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a CIMMYT, Mexico
b Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105-5051
c Dep. of Plant Pathology, 495 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108
d Dep. of Cereal and Food Sciences, North Dakota State Univ., Fargo, ND 58105
* Corresponding author (Richard.Horsley{at}ndsu.nodak.edu.)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Tillage, crop rotation, and chemical control have been suggested as methods for reducing FHB severity; yet, success with these methods has been limited. Genetic resistance offers the greatest potential for reducing FHB. Nearly 40 barley genotypes have been identified with partial resistance to FHB (Prom et al., 1996). One of the most resistant two-rowed barley accessions identified is CIho 4196 (Prom et al., 1996; Takeda, 1992). This accession originates from China and is being used extensively as a parent by the six-rowed barley breeding program at North Dakota State University (NDSU). In general, CIho 4196 and most of the other resistant genotypes identified by Prom et al. (1996) tend to head and mature later, and are taller than cultivars currently grown in the upper U.S. Midwest.
To design an efficient breeding strategy for developing FHB resistant cultivars, we need to know several things about the accessions used as parents; particularly we need to know the heritability of FHB resistance and DON accumulation. Information on the heritability of FHB resistance and DON accumulation in barley is limited. Takeda (1990) studied the genetic behavior of FHB resistance in five crosses between two-rowed and six-rowed barley in the F2, F3, and F4 generations. He suggested that resistance to FHB is under the control of quantitative genes. Heritability estimated from the genetic gain:selection differential ratio was 0.25 in the F2–F3 selection response and 0.33 in the F3–F4. Takeda (1992) reported that heritability for FHB resistance in barley was 0.6 in the broad sense and 0.4 in the narrow sense, and that resistance was under the control of predominantly additive genes.
Development of a suitable breeding strategy is also dependent on knowing the correlated responses of selecting for reduced FHB severity on DON accumulation, days to heading, and plant height. Limited information is available on this topic for these traits. The objectives of this study were (i) to estimate the heritability of FHB resistance and DON accumulation and (ii) to evaluate the selection for reduced FHB severity at an intensity of 40% and to determine the correlated responses on DON accumulation, days to heading, and plant height in a population derived from the cross Foster/CIho 4196.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Field Experiments
One hundred fifty of the 250 F4:5 recombinant inbred lines were randomly selected and sown at Langdon, ND, on 17 May 1997 in an FHB disease nursery. The soil at this location is a fine, montmorillonitic Utric Natriborroll. Experimental units were one, 1-m row. Entries were assigned to experimental units by an augmented block design (Federer, 1993), and each experiment was repeated twice. Block size in each experiment was 25 entries, and each block included the parents Foster and CIho 4196. The FHB epidemic nursery was inoculated four times with F. graminearum, beginning 1 wk before heading, and once a week for four consecutive weeks by the method of Prom et al. (1996).
This experiment was repeated in spring 1998 with F4:6 seed harvested from the 150 families the previous year. Experiments were sown in FHB disease nurseries located at Fargo on 15 May, at Langdon on 27 April, and at Osnabrock, ND, on 15 May. The same experimental design, plot size, and FHB inoculation methods were used as in 1997. The soil at Langdon and Osnabrock was a fine, montmorillonitic Utric Natriborroll. At Fargo, the soil was a fine montmorillonitic, frigid Typic Haploboroll.
Inoculum Preparation
Fusarium graminearum inoculum was prepared according the methods of Xia (1956) as modified by Prom et al. (1996). First, equal parts, by volume, of barley and maize (Zea mays L.) grain are soaked separately in water for 48 h. After soaking, the barley and maize are put in stainless steel pans and covered with aluminum foil. Next, the barley and maize grain are autoclaved for 20 min at 121°C, in each of two consecutive days, to sterilize the grain substrate. Pieces of agar containing five isolates of F. graminearum (KB-172, KB-173, KB-176, KB-582, and KB-672) (Salas et al., 1999) then are added individually to each pan containing the sterilized grain, and the grain is incubated at 25°C for 14 d under complete darkness. Ten days later, the inoculum is mixed by hand in each pan.
Prior to inoculation, the colonized seeds of barley and maize are mixed in equal proportions. The mixed grain then is spread over the barley plots at a rate of 50 g colonized grain m-2 for four consecutive weeks, beginning 1 wk before heading of the earliest entries. At Osnabrock in 1998 and 1999, plots were irrigated at a rate of 1.20 L hr-1 with a model XS-360 F xeri-spray sprinklers (Rain Bird, Glendora, CA). Sprinkler heads were spaced 4 m apart and 120 cm above the ground. Irrigation was done early in the morning (0600–0800 h) and late in the afternoon (1600–1800 h) to provide favorable conditions for ascospore liberation and infection. The irrigation was done for 30 s every 30 min during this period. At Fargo and Langdon in 1997 and 1998, plots were irrigated at a rate of 0.19 L h-1 with 1800-SAM-PRS sprinklers (Rain Bird, Glendora, CA). Sprinkler heads were spaced 3 m apart and 120 cm above the ground. Irrigation was done once for 10 min at 0700 and 1700 h.
Morphological, Agronomic, and FHB Data Collected
Data on morphological and agronomic characteristics that were variable in the parents (Foster and CIho 4196) were collected. Data collected in the field were days to heading (number of days after 31 May when 80% of spikes were fully emerged from the boot), plant height (distance from soil surface to the tip of spikes, excluding awns), spike type (vrs1vrs1Int-cInt-c = normal six-rowed, Vrs1Vrs1int-cint-c = normal two-rowed, and Vrs1Vrs1Int-cInt-c = hybrid two-rowed), presence of lemma awn spiculation (smooth, semi smooth, and rough), spike angle (1 = erect spike, 90° from horizontal; 5 = bent spike, 0° from horizontal), and spike density (number of rachis nodes per cm of rachis).
Disease readings were done at the soft dough stage (Zadoks 85) of development. Fifteen spikes within each row were harvested at random and the number of infected kernels per spike was counted. The percent FHB severity was calculated by dividing the total number of infected kernels by the total number of kernels and multiplying by 100.
At maturity, each plot was harvested with hand shears. Grain samples were dried at 35°C in a forced dryer to approximately 100 g kg-1 moisture, deawned, and cleaned. Deoxynivalenol accumulation (µg g-1) of harvested grain was determined by the gas chromatograph method of Tacke and Casper (1996).
Statistical Analyses
Data for parents and progeny in each experiment in an environment were analyzed as an augmented block design. Adjusted means (i.e. least square means) were calculated for each line, and these means were used to perform the analysis of variance across experiments as a randomized complete block design. Within a year, each experiment at an environment was considered a replicate. Combined analyses across environments within a year were done for all traits in which the error mean squares were homogeneous. In the combined analyses, environments and entries were considered random effects. Mean separation between parents was done by t-tests. F-tests and t-tests were considered significant at P 
0.05.
The adjusted means for the progeny from each environment for FHB severity and DON accumulation were used to estimate heritability. Heritability of FHB severity and DON accumulation were estimated using the standard unit method of Frey and Horner (1957). The parent-offspring regression of data coded in terms of standard units is equivalent to the coefficient from simple parent-offspring correlation (r). An r value was calculated for each parent-offspring combination in 1997 and 1998 (F4:5 and F4:6 generations, respectively). In the analyses, the F4:5 and F4:6 generations corresponded to parent and offspring, respectively. The homogeneity of heritability values across different environments was tested by the chi-square test of Edwards (1976). Pooled heritability values across all environments for FHB severity and DON accumulation were calculated by the method described by Edwards (1976).
The correlation between FHB severity and DON accumulation, and the correlation of FHB severity with all morphological and agronomic data collected were determined. To facilitate the calculation of the correlation between FHB severity and spike type, six-rowed lines were assigned a value of 1 and two-rowed lines were assigned a value of 2. Correlation values were calculated for each environment, and homogeneity of correlation values across environments were tested. Pooled correlation values were calculated as specified in Edwards (1976) for correlation values deemed to be homogenous.
The correlated response of selecting for reduced FHB severity on DON accumulation, days to heading, and plant height was determined by calculating the selection differential between FHB severity and each of the traits. The selection differential was the difference between the mean of the 40% most resistant F4:6 lines and the mean of the unselected population comprised of 150 F4:5 lines (Helms and Orf, 1998). All statistical analyses were done by SAS (SAS, 1992).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Spike angle and kernel density were measured only in 1998. We began collecting data on these traits on all our breeding lines with putative FHB resistance in 1998 because it appeared that these traits could be associated with FHB resistance. Significant differences were observed between the spike angle and kernel density of the two parents (Table 2) . The spike of Foster was more erect and had a lower kernel density than the spike of CIho 4196. This observation differs from that of Zhu et al. (1999) in which dense spikes were observed to have greater FHB susceptibility. This suggests that development of lax spikes in itself will not result in lower levels of FHB. Mean spike angle and density of the F4:6 was intermediate to that of the two parents. Transgressive segregation in both directions was observed in the F4:6 population for both traits.
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Heritability of FHB Severity and DON Accumulation
The heritability values for FHB severity and DON accumulation from each of the environments were homogenous (P < 0.05); thus, pooled heritability values were calculated. The estimated narrow-sense heritabilities were 0.65 for FHB severity and 0.46 for DON accumulation. These results agree with Takeda (1992) who reported that heritability for barley FHB resistance was 0.6 in the broad sense and 0.4 in the narrow sense, and that resistance was under the control of predominantly additive minor genes. Our results also agreed with those of Zhu et al. (1999) and differed with those of Ma et al. (2000). Zhu et al. (1999) observed narrow-sense heritabilities calculated on an entry-mean basis ranging from 0.50 to 0.81. Ma et al. (2000) found a narrow-sense heritability of 0.31. The moderate heritability values found in this study suggest that FHB resistance and DON accumulation are multigenic traits that may be strongly influenced by the environment. Thus, selection for FHB resistance and DON accumulation needs to be based on observations from many environments. In our breeding program, we require favorable FHB data from a minimum of four FHB screening nurseries before we can confidently state a line is resistant. This stringent requirement results in delaying selection for FHB resistance and DON accumulation until later generations.
Correlation between Traits
Days to heading, plant height, spike angle, spike density, and spike type were negatively associated with increased FHB severity (Table 3)
. Other researchers made similar observations using different sources of FHB resistance (Ma et al., 2000; de la Pena et al., 1999; Zhu et al., 1999). On the basis of a cross between six-rowed genotypes (M69/'Chevron'), de la Pena et al. (1999) stated that the undesirable linkage between FHB resistance, and late heading and taller plants need to be broken. In another study using Chevron as the FHB resistant parent, Ma et al. (2000) found that many of the quantitative trait loci (QTL) for FHB severity and DON accumulation coincided with QTL for heading date, plant height, and spike morphology traits. On the basis of their observations, they stated that QTL with major effects for FHB severity and DON accumulation probably do not exist in Chevron. In a study involving a cross between two two-rowed genotypes, Zhu et al. (1999) found FHB resistance to be associated with taller plants, more seeds per spike, low-density spikes, and small lateral florets.
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Other researchers found no associations between FHB severity, and days to heading and plant height. Zhu et al. (1999) found no associations between days to heading and FHB severity in barley. Hilton et al. (1999) suggested that there are independent genes affecting FHB and plant height in wheat that may allow plant breeders to select resistant cultivars of any height. Deoxynivalenol accumulation was positively associated with FHB severity (Table 3). This association indicates that if we select for lower FHB severity, we also select for lower DON accumulation. Using the mapping population derived from the cross M69/‘Chevron’, de la Pena et al. (1999) found DON to be positively correlated to FHB severity at Crookston, MN, in 1995, but no association was found between these two traits at Hangzhou, China in 1997.
Presence of lemma awn spiculation was not associated with FHB severity (Table 3). This indicates that awn roughness will not affect FHB severity, allowing development of the smooth awn FHB-resistant cultivars preferred by producers. Spike angle and density were both negatively associated with FHB severity (Table 4) . The negative correlation between FHB severity and spike density in this study may be surprising to some since the resistant parent CIho 4196 had a denser spike than Foster; however, the correlation was weak (-0.20). Thus, as stated earlier, development of cultivars with lax spikes will not ensure greater FHB resistance.
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Because heading date and FHB severity are associated, we need to determine if field resistance is due to genes conferring FHB resistance or a pleiotropic effect of heading date. We attempted to reduce the confounding effect of heading date in the field by ensuring there was inoculum present from 1 wk before the earliest genotypes headed until maturity was reached by all genotypes. Mist-irrigation also was provided for the same time frame. However, there are conditions other than heading date that can confound FHB severity in field nurseries. These conditions include the time needed for the colonized grain to form perithecia and release ascospores, and the fluctuations in temperature, humidity, and precipitation.
To remove completely the effects of heading date and other environmental factors on FHB severity, it probably is necessary to evaluate plants in the greenhouse. In our greenhouse screening of breeding lines, we routinely sow checks weekly to ensure there are always ones at the proper growth stage when we inoculate the progeny. While screening plants for FHB resistance in the greenhouse is ideal from the standpoint of removing the possible confounding effects of heading date and environment, a limiting factor in greenhouse assessments is space because evaluations are done on adult plants.
Correlated Responses When Selecting for FHB Severity
To determine the effect that selection for reduced FHB severity had on other traits, the selection differential method of Helms and Orf (1998) was used. Values for differential selection in FHB resistance have not been previously reported. A selection intensity of 40% was chosen to select the 60 most FHB resistant F4:6 lines. The selected lines had an adjusted mean FHB severity ranging from -13.4 to 14.0%. Less than 14% of the selected lines had six-rowed spikes. The difference in FHB severity, DON accumulation, days to heading, and plant height between the mean of the 60 selected F4:6 families and the mean of unselected F4:5 population were then calculated and are presented in Table 4.
Mean FHB severity, DON accumulation, and days to heading of the selected lines were all much lower than those observed in the mean of the unselected population. Fusarium head blight severity and DON accumulation were reduced by 28.6 and 58.7%, respectively, in the selected lines compared with the unselected lines. Days to heading were decreased by 37% in the selected lines even though there was a negative relationship between FHB severity and days to heading. This is not surprising since the correlation between these two traits was moderately weak (-0.43). Plant height was the only trait where the mean between the selected and unselected families did not differ. In fact, none of the selected plants had plant height approaching that of Foster.
A challenge U.S. Midwest barley breeders have had is transferring FHB resistance from unadapted two-rowed barley accessions to six-rowed malting barley. This challenge was made more difficult becaue of the negative linkages in chromosome 2H between FHB resistance, row type, days to heading, and plant height described earlier. However, the linkage between row-type and resistance appears to have been broken and an FHB resistant six-rowed germplasm line (6NDRFG-1) from the cross Foster/CIho 4196 was recently released (Urrea et al., 2002). However, the linkage between FHB resistance, plant height, and, days to heading has not been broken. Plant height and days to heading of 6NDRFG-1 are similar to CIho 4196.
The differential selection values calculated for FHB severity, DON accumulation, and days to heading in this study indicate that development of FHB-resistant lines with acceptable DON accumulation and days to heading should be possible. However, when the failure to reduce plant height is considered, the difficulty in developing resistant plants with height similar to currently grown cultivars is quite evident. Thus, results from this study suggest that development of FHB-resistant lines with acceptable height from a cross with CIho 4196 as a parent will be difficult.
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ACKNOWLEDGMENTS |
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Received for publication July 15, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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