Crop Science 41:549-552 (2001)
© 2001 Crop Science Society of America
PLANT GENETIC RESOURCES
Resistance to Frogeye Leaf Spot in Maturity Groups VI and VII of Soybean Germplasm
Wencai Yang and
David B. Weaver
Dep. of Agronomy and Soils, Auburn Univ. and Alabama Agric. Exp. Stn., Auburn, AL 36849
Corresponding author (dweaver{at}acesag.auburn.edu)
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ABSTRACT
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Frogeye leaf spot (FLS) (caused by Cercospora sojina Hara) is an important foliar disease in many soybean [Glycine max (L.) Merr.] production areas of the world. It can cause severe yield losses, but can be completely managed with genetic resistance. Three dominant genes for resistance have been reported. However, the fungus is known to be genetically variable, and in the past, new races have developed that were capable of infecting previously resistant genotypes. Finding additional sources of resistance may reduce the impact of new virulent races. The objective of this study was to evaluate all available soybean plant introductions in maturity groups VI and VII for resistance to FLS. Plants were inoculated with C-32, an isolate of C. sojina, in the greenhouse. Genotypes were rated on a scale of 0 to 6 on the basis of percent leaf area infected with lesions. Six hundred sixty (39.3%) of the accessions received a rating of 3 or lower, and were identified as resistant. Twelve accessions remained completely disease free after repeated inoculation, thus were considered highly resistant or immune. There appears to be adequate resistance to FLS within the germplasm collection, and many of these may have unique resistance genes.
Abbreviations: FLS, frogeye leaf spot • MG, maturity group
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INTRODUCTION
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FROGEYE LEAF SPOT is a common foliar disease of soybean. It occurs worldwide and is most severe in warm, humid climates (Athow, 1973; Ma, 1994). This disease occurs mainly on foliage but can also occur on the stems, pods, and seeds (Lehman, 1934). Its occurrence in the USA was first reported in 1924 (Melchers, 1925). Most studies have found yield reductions due to FLS in the range of 10 to 50% (Laviolette et al., 1970; Bernaux, 1979; Ma, 1994; Mian et al., 1998). Greater than 60% yield loss due to the disease has been reported in the tropical environment of Nigeria (Dashiell and Akem, 1991). Although fungicides are effective in FLS control and can increase yield (Horn et al., 1975; Sloane et al., 1975; Akem, 1995), the use of resistant cultivars is an inexpensive and environmentally friendly approach to management of the disease (Bernaux, 1979; Ma, 1994; Akem, 1995).
Three dominant resistance genes have been identified in the USA. Rcs1 in ‘Lincoln’ confers resistance to Races 1 and 5 (Athow and Probst, 1952; Probst and Athow, 1958; Phillips and Boerma, 1982). Rcs2 in ‘Kent’ confers resistance to Race 2 (Probst et al., 1965). Rcs3 in ‘Davis’ confers resistance to all described races in the USA and all Brazilian isolates tested (Yorinori, 1980; Phillips and Boerma, 1982; Boerma and Phillips, 1983). Some additional genes for resistance to Race 5 have been identified in ‘Ransom’, ‘Stonewall’, and ‘Lee’, and each of these genes was non-allelic to Rcs3 and to each other (Pace et al., 1993). However, these are not considered to be important sources of resistance because Race 5 is no longer considered to be an economic threat in the southern USA (Baker et al., 1999). Resistance in ‘Peking’ to many isolates is also controlled by one single dominant gene that is non-allelic to Rcs3 (Baker et al., 1999). The gene symbol HRCS7 was assigned to a gene for resistance to Chinese Race 7 in ‘Hardison’, ‘Dong Nong 84-898’, and ‘Ozzie’ (Yang et al., 1995). The allelism between HRCS7 and other genes has not been studied. There are other genetic studies on the resistance of soybean cultivars to FLS in China (Liu and Huang, 1986; Zhang et al., 1990), however, genes conferring the resistance have not been designated.
Cercopora sojina appears to be genetically variable. Twenty-two races of the fungus have been identified in Brazil (Yorinori, 1992). Fourteen races have been identified in China (Ma and Li, 1997). In the USA, at least 12 races have been described (Athow et al., 1962; Ross, 1968; Yorinori, 1980; Phillips and Boerma, 1981). A recent study designed to further elucidate the extent of pathogen variability and define a set of differential cultivars to aid in race identification of the fungus reported 44 races of C. sojina on a set of 38 differential genotypes (D.V. Phillips, 1998, personal communication). Relationships among races have been difficult to establish because of lack of a uniform set of differential cultivars with known unique genes for resistance.
Breeding for resistance is a long-term disease-management strategy. The use of resistance resources depends on population dynamics of races. There is concern that the widespread use of a few resistance genes might accelerate the selection of new pathogenic races and consequently result in destabilization of crop production (Browning and Frey, 1969; National Academy of Sciences, 1972). The appearance in the midwestern USA of new highly virulent races of C. sojina such as Race 2 in the late 1950s (Athow et al., 1962), Races 3 and 4 in the mid 1960s (Ross, 1968), and Race 5 in the late 1970s (Phillips and Boerma, 1981), and the breakdown of resistance (Yorinori, 1992) have demonstrated the need for the identification of alternative sources of genetic resistance to this pathogen. The objective of this study was to identify resistance to FLS in maturity groups (MG) VI and VII of the USDA soybean germplasm collection.
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MATERIALS AND METHODS
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Plant Materials and Experimental Design
All available plant introduction accessions (PIs) in MG VI and VII from the USDA Soybean Germplasm Collection were obtained from USDA-ARS (Urbana, IL). The 1686 PI accessions consisted of 1024 in MG VI and 662 in MG VII. One thousand six hundred fifty-six accessions were introduced from 32 countries or regions, with about 80% of them from three countries: China (29.2%), Japan (36.4%), and South Korea (14.0%). The origin of the remaining 30 accessions was not clear (Table 1).
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Table 1. Frogeye leaf spot resistant and total soybean accessions in maturity groups VI and VII introduced from 32 countries or regions
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Accessions were divided into 24 groups (experiments) because of limitations of greenhouse space and humidity chamber size. Each experiment was arranged as a completely randomized design with three replications. The experimental unit was a single 10-cm pot containing four plants. Davis and Lee were used as resistant and susceptible controls, respectively. Plants were grown in Pro-mix (Premier Brands Inc., Yonkers, NY) and fertilized and watered as necessary.
Media and Inoculum Preparation
Soybean stem lima bean agar (SSLBA) medium composed of equal parts of soybean stem agar and lima bean agar (Difco, Detroit, MI) was used to culture the fungus. To prepare the medium, 50 g of frozen soybean stems were placed in 500 mL of distilled H2O and blended at high speed until the stems were macerated. The resulting homogenate was transferred to a 1-L beaker, cooked in the microwave for 5 min, then boiled for 20 min. Lima bean agar (5.75 g), bacto-agar (4.0 g), and 100 mL distilled H2O were added and mixed to suspend the agar. The homogenate was then filtered through a double layer of cheesecloth, and the total volume brought up to 1 L by adding distilled H2O. The suspension was autoclaved for 20 min, and allowed to cool at room temperature for 1 h. Fifteen milliliters was added to each 9-cm petri dish.
A virulent culture of C. sojina was obtained from Dr. D.V. Phillips, University of Georgia (Phillips, 1998, personal communication) designated as C-32. This isolate is the same as that identified by Baker et al. (1999) as AU-05, and was the most virulent isolate tested in that experiment and the only isolate to elicit a susceptible reaction in Peking. The isolate was collected at Jay, FL, from ‘Thomas’ soybean, and is indistinguishable from Race 4 by the differentials of Ross (1968). The inoculum was grown on SSLBA at room temperature (25°C) in an incubator. Spores were harvested by pouring 10 mL of distilled water in the petri dish and scraping the spores off with a 2.5-cm-wide paint brush. The spore suspension was filtered through two layers of cheesecloth to remove debris and the spores in the filtrate were counted with a hemocytometer. The spore (conidial) suspension was standardized to a concentration of 4 to 6 x106 spores mL-1. Tween 20 [polyoxyethylene (20) sorbitan monolaurate] was added (1 mL/L) as a surfactant.
Inoculation, Rating, and Data Analysis
Seedlings were inoculated at V2 to V4 developmental stage (Fehr and Caviness, 1977) by spraying the abaxial and adaxial surfaces of all leaves with the conidial suspension, using The Power Garden Sprayer (Sprayco, Detroit, MI). Inoculated plants were immediately placed in a humidity chamber for 72 h at 25 to 30°C, and then transferred to a greenhouse bench where they were kept at 25 to 30°C. Ratings were made 14 d after inoculation. Three leaflets showing disease symptoms were randomly selected from each plant. Plants with no disease symptoms were assigned a value of 0% leaf area infected. Percent leaflet area infected of plants with symptoms was determined using a standard area diagram (Liu et al., 1991; Mian et al., 1998). Average percentage of lesion area of four plants in each pot was calculated. For analysis, these data were then converted to a scale of 0 to 6 (Liu et al., 1991), where 0 = no visible signs of infection; 1 = lesion area less than 0.6%; 2 = 0.6 to 2.5% lesion area; 3 = 2.5 to 5.2% lesion area; 4 = 5.2 to 9.9% lesion area; 5 = 9.9 to 16.4% lesion area; and 6 = lesion area greater than 16.4%. Accessions falling in classes 0 to 3 were classified as resistant, while those falling in classes 4 to 6 were classified as susceptible. Data were analyzed by SAS (SAS institute, 1987). Analysis of variance was conducted within each experiment. All accessions that showed no disease symptoms (0 rating) in the initial series of experiments were evaluated again in a final experiment by procedures identical to previous experiments, with the exception that control genotypes were replicated six times instead of three.
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RESULTS AND DISCUSSION
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Variation in percentage of lesion area in resistant control Davis ranged from 1.2 to 2.7% (scores from 2–3) with a mean of 1.8% and in susceptible control Lee ranged from 13.2 to 18.1% (scores from 5–6 with a mean of 15.3% among the 24 experiments. Because Davis was the resistant control, and often rated 3, a score of 3 was determined to be the cutoff point for resistance. Variation in disease expression may be due to minute differences in inoculum (concentration and vigor) or in leaf developmental stages of plants (Phillips and Boerma, 1981). Differences among accessions were significant, while no significant difference was observed among the three replications. The percent leaf lesion area on all accessions ranged from 0 to 22.6%. This is in agreement with previous reports that the percent lesion area is normally less than 25% under greenhouse inoculation conditions (Liu et al., 1991). On the basis of the 0-to-6 scale, 393 (38.4%) in MG VI and 267 (40.3%) in MG VII accessions had ratings of 3 or less and were considered resistant to C-32 (Table 1). Almost 50% of the accessions introduced from South Korea, 41% of the accessions introduced from Japan, and 33% of the accessions introduced from China were resistant to this isolate. Among the resistant accessions, 20 received a score of 0 and displayed no disease symptoms. Upon subsequent reevaluation, 12 of these were again completely disease free and thus were scored 0 (Table 2). None of these were significantly different from Davis (score of 2 with 0.8% leaf area infected). Four accessions had very low disease development (<0.1–0.2% lesion area) and scored 1. The other four had lesion area ranging from 1.5 to 4.3%, thus were still considered resistant. Lee received a rating of 4 in this experiment with a 9.0% leaf lesion area. This is a somewhat lower level of symptoms than in the initial 24 trials, and may be attributed to different personnel conducting the experiment, and a different time of year (winter for the initial trials, summer for the reevaluation). Davis has been an excellent source of resistance to FLS for many years. Thus, many of the PIs that were found resistant in these experiments may be potential new sources of resistance. In particular, those 12 that failed to show any disease symptoms after repeated inoculation are potential sources of new resistance genes for developing resistant soybean cultivars.
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Table 2. Country of origin, maturity group, and final disease rating of 16 soybean accessions with a rating of 0 (no disease symptoms) in the initial screening and a rating of 0 or 1 in the reevaluation for resistance to frogeye leafspot
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It is difficult to separate genotypes into discrete resistant and susceptible classes when rating for resistance on a continuous scale. Several criteria, including number of lesions, size of lesions, or percent leaflet area infected have been used as an index in evaluating resistance to FLS (Phillips and Boerma, 1981; Li and Yang, 1996, Dashiell and Akem, 1991; Liu et al., 1991). However, in evaluating a large number of genotypes, we found conflicting results when number of lesions and size of lesions were used as index (Yang and Weaver, 1998, unpublished data). Some accessions had numerous small sterile flecks, similar to those described by Boerma and Phillips (1983). They might be classified as susceptible according to number of lesions, or classified as resistant according to size of lesions. Some accessions had a few large, spreading, fertile lesions. They might be classified as susceptible accessions according to the size of lesions, or classified as resistant accessions according to number of lesions. To eliminate conflicting results, we used percent leaflet area infected converted to a 0-to-6 scale as an index in our evaluations.
Successful development of varietal resistance depends on an adequate understanding of inherent variability as well as shifts of pathogen populations. Little information is available on these aspects of FLS research. The soybean germplasm collection is an invaluable gene pool resource for pest resistance and genetic studies. This is the first attempt to evaluate resistance to FLS in MG VI and VII of soybean germplasm and will provide information for soybean breeders who may wish to use these accessions in a soybean disease resistance breeding program. Further inheritance studies are needed to understand the genetic relationships among the resistant accessions.
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ACKNOWLEDGMENTS
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The authors thank Dr. D.V. Phillips, the University of Georgia, Griffin, GA for providing the culture of C-32 and technical advice regarding preparation of the inoculum, and Randy Nelson, curator of the USDA Germplasm Collection for providing the seeds.
Received for publication October 3, 2000.
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REFERENCES
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- Akem, C.N. 1995. The effect of timing of fungicide applications on control of frogeye leaf spot and grain yield of soybeans. Eur J. Plant Pathol. 10(2):183–187.
- Athow, K.L. 1973. Fungal diseases. In B.E. Caldwell et al. (ed.) Soybean: Improvement, production, and uses. ASA, Madison, WI.
- Athow, K.L., and A.H. Probst. 1952. The inheritance of resistance to frogeye leaf spot of soybeans. Phytopathology 42:660–662.[ISI]
- Athow, K.L., A.H. Probst, C.P. Kartzman, and F.A. Laviolette. 1962. A newly identified physiological race of Cercospora sojina on soybean. Phytopathology 52:712–714.[ISI]
- Baker, W.A., D.B. Weaver, J. Qiu, and P.F. Pace. 1999. Genetic analysis of frogeye leaf spot resistance in PI54610 and Peking soybean. Crop Sci. 39:1021–1025.[Abstract/Free Full Text]
- Bernaux, P. 1979. Identification of some soybean diseases in Cameroon. Agronomie Tropicale 34(3):301–304.
- Boerma, H.R., and D.V. Phillips. 1983. Genetic implication of the sensitivity of Kent soybean to Cercospora sojina. Phytopathology 73:1666–1668.[ISI]
- Browning, J.A., and K.J. Frey. 1969. Multiline cultivars as a means of disease control. Annu. Rev. Phytopathol. 7:355–382.[ISI]
- Dashiell, K.E., and C.N. Akem. 1991. Yield losses in soybeans from frogeye leaf spot caused by Cercospora sojina. Crop Protection 10:465–468.
- Fehr, W.R., and C.E. Caviness. 1977. Stages of soybean development. Iowa Coop. Ext. Serv. Spec. Rep. 80.
- Horn, N.L., F.N. Lee, and R.B. Carver. 1975. Effects of fungicides and pathogens on yield of soybeans. Plant Dis. Rep. 59(9):724–728.
- Laviolette, F.A., K.L. Athow, A.H. Probst, J.R. Wilcox, and T.S. Abney. 1970. Effect of bacterial pustule and frogeye leaf spot on yield of Clark soybean. Crop Sci. 10:418–419.[Abstract/Free Full Text]
- Lehman, S.G. 1934. Frog-eye (Cercospora diazu miura) on stems, pods, and seeds of soybean and the relation of these infections to recurrence of the disease. J. Agric. Res. 48(2):131–147.
- Li, H.Y., and Q.K. Yang. 1996. Screening for resistance of soybean varieties to races of Cercospora sojina Hara. Crop Genetic Resources 1:6–8.
- Liu, X.M., C.Y. Li, and M.H. Zhang. 1991. The grading standard of disease severity on leaf spot of frogeye leaf spot of soybean. Soybean Sci. 10(4):330–334.
- Liu, Z.T., and G.C. Huang. 1986. Breeding of soybean cultivars resistant to frogeye leaf spot. Scientia Agricultura Sinica 19(3):26–31.
- Ma, G.Z. 1994. Review and forecast of study on frogeye leaf spot. Soybean J. 1:6–7.
- Ma, S.M., and B.Y. Li. 1997. Primary report on the identification for physiological races of Cercospora sojina Hara in Northeast China. Acta Phytopathologica Sinica 27(2):180.
- Melchers, L.E. 1925. Diseases of cereal and forage crops in the United States in 1924. Plant Dis. Rep. Suppl. 40:186.
- Mian, M.A.R., H.R. Boerma, D.V. Phillips, M.M. Kenty, G. Shannon, E.R. Shipe, A.R. Soffes Blount, and D.B. Weaver. 1998. Performance of frogeye leaf spot-resistant and -susceptible near-isolines of soybean. Plant Dis. 82:1017–1021.
- National Academy of Sciences. 1972. Genetic vulnerability of major crops. National Academy of Sciences USA, Washington, DC.
- Pace, P.F., D.B. Weaver, and L.D. Ploper. 1993. Additional genes for resistance to frogeye leaf spot race 5 in soybean. Crop Sci. 33:1144–1145.[Abstract/Free Full Text]
- Phillips, D.V., and H.R. Boerma. 1981. Cercospora sojina race 5: a threat to soybean in the southeastern United States. Phytopathology 71:334–336.[ISI]
- Phillips, D.V., and H.R. Boerma. 1982. Two genes for resistance to race 5 of Cercospora sojina in soybeans. Phytopathology 72:764–766.[ISI]
- Probst, A.H., and K.L. Athow. 1958. Additional studies on the inheritance of resistance to frogeye leaf spot of soybeans. Phytopathology 48:414–416.[ISI]
- Probst, A.H., K.L. Athow, and F.A. Laviolette. 1965. Inheritance of resistance to race 2 of Cercospora sojina in soybeans. Crop Sci. 5:332–338.[Free Full Text]
- Ross, J.T. 1968. Additional physiologic races of Cercospora sojina on soybean in North Carolina. Phytopathology 58:708–709.[ISI]
- SAS Institute. 1987. SAS/STAT guide for personal computers, Version 6 Edition. SAS Institute, Cary, NC.
- Sloane, L.W., S.H. Crawford, N.L. Horn, and R.B. Carver. 1975. Soybean yields increased by foliar fungicides. Louisiana Agric. 18(4):4–5, 16.
- Yang, Q.K., X.G. Zhang, and J.L. Wang. 1995. Inheritance of resistance of soybean to race 7 of Cercospora sojina Hara. Soybean Sci. 14(1):80–82.
- Yorinori, J.T. 1980. Cercospora sojina: Pathogenicity, new races and seed transmission in soybean. Ph.D. Diss. (Diss. Abstr. 81-14505), Univ. Of Illinois, Urbana-Champaign, IL.
- Yorinori, J.T. 1992. Management of foliar fungal diseases in Brazil. p. 185–193. In L.G. Copping et al. (ed.) Pest management in soybean. Elsevier Applied Science, London, England.
- Zhang, X.G., Q.K. Yang, Y.G. Qi, T.L. Wu, and J.L. Wang. 1990. Genetic implication of soybean resistance to Cercospora sojina race 1. Soybean Genetics Newsl. 17:21–26.
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