. Amer. Soc.
Hort. Sci. 107:
Kolkman J.M. and James D. Kelly An indirect test using oxalate to determine physiological resistance to white mold in common bean. Crop Science 40:
Kull, L.S., T.D. Vuong, K.S. Powers, K.M. Eskridge, J.R. Steadman, and G.L. Hartman Evaluation of resistance screening methods for Sclerotinia stem rot of soybean and dry bean. Plant Dis.
87:
Petzoldt, R. and M.H. Dickson Straw test for white mold disease in common beans. Annu. Rep. Bean Improv. Coop. 39:
Purdy, L.H Sclerotinia sclerotiorum: history, diseases and symptomatology, host range, geographic distribution, and impact. Phytopathology 69: