Download Genetic Analyses of Agronomic Traits Controlled by Wheat

Genetic Analyses of Agronomic Traits Controlled by Wheat Chromosome 3A
M. M. Shah, P. S. Baenziger,* Y. Yen, K. S. Gill, B. Moreno-Sevilla, and K. Haliloglu
ABSTRACT
tween a chromosome substitution line having the chromosome of interest and the parent cultivar having the
homologue for the chromosome of interest (Law, 1966;
Yen and Baenziger, 1992). Trait comparisons can be
made and the genes controlling these traits can further
be identified and mapped by conventional biometrical
approaches or by linkage studies with molecular markers (Lander and Botstein, 1989; Paterson et al., 1990;
Joppa et al., 1997).
Law and coworkers (1966, 1967, and 1976) and Snape
et al. (1985) were able to identify segregating loci and
their gene action for important agronomic characters
on wheat chromosomes 7B, 5A, and 5D using RICL
populations developed between a wheat cultivar (often
‘Chinese Spring’) and the chromosome substitution
lines for the respective chromosomes. However, these
studies were limited because they generally were conducted in only a single or few environments and did not
estimate genotype 3 environment interactions, which
are known to affect measurements of quantitative traits
such as grain yield in wheat (Carver et al., 1987; Sharma
et al., 1987; Mahal et al., 1988; Silis et al., 1989). The
mapping of genes controlling agronomic traits on specific chromosomes in the above studies was possible
because monogenically inherited phenotypic markers
were available. An adequate number of these phenotypic markers often are not found in conventional breeding populations involving two parental cultivars. Hence,
little is known concerning the number and nature of the
genes responsible for important agronomic characters
on specific chromosomes of wheat. The knowledge of
major gene(s) (one or few genes having large effects on
the expression of a trait) vs. minor genes (many genes
having small and equal effects on a trait) and their
relationship on a chromosome is important to develop
efficient breeding strategies. The objectives of this study
were to determine if previously identified important
agronomic traits on chromosome 3A were controlled
by major or minor gene(s) and if those genes were
pleiotropic, linked, or independent.
Previous studies with chromosome substitution lines between hard
red winter wheat (Triticum aestivum L.) cultivars Cheyenne (CNN)
and Wichita (WI) identified genes on chromosome 3A of WI which
affect grain yield, yield components, grain volume weight, plant height,
and anthesis date. This study was conducted to determine if the trait
variation caused by chromosome 3A could be explained by major or
minor gene segregation and if these genes are pleiotropic, linked, or
independent on the chromosome. A population of recombinant inbred
chromosome lines for chromosome 3A (RICLs-3A), developed between CNN and a chromosome substitution line CNN(WI3A), was
evaluated in multi-location field trials in 3 yr. Our results indicate
significant differences (P # 0.05) between parental lines and among
RICLs for grain yield, 1000-kernel weight, plant height, and anthesis
date, but not for kernel number per spike, spike number per square
meter, and grain volume weight. A 1:1 genetic ratio for anthesis date
suggested the presence of a single segregating locus controlling the
trait. None of the other agronomic traits could be separated into
unequivocal groups and hence, major genes were not detected. This
indicates that the traits were controlled either by several genes or
few genes with enough environmental influence, or both, to obscure
their effects. Significant correlations and possible crossover products
between anthesis date, plant height, and 1000-kernel weight suggest
that these traits were controlled either by linked gene(s) or by pleiotropic genes with additional genes affecting one of the traits.
P
revious research with reciprocal chromosome substitution lines between bread wheat cultivars Cheyenne (CNN) and Wichita (WI) determined that WI
chromosome 3A contained gene(s) that significantly increased grain yield and 1000-kernel weight, decreased
plant height, and delayed anthesis when substituted in
CNN (Berke et al., 1992a). In the reciprocal substitution
line, WI(CNN3A), CNN chromosome 3A was identified
to be the location of gene(s) that significantly decreased
grain yield and spike number per square meter, but it
had no significant effect on anthesis date and plant
height (Berke et al., 1992a).
Reciprocal chromosome substitution lines can only
be used to determine the effects of a whole chromosome. To detect the number and relative location of
genes on a specific chromosome, chromosomal effects
need to be partitioned into those of chromosome segmental effects (Paterson et al., 1990; Yen et al., 1992;
Joppa et al., 1997). A classical approach to identify chromosomal segments is the development and evaluation
of recombinant inbred chromosome lines (RICLs) be-
MATERIALS AND METHODS
Development of 3A Recombinant Inbred Chromosome
Line Population
A population of 50 RICLs-3A was developed by R. Morris
and Y. Yen following the method described by Law (1966),
after crossing the hard red winter wheat cultivar CNN with
the chromosome substitution line CNN(WI3A). In the F1,
meaningful recombination only occurs between WI3A and
CNN3A chromosomes, as all the other chromosomes are from
CNN parent. The F1 was crossed as a male to CNN-monosomic
M.M. Shah, Dep. of Agronomy, Iowa State Univ. of Science and
Technology, Ames, IA 50011; P.S. Baenziger, K.S. Gill, and K. Haliloglu, Dep. Agronomy, Univ. of Nebraska, Lincoln, NE 68583; Y.
Yen, Dep. of Biology/Microbiology, South Dakota State Univ., Brookings, SD 57006; B. Moreno-Sevilla, HybriTech Seed International,
Boise, ID 83713. Nebraska Agric. Res. Div., J. Series No. 12258.
Received 5 June 1998. *Corresponding author (pbaenziger1@
unl.edu).
Abbreviations: ANOVA, analysis of variance; CNN, ‘Cheyenne’;
RICLs-3A, recombinant inbred chromosome lines for chromosome
3A; WI, ‘Wichita’.
Published in Crop Sci. 39:1016–1021 (1999).
1016
SHAH ET AL.: GENETIC ANALYSIS OF AGRONOMIC TRAITS IN WHEAT
3A. Monosomic plants (2N 2 1 5 41) from the resulting
progeny were selected by cytological examination of root-tip
cells. The hemizygous chromosome in these plants should
either be identical to the parental chromosome (therefore
non-recombinant) or will have resulted from a crossover event
(therefore recombinant). The male parent (F1 in this case)
transmits a euploid gamete (1N 5 21) and the female monosomic plant produces aneuploid (1N 2 1 5 20) or euploid (1N 5
21) gametes. The recombinant monosomic progeny resulting
from the fertilization of a male euploid gamete with a female
aneuploid gamete were selected. The disomic progeny (2N 5
42) resulting from the fertilization of a male euploid gamete
with a female euploid gamete were discarded. Upon selfing
these monosomic plants and selecting the disomic progeny,
again by cytological examination of root-tip cells, 50 homozygous RICLs-3Awere developed in the CNN background.
The seed of each individual line was increased in the greenhouse and the lines were then grown in the field trials. Each
RICL should differ from another unless no crossover occurred
or the identical crossover occurred. The possibility of univalent
shifts occurring during RICL development was not tested, but
considered low with only two generations of aneuploidy. The
common background, except for the substituted chromosome,
should have promoted chromosome pairing (Person, 1956).
Previous work of developing the CNN monosomic series had
indicated few univalent shifts (R. Morris, 1998, personal communication).
Field Trials
In 1994, 50 RICLs-3A, CNN, and CNN(WI3A) were grown
at Lincoln, NE (Sharpsburg silty clay loam, a fine, montmorillonitic, mesic Typic Argiudoll) and North Platte, NE (Holdrege silt loam, a fine-silty, mixed, mesic Aridic Argiustoll).
In 1995 and 1996, the 50 RICLs-3A, CNN, and CNN(WI3A)
were grown at Lincoln, Mead (Sharpsburg silty clay loam, a
fine, montmorillonitic, mesic Typic Argiudoll), North Platte,
and Sidney, NE (Holdrege silt loam, a fine-silty, mixed, mesic
Aridic Argiustoll). The parent cultivar, CNN, and the substitution line, CNN(W13A), were included in this study to determine if the previously observed chromosome 3A effects could
be observed in these environments. The trial at North Platte
(1996) was lost due to hail and high winds and no data were
collected. Trials at Lincoln (1996) and Sidney (1995) were
also damaged severely by adverse weather (heavy rain and
hail) and data from these trials were excluded from analyses
for grain yield, its component traits, and grain volume weight.
However, data from these trials were included for plant height
and anthesis date.
A randomized complete block design with two replications
in 1994 and 1995, or three replications in 1996 was used. Each
experimental unit consisted a plot of four 2.4-m rows with
0.3 m between rows. Seeding rate was 68 kg ha21 at Lincoln
and Mead, and 50 kg ha21 at North Platte and Sidney. All
trials were planted on fallowed ground the previous year
and recommended production practices were used. All trials
were kept disease free with two applications of the fungicide
Tilt (1-[[2-(2,4-dichlorophenyl)-4-propyl-1, 3-dioxlan-2-yl]
methyl-1H-1, 2,4 triozole; Novartis, Greensboro, NC).
Except where noted, data for grain yield, kernel number
per spike, 1000-kernel weight, spikes per square meter, and
plant height was collected from all replications at Lincoln,
Mead, North Platte, and Sidney. Grain volume weight was
measured from Lincoln, Mead, and Sidney, whereas anthesis
date, which required daily measurements, was measured only
at Lincoln and Mead. Grain yield was measured in Mg ha21
by harvesting the middle two rows of each plot at Lincoln and
1017
Mead (for pure seed) and by harvesting the entire plot with
a small plot combine harvester at North Platte and Sidney.
Ten spikes were harvested from the outer two rows at Lincoln
and Mead, or from the four rows at North Platte and Sidney,
and threshed to determine kernel number per spike and 1000kernel weight. Spikes per square meter were estimated by
dividing plot grain yield by the multiplication product of kernel
number per spike and kernel weight. Plant height (measured
from the ground to the tip of the spike, awns excluded) was
the visual average of a random sample of spikes from the
middle two rows of each plot. Grain volume weight was measured in kg hL21 with Seedburo volumetric scale (Seedburro
Equipment Co., Chicago, IL). Anthesis date (days after 1
May) was visually estimated as the date when 50% of the
spikes in a plot had extruded anthers.
Data Analyses
A combined analysis of variance (ANOVA) was conducted
across environments by the SAS procedure GLM and the
RANDOM statement (SAS Inst., 1990) to test significant (P #
0.05) differences between parents, parents and RICLs-3A, and
among RICLs-3A for all traits. Environments, replications,
and RICLs were considered as random effects while CNN
and CNN(WI3A)) were considered as fixed effects. The
genotype 3 environment interaction (G 3 E) or the partition
of the G 3 E was used to test genotypic effects or their
partitions in the ANOVA.
The data were analyzed for normality with PROC UNIVARIATE and the NORMAL statement (SAS Inst., 1990)
on the genotypic means across environments. The frequency
histograms for each trait were obtained by using the range of
genotypic means separated by 0.5 standard error units. Traits
that showed a bimodal distribution were divided into two
distinct phenotypic classes and were analyzed for deviation
from the expected Mendelian segregation ratio (1:1) by a
chi-square test. A 1:1 ratio indicated single-locus control. A
protected LSD was calculated for each significant trait from
the combined ANOVA with synthesized G 3 E error term
used to identify transgressive segregates for each trait.
Correlation analysis for the 50 RICLs with genotypic means
across environments was conducted between pairs of traits
exhibiting significant variation to determine if the gene(s)
controlling two agronomic traits were coincident or independent. The procedure CORR (SAS Inst., 1990) was used for
this analysis. A nonsignificant correlation between two traits
was interpreted as the genes were segregating independently
or environmental effects masked the effects of these genes.
A significant (P # 0.05) correlation was considered an indication of the presence of linkage or pleiotropy. Scatter diagrams
were plotted between statistically correlated traits with genotypic means used to determine if distinctive groups of the
RICLs were observed in relation to the parental values, i.e.,
if two parental classes determined the association. Scatter
diagrams also were used to determine if the putative recombinant products or additional independent genes existed. Two
traits were considered as linked or having additional independent gene(s), and not pleiotropic, if in the scatter diagram
putative recombinant lines (lines exhibiting the phenotype of
one parent for the first trait and the phenotype of the other
parent for the second trait) could be identified.
RESULTS AND DISCUSSION
The F-tests from the ANOVA across environments
detected significant (P # 0.05) differences between
CNN and CNN(WI3A) and among the RICLs for grain
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CROP SCIENCE, VOL. 39, JULY–AUGUST 1999
Table 1. Overall means and mean squares (MS) from the analysis of variance of seven traits for Cheyenne (CNN), chromosome
substitution line CNN(W13A), and 50 Cheyenne-Wichita recombinant inbred chromosome 3A lines (RICLs-3A), combined across
a variable number of Nebraska environments.
Grain yield
Source
df
MS
ha21)2
Environments (E)
Genotypes (G)
CNN vs. CNN(W13A)
Parents vs. RICLs-3A
RICLs-3A
G3E
CNN vs. CNN(W13A) 3 E
Parents vs. RICLs-3A 3 E
RICLs-3A 3 E
Pooled Error
Mean
CV%
6
51
1
1
49
306
6
6
294
459
(Mg
141.9
0.233**
0.799**
0.489
0.22*
0.144**
0.03
0.18
0.15**
0.113
2.68
13.5
Kernel no.
spike21
1000-Kernel
weight
Spike no.
m 22
MS
MS
MS
2
no.
4503.6
10.2
1.2
0.5
10.5
9.8
4.8
8.8
9.9
8.8
28.5
10.4
2
g
3259.3
12.5**
42.1**
3.2
12.1**
7.9**
3.8
9.4
8.0*
6.3
29.9
8.3
Grain volume
weight
Plant height
Anthesis date
(d after May 1)
df
df
df
no.
1 394 944
4 957
5
7 291
5 008
4 938**
3 501
4 246
4 981**
3 307
312
18.4
MS
hL21)2
2
5
51
1
1
49
250
5
5
240
376
(kg
139.8
0.7
1.3
1.9
0.7
0.8
1.0
1.1
0.8
0.8
62.2
1.4
MS
2
8
51
1
1
49
408
8
8
392
610
cm
11 250
94**
177*
4
94**
39
48
9
39
36
102
5.9
3
51
1
1
49
153
3
3
147
306
MS
d2
8656.4
27.6**
56.1**
8.8
27.4**
2.3*
0.5
3.6
2.3
1.8
37.8
3.5
*, ** Significant at the 0.05 and 0.01 levels of probability, respectively.
yield, kernel weight, plant height, and anthesis date,
but not for kernel number per spike, spikes per square
meter, and grain volume weight (Table 1). The substitution line CNN(WI3A) had significantly higher grain
yield and kernel weight, was earlier for anthesis, and
was shorter than CNN (Table 2). These results are in
agreement with previous reports of chromosome substitution line analyses involving chromosome 3A (Berke
et al., 1992a; Yen et al., 1997) with the exception of
grain volume weight, which was significant in Berke et
al. (1992a), but was non-significant in Yen et al. (1997).
Significant differences were identified among the RICLs
for those traits where the parents were significantly different, which indicated that the parental genes segregated among the RICLs. As expected, the RICL-3A
population mean was not significantly different from
the mid-parental value for any trait, indicating primarily
additive gene action (Yen et al., 1997).
Genotype 3 environment interactions for the two
parents were non-significant (Table 1) for all the traits
indicating that they responded similarly across environments. The G 3 E interactions among RICLs were
significant for some traits (Table 1), but they appeared
to be due to changes in magnitude and not reversal
in order. Therefore we present RICL means averaged
across environments.
As determined by the normality test, all traits statistically fit the normal distribution except anthesis date
(P # 0.001), whose distribution was bimodal (Fig. 1 a).
When grouped by the parental values, a Chi-square test
(x2 5 2.88, P 5 0.09) indicated 1:1 segregation for a
single locus controlling anthesis date. Previous research
with substitution or aneuploid lines (Zemetra et al.,
1986; Miura and Worland, 1994), has shown that chromosome 3A contained gene(s) controlling earliness as
determined by heading date; however, none of the previous research was designed to determine the number of
loci affecting the trait. A unimodal normal distribution
was observed for grain yield, kernel number per spike,
kernel weight, spike number per square meter, grain
volume weight, and plant height (Fig. 1), so it was not
possible to ascertain as how many genes controlled these
traits. The traits are either genetically complex or highly
environmentally sensitive, or both. We believe the observed segregation patterns as influenced by chromosome 3A were due to genes at multiple loci, each having
a small effect that were environmentally sensitive. The
RICLs represented a distribution of genotypes, that
along with environmental influences, could give rise to
continuous variation. No statistically significant transgressive segregation was observed for any of the traits
under investigation, which would suggest if multiple loci
controlled these traits, the genes were in coupling phase.
Significant correlations were observed between anthesis date and plant height (r 5 0.59**), between anthesis date and kernel weight (r 5 20.56**), and between
plant height and kernel weight (r 5 20.32**). These
correlations may indicate that the genes on chromosome
3A controlling these traits may be linked or are pleiotropic. Because the number of environments that anthesis
Table 2. Mean agronomic performance for Cheyenne (CNN), chromosome substitution line CNN(W13A), and 50 Cheyenne-Wichita
recombinant inbred chromosome 3A lines (RICLs-3A), combined across a variable number of Nebraska environments.
Seven environments
Entry
Grain yield
ha21
CNN
CNN(W13A)
RICLs-3A
Range
LSD0.05†
Mg
2.63
2.94
2.69
2.44–2.97
0.26
Kernel no.
Spike21
no.
28.5
28.3
28.4
26.7–30.3
ns‡
1000-kernel
weight
g
28.4
30.8
30.0
28.3–32.7
2.0
† LSD is for the comparison of individual line mean values.
‡ Nonsignificant (P . 0.05) genotypic variation.
Six environments
Spike no. m22
no.
331
326
312
276–363
ns‡
Grain volume
weight
hL21
kg
62.2
62.7
62.2
61.6–62.8
ns‡
Nine
environments
Four
environments
Plant height
Anthesis date
cm
104
99
102
97–107
4
d post May 1
40.3
36.8
37.8
35.1–41.1
1.3
SHAH ET AL.: GENETIC ANALYSIS OF AGRONOMIC TRAITS IN WHEAT
1019
Fig. 1. Frequency distribution for anthesis date (a), plant height (b) , grain yield (c), kernel number per spike (d), 1000-kernel weight (e), spike
number per square meter (f), and grain volume weight (g) of 50 recombinant inbred chromosome lines-3A population derived from CNN
and CNN(WI3A) parents of hard red winter wheat. The phenotype of the CNN and CNN(WI3A) parents are shown by arrows. The values
indicated on the X-axis are the lower limits of each group.
date (4), plant height (9), and kernel weight (7) were
measured in are different, it is not possible to exclude
the possibility that the correlations may be due to an
environmental covariate. However, our experience is
that early lines tend to be early everywhere in Nebraska,
as are tall lines like CNN. None of the traits for which
the RICLs exhibited significant variation (anthesis date,
plant height, and kernel weight) were correlated with
grain yield, indicating that the contributions of genes
controlling these traits may be too small or modified by
the environment to detect any correlation with grain
yield. It was also possible that grain yield may be determined by independent genes on chromosome 3A. Law
(1967) described the difficulties in identifying genes controlling complex characters such as grain yield and suggested the genes affecting yield could be better determined by identifying the genes affecting the components
of yield.
The scatter diagram (Fig. 2a) for anthesis date and
plant height indicates that most of the early flowering
lines were also short statured [similar to CNN(WI3A)],
while most of the late flowering lines were also taller
(similar to CNN). However, two lines had plant heights
similar to CNN(WI3A) and shorter than CNN, but were
as late flowering as CNN (Fig. 2a). Either a crossover
had occurred between the genes controlling anthesis
date and plant height or additional gene controls one
of the traits. This pattern was also observed in the scatter
diagram between anthesis date and kernel weight (Fig.
2b). For example, while most of the early lines had
heavier kernel weights which is characteristic of
CNN(WI3A), most of the late lines had lower kernel
weights which is characteristic of CNN. Three lines were
late flowering (similar to CNN) but had significantly
heavier kernel weight [similar to CNN(WI3A)]. The
lower correlation between plant height and kernel
weight (Fig. 2c) showed little relationship between
these traits.
This study demonstrates the power of single-chromosome recombinant inbred lines as a means of locating
and identifying loci controlling quantitative agronomic
traits and also as a means of chromosome engineering.
The field evaluation of RICLs-3A provided the strong
evidence that previously identified genes for important
agronomic traits on chromosome 3A were also present
and segregating in the RICLs population, despite having
large G 3 E interactions for some traits. Information
from this population is more relevant than interspecific
hybrid populations to plant breeders because of the
past commercial importance and the significant genetic
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CROP SCIENCE, VOL. 39, JULY–AUGUST 1999
Fig. 2. Scatter diagram showing the relationship between mean plant height and anthesis date (a), 1000-kernel weight and anthesis date (b), and
1000-kernel weight and plant height (c) for 50 recombinant inbred chromosome lines-3A derived from CNN and CNN(WI3A) parents of
hard red winter wheat. The CNN and CNN(WI3A) phenotypes are shown by arrows, and the degree of relationship is indicated by the
correlation coefficient.
contribution of CNN and WI to many current winter
wheat cultivars (Berke et al., 1992b). Cheyenne has a
coefficient of parentage of 0.33 with Wichita, 0.42 with
‘Colt’, 0.81 with ‘Lancer’, 0.16 with ‘Brule’, 0.43 with
‘Siouxland’, and 0.41 with ‘Centurk’. Wichita has a coefficient of parentage of 0.29 with Colt, 0.33 with Lancer,
0.13 with Brule, and 0.25 with Siouxland (Cox et al.,
1985). In addition, Brule is the main parent of the cultivars ‘Arapahoe’, ‘Vista’, and ‘Niobrara’ which are
widely grown in Nebraska and South Dakota. Hence,
many genes from CNN and WI are likely present in
currently grown cultivars in the Great Plains, and thus
of interest to plant breeders today for wheat cultivar development.
In conclusion, anthesis date was found to be simply
inherited by detecting a single locus segregating in the
RICLs-3A. Other traits did not fit simple monogenic
ratios. They were either controlled by genes at multiple
loci, influenced by the environment, or both, so that
RICLs had a continuous distribution. It is interesting
to note that the loci may be dispersed sufficiently far
apart so that they appear to segregate independently
and when coupled with environmental influences, produce a continuous distribution. Anthesis date, plant
height, and kernel weight appeared to be controlled
either by linked gene(s), or by pleiotropic genes with
additional genes affecting one of the traits. The loci can
further be mapped on the chromosome by the use of
molecular marker techniques.
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Genetic Analysis of Frogeye Leaf Spot Resistance in PI54610 and Peking Soybean
William A. Baker, David B. Weaver,* Jiansheng Qiu, and Patrick F. Pace
ABSTRACT
Frogeye leaf spot (FLS) of soybean [Glycine max (L.) Merr.] is
caused by the fungus Cercospora sojina Hara. The fungus is ubiquitous, but only problematic in hot humid soybean-producing regions
such as Brazil, China, Nigeria, and the southern USA. Significant
yield losses (10–50%) are commonly associated with FLS epidemics.
The quantification of unique alleles for resistance within the southern
germplasm pool is an essential step toward developing a more usable
set of differential genotypes and thereby clarifying the race situation
within the C. sojina–soybean interaction. Our objective was to determine the inheritance of resistance to FLS in PI54610 and Peking and
their allelic relationship to Rcs3. ‘Lee’ soybean was used as a susceptible parent for crosses and control in all experiments. Parents and F2
seedlings were inoculated with a C. sojina spore suspension in the
greenhouse or field and then rated for disease development 14 to
21 d later. On the basis of segregation ratios (3:1 resistant/susceptible
in Peking 3 Lee and PI54610 3 Lee, and 15:1 in ‘Davis’ 3 Peking
and PI54610 3 Peking), we found resistance in Peking was determined
by a single dominant gene nonallelic to Rcs3. We also found, based
on nonsegregation of resistance within the Davis 3 PI54610 population, that PI54610 has the same gene as in Davis (Rcs3). Resistance
in Peking should be considered unique for the purpose of race differentiation and as a commercial source of resistance to FLS should
Rcs3 fail.
F
rogeye leaf spot of soybean is caused by the phytopathogenic fungus C. sojina Hara (Hara, 1915). Cercospora sojina infects leaves, stems, and seeds of soybean (Sinclair and Backman, 1989). The pathogen has
a worldwide distribution, with significant yield losses
reported in China (Ma, 1994), Brazil (Yorinori, 1992),
Nepal (Manandhar and Sinclair, 1982), Nigeria (Akem
et al., 1992), and the USA (Laviolette et al., 1970; Hartwig, 1990). The severity of FLS infection is highly dependent on the environmental conditions (Akem and Dashiell, 1994) and disease development is favored in warm
humid environments.
William A. Baker and David B. Weaver, Dep. of Agronomy and
Soils, Auburn, AL 36849; Jiansheng Qiu, Dep. of Plant Pathology,
Auburn, AL 36849; and Patrick F. Pace, DEKALB Genetics Corporation, 3100 Sycamore Road, DeKalb, IL 60115. Contribution from the
Dep. of Agronomy and Soils, Auburn Univ., Auburn, AL 36849.
Received 3 Aug. 1998. *Corresponding author (dweaver@acesag.
auburn.edu).
Published in Crop Sci. 39:1021–1025 (1999).
The fungus was first described in Japan (Hara, 1915).
The first report of FLS in the USA occurred in 1924
(Melchers, 1925) with significant yield reductions in the
Midwest attributed to the pathogen by the late 1940s
(Athow and Probst, 1952). Genetic resistance is the most
stable, economical, and environmentally sound strategy
for control of C. sojina (Sinclair and Backman, 1989);
however, the appearance of new highly virulent races
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, 1989) has
demonstrated the need for the identification of alternative sources of genetic resistance to this pathogen.
Studies involving yield loss assessments have been
performed in Brazil, Nigeria, and the USA. Yield reduction due to FLS has been attributed to reduced photosynthetic capacity and premature defoliation of infected
leaves (Akem and Dashiell, 1994), causing a reduction
in seed size rather than seed number (Horn et al., 1975;
Dashiell and Akem, 1991). A 3-yr study in Indiana with
the susceptible ‘Clark’ reported yield losses of up to
21% due to infection by C. sojina (Laviolette et al.,
1970). A study in Brazil, involving six cultivars, showed
inoculated plots yielded 14% less than noninoculated
plots (Yorinori, 1992). Studies in Nigeria, designed to
elucidate the interaction between fungicides and natural
inoculum levels on three different cultivars, reported a
yield loss of 66% in a susceptible genotype (Dashiell
and Akem, 1991). Comparison of yields between lines
resistant and susceptible to FLS in the USDA Uniform
Tests indicated losses ranged from 10% at Quincy, FL
(Hartwig and Edwards, 1989) to 30% at Tallassee, AL
(Hartwig, 1990). A comparison of near isogenic lines
(NILs) resistant or susceptible to FLS was performed
in multiple locations across three growing seasons. A
significant yield loss of up to 31% was found between
the resistant and susceptible NILs (Mian et al., 1998).
Twenty-two races of C. sojina occur in Brazil (Yorinori, 1992). Some of the recently identified races are
virulent against cultivars that previously had shown resistance (Yorinori, 1992). A report from China identified eleven races of C. sojina (Hu et al., 1995). At least
Abbreviations: FLS, frogeye leaf spot; NIL, near isogenic line; PDA,
potato dextrose agar.