Crop Science 42:159-164 (2002)
© 2002 Crop Science Society of America
CROP ECOLOGY, MANAGEMENT & QUALITY
The Effect of Plant Height on Tan Spot on Durum Wheat in Southern Saskatchewan
M. R. Fernandez*,
J. M. Clarke and
R. M. DePauw
Semi-arid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1030, Swift Current, SK, Canada S9H 3X2
* Corresponding author (fernandezm{at}em.agr.ca)
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ABSTRACT
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Tan spot [caused by Pyrenophora tritici-repentis (Died.) Drechs.] is the most important leaf disease of durum wheat (Triticum turgidum L. var. durum). Most of the durum cultivars in western Canada are of conventional height but development of shorter cultivars is desirable. Changes in microenvironment resulting from a height reduction might cause greater leaf spot development. The effect of plant height on tan spot of durum wheat was determined in five pairs of genotypes near-isogenic for plant height and five randomly selected populations, each with a semidwarf and a conventional height parent. These were grown in five environments in Saskatchewan (1996–1998 at Swift Current, 1997–1998 at Indian Head), and evaluated for tan spot reaction in a growth chamber. There was no difference in tan spot reaction under controlled conditions between short and tall genotype(s) in each pair or population, although there were differences within each height category. Under field conditions, short genotypes had either equal or greater disease than tall genotypes. Contrasts between all short and tall genotypes also showed that the tall had less disease in 1998 at Indian Head (9.8 for tall, 10.2 for short) and in all years at Swift Current (9.5 and 9.9 in 1996, 8.1 and 8.7 in 1997, 8.5 and 8.9 in 1998, for tall and short, respectively). However, there was genetic variability within populations, with some short having equal or less disease than tall genotypes. Some of these short genotypes also had a low tan spot lesion type under controlled conditions. We conclude that plant height could affect tan spot development in durum wheat under conditions prevalent in southern Saskatchewan, and that this is probably mediated by canopy density.
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INTRODUCTION
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MOST OF THE DURUM WHEAT CULTIVARS currently registered in western Canada are of conventional height. Durum production is restricted in high moisture areas and under irrigation by lodging, foliar diseases, and grain quality. Short, strong-strawed durum wheat cultivars would reduce the risk of lodging in high moisture or irrigated environments. The reduced amount of straw residue is also desirable in high moisture environments and in dryland continuous cropping systems.
Durum wheat cultivars currently grown in western Canada are moderately to highly susceptible to leaf spots (Varieties of Grain Crops, 2001). Tan spot, caused by Pyrenophora tritici-repentis (Died.) Drechs. [anamorph Drechslera tritici-repentis (Died.) Shoemaker] is the most important leaf spotting pathogen in durum wheat production (Fernandez et al., 1996) and the most prevalent in Saskatchewan (Fernandez et al., 1999a, 2000). High tan spot disease levels were shown to occur even under relatively dry conditions in southern Saskatchewan (Fernandez et al., 1996, 1998b, 1999b). Pyrenophora tritici-repentis also causes the kernel disease red smudge, which is an important downgrading factor in durum wheat (Fernandez et al., 1998a; Francl and Jordahl, 1993).
Studies on the relationship between leaf spot disease severity of wheat (Triticum spp.) and plant height or maturity have shown negative associations for Mycosphaerella graminicola (Fuckel) J. Schröt. in Cohn (anamorph Septoria tritici Roberge in Desmaz.) and Phaeosphaeria nodorum (E. Müller) Hedjaroude [anamorph Stagonospora nodorum (Berk.) Castellani & E.G. Germano (Danon et al., 1982; Scott et al., 1982; Scott and Benedikz, 1985; Baltazar et al., 1990; Camacho-Casas et al., 1995]. Mechanisms ranging from linkage to pleiotropy have been suggested. For a single durum wheat population, Elias et al. (1989) reported a negative correlation between tan spot severity and days to heading but not with plant height. Fernandez et al. (1993) reported a negative correlation between leaf spot severity and plant height for durum wheat genotypes grown under irrigated conditions in southern Saskatchewan in 1991 and 1992.
Most of these field studies used artificial inoculation from the top of the plant canopy. This approach would not make it possible to determine the effect that plant height and plant canopy per se might have on the development of leaf spots under natural field conditions, independent of genetic resistance. In addition, many of the above studies used genetically diverse genotypes which would confound genotypic with phenotypic effects. Investigation of genotypes genetically similar except for stature is necessary to separate effects of plant height and canopy differences from genotypic differences. The possible association between leaf spotting severity and plant stature is of value when setting targets for incorporation of genetic resistance to leaf spots in short-stature durum wheat.
The objective of this study was to determine the effect of plant height in near-isogenic and randomly selected durum wheat genotypes on the development of tan spot in dryland environments in southern Saskatchewan.
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MATERIALS AND METHODS
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Pairs of durum wheat genotypes near-isogenic for plant height were all derived from the semidwarf DT369 (Wascana/Quilafen, PI546362) (McLeod et al., 1991). The pairs from the 8667 population were selected at the F5 from the cross Fanfarron/DT369. Fanfarron (PI221411) is a conventional height (tall) cultivar. The pair from the 9064 population was a F6-derived F8 selected from the cross Plenty/8361-BR1A. Plenty (Knott, 1991) is a conventional-height cultivar and 8361-BR1A is a semi-dwarf derived from the cross DT369/Golden Ball.
Randomly selected durum wheat populations were derived from the crosses 8670 (Tschernovska/DT369), 8674 (STD40/DT369), 8981 (8560-150D/Wascana), 9066 (8363-BB3D/STD65), and 8982 (Kyle/Nile). Tchernovska, Nile, STD40, and STD65 are from ICARDA; 8560-150D is from CIMMYT; Kyle (Townley-Smith et al., 1987), Wascana (Hurd et al., 1972), and 8363-BB3D (Golden Ball/DT379) originate from the Semiarid Prairie Agricultural Research Centre breeding program. The populations were advanced from F2 through F6 (F7 in the case of 8982) by a random ear-row procedure. Genotypes were classified as either gibberellic acid-insensitive (reduced height) or -sensitive (conventional height) (Gale and Gregory, 1977).
These genotypes were grown in four-row plots 3 m long with 0.23-m row spacing at Swift Current and Indian Head, Saskatchewan in 1996 (Swift Current only), 1997, and 1998. The experimental design was a randomized complete block with three replications. Plots were seeded in early May of each year, and combine-harvested at maturity in late August to early September. Measurements taken in each plot included height from the soil surface to the tip of the tallest spike, excluding awns, at plant maturity; days to 50% heading and days to physiological maturity; grain yield, and 1000-kernel weight.
Leaf area index was measured at the milk stage of plant development (Zadoks' GS 73-77, Zadoks et al., 1974) in 1998 at the Swift Current site with a leaf area index analyzer (LAI-2000 plant canopy analyzer, LI-COR Inc., Lincoln, NE). Meteorological data (precipitation and temperature) were collected on site at both locations.
The severity of leaf spots was measured at the milk stage by a 0-to-11 scale (Fernandez et al., 1998b). Each leaf spot score was the average of the scores of approximately 50 plants from the center rows of each plot. Flag leaves with leaf spot lesions were randomly collected immediately after rating from each of the plots. Diseased leaf pieces, about 1 cm2, were surface disinfested in 0.06% (v/v) sodium hypochlorite for 1 min, rinsed three times in sterile distilled water, plated on water agar, and incubated for 7 d under near-UV lights (12 h light at 22°C, 12 h dark at 15°C). The percent leaf area covered by each pathogen was calculated relative to its frequency of isolation.
The genotypes used in this study were tested for tan spot reaction under controlled conditions by inoculation at the flag leaf stage with P. tritici-repentis following the procedure of Fernandez et al. (1994). There were three replications in a randomized complete block design. Twelve isolates collected from different field locations in Saskatchewan were used (Fernandez et al., 1996). At 7 and 14 d after inoculation, flag leaves were rated according to percent area infected and lesion type by a scale of 1 (small, dark lesions) to 3 (large, tan lesions) which was adapted from Lamari and Bernier (1989).
Analysis of variance was performed on leaf spot data by GLM procedures (SAS Institute, Inc., 1985). When F values were significant (P < 0.05), least significant differences were calculated. Single degree of freedom orthogonal contrasts were also performed to determine significant differences between height groups. Simple correlations were used to compare incidence and severity values among environments and between height groups and between tan spot reaction under artificial conditions and tan spot severity in the field.
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RESULTS
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None of the durum wheat genotypes tested showed resistance to leaf spots under natural field conditions in southern Saskatchewan. Leaf spotting scores ranged from 6 to 11, 6 being no lesions on flag leaf, and 6 to 10% of penultimate leaf area covered with spots; 7 = trace to 1% of flag, and 6 to 10% of penultimate leaves; 8 = 2 to 5% of flag, and 11 to 25% of penultimate; 9 = 6 to 10% of flag, 26 to 50% of penultimate; 10 = 11 to 25% of flag, >50% of penultimate; and 11 = 26 to 50% of flag, >50% of penultimate. Severity of leaf spots on lower leaves was >50% in all cases. Analysis of infected leaf tissue samples showed that 82 to 100% of the lesions developed on flag leaves of all genotypes used in this study were attributed to P. tritici-repentis. Tan spot severity was greatest (P < 0.01) at Indian Head in 1998 and Swift Current in 1996 (Table 1). The lowest (P < 0.01) disease levels were recorded at Indian Head in 1997. Environments favorable for plant growth were Indian Head in 1998 and Swift Current in 1997 as shown by grain yield, days to maturity, and plant height. The lowest (P < 0.01) grain yield was at Indian Head in 1997. The highest amount of precipitation over the growing season (May–August), especially in late spring, was at Indian Head in 1998. Indian Head in 1997 had the lowest amount of precipitation throughout the season. July was somewhat wetter at Swift Current in 1997 and 1998 than at the other environments, but Swift Current in 1996 and Indian Head in 1998 had the greatest number of days with precipitation in July. Days to heading were significantly correlated with days to maturity at Swift Current (r = 0.84 (P < 0.01) in 1996, r = 0.55 (P < 0.01) in 1997, and r = 0.66 (P < 0.01) in 1998). Therefore, only maturity ratings, which were taken in all environments, will be considered in our discussion.
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Table 1. Average leaf spot scores, height, days to heading and maturity, grain yield and 1000-kernel weight of all lines tested and rain (mm), days to maturity, and average minimum and maximum temperature (°C) in the growing season, for each of the environments.
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There was environmental variation for plant height, but height differences among genotypes were consistent across environments. There was no overlap in plant height between the short and tall genotype groups within each of the random populations or near-isogenic pairs in any of the environments or for all environments combined (Table 2). The short genotypes were an average of 22 to 36 cm shorter than the tall genotypes for the near-isogenic pairs and 17 to 37 cm for the random populations. The smallest difference between the tall and short categories was observed at Swift Current in 1996 and Indian Head in 1997, whereas the largest difference was at Swift Current in 1997 and Indian Head in 1998, the most favorable environments for plant growth. There was also a range in plant height within the short and tall genotypes for most of the populations. In some cases, the difference in height between the shorter and the taller short genotypes, or between the shorter and taller tall genotypes, were as great as between the two height categories.
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Table 2. Leaf spot severity (0–11) and plant height (cm) of durum wheat pairs near-isogenic for plant height and of randomly selected populations in five environments in southern Saskatchewan.
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A combined analysis of variance showed that genotype x environment interactions were significant (P < 0.05) for all parameters measured, therefore each environment was analyzed separately. The relative ranking of genotypes based on leaf spotting disease severity varied among environments, even for the same location (Table 2; data for individual genotypes in the randomly selected populations not presented). For example, at Swift Current, 8667-1-09A had a lower severity than did 9064-DE1B in 1996 but it was higher in 1998. The range in tan spot severity ratings was in general wider in the Swift Current environments than at Indian Head.
Single degree of freedom contrasts indicated a significant (P < 0.10–0.01) difference in tan spot severity between the tall and the short genotype in two to four of the five environments for the pairs of near-isogenic genotypes and in three to five of the random populations (Table 2). Only two of the pairs and two of the random populations were significantly different at Indian Head in 1997. In all cases, the short genotypes had a greater tan spot severity than the tall ones. The magnitude of the differences in disease severity between the short and the tall genotypes appeared to be greater at Swift Current than at Indian Head. Single degree of freedom contrasts between all short and tall genotypes used in this study showed that the latter were significantly (P < 0.01) less diseased than the former in all environments, except for Indian Head in 1997 where there was no difference between the two height categories.
Because the height of the genotypes in each of the height groups in the random populations varied, correlations were performed between plant height and leaf spot severity. There was a negative correlation (P < 0.10–0.01) between tan spot severity and plant height for each of the randomly selected populations (Table 3), except for 8670, in three out of five environments. Swift Current in 1998 was the environment with the greatest number of populations where tan spot severity was correlated negatively with plant height. Only two populations showed significant correlations at Indian Head in 1997 and Swift Current in 1996, and one at Swift Current in 1997.
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Table 3. Simple correlation coefficients of leaf spotting ratings at milk stage and plant height, days to physiological maturity, grain yield and 1000-kernel weight of randomly selected populations of durum wheat at five environments in southern Saskatchewan from 1996 to 1998.
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Single degree of freedom contrasts performed on percent leaf area infected and lesion type after inoculation with P. tritici-repentis under controlled conditions showed that there was no difference between the short and tall genotype(s) in the near-isogenic pairs or randomly selected populations. There was also no correlation (P > 0.05) between tan spot reaction under controlled conditions and leaf spot severity in the field in any of the environments for any of the randomly selected populations. However, some significant differences in lesion type were found within the height categories for each of the populations (Table 4).
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Table 4. Lesion type on flag leaves of plants inoculated with P. tritici-repentis in the growth chamber, and mean plant height and leaf spot scores from five environments in SK.
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Although in all populations the most susceptible genotypes in the field were among the short genotypes and the least susceptible among the conventional height genotypes, there were exceptions. Within most populations, a tall genotype(s) showed a greater leaf spot severity than the short genotypes, and/or a short genotype(s) had a lower disease severity than the tall genotypes (Table 4). Some of the short genotypes with low leaf spot severity in the field also showed a lower leaf spot reaction under controlled conditions, in particular 8674-RT and 9066-RV1A.
In most cases, there was no correlation between leaf spot severity and days to maturity (Table 3). The largest negative correlation between tan spot and days to maturity was at Swift Current in 1996 and Indian Head in 1997 for 8982. This population also had a positive correlation between plant height and days to maturity at Swift Current in 1996 (r = 0.62, P < 0.10).
Grain yield and 1000-kernel weight were in some cases negatively correlated with tan spot severity (Table 3), except for 8670. In about half of the cases where there was a negative correlation between leaf spot severity and grain yield or 1000-kernel weight, there was also a positive correlation between the latter two and plant height (data not presented), which suggests that the correlation between leaf spot severity and grain yield might be at least partly related to the association of these parameters with plant height. Only 8674 at Swift Current in 1996 and Indian Head in 1998 had a strong negative association between leaf spot severity and grain yield (Table 3) but no correlation between grain yield and plant height.
Single degree of freedom contrasts for leaf area index measured at Swift Current in 1998 between the short and tall genotypes indicated a significant difference (P < 0.05) for three out of the five near-isogenic pairs and four out of the five populations. The short genotypes had a higher leaf area index (lai) than the tall genotypes (lai = 2.8 m2 m-2 for short and 2.2 m2 m-2 for tall genotypes in the randomly selected populations, lai = 3.0 m2 m-2 for short and 2.4 m2 m-2 for tall genotypes in the near-isogenic populations). In addition, there was a significant negative correlation between plant height and leaf area index for all genotypes (r = -0.40, P < 0.01).
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DISCUSSION
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There was no apparent association between tan spot reaction and the semidwarf trait for any of the near-isogenic durum wheat pairs or randomly selected populations when tested under controlled conditions. However, our field observations of durum wheat genotypes in five environments indicated a phenotypic association between plant height and tan spot severity under conditions prevalent in southern Saskatchewan from 1996 to 1998.
In all cases where there was a significant difference in tan spot severity in a near-isogenic pair or random population, the short genotype(s) was more severely diseased than the tall genotype(s). For most of the randomly selected populations, there was also a significant negative correlation between tan spot severity and plant height in most of the environments. Because there was no difference in tan spot reaction under controlled conditions within any of the pairs, or between the short and tall genotypes for any of the random populations, the greater disease severity of the short genotypes might be attributed to the close proximity of the leaves to the ground and to each other. The denser canopy of the short than of the tall genotypes in most of the near-isogenic pairs and randomly selected populations, as indicated by leaf area index measurements, must have provided a more suitable environment for the development of leaf spots. Bahat et al. (1980) suggested an association between the closer proximity of leaves in a shorter than a taller genotype and development of S. tritici. The more favorable environment for disease development in short than tall genotypes could be attributed to differences in moisture. Scott et al. (1985) reported that duration of leaf surface wetness was negatively correlated with plant height and positively correlated with amount of S. nodorum on the leaves.
Differences in disease severity between the tall and short genotypes were observed in all environments including the least (Indian Head in 1997) and most favorable (Indian Head in 1998) environments for disease development, although the association between tan spot severity and plant height did not occur in all environments or populations. The negative correlation between tan spot severity and plant height was the lowest at Indian Head in 1997. No environment(s) favored a consistent significant difference in tan spot severity between the short and tall genotypes in all randomly selected populations and near-isogenic pairs.
Although in most cases tan spot severity did not appear to be associated with days to plant maturity, there appeared to be some environmental influence on the relationship between disease development and plant maturity. The greater number of correlations (positive and negative) of tan spot severity with days to maturity were found at Indian Head in 1997, the least favorable environment for disease development. Tan spot appears to have lower requirements for high humidity than other leaf spotting pathogens (Fernandez et al., 1996). Differences in days to maturity among genotypes would thus not be expected to affect the development of this disease as much as that of other leaf spotting pathogens, especially when these differences are of small magnitude.
Negative correlations between tan spot severity and grain yield or 1000-kernel weight appeared for the most part to be spurious, and could be attributed to the association between these plant growth parameters and plant height. Reduced kernel size in semidwarf than in tall durum genotypes was reported by Fernandez et al. (1993), McClung et al. (1986), and Joppa (1973).
These observations indicate that plant height could affect tan spot development in durum wheat under conditions prevalent in southern Saskatchewan, and that this effect is probably mediated by canopy density. The effect of plant height on tan spot development was influenced little by environmental conditions typical of southern Saskatchewan. The observation that in some of the populations there was at least one short genotype that was equally or less severely diseased than the tall genotypes under field conditions indicates genetic variability among the progeny of these crosses which would allow for selection of short genotypes with better resistance to tan spot. In some cases, these genotypes also had a lower tan spot lesion type under controlled conditions.
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ACKNOWLEDGMENTS
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The technical assistance of D. Green, Y. Chen, R. Dunbar and B. Bradley is gratefully appreciated.
Received for publication January 16, 2001.
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ABSTRACT
INTRODUCTION
