Published in Crop Sci. 44:792-796 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
CROP BREEDING, GENETICS & CYTOLOGY
Leaf Tip Necrosis
A Phenotypic Marker Associated with Resistance to Spot Blotch Disease in Wheat
A. K. Joshi*,a,
R. Chandb,
S. Kumara and
R. P. Singhc
a Department of Genetics and Plant Breeding
b Department of Mycology and Plant Pathology, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi 221 005, India
c International Maize and Wheat Improvement Centre (CIMMYT), Apdo Postal 6-641, 06600, Mexico, D.F., Mexico
* Corresponding author (joshi_vns{at}yahoo.co.in).
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ABSTRACT
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Spot blotch, caused by Bipolaris sorokiniana (Sacc.) Shoem. syn. Drechslera sorokiniana (Sacc.) Subrm and Jain (syn. Helminthosporium sativum, teleomorph Cochliobolus sativus), is an important disease of wheat (Triticum aestivum L.) in warmer and humid regions of the world. To date, no morphological marker is known to be associated with resistance to this disease. The purpose of this study was to find out the association of leaf tip necrosis (Ltn) with resistance to spot blotch disease. A total of 1407 spring wheat genotypes that originated from the Indian and CIMMYT wheat breeding programs were evaluated for Ltn and resistance to spot blotch for three seasons (1994–1995, 1995–1996, and 1996–1997) under field conditions. Disease severity was recorded at six growth stages under artificially created epidemics. About 75% of the genotypes showing Ltn (Ltn+) were resistant or moderately resistant, whereas 82% not showing it (Ltn–) were moderately susceptible or susceptible. Mean spot blotch rating of the Ltn+ genotypes was significantly lower than the Ltn– genotypes at all growth stages and the genotype x environment interaction was nonsignificant. To confirm the association of Ltn with resistance, individual F2–derived F3, F4, F5, and F6 progenies from the cross of the ‘HUW234’ near-isogenic pair for Ltn were evaluated for spot blotch severity. In each generation, the Ltn+ homozygous progenies had significantly less disease than those homozygous Ltn–. These results confirm that leaf tip necrosis is associated with moderate resistance to spot blotch and can be used as a morphological marker to facilitate selection for resistance.
Abbreviations: AUDPC, area under disease progress curve • Ltn, Leaf tip necrosis • NIL, Near-isogenic line
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INTRODUCTION
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SPOT BLOTCH is a wheat disease of concern in warmer and humid regions of the world (Dubin and Van Ginkel, 1991; Duveiller et al., 1998; Chaurasia et al., 2000; Joshi et al., 2002). Estimates of yield losses due to spot blotch are reported to vary from 15.5 to 19.6% (Dubin and Van Ginkel, 1991), 20 to 80% (Duveiller and Gilchrist, 1994), and up to 100% under severe infection conditions (Srivastava et al., 1971; Mehta, 1994). An integrated approach, with host-plant resistance as its major component, is necessary to achieve best control of the disease in environments highly favorable to disease development (Mehta et al., 1992; Joshi and Chand, 2002).
Past efforts to breed spot blotch resistant wheat cultivars have been unsatisfactory (Saari, 1998) and the importance of disease has increased in the Indian Subcontinent in recent years because of cultivation of susceptible cultivars (Duveiller et al., 1998; Chaurasia et al., 1999). Because of lower heritability of resistance to this disease (Sharma et al., 1997), effectiveness of selection in segregating breeding populations is often low. Furthermore, the absence of a suitable marker for resistance has made the task of selection more difficult (Joshi and Chand, 2002; Joshi et al., 2002). Although attempts were made to screen for resistant genetic stocks (Chaurasia et al., 1999), no attempt has been made to identify a marker trait associated with resistance to this disease. Plant height and days to maturity, known to influence the expression of spot blotch resistance (Dubin et al., 1998), were recently shown to have little or no genetic relationship with resistance (Joshi et al., 2002). However, erect leaf posture was shown to reduce spot blotch incidence in wheat (Joshi and Chand, 2002).
Leaf tip necrosis of wheat, which is known to be due to the presence of gene Ltn, is reported to be linked with genes Lr34 and Yr18 involved in slow rusting resistance to leaf [Puccinia recondita Roberge ex Desmaz. f. sp. tritici Eriks & E. Henn.) D.M. Henderson] and stripe rusts (Puccinia striiformis Westend.), respectively (Dyck, 1991; McIntosh, 1992; Singh, 1992a, 1992b). Many high yielding wheat varieties possess this trait (Singh and Rajaram, 1992) and Ltn has been suggested to be used as a marker for selection for Lr34/Yr18 genes known to be present in chromosome 7DS (Dyck, 1987). Singh (1993) reported that genes Ltn/Lr34/Yr18 were also linked/pleiotropic to gene Bdv1 that conferred slow yellowing to barley yellow dwarf virus. In our routine field trials at Banaras Hindu University, Varanasi, India, we observed that genotypes that had leaf tip necrosis showed lesser spot blotch development than those without this trait (Joshi and Chand, unpublished data, 1998). On the basis of this initial observation, two experiments were planned with the objective of determining the association between the leaf tip necrosis and resistance to spot blotch of wheat.
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MATERIALS AND METHODS
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Experiment 1: Evaluation of Wheat Germplasm for the Presence–Absence of Leaf Tip Necrosis (Ltn) Trait and Spot Blotch Severity
The 1407 wheat lines included in the first experiment were previously studied to find sources of resistance to spot blotch (Chaurasia et al., 1999) and the association of plant height, days to maturity (Joshi et al., 2002), and leaf angle (Joshi and Chand, 2002) with spot blotch severity. The 1407 lines were obtained from CIMMYT (International Maize and Wheat Improvement Center, Mexico), DWR (Directorate of Wheat Research, Karnal, India), and NBPGR (National Bureau of Plant Genetic Resources, New Delhi, India). These lines included cultivars, genetic stocks for different traits, and advanced lines being tested in different trials and nurseries viz. Crossing Block, National Genetic Stock Nursery, Leaf Blight Screening Nursery, Drought and Heat Tolerance Nursery, Initial Varietal Trials, Advanced Varietal Trials, International Bread Wheat Screening Nursery, Elite International Germplasm Nursery, International Drought and Heat Tolerance Nurseries.
Each line was hand sown in three replications of a randomized complete block design in a paired row plot of 3-m length with 25-cm spacing between the rows and 5 cm between seeds at the research station of Banaras Hindu University, Varanasi, India (North Eastern Plains Zone, 25.2°N and 83.0°E) for three consecutive seasons, 1994–1995, 1995–1996, and 1996–1997. This area is known for an environment that is highly favorable for spot blotch development. Same agronomic practices recommended for normal fertility (120 kg N: 60 kg P2O5: 40 kg K2O ha–1) were followed in all the three years. Full doses of K2O and P2O5 were applied at the time of sowing. Nitrogen was supplied in split applications, 60 kg N ha–1 at sowing, 30 kg N ha–1 at the first irrigation (21 d after sowing) and 30 kg N ha–1 at the second irrigation (45 d after sowing).
Two rows of susceptible disease spreaders, ‘Sonalika’ (a bread wheat cultivar) and A-9-4-1 (a durum wheat, Triticum durum Desf.), were included after every tenth entry to enhance disease development and spread. To achieve high disease pressure, planting in each year was done during the second week of December rather than the normal recommended date of 15 to 25 November. This allowed the post anthesis stages to coincide with warmer temperatures during March, which enhance the disease development. Spreader rows of A-9-30-1 were also planted in the alleyways of the experimental plots 2 wk before sowing the experiment to induce disease development.
Presence or absence of Ltn was recorded for all wheat lines through visual observation at anthesis (Singh, 1992a). This stage corresponds to the stage 65-69 in the Zadoks scale (Zadoks et al., 1974). The status of Ltn can be easily determined at this stage. Ltn data were used to classify the lines in two groups: Ltn present (Ltn+) and Ltn absent (Ltn–).
A pure culture of a local isolate of B. sorokiniana identified at this center (registered in Auckland, New Zealand, No. ICMP 13584) and known to be most aggressive, was used in all the studies. The inoculum was multiplied on wheat grains and spores were harvested in water (Misra, 1973). The disease was initiated by uniformly spraying the spreader rows and all other plots during the evening hours with a spore-water suspension of fixed concentration (104 spores mL–1) at three growth stages: tillering, flag leaf emergence, and anthesis (Chaurasia et al., 1999; Joshi et al., 2002). The field was frequently irrigated to maintain high humidity and promote disease development.
Spot blotch disease for each line was evaluated on 10 random plants following the 0-to-9 scale (Saari and Prescott, 1975) at six growth stages, viz. 25, 37, 47, 57, 69, and 77 on Zadoks scale (Zadoks et al., 1974) as described in Chaurasia et al. (1999) and Joshi and Chand (2002). Disease severity (%) was also recorded for each genotype. Disease evaluation was done on the same 10 plants throughout each season. The average of the 10 individual scores was used as the score for each genotype. Genotypes scoring 1 to 3 were considered to be resistant, 4 to 5 as moderately resistant, 6 to 7 as moderately susceptible, and 8 to 9 as susceptible. Area under the disease progress curve (AUDPC), which is a better indicator of disease expression over time (Van der Plank, 1963; Chaurasia et al., 1999), was calculated by the formula given in Joshi et al. (2002) and Joshi and Chand (2002).
where, Yi = disease level at time ti, t(i + 1) – ti = Time (days) between two disease scores Statistical analysis of 3 yr disease score of Ltn+ and Ltn– lines was done in a split plot analysis in which year was the main-plot factor and Ltn as subplot factor.
Experiment 2: Genetic Linkage between Leaf Tip Necrosis and Resistance to Spot Blotch
We used two reselections, HUW234Ltn+ and HUW234 Ltn–, of the heterogeneous Indian wheat cultivar HUW234 for this purpose. The reselections, made by the last author in Mexico, were based on the presence or absence of leaf tip necrosis and slow rusting resistance to leaf rust. These reselections could be considered near-isogenic lines (NILs) for genes Ltn and Lr34 because cultivar HUW234 originated from a single back cross (HUW12/Sparrow//HUW12) followed by pedigree selection up to the F8 generation. The cultivar HUW234, developed at Banaras Hindu University, Varanasi, India, is the most popular cultivar in the North Eastern Gangetic Plains of India for last 15 yr. In our initial studies during 1997-1998 crop season, HUW234Ltn+ showed significantly less spot blotch severity compared with HUW234 Ltn– indicating that genes Ltn/Lr34 could be associated with resistance. The Ltn NILs were further tested for next four years (1998-1999 to 2001-2002) in two dates of sowings in each of the years for their response to spot blotch disease. Statistical analysis using AUDPC values of 5 yr of Ltn+ and Ltn– lines was done in multiple factor split plot analysis in which year was taken as the main-plot factor, sowing date as the subplot factor, and Ltn as sub-subplot factor. The paired t test was also done for comparing the AUDPC values of the NILs.
To confirm the suspected association of Ltn with the spot blotch disease, individual plant selections of the near-isogenic HUW234 reselections were crossed during the 1997–1998 crop season. Twenty F1 seeds were multiplied in an off-season nursery during 1998 to obtain F2 populations by harvesting three individual F1 plants. The F3 generation was obtained by harvesting 180 space sown random F2 plants during the 1998–1999 season. The F4, F5, and F6 generations were derived by growing small seed samples of each line and by harvesting one random plant from each line in each generation (Singh and Rajaram, 1992; Joshi et al., 2002). Off-season nurseries were again used to expedite generation enhancement.
One hundred sixty F3 and F4 lines were evaluated during the 1999–2000 crop season in three replications and two dates of sowing (30 Nov. and 15 Dec. 1999) at Varanasi following split plot design as previously described. The 160 F5 and the same number of F6 lines were also evaluated during the 2001–2002 crop season in two dates of sowing (28 Nov. and 20 Dec. 2001) following the experimental design as followed in the case of F3 and F4. In each of the four generations (F3, F4, F5, and F6) the plots of each line consisted of a single 3-m row with 30 cm between plots and approximately 40 to 50 plants per row. One row of the highly susceptible spreader cultivar Sonalika was sown in the middle of the alleyways and after every tenth entry to promote disease development and spread. Parental lines, HUW234Ltn+ and HUW234 Ltn–, were included at the beginning and end of the experiment and after every 40th plot as checks for comparison. Disease severity and presence of Ltn were recorded for the parental genotypes and all the progeny rows in the four generations (F3, F4, F5, and F6) derived from HUW234 NILs cross. For each line, the disease scores of all the plants in all plots were recorded.
Procedures involving the initiation of spot blotch epidemic, disease evaluation, AUDPC calculation, etc. were similar to those described in Exp. 1. Each of the lines was characterized as either homogeneous or homozygous for the presence or absence or segregating for leaf tip necrosis in each generation. The 
2 analysis was done to test the number of genes controlling Ltn trait. Statistical analysis using AUDPC values of the homozygous or homogeneous Ltn+ and Ltn– progeny rows of four generations (F3, F4, F5, and F6) was done in multiple factor split plot analysis in which generation was taken as the main-plot factor, date as the subplot factor, and Ltn as sub-subplot factor. The paired t test was also done to compare the AUDPC of the Ltn+ and Ltn– homogeneous or homozygous progeny lines in four generations.
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RESULTS
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Experiment 1: Evaluation of Wheat Germplasm for the Presence–Absence of Leaf Tip Necrosis Trait and Spot Blotch Severity
The analysis of variance of the 1407 germplasm and elite breeding lines indicated that the differences between Ltn+ and Ltn– lines were significant for spot blotch severity (0–9 scale) and AUDPC values (Table 1). These lines could be split into two groups, 458 (32.6%) lines with leaf tip necrosis and 949 (67.4%) without it (Table 2). None of the lines appeared to be heterogeneous for Ltn. Although the resistance responses of the Ltn+ lines varied, 75.5% were either resistant or moderately resistant to spot blotch. None of the Ltn+ lines was susceptible to spot blotch. In contrast, despite the fact that the resistance responses of Ltn– lines also varied, 82.9% of the Ltn– lines were moderately susceptible or susceptible to spot blotch. We therefore concluded that the presence of leaf tip necrosis in wheat lines was associated with reduction in spot blotch progress and final disease ratings.
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Table 1. Mean squares for the effect of presence/absence of leaf tip necrosis (Ltn) in 1407 germplasm/elite breeding lines tested for spot blotch severity (0–9 scale) and AUDPC for 3 yr.
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Table 2. Mean spot blotch response of Ltn+ and Ltn– germplasm/elite breeding lines of wheat at late milk (GS77) stage tested under field conditions for 3 yr.
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Experiment 2: Genetic Linkage between Leaf Tip Necrosis and Resistance to Spot Blotch
The analysis of variance of the mean spot blotch AUDPC of the near-isogenic reselections of HUW234 (HUW234Ltn+ and HUW234Ltn–) evaluated for 5 yr in two planting dates indicated that there was significant difference for Ltn in displaying association with spot blotch resistance (Table 3). The t test clearly indicated that the HUW234Ltn+ reselection had significantly less spot blotch than the HUW234Ltn– in each planting date (Table 4). The analysis of variance of the spot blotch AUDPC of the four segregating generations (F3, F4, F5, and F6) also indicated that the differences between Ltn+ and Ltn– homogeneous or homozygous progeny lines were significant (Table 5). Similarly, paired t test of the mean AUDPC values of Ltn+ and Ltn– homozygous progeny rows for each of the four segregating generation showed that Ltn+ lines were significantly superior to Ltn– lines under both sowing dates (Table 6). The observed frequencies of the homozygous Ltn+ and Ltn– lines in each generation were in agreement to the frequencies expected for segregation of a single gene (Table 6).
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Table 3. Mean squares for the effect of the presence/absence of leaf tip necrosis (Ltn) in HUW234 near-isogenic reselections on spot blotch AUDPC in two sowing dates during 5 yr in field trials at Varanasi, India.
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Table 4. Mean spot blotch AUDPC of the HUW234Ltn+ and HUW234Ltn– near-isogenic reselections in two planting dates during five crop seasons at Varanasi, India.
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Table 5. Mean squares for the effect of leaf tip necrosis (Ltn) on spot blotch AUDPC of the F3, F4, F5, and F6 lines homozygous for the presence and absence of Ltn in the cross ‘HUW234Ltn+’ x ‘HUW234Ltn–’ tested in two planting dates.
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Table 6. Comparison of mean spot blotch AUDPC of Ltn+ and Ltn– progeny rows in F3, F4, F5, and F6 generations obtained by crossing HUW234 near-isogenics reselections for Ltn (Ltn+ and Ltn–) in two sowing dates.
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DISCUSSION
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The evaluation of 1407 germplasm and elite breeding lines showed that Ltn+ genotypes, on the average, possessed better resistance to spot blotch. This was reflected as greater proportion of Ltn+ lines exhibiting resistant or moderately resistant response with no line scored as susceptible (Table 2), maintenance of significantly superior resistance by Ltn+ lines throughout all growth stages (Fig. 1)
and, significantly lower AUDPC values of Ltn+ lines in comparison with Ltn– lines in progenies derived from the cross of near-isogenic HUW234 reselections for Ltn (Table 6). Further, the analysis of variance for spot blotch response of germplasm and elite breeding lines, HUW234 NILs, and progeny rows homogeneous or homozygous for Ltn+ or Ltn– in each of the four segregating generations (F3, F4, F5, and F6) showed significant differences for Ltn. Although the presence of Ltn was associated with resistance to spot blotch, its absence was not always associated with susceptibility. This indicated that although the presence of Ltn is associated with reduced spot blotch severity, presence of additional genes is necessary to achieve relatively higher levels of resistance. Incidentally, some lines having Ltn+ showed moderately susceptible reaction. Our observation showed that all these lines possessed pseudo-black chaff spike phenotype known to be associated with stem rust resistance gene Sr2 (Roelfs, 1988). Another study has shown that the presence of pseudo-black chaff enhanced the susceptibility to spot botch disease (Joshi and Chand, unpublished data, 1998).
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Fig. 1. Mean score (0–9 scale) of spot blotch at six growth stages of 1407 Ltn+ and Ltn– germplasm/elite breeding lines in three years of testing. **Indicates significant t values at P = 0.01. Bars represent standard errors; however, some error bars are smaller than the symbol and may not be visible.
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Leaf tip necrosis of adult plants in wheat is known to be due to the presence of gene Ltn, which is linked with genes Lr34 and Yr18, involved in slow rusting resistance to leaf and stripe rusts, respectively (Dyck, 1991; McIntosh, 1992; Singh, 1992a, 1992b). Although the expression of Ltn is not an attractive trait and may have slight effect in reducing grain yields in some environments (Ma and Singh, 1996; Singh and Huerta-Espino, 1997), its role as a marker of durable resistance to both leaf and stripe rusts cannot be undermined. Many high yielding wheat cultivars possess this trait (Singh and Rajaram, 1992) and Ltn can be used as a marker for selecting Lr34/Yr18 genes known to be present in chromosome 7DS (Dyck, 1987). Singh (1993) also reported that Ltn was also associated to gene Bdv1 that conferred slow yellowing to Barley yellow dwarf virus. For each of these diseases, the effect of individual genes was found to be partial and unsatisfactory under high disease pressure; however, their combination with the Lr34/Yr18 complex was able to confer high levels of adult-plant resistance (Singh and Rajaram, 1992).
Our results establish that the chromosomal region in wheat carrying the linked genes Ltn/Lr34/Yr18/Bdv1 also carries a gene that has association with moderate levels of resistance to spot blotch disease. We failed to identify recombinants between Ltn and spot blotch susceptibility in the cross involving the near-isogenic HUW234 reselections for Ltn/Lr34 indicating that the gene that conferred spot blotch resistance is either very closely linked to other genes mentioned above or the spot blotch and resistance to other diseases may be due to the pleiotropic effect of the same gene.
The inheritance of Ltn has been reported to be under the control of one gene (Singh, 1992a). The frequencies of homozygous Ltn+ and Ltn– lines in the four generations from the cross of near-isogenic HUW234 reselections in our study were in accordance to monogenic segregation ratios. Being monogenic, selection for Ltn can help provide moderate levels of resistance to several diseases, including leaf rust, stripe rust, and spot blotch, three of the most important diseases of wheat in the Indian subcontinent.
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ACKNOWLEDGMENTS
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We appreciate financial support from the Council of Scientific and Industrial Research, New Delhi, India and scientific review of the paper by Dr. O. Ferrara and Dr. E. Duveiller, CIMMYT Regional Office, Kathmandu, Nepal.
Received for publication April 25, 2003.
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ABSTRACT
INTRODUCTION
