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Published in Agron J 99:1463-1470 (2007)
DOI: 10.2134/agronj2007.0105
© 2007 American Society of Agronomy
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Legumes

Cultivar Type, Plant Population, and Ascochyta Blight in Chickpea

Yantai Gana,*, Bruce D. Gossenb, Lin Lic, Greg Forda and Sabine Bannizac

a Agric. and Agri-Food Canada, Swift Current Research Centre, SK, S9H 3X2, Canada
b Agric. and Agri-Food Canada, Saskatoon Research Centre, Saskatoon, SK S7N 0X2, Canada
c Dep. of Plant Sci., Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada

* Corresponding author (gan{at}agr.gc.ca)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Integrated management strategies are required to minimize ascochyta blight, a fungal disease caused by Ascochyta rabiei (Pass.) Labrousse [teleomorph, Didymella rabiei (Kovachevski) v. Arx] in chickpea (Cicer arietinum L.). This study determined the effect of cultivars varying in plant architecture and plant population density (PPD) on the severity of ascochyta blight. Four desi chickpea (with pinnate leaves) and four kabuli chickpea (two with pinnate leaves and two with unifoliate leaves) were grown at 25, 36, 44, 53, and 62 plants m–2 (actual counts 3 wk after initial seedling emergence) at Swift Current from 2002 to 2005 and at Saskatoon in 2004 and 2005. Site-years had a significant effect on ascochyta blight epidemics, with the highest severity at Swift Current in 2005 and lowest at Saskatoon in 2004. Across site-years, ascochyta blight was most severe on ‘Evans’, followed by ‘CDC Xena’, and lowest on ‘222B-11’. Cultivars with pinnate leaves had lower blight severity than those with unifoliate leaves during all growth stages. At the late-pod stage, severity in cultivars with pinnate leaves averaged 15% compared with 48% in unifoliate cultivars. Kabuli cultivars had higher severity than desi cultivars throughout the growing season, and at the late-pod stage, severity was 13% for the desi and 33% for the kabuli. There was a significant interaction between cultivar and PPD for blight severity. Ascochyta blight increased as PPD increased for the majority of the cultivars tested, with a few exceptions. Site-year accounted for the largest portion of the treatment variance in blight severity (69%), followed by cultivar type (25%), and then PPD (6%). Increasing PPD consistently increased seed yield per unit area, despite more disease on plants at higher PPD. Identifying optimum plant populations for groups of cultivars with similar plant architecture should be a component in an integrated strategy to minimize ascochyta blight in chickpea.

Abbreviations: PPD, plant population density

Cultivar Type, Plant Population, and Ascochyta Blight in Chickpea

Yantai Gana,*, Bruce D. Gossenb, Lin Lic, Greg Forda and Sabine Bannizac

a Agric. and Agri-Food Canada, Swift Current Research Centre, SK, S9H 3X2, Canada
b Agric. and Agri-Food Canada, Saskatoon Research Centre, Saskatoon, SK S7N 0X2, Canada
c Dep. of Plant Sci., Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada

* Corresponding author (gan{at}agr.gc.ca)

Received for publication March 25, 2007.
Integrated management strategies are required to minimize ascochyta blight, a fungal disease caused by Ascochyta rabiei (Pass.) Labrousse [teleomorph, Didymella rabiei (Kovachevski) v. Arx] in chickpea (Cicer arietinum L.). This study determined the effect of cultivars varying in plant architecture and plant population density (PPD) on the severity of ascochyta blight. Four desi chickpea (with pinnate leaves) and four kabuli chickpea (two with pinnate leaves and two with unifoliate leaves) were grown at 25, 36, 44, 53, and 62 plants m–2 (actual counts 3 wk after initial seedling emergence) at Swift Current from 2002 to 2005 and at Saskatoon in 2004 and 2005. Site-years had a significant effect on ascochyta blight epidemics, with the highest severity at Swift Current in 2005 and lowest at Saskatoon in 2004. Across site-years, ascochyta blight was most severe on ‘Evans’, followed by ‘CDC Xena’, and lowest on ‘222B-11’. Cultivars with pinnate leaves had lower blight severity than those with unifoliate leaves during all growth stages. At the late-pod stage, severity in cultivars with pinnate leaves averaged 15% compared with 48% in unifoliate cultivars. Kabuli cultivars had higher severity than desi cultivars throughout the growing season, and at the late-pod stage, severity was 13% for the desi and 33% for the kabuli. There was a significant interaction between cultivar and PPD for blight severity. Ascochyta blight increased as PPD increased for the majority of the cultivars tested, with a few exceptions. Site-year accounted for the largest portion of the treatment variance in blight severity (69%), followed by cultivar type (25%), and then PPD (6%). Increasing PPD consistently increased seed yield per unit area, despite more disease on plants at higher PPD. Identifying optimum plant populations for groups of cultivars with similar plant architecture should be a component in an integrated strategy to minimize ascochyta blight in chickpea.

Abbreviations: PPD, plant population density


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CHICKPEA is one of the world's most important grain legumes; in 2003 the world production reached 7.1 million MG, ranking third behind dry bean (Phaseolus vulgaris L.), and dry pea (Pisum sativum L.) (FAO, 2004). This annual legume is a significant contributor to agriculture, because its N2–fixation reduces requirements for N fertilizer inputs, and also it contributes to the diversification of cereal-based cropping rotations. Seed yield of chickpea can vary from zero to >3600 kg ha–1, depending on the management of biotic and abiotic constraints.

Ascochyta blight is the most important constraint to chickpea production worldwide. The disease occurs in the major chickpea growing areas of the world (Akem, 1999; Khan et al., 1999; Kaiser et al., 2000; Chongo et al., 2003b). Buildup of inoculum in areas with intensive production of chickpea or with short crop rotations contributes to the development of blight epidemics. Chickpea infected by Ascochyta rabiei produces low seed yield with poor quality, and yield losses in susceptible cultivars can reach 100% (Reddy and Singh, 1990, p. 134). Economic losses due to the disease are substantial in many regions including Australia (Knights and Siddique, 2002), Canada (Chongo et al., 2003a), Latin America (Kaiser et al., 2000), southern Europe (Trapero-Casas and Jiménez-Díaz, 1986), the USA (Kaiser and Muehlbauer, 1989), and West Asia (Akem, 2001).

The development of integrated disease management strategies is the key for successful chickpea production (Gan et al., 2006). High levels of blight resistance are not available in current cultivars (Pande et al., 2005). Some cultivars have moderate resistance at the seedling stage, but the resistance declines as plants grow older (Chongo and Gossen, 2001). Also, the pathogen is genetically diverse (Chongo et al., 2004), and new pathotypes of the pathogen can appear that render moderately resistant cultivars susceptible (Akem, 1999; Pande et al., 2005).

Plant population density is a key component in optimizing the productivity of chickpea. The use of high PPD usually increases seed yield of chickpea in areas with a short growing season (Gan et al., 2003a; Regan et al., 2003), but the magnitude of the yield increase depends on environmental conditions (Lamb and Podder, 1998; Singh and Saxena, 1999). In areas where the growing season is short, the increased seed yield with increasing PPD is largely due to increased light interception of the crop canopy (Li, 2006). In arid to semiarid environments, the increased seed yield with higher PPD is largely due to improved water use and water use efficiency (Leach and Beech, 1988). The use of high PPD in chickpea production decreases soil water evaporation early in the growing season when plant canopy closure is low (Turner et al., 2001). In contrast, low PPD may allow weeds to develop more aggressively and limit crop yield potential. Plants grown at lower PPD are usually shorter and branchy, which increases losses during combine harvest (Jettner et al., 1999).

Plant density also affects disease risk. A high PPD produces a dense plant canopy that reduces air movement and evaporation, and increases shading and leaf wetness periods within the canopy (Siddique and Bultynck, 2004). Also, changes in row spacing and seeding rate change the proximity of individual plants and plant parts, which may influence the movement of pathogens from plant to plant. For example, narrower row spacing coupled with heavier canopies resulted in higher levels of damage by sclerotinia white mold or botrytis gray mold in chickpea (Bakr et al., 2002). Chickpea grown in wider rows had less ascochyta blight than when grown in rows with a narrower spacing (Akem, 2001). In a recent study, Chang et al. (2007) found that low PPD in chickpea reduced ascochyta severity and increased seed yield on a per-plant basis, and that the seed yield per unit area was lower with lower PPD due to a fewer seeds per unit area. However, little is known about the effect of PPD on ascochyta blight in chickpea cultivars with different leaf types and growth habits.

We hypothesized that the occurrence of ascochyta blight in a chickpea crop was influenced by environmental conditions, and that the severity of the disease depended on the interactive effects of cultivar type and PPD. Therefore, the objectives of this study were to determine (i) the effect of plant growth habit and cultivar type (i.e., cultivars with pinnate vs. unifoliate leaves; cultivars with branchy vs. erect growth habit, and desi vs. kabuli types) on ascochyta blight severity, and (ii) the response of ascochyta blight to varying PPD of chickpea in the short-season areas of the northern Great Plains.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sites and Experimental Design
Field experiments were conducted on an Aridic Haploboroll (Orthic Brown Chernozem silt loam) soil at the Agriculture and Agri-Food Canada Research Centre, Swift Current, SK, Canada (50.2° N, 107.4° W) from 2002 to 2005, and on a Typic Boroll (Dark Brown Chernozem clay loam) soil at Agriculture and Agri-Food Canada Research Centre, Saskatoon, SK, Canada (52.1° N, 106.41° W) in 2004 and 2005. The genotypes studied were four desi chickpea cultivars (CDC Anna, CDC Cabri, Myles, 222B-11), two kabuli cultivars with unifoliate leaf types (CDC Xena and Evans), and two kabuli cultivars with pinnate leaf types (Amit, CDC ChiChi). The main characteristics of these genotypes are summarized in Table 1 . The genotypes used in the study are those most popularly grown in the northern Great Plains (Saskatchewan Agriculture and Food, 2006). Seeds of all cultivars were from a Saskatchewan seed grower in the winter of 2001 except the line 222B-11 which was from the University of Saskatchewan chickpea breeding program. Laboratory tests showed <0.8% of ascochyta blight infection on the seed. A preseeding seed treatment was applied to all the seeds with 600 g a.i. each of carbathiin and thiabendazole, and 16 g a.i. of metalaxyl per 100 kg seed to minimize the impact of seed- and soilborne pathogens.


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  		Table 1. Characteristics of chickpea cultivars used in field experiments at Saskatoon and Swift Current, SK, 2002 to 2005.

 
In each of the six site-years, the experiment was laid out as a factorial, randomized complete block design with four replicates. The target PPDs were 20, 35, 50, 65, and 80 plants m–2. Seeding rates to achieve the target PPDs were determined based on preseeding germination, seed size, and an estimated field emergence rate of 75%. Plots were 1.2 x 6.0 m in size, consisting of eight rows with 15 cm between rows. A 6-m border of cultivated soil between blocks and a 2-m border between plots within a block were included to minimize interplot interference. The plot area was within 150 m of a field in which the blight-infected straw of a susceptible chickpea cultivar grown in the previous year had been left on the soil surface. This ensured that epidemics of ascochyta blight occurred without artificial inoculum of the pathogen. Air temperatures and precipitation were recorded with automatic weather stations located about 200 m from the plot area (Table 2 ).


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  		Table 2. Rainfall and mean air temperatures during the growing season at the six experimental sites in Saskatchewan.

 
Seeding and Plot Management
Seed was directly sown into standing stubble of spring wheat (Triticum aestivum L.) at a depth of 4 cm on dates when noon soil temperature at a 10-cm soil depth was between 10 and 14.6°C (Table 3 ). Plots were seeded with a disc drill equipped with a seed splitter. All plots received an application of 20 (Saskatoon 2004) to 35 (Swift Current 2005) kg ha–1 of 11–51–0–2 (N–P–K–S) fertilizer before seeding, and 5.5 kg ha–1 of Nitragin GC (LiphaTech Inc., Milwaukee, WI), a soil Rhizobium inoculant at seeding. Weed control was achieved using a preseeding burn-off treatment with glyphosate at a rate of 200 g a.i. ha–1, along with postemergent applications of sethoxydim at the rate of 0.21 kg ha–1 and metribuzin at the rate of 0.16 kg ha–1. Ascochyta blight was noted during the seedling stage in each year. To minimize the probability of a total crop failure due to the disease, azoxytrobin (Quadria, Syngenta Crop Protection Canada, Inc., Guelph, ON) was applied at 125 g a.i. ha–1 at the seedling stage, followed 10 to 14 d later by pyraclostrobin (Headline, BASF Canada, Mississauga, ON) at 150 g a.i. ha–1. The foliar fungicides were applied to the crop plots using 220 L ha–1 of water at 275 kPa pressure with standard flat fan nozzles. All crops received the same fungicide treatments to minimize any possible confounding effect from fungicide application.


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  		Table 3. Seeding date and soil temperature at seeding, and phenological data for chickpea cultivars grown at six experimental sites in Saskatchewan.{dagger}

 
Data Collection and Statistical Analysis
Stand establishment was assessed 3 wk after initial emergence by counting the number of plants in two randomly selected two-row sections of 50-cm length in each plot. Julian dates were recorded for seedling emergence (about 45–50% of seedlings emerged in a plot when the rows of plants were visible), flowering (about 15–20 counts of open flowers in a plot), and plant maturity (when >90% of the pods in a plot had turned brownish tan color, and seed moisture was 300–350 g kg–1). Days to emergence, flowering, and maturity were then calculated based on the Julian dates. Blight severity was assessed at the seedling, early flowering, late-flowering, and late-podding stages on 10 plants at each of five places in every plot. Assessments were based on the Horsfall-Barratt scale (Horsfall and Barratt, 1945), where 0 = no symptoms and 11 = 97 to 100% of leaf area affected. Ratings were converted to percentage area affected, and mean plot ratings at each rating date were used for statistical analysis. Disease incidence was not recorded because incidence for ascochyta blight in chickpea is highly correlated with severity (Gossen and Miller, 2004). About 7 to 10 d after plant maturity, or when the seed moisture was 160 to 180 g kg–1 (based on 20–25 bundles of hand-harvested plant samples), the center six rows of each plot (5.4 m2) were harvested with a plot-scale combine. Harvested seed samples were air-dried to consistent moisture, weighed, and seed yield per unit area presented on a dry-weight basis.

There was no animal interference or insect damage noticed on chickpea crop in any site-year except two plots at Swift Current in 2004, in which a gopher hole of {approx}80-cm diameter was noticed at the stage of plant flowering. Control measure was taken immediately and the plants in the affected spots were discarded and seed yield calculated based on the reduced plot size.

Data were analyzed using the MIXED procedure of SAS (SAS Institute, 1999). The fixed effects were cultivar, PPD, and their interactions, and the random effects were replicates and site-years. Variance components were estimated for each factor by analyzing the data set across site-years. Percentage variance estimate was calculated for the effects of site-years, cultivar type, and PPD; this giving rise to a relative assessment for each of the effects. In this calculation, variance estimates for blocks and experimental errors was excluded. A single degree of freedom contrast was used to determine the difference between pinnate and unifoliate leaf types, between desi and kabuli chickpea, and between cultivars with branchy and erect growth habit. Simple linear regression was used to describe the relationship between PPD and blight severity for site-years with the low-, moderate-, and high-blight infection separately. The same regression was also used to determine the relationship between PPD and seed yield per unit area for site-years with overall low-, moderate-, and high-yielding potential separately.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site and Year Effect
The growing season (May–August) precipitation (GSP) at Swift Current in 2003 was 30% below the 45-yr average (Table 2), whereas the GSP at Swift Current in 2002 and at Saskatoon in 2005 was 65 and 50% above the long-term average, respectively. At the rest of the sites, GSP was near normal, with one or more months varied from the long-term average. Chickpea plants at Swift Current in 2002 required an average of 135 d to reach full maturity from seeding, the longest among the six sites (Table 3). In contrast, chickpea at Swift Current in 2003 matured in <85 d. The large variations in environmental conditions experienced during the course of the study allowed the assessment of ascochyta blight responses to cultivar types and population densities under a wide range of environments.

On average, plant stand was greatest at Swift Current in 2003 where plants were seeded into a warmer seedbed (Table 2) and adequate soil moisture was available during the early growing season. Plant establishment was lowest at Saskatoon in 2004 (Table 4 ), due to dry soil surface at the time of seeding. At all site-years, actual plant stands (ranging from 21–70 plants m–2) were lower than planned targets (20–80 plants m–2); particularly, the high-density targets were undershot. Nevertheless, a wide range of plant densities was achieved for the purpose of this study. The optimum plant population for chickpea production is between 38 and 45 plants m–2 for the northern Great Plains (Gan et al., 2003a).


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  		Table 4. Mean plant population density (± SE) measured 3 wk after initial seedling emergence, across all cultivars per site.

 
Trial site and year also had a major influence on the severity of ascochyta blight (Fig. 1A ) and seed yield (Fig. 1B). Overall, the ratings of ascochyta blight were highest at Swift Current in 2005 and lowest at Saskatoon in 2004 (Fig. 1A). At Swift Current in 2005, the initial symptoms of ascochyta blight were observed 10 d before flowering on highly susceptible cultivars such as Evans and CDC Xena. At the other sites, the initial symptoms were not seen until the plants were approaching first flowering. Seed yield was highest at Swift Current in 2004 (Fig. 1B). The growing season at Swift Current in 2004 was long (Table 3) with adequate moisture available during the entire growth period (Table 2). At Saskatoon in 2004, the crops did not reach full maturity before the onset of late-fall frost due to exceptionally late seeding (Table 3), and thus no yield data were obtained.


Figure 1
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  		Fig. 1. (A) The severity of ascochyta blight and (B) seed yield in eight chickpea cultivars tested in Saskatchewan. Data are averaged across the five plant densities, bars are standard errors. (No yield data were obtained at Saskatoon in 2004 due to plant immaturity before the onset of late-fall frost.)

 
Cultivar Type
There were significant differences among cultivars in blight severity and seed yield (Fig. 1 and 2 ). Across site-years, ascochyta blight was most severe on Evans, followed by CDC Xena, and lowest on the genotypic line 222B-11 and CDC Cabri (Fig. 2). Differences in blight severity among cultivars were greatest at Swift Current in 2005, where severity reached 75% on CDC Xena and Evans (Fig. 1). Differences in blight severity were smallest at Saskatoon in 2004, where severity on all cultivars was <20% (Fig. 1). Across site-years, seed yields of the highly susceptible cultivars (Evans, CDC Xena) were significantly lower than more resistant cultivars (Fig. 2).


Figure 2
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  		Fig. 2. The relationship between the severity of ascochyta blight and seed yield for eight chickpea cultivars tested in Saskatchewan. The data were averaged across five site-years and five plant densities, with the bars being standard errors. (No yield data were obtained at Saskatoon in 2004 due to plant immaturity before the onset of late-fall frost).

 
On average, cultivars with pinnate (i.e., fern) leaves had significantly lower blight severity than those with unifoliate leaves at all the growth stages (Fig. 3A ). The difference between the two leaf types in the severity of ascochyta blight was smallest at the seedling stage, but increased as the plants developed. By the late-pod stage, blight severity in cultivars with pinnate leaves was 15%, but averaged 48% in unifoliate cultivars. Severity peaked at the late-flowering stage for the pinnate leaf cultivars, and then leveled off. In contrast, blight severity did not peak until the late-pod stage for the unifoliate cultivars.


Figure 3
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  		Fig. 3. Disease progress of ascochyta blight on (A) cultivars with pinnate vs. unifoliate leaf traits, and (B) desi vs. kabuli type of chickpea. Data were averaged across five plant densities (** indicates that the blight difference was significant at P ≤ 0.01 at a given growth stage).

 
Kabuli chickpea cultivars had a significantly higher severity of ascochyta blight than desi cultivars throughout the growing season (Fig. 3B). This difference in blight severity between the two chickpea types was consistent from seedling to late flowering stages. At the late-pod stage, mean blight severity was 13% for desi cultivars and 32% for kabuli cultivars. Also, blight severity was often lower in cultivars with an erect growth habit than those with a branchy growth habit, but it was inconsistent across site-years (data not shown).

Plant Population Effects
Simple linear regressions of ascochyta blight severity to PPD (actual plant counts ranging from 21–70 plants m–2) were made for all chickpea cultivars under conditions with low-, moderate-, and high-severity of ascochyta blight. There were significant interactions between cultivar and PPD in affecting blight severity. Under low blight severity conditions (i.e., at Swift Current 2002, Saskatoon 2004) where mean severity was about 12%, the severity did not have any response to PPD for all cultivars except Evans which had a negative association between blight severity and PPD (y = 25.5–0.1100x, r2 = 0.43**, where y is the disease severity in %, x is PPD, and ** represents the significant regression at P < 0.01).

Under moderate blight severity conditions (i.e., at Swift Current 2003 and 2004, and Saskatoon 2005) where mean severity was {approx}18%, six of the eight cultivars showed linear responses of blight severity to PPD as shown in Table 5 . Among the cultivars, CDC Cabri and CDC Xena had the greatest responses to PPD (largest b values). The cultivars CDC ChiChi and Myles did not have any response to PPD (data not shown).


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  		Table 5. Summary of linear regression of ascochyta blight severity on plant density in chickpea cultivars under conditions with moderate and high disease severity in Saskatchewan, Canada.

 
Under high blight severity conditions (i.e., at Swift Current 2005) where mean severity reached 35%, there were large differences among cultivars in response to PPD as shown in Table 5. These regressions showed that as PPD increased, the disease severity increased for cultivars Amit, CDC Anna, CDC Xena, Evans, and Myles, but decreased for CDC Cabri. There was no effect of PPD on blight severity for Genotypes 222B-11 or CDC ChiChi (data not shown). Evans had the greatest response to PPD.

Means across site-years showed that blight severity generally increased as PPD increased from 21 to 70 plants m–2, but the magnitude of the responses differed under varying growing conditions. For example, CDC ChiChi increased blight severity with increasing PPD at Swift Current in 2003, had no effect in 2004, and declined in 2005 (data not shown). Analysis of variance across site-years revealed that site-years accounted for the largest portion of the variance in ascochyta blight severity (69%), followed by cultivar type (25%), and then PPD (6%). Once symptoms of ascochyta blight developed in a field, susceptible cultivars quickly became infected, regardless of plant density.

Similarly, seed yield per unit area was a linear function of PPD for the majority of the cultivars studied, but the magnitude of the responses varied between cultivars. To illustrate the differences in the magnitude of the linear responses, four cultivars were selected from the Swift Current 2005 site and their seed yields (y) were plotted against PPD (x) (Fig. 4 ). It is apparent that Evans had overall lowest seed yield, but strongest response to PPD (greatest b value in the regression: y = 338 + 28.02x, r2 = 0.91). CDC Anna (y = 2842 + 23.07x, r2 = 0.79) and CDC Cabri (y = 2677 + 16.57x, r2 = 0.69) had greater linear responses with overall higher seed yields than CDC Xena (y = 1919 + 10.01x, r2 = 0.53).


Figure 4
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  		Fig. 4. Simple linear regression of seed yield to plant population (actual plant counts) for four chickpea cultivars selected from Swift Current in 2004, illustrating the differences in the magnitude of the linear responses among cultivars.

 
Growing conditions influenced the relationship between seed yield and PPD. At low-yielding environments (i.e., Swift Current 2003 and Saskatoon 2005, where the mean seed yield was 1040 kg ha–1), the linear regression was significant for the cultivars Amit, CDC Cabri, CDC Xena, and Myles, but was insignificant for the four others (data not shown). At moderate-yielding (i.e., Swift Current 2002 and 2005, where the mean seed yield was 1990 kg ha–1) and high-yielding (i.e., Swift Current 2004, with the mean seed yield of 3230 kg ha–1) environments, the linear responses of seed yield to PPD were significant for seven of the eight cultivars. The cultivar CDC ChiChi was the only one that did not show any yield response to PPD regardless of growing conditions.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
In the present study, nearly 70% of the explained variance in the blight severity of chickpea was associated with growing environments, 25% by cultivar type, and only a small (although often significant) proportion by plant population. Ascochyta blight severity was highest at Swift Current in 2005 (35%, averaged across all cultivars). Heavy rain and moderate temperatures during plant flowering (in June) may have shortened the time required to complete the pathogen's infection cycle and thus increased the number of cycles that occurred during the growing season. Moisture during the flowering stage of chickpea has been identified as the key factor affecting infection cycles of ascochyta blight (Trapero-Casas and Kaiser, 1992) and the severity of the disease (Armstrong-Cho et al., 2004).

Two applications of foliar fungicides were used in the present study; this decision was based on previous studies conducted in Saskatchewan, where the incidence of ascochyta blight on susceptible cultivars was as high as 100% (Chongo et al., 2003a). Many Saskatchewan producers experienced 100% crop losses in susceptible cultivars (Chris Stewart, Saskatchewan Crop Insurance Corporation, personal comm., 2006). Despite the two application of foliar fungicides which may have reduced overall blight infection substantially, large variability in blight severity existed among site-years and among cultivars. Chongo et al. (2003a) also demonstrated that cultivar differences in blight severity were intact even with multiple sprays of foliar fungicides. Under conditions of moderate to high inoculum pressure and growing seasons favorable for disease development, the spray of foliar fungicides may help minimizing the damage caused by the blight but may not provide total control of the disease (Gan et al., 2006).

Severity of ascochyta blight was consistently lower on cultivars with pinnate leaves than those with unifoliate leaves under the same growing conditions. Our results support previous observations that chickpea cultivars with compound leaves are more resistant than those with unifoliate leaves (Chongo et al., 2003a; Ahmed et al., 2006). In Australia, nearly all unifoliate genotypes have been dropped from production systems due to their high susceptibility to ascochyta blight (K.H.M. Siddique, 2007, personal communication). Plant architecture may play an important role in the difference between the two leaf types of chickpea in response to ascochyta blight. Cultivars with pinnate leaves have greater solar interception and higher biomass production than unifoliate lines (Li, 2006), which may help the crop compensate for damage caused by the blight disease. Also, temperature and relative humidity have a large influence on epidemics of ascochyta blight (Navas-Cortés et al., 1998; Armstrong-Cho et al., 2004). Leaf shape and orientation, together with growth habit, may influence the microclimate of plant canopy, which could affect pathogen infection and colonization. However, we did not measure microclimate of the canopy in this study. Cultivars with erect growth habit generally had lower blight severity than those with a branchy growth habit, but the response was not consistent across site-years.

Some researchers speculated that ascochyta blight resistance in unifoliate leaf genotypes of chickpea could be improved by selecting those lines that secrete certain types of acids such as malic acid at a lower level on leaves and therefore would act less like an agar plate in trapping ascospores from the atmosphere. However, Armstrong-Cho and Gossen (2005) demonstrated that these acids accumulated on leaves of chickpea affected spore germination, but they did not play an important role in impacting ascochyta blight colonization. These acids were easily washed off by rain drops and became absent when infection actually occurred.

In the present study, blight severity was lower in desi than in kabuli cultivars. This difference might be related to plant architecture. Desi cultivars tended to grow more erectly with more open spacing within a plant canopy, whereas kabuli cultivars, especially those with unifoliate leaves, tended to shade more between leaves. Differences in leaf arrangement, leaf angle, and petiole length allow greater light penetration into the canopy in desi than in kabuli cultivars (Li, 2006). More research is needed to examine possible genetic linkages between plant architecture and susceptibility to ascochyta blight in chickpea.

In some cultivars, ascochyta blight severity was higher at high population densities, but there was no association between blight severity and plant population for other cultivars, especially when overall blight infection was low. These results indicate that genetic differences in plant architecture and canopy characteristics in chickpea may play a role in influencing ascochyta blight severity. Plant populations required for minimizing disease severity and maximizing yield potential may vary depending on plant architecture for specific cultivar type. Optimal plant density should be identified for groups of cultivars with similar plant architecture. At high population densities, the close proximity of host plants increases the number of plants available to intercept inoculum and reduces the chance of inoculum being lost to the ground (Burdon and Chilvers, 1982). This can be the case where the main source of inoculum is from ascospores which often spread across fields. Also, high population density tends to maintain a more humid microclimate within the crop canopy, especially under subhumid to humid environments, which favors blight development (Siddique et al., 1998). However, in the semiarid environment of the northern Great Plains, rainfall during flowering is generally required to trigger new infection cycles of ascochyta blight in chickpea.

In the present study, seed yield per unit area was linearly associated with plant population for the majority of the cultivars studied. These results agree with previous findings in chickpea (Jettner et al., 1999; Gan et al., 2003a; Regan et al., 2003). The increased seed yield with high plant population is attributable to the production of more pods (Gan et al., 2003b) and more seeds per unit area (Regan et al., 2003), despite more disease on individual plants (Chang et al., 2007). Pulse plants grow slowly during the early part of the seedling growth period when soil water losses through evaporation can be substantial. Increasing plant population may help reduce early season evaporation losses due to fast ground cover (Martin et al., 1994). Moreover, pulse plants at a high population density intercept more light, produce more biomass, and remobilize more photosynthates to the seed during the reproductive growth period compared with plants grown at lower density (Leach and Beech, 1988). Lower seed yield at lower population density was probably due to limited elasticity of chickpea for compensatory growth. Seed yield on a single plant basis usually increases when plant population was low, but the increases will be limited due to limited growth resources such as soil water and nutrients. Also, chickpea is a relatively weak competitor with weeds, and the crop at low plant populations often has a reduced yield potential due to heavy weed infestation.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The study at six site-years with highly variable growing conditions revealed that environmental conditions had the largest influence on ascochyta blight severity in chickpea, followed by cultivar type and then PPD. Under the same growing conditions, cultivars with compound leaves had consistently lower blight severity than cultivars with unifoliate leaves. Similarly, desi-type cultivars had lower severity of ascochyta blight than kabuli-type cultivars. These differences may not be due solely to inherent disease resistance. Rather, it may be the result of slower disease spread associated with differences in plant canopy structure. Plant architecture may play an important role in the severity of ascochyta blight in chickpea, but further research is needed to quantify this effect and to examine whether it is solely based on disease escape or whether different resistance mechanisms are involved. High plant population densities are required to maximize seed yield in chickpea, even though this study showed that the severity of ascochyta blight increased with increasing population density for some cultivars. Recommendations on optimum plant population to minimize blight severity and maximize seed yield need to be established for groups of cultivars with similar plant architecture. Adoption of an integrated management strategy that includes the selection of field sites with minimum levels of blight inoculum, choice of genotypes with favorable plant architecture, and use of optimal plant densities, will help minimize losses caused by ascochyta blight in chickpea.


   ACKNOWLEDGMENTS
 
The authors acknowledge the financial support of the project by the Saskatchewan Pulse Growers, Agricultural Development Fund of Saskatchewan Agriculture and Food, and Agriculture and Agri-Food Canada Matching Investment Initiative. We also thank Cal McDonald, Lee Poppy, Ray Leshures, and Ken Bassendowski for expert technical assistance.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution no. MS20070201.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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