Published in Crop Sci. 43:2291-2294 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
NOTES
SEVERITY OF ASCOCHYTA BLIGHT IN RELATION TO LEAF TYPE IN CHICKPEA
Yantai Gan*,a,
Puhai Liub and
Calvin McDonalda
a Semiarid Prairie Agric. Res. Centre, Agric. and Agri-Food Canada, Swift Current, SK S9H 3X2, Canada
b Dep. of Water Resources Engineering, Gansu Agric. Univ., Lanzhou, Gansu, 730070. P. R. China
* Corresponding author (gan{at}agr.gc.ca).
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ABSTRACT
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Severity of Ascochyta blight (Ab) [caused by Ascochyta rabiei (Pass.) Labrousse] in chickpea (Cicer arietinum L.) can be reduced by use of cultivars with a desirable plant architecture. Field experiments were conducted in semiarid southwestern Saskatchewan in 2001 and 2002 to determine the relationship between Ab severity and leaf shape in chickpea. Seed was treated with thiabendazole [2-(thiazol-4-yl)benzimidazole], and metalaxyl [N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-DL-alanine methyl ester] to reduce seed-borne diseases, and plots were sprayed with chlorothalonil (tetrachloroisophthalonitrile) and azoxystrobin [methyl (E)-2-{2-[6-(2-cyanophenoxy) pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate] to minimize leaf diseases. Chickpea with fern leaves were compared with those with unifoliate leaves for their susceptibility to Ab. No diseases were observed during the seedling stage. As the crop approached preflowering, Ab symptoms became evident and the differences in Ab severity became great between fern- and unifoliate-leafed chickpea. Measured at the bloom stage in 2001, Ab severity was 36% for unifoliate-leafed chickpea and 12% for fern-leafed chickpea. In the cool, moist year of 2002, the disease was severe with the unifoliate-leafed chickpea having an Ab severity of 95%, compared with 35% for the fern-leafed chickpea. The relative rankings of Ab severity between the two leaf types remained the same even when plant population was increased from 21 to 76 plants m-2. Chickpea producers in the semiarid northern Great Plains should select cultivars with a fern leaf shape to reduce the Ascochyta disease pressure and minimize disease damage to the crop.
Abbreviations: Ab, Ascochyta blight
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INTRODUCTION
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EMPHASIS ON SOIL HEALTH, environmental quality, and economic innovation has stimulated significant changes in cropping systems throughout the semiarid northern Great Plains (Gan et al., 2002; Miller et al., 2002). In these regions, producers have been diversifying their cropping systems by including nontraditional crops such as chickpea. The area seeded to chickpea in Saskatchewan, for example, has increased from about 6000 ha in 1995 to about 400000 ha in 2001 (Anonymous, 2001). The favorable climate and soil conditions in these regions make chickpea a good alternative for crop diversification (Gan et al., 2002). However, chickpea production is facing a significant challenge in the management of Ascochyta blight caused by Ascochyta rabiei (Pass.) Labrousse (Nene and Reddy, 1987). This disease has become a major threat to chickpea production in the last few years. The fungus is well established in many local chickpea fields. Seed lots, crop residues, and volunteer chickpea plants also spread the pathogen. The widespread occurrence of this disease causes chickpea crop losses as high as 25% in 2000 and >30% in 2001 in Saskatchewan (Chris Steward, 2002, Personal Communication).
Initial infections by this disease may come from seed-borne inocula, overwintered infested residues, or spread by wind-borne ascospores (Armstrong et al., 2001). In the field, infections begin low in the crop canopy during the early growing season but plants can be infected at any growth stage (Chongo and Gossen, 2001). All plant parts above the soil surface can be infected and may develop elongated, sunken, dark lesions. Lesions often girdle stems, weaken branches and petioles, and cause breakages. The disease may kill all plant parts above the girdled stem. The fungus produces fruiting bodies (i.e., pycnidia) that appear as tiny, black spots, often arranged in concentric rings within the lesions. These pycnidia produce conidia that are water-splashed and inoculum for the current season or the subsequent seasons. Sometimes, the blight first appears in small patches of plants in the field, and these patches rapidly increase in size with lesions throughout the crop canopy. Sometimes, the disease is evenly distributed throughout the field from the very start of the disease infection. Under favorable conditions, the pathogen infects the leaves, stems, and pods. Pod infections ultimately lead to seed infection with heavily infected seeds bearing visible symptoms such as small size, wrinkles, or dark discoloration. Cool, moist conditions accelerate infection and the spread of the disease throughout the entire field.
This devastating disease is of concern to chickpea producers in the semiarid northern Great Plains. Systematic management practices are needed to minimize the disease pressure and maximize production returns. Two leaf types of chickpea are currently grown in these regions: one has ‘fern’ leaves with multiple leaflets attached to a leaf petiole, and the other has ‘unifoliate’ leaves in which a single, large leaf is attached to the leaf petiole. A disease survey conducted in Saskatchewan, 2002, indicated that cultivars with fern leaves were less susceptible to Ab than those with unifoliate leaves (Chongo et al., 2002). However, these observations were made from producer's fields and it was impossible to discriminate the confounding effects of cultivar type from management practices, weather conditions, or field locations. Information is needed to determine whether Ab severity in chickpea is related to morphological characteristics such as leaf shape. The objective of this study was to determine the relationship between leaf type and the Ab severity of chickpea grown in a semiarid environment in the northern Great Plains.
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Materials and Methods
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The field experiment was conducted in 2001 and 2002 at the Semiarid Prairie Agricultural Research Centre, near Swift Current, SK. The study included eight chickpea cultivars; four of them being fern leaf type (cv. Amit, CDC ChiChi, CDC Chico, and CDC Yuma) and the four others with unifoliate leaf types (cv. CDC Diva, CDC Xena, Evans, and Sanford). These cultivars were chosen because of their popularity among chickpea growers in the semiarid northern Great Plains (Anonymous, 2002). The experiments were planted in a randomized complete block design with four replications.
Before planting, seed was treated with 600 g a.i. each of carbathiin (5,6-dihydro-2-methyl-N-phenyl-1,4-oxathiin-3-crboxamide) and thiabendazole, and 16 g a.i. of metalaxyl per 100 kg seed to minimize infections by seed-borne and soil-borne pathogens. Seed was sown at a depth of 3 to 4 cm on 2 May 2001 and 14 May 2002, when the noon soil temperature at a 10-cm depth was between 9 and 13°C. Seeding rate was 60 viable seeds m-2, targeting a plant population of 45 plants m-2, the recommended plant population density for chickpea (Gan et al., 2003). Plots were planted with a 2-m-wide disc drill equipped with a seed splitter. Each plot consisted of four rows, 6 m in length with 29-cm-row spacing. All plots received 26 kg ha-1 (in 2001) and 33 kg ha-1 (in 2002) of 11-51-0-2 fertilizer approximately 2 wk before seeding, and also received 5.5 kg ha-1 (in both years) of ‘Nitragin’, an appropriate soil implant Rhizobium inoculant (a granular form) for symbiotic N fixation (Lipha Tech Inc. Saskatoon, Canada) at seeding. The Rhizobium inoculant was applied in the seed rows. Weeds were controlled using a preplanting application of ethalfluralin [N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoro-methyl)benzenamine], a preemergent application of glyphosate [N-(phosphonomethyl)glycine], and a post emergence application of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one} applied at recommended rates. Before flower, the plants were sprayed with the fungicides chlorothalonil and azoxystrobin (Syngenta, Canada) with the recommended rate at 222 L ha-1 water volume with 275 kPa pressure with standard flat fan nozzles.
The severity of Ab was assessed at the full flower stage, by the Horsfall-Barratt (HB) scale (Horsfall and Barratt, 1945). Disease severity was rated in a scale from 0 to 11, with 0 representing no symptoms and 11 representing the whole plant diseased. These ratings were then converted to percentages with a disease severity rating of 1 being converted to 2.3% and 11 to 99%. Disease severity ratings were conducted in two (2001) to five (2002) spots in each plot and average was calculated for each plot.
A second experiment was conducted at the same site on an adjacent field to examine whether the relative rankings of Ab severity between the two leaf types found in the main experiment would hold true when plant population density was increased. The same set of eight cultivars used in the first experiment was planted at 30, 45, 60, 75, and 85 viable seeds m-2, with the last two planting rates being 25 and 42% higher, respectively, than the recommended rate. The same fungicides used in the first experiment were applied three times (before flower, at full bloom, and early podding stages) to minimize leaf blight. The Ab severity was assessed by the same method as in the main experiment. In addition, dry matter yield was determined at preflower, full-flower, and late-podding stages to determine the productivity of crops with different leaf type. Analysis of variance was performed on the data set using the GLM procedure of SAS (SAS Inst., 1996), with blocks as a random effect and leaf types as fixed effects. Means were separated by Fisher's protected least significant difference (LSD) test at the P = 0.05 level. A single degree-of-freedom contrast was used to determine significance between the two leaf types.
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Results and Discussion
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The growing season (May–August) precipitation in 2001 was 121 mm, which is lower than the 40-yr (1961–2000) average (204 cm), whereas the precipitation in 2002 (340 mm) was 60% greater than the long-term average. Mean air temperature during the 2001 growing season was 16.9°C, 1.2°C higher than the 40-yr average, whereas the 2002 mean temperature was 0.9°C lower than the long-term average. The cool and moist growing conditions of 2002 provided favorable conditions for disease development. Despite the application of fungicides, Ab infection in chickpea was moderate in 2001 and severe in 2002 with no artificial inoculation to the plots. The two contrasting growing seasons provided an excellent environment for evaluating the susceptibility to Ab of the two leaf types (fern and unifoliate) of chickpea.
In both years, there were no differences in Ab severity between the two leaf types at the seedling stage (data not shown). As the crop approached the preflowering stage, the differences in Ab severity gradually became evident. Initial symptoms of Ab were present on plants approximately 45 d after seeding in 2001 and 54 d in 2002. As the crop reached full bloom, the Ab disease became severe. In 2001, the average Ab severity was 36% for chickpea with unifoliate leaves, which was significantly greater than that for fern leaf chickpea (12%) (Table 1)
. The Ab severity was much greater under the cool, moist growing conditions of 2002 compared to 2001, but the relative ranking of Ab severity between the two leaf types was the same in 2002 as in 2001. In 2002, fern leaf chickpea had an average Ab severity of 35%, whereas the Ab severity for the unifoliate leaf chickpea was 95%. Among the fern leaf cultivars studied, Amit had the lowest Ab severity in both years. Among the unifoliate leaf cultivars, CDC Diva had lower Ab severity than Evans or Sanford. Although variations in Ab severity existed among individual cultivars within a leaf type, these variations were much smaller than variations found between the two leaf types.
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Table 1. Ascochyta blight (caused by Ascochyta rabiei) severity at the bloom stage, days from seeding to flowering, and plant height for chickpea with fern and unifoliate leaf shapes, grown in southwestern Saskatchewan, 2001-2002.
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Both fern and unifoliate leaf chickpea required a similar number of days from seeding to flowering, and had a similar plant height (Table 1). Neither time of flowering nor plant height influenced the relative ranking of Ab severity between the two leaf types.
A second experiment was conducted in 2002 only in which two leaf types of chickpea were compared for their susceptibility to Ab under different plant population densities. This experiment was to determine whether the relative rankings of Ab severity between the two leaf types found in the main experiment remained true under various plant population densities. The results of the second experiment (Table 2)
were similar to those obtained in the first experiment (Table 1). The fern leaf chickpea had significantly lower Ab severity than the unifoliate leaf chickpea measured at both flowering and late-podding stages at all plant population densities (Table 2). Actual plant population ranged from 29 to 76 plants m-2 in the second experiment. The changes of plant population did not alter the relationship between Ab severity and leaf type, even though dry matter yield differed significantly between the two leaf types at different growth stages. Worth noting is that the Ab severity of unifoliate leaf chickpea was significantly greater at the lower plant population density at the podding stage, suggesting that Ascochyta disease on unifoliate leaf chickpea is more destructive when plant population is low. Although high plant population reduced Ab severity at the late-podding stage for the unifoliate leaf chickpea, such a reduction in Ab severity did not alter the overall rankings of Ab severity between the two leaf types. The Ab severity was noticeably lower in the second experiment compared with the first experiment, because fungicides were applied three times in the second experiment but one in the first.
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Table 2. Ascochyta blight (caused by Ascochyta rabiei) severity and dry matter yield of chickpea with fern or unifoliate leaves grown at different plant population densities in southwest Saskatchewan, 2002.
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Results of this research show chickpea with fern leaves had significantly lower Ab severity than those with unifoliate leaves under semiarid growing conditions. These results agree with the observations made on producers' fields (Chongo et al., 2002). In semiarid Australia, most unifoliate types of chickpea have been discarded from production systems because of their high susceptibility to Ab (Kana and White, 2002, personal communication). In our study, the large differences in Ab severity between the two leaf types were consistent regardless of precipitation levels or changes in plant population density. We did not examine the mechanisms responsible for the large differences in Ab severity between the two leaf types, nor did we make any conclusions on the effect of plant population density on Ab severity. We speculate that the plant architecture may differ between the two leaf types of chickpea, resulting in differences in canopy microclimate. The latter would affect radiation interception, relative humidity, and thereby disease development. Navas-Cortés et al. (1998) observed that temperatures and relative humidity had a large influence on D. rabiei on chickpea. Detailed research is needed to elucidate the effect of plant architecture and canopy structure on Ab in chickpea. Seed yield was not measured in this study, because a negative correlation between Ab severity and seed yield in chickpea has been widely realized in producers' communities. Our results were based on phenotypic observations on a limited number of cultivars only. Further research is needed to examine possible genetic linkages between leaf shape and Ab susceptibility in chickpea. To study the possible genetic linkage between leaf shape and Ab susceptibility, one could cross two genotypes with different leaf types and Ab resistance levels and then study the segregation of parental types and hybrids with regard to the two traits. However, this type of study is far beyond the scope of the experiment being reported in this paper. Our results strongly suggest that chickpea breeders need to focus more on fern leaf genetic materials for developing Ascochyta resistant cultivars. Producers are strongly encouraged to select chickpea cultivars with a fern leaf type to reduce Ascochyta disease pressure and minimize disease damage to the crop.
Received for publication January 20, 2003.
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REFERENCES
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- Anonymous. 2001. Chickpea: Situation and outlook. Biweekly Bull., Vol. 14, No.3. Agriculture and Agri-Food Canada, Ottawa, ON.
- Anonymous. 2002. Varieties of grain crops. In 2002 Saskatchewan Seed Guide. Saskatchewan Agriculture, Food, and Rural Revitalization, Regina, SK.
- Armstrong, C.L., G. Chongo, B.D. Gossen, and L.J. Duczek. 2001. Mating type distribution and incidence of the teleomorph of Ascochyta rabiei (Didymella rabiei) in Canada. Can. J. Plant Pathol. 23:110–123.
- Chongo, G., and B.D. Gossen. 2001. Effects of plant age on resistance to Ascochyta rabiei in chickpea. Can. J. Plant Pathol. 23:358–363.
- Chongo, G., S. Banniza, and T. Warkentin. 2002. Occurrence of Ascochyta blight and other diseases in Saskatchewan in the 2001 drought year. Can. Plant Dis Surv. 83:85–89.
- Gan, Y., P.R. Miller, P.H. Liu, F.C. Stevenson, and C.L. McDonald. 2002. Seedling emergence, pod development, and seed yields of chickpea and dry pea in a semiarid environment. Can. J. Plant Sci. 82:531–537.
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- Nene, Y.L., and M.V. Reddy. 1987. Chickpea diseases and their control. p. 233–270. In M.C. Saxena and K.B. Singh (ed.) The chickpea. C.A.B. International, Oxfordshire, UK.
- SAS Institute, Inc. 1996. SAS/STAT user's guide. Version 6, the 4th ed. SAS Institute, Inc., Cary, NC.
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ABSTRACT
INTRODUCTION
