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a Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1030, Swift Current SK, S9H 3X2
b Indian Head Research Farm, Agriculture and Agri-Food Canada, P.O. Box 760, Indian Head SK, S0G 2K0
c 142 Rogers Rd., Saskatoon, SK S7N 3T6
* Corresponding author (fernandezm{at}agr.gc.ca).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: C, cereal • CRR, common root rot • CRRI, common root rot index • CT, conventional-till • F, summer fallow • FHB, Fusarium head blight • MT, minimum-till • NC, noncereal • SI, subcrown internode • ZT, zero-till
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Fusarium head blight (FHB) in barley has been established in the eastern Prairies for the last decade (Tekauz et al., 2000). Fusarium graminearum and F. avenaceum were the most common FHB pathogens of barley crops grown in 2005 in Manitoba (Tekauz et al., 2006), whereas F. avenaceum has consistently been one of the most commonly isolated species from FHB-affected barley crops in Saskatchewan, where this disease has occurred at lower levels than in eastern regions of the Prairies (Pearse et al., 2006). Because of concerns regarding increasing FHB development on the eastern Canadian Prairies and its apparent spread westward, it is essential to put in place a comprehensive strategy to stop or reduce the rate of spread of this disease and to decrease the damage it has been causing to the barley industry in areas where it is already well established. To this end, there is a need for more information on the epidemiology of FHB in Saskatchewan so that the risk factors associated with its spread and development can be better understood. A comprehensive approach needs to include an examination of crown and root rot caused by Fusarium spp. in this region. Fusarium infection of ground and underground barley tissue could result in higher fungal levels in crop residues and thus be a source of inoculum for spike infection and fungal carryover from one season to the next. A better understanding of all factors affecting Fusarium inoculum and infection of barley tissue may help in devising a more effective strategy aimed at reducing inoculum levels and disease development and preventing the further spread of important cereal diseases caused by Fusarium spp.
Few studies have been conducted on the impact of agronomic practices, such as tillage system and crop rotation, on CRR of barley and associated fungal populations. Windels and Wiersma (1992) reported higher levels of C. sativus, and lower levels of F. avenaceum and F. graminearum, going from less to more intensive tillage. Common root rot in barley decreased when grown after 2 yr of oilseed crops (Conner et al., 1996; Piening and Orr, 1988), whereas barley grown following summer fallow had higher CRR, mostly attributed to C. sativus (Piening et al., 1976), and lower levels of Fusarium spp. in crowns than when grown following a crop (Sturz and Johnston, 1985). In recent years, Canadian Prairie producers have become more reliant on noncereal crops, including oilseeds and pulses, and have increasingly adopted more continuous cropping and greater use of conservation tillage practices. It is therefore of interest to determine the impact of currently popular cropping sequences and tillage systems on fungal populations in underground tissue of barley crops.
The objective of the present study was to determine CRR levels in barley crops grown in eastern Saskatchewan, identify and quantify fungal species from infected tissue, and determine the association between disease and fungal levels and crop production systems, with the aim of determining what crop production factors might reduce Fusarium infections in barley crops.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Sampling for Common Root Rot Evaluation
In late July to early August, a total of 35 to 50 plants at approximately the mid-milk to dough stage (growth stages 75–83, Zadoks et al., 1974) were carefully pulled from at least 15 spots randomly selected within each field following a large circular pattern, starting about 40 m from the edge of the field. Each plant sampled had an SI of at least 2 cm. Samples were washed under tap water, thoroughly dried, and kept at room temperature until analyzed. Subcrown internodes were carefully removed and rated for extent of brown to black discoloration on a 0 to 3 scale (0 = no discoloration, 1 = slight with <25% of the surface area discolored, 2 = moderate with 25 to 50% discoloration, and 3 = severe with >50% discoloration (Ledingham et al., 1973). An SI discoloration index (common root rot index; CRRI) was calculated for each field based on the incidence and severity of the discoloration, as follows: [(
category value x plants in category)/total number of plants sampled)] x 100. The most discolored segment (about 1 cm2) of each SI was then excised, surface-disinfested for 1 min in 0.6% NaOCI, and rinsed in sterile distilled water twice. Tissue pieces were then plated on modified potato dextrose agar (Burgess et al., 1988; Fernandez and Chen, 2005) and incubated under cool-white fluorescent and near-UV lights (16 h light/8 h dark) (light intensity of 110 µmol s–1 m–2) for about 7 d. Fungi growing out of the tissue pieces were identified on the basis of colony and spore morphology using descriptions and keys in Samson et al. (2002) and Watanabe (2002). All isolates identified as F. graminearum produced perithecia in culture. Percentage isolation of each fungus was calculated based on the total number of isolates in each field.
Categorization of Barley Crops/Fields into Crop Production Factors
At the end of the growing season, each producer supplied information regarding grain yield and the agronomic practices related to the crop(s) sampled, such as cultivar, crop history, tillage method, and pesticide use. This information was used to categorize the crops/fields according to crop production factors, which was then used for further analysis to determine the association of these factors with CRRI and percentage fungal isolation.
For tillage system, fields were categorized on the basis of the total number of tillage operations performed in the previous 3 yr. Fields under conventional till (CT) had a total of seven or more tillage operations, and those under minimum till (MT) had one to six operations (i.e., up to two tillage passes per year). There were no tillage operations in fields under zero-till (ZT) management during the same period of time. Residue cover was not estimated for any field. The average number of tillage operations in the previous 3 yr was 3.4 for fields under MT and 8.2 for fields under CT. Herbicide applications were categorized according to whether the fields had received any of the Group 1, 2, 4, or 9 herbicides (Saskatchewan Agriculture and Food, 2006) in the previous 18 mo.
For previously grown crops, fields were categorized according to the crop, if any, grown the previous year: cereal, oilseed, pulse, or summer fallow. Fields were also categorized according to the crops, if any, grown the previous 2 yr, regardless of the order in the sequence: two cereal (C) crops (C–C), two noncereal (NC) crops (NC–NC), a cereal and a noncereal crop (C–NC), or summer fallow (F) and a crop (C–F or NC–F). In addition, fields were also categorized according to whether the first crop in the C–NC sequence was a cereal or an oilseed (O) crop (i.e., C*–NC for cereal as the first crop, O*–C for oilseed as the first crop).
The most common cropping practice the year before barley was to grow oilseed crops (42% of all fields), namely, canola (Brassica spp.) (35%) and flax (Linum usitatissimum L.) (6%). This was followed by cereal crops (38%), the most common of which were common wheat (Triticum aestivum L.) and durum wheat (T. turgidum L. var. durum) (15%), barley (13%), and oat (Avena sativa L.) (9%), followed by summer fallow (11%), and pulse crops (8% of all fields). The most common cropping practice 2 yr previous to the sampling was to grow cereal crops, namely, common and durum wheat (29% of all fields), barley (12%), or oat (9%). This was followed by oilseed crops, canola (20%) or flax (7%), summer fallow (15%), and pulse crops (4% of the fields).
Most barley crops grown after a cereal or oilseed crop were under MT management (60–65%), those after a pulse crop were under either MT (33%) or ZT (42%), and most crops grown after summer fallow were under CT (44%) or MT (56%). When fields were categorized according to the two previously grown crops, most barley crops grown after C–C or C–NC were under MT (61–75%), most grown after NC–NC were under ZT (64%) or MT (36%), and most grown after C–F or NC–F were under MT (53–60%) or CT (35–47%).
Based on the N fertilizer input in the spring and/or previous fall, on average, barley crops preceded by a cereal or an oilseed crop received the highest N rate (mean of 65–66 kg ha–1), followed by those grown after a pulse crop (56 kg ha–1), while crops grown after summer fallow received the least N (33 kg ha–1). Based on the categorization of fields according to the previous two crops, barley grown after C–NC received the most N (mean of 69 kg ha–1), followed by C–C, NC–NC, and NC–F (53–55 kg ha–1), with crops grown after C–F receiving the least N (36 kg ha–1). No soil N testing was performed on any of the fields sampled.
Statistical Analyses
Disease-, fungal- and grain yield–related responses were compared with SAS's SURVEYREG procedure, and means were estimated with the SURVEYMEANS procedure (SAS Institute, 1999). The analysis included the barley crop type (two- or six-rowed) cross-classified with previous crop(s) or tillage systems. Data collected for each year were assumed to be stratum for the analysis. The rate at which each stratum was sampled was based on the actual number of samples divided by the total number of barley producers in Saskatchewan (assumed to be one half of 11130 for barley; derived from http://www.statcan.ca/english/Pgdb/agrc22i.htm). Treatment effects were declared significant at P 
0.10. Contrasts were performed among cropping sequences and tillage systems for total disease level (CRRI) and percentage of the most commonly isolated fungi. Only the contrasts that showed significant effects for at least one of the fungi or CRRI are presented in the tables. Fungi for which no significant effects of previous crop(s) or tillage method were determined are also not included in the tables.
The impact of previous application of herbicide Groups 1, 2, 4, and 9 in each of the tillage systems was further examined for disease-, fungal- and grain yield–related variables. Differences among the herbicide treatments (applied: yes/no) for the 18-mo data were analyzed separately with SAS's SURVEYREG procedure, and means were estimated with the SURVEYMEANS procedure (SAS Institute, 1999). Effects were again declared significant at P 0.10.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Effects of Crop Production Factors on Common Root Rot and Fungal Isolations
The CRRI was significant for crop type, being higher in two- than in six-rowed barley cultivars (Table 3). However, there was little difference among crop types for percentage fungal isolations. Fusarium culmorum was the only species present at significantly higher levels in six-rowed barley, whereas F. equiseti was isolated most frequently from two-rowed barley. There were no significant interactions of crop type with previous crop or summer fallow, whereas there were some significant interactions of previous 2-yr cropping sequence with crop type; however, these did not result in changes in rank order. Because of the small sample size for some of the cropping sequences for two- and/or six-rowed barley, only the analysis based on all crops is presented.
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Overall, growing a noncereal crop the previous year resulted in higher relative levels of Fusarium spp. in barley SIs than growing a cereal crop, with barley grown after a pulse crop having the highest levels (Table 3). Fusarium spp. were also most frequently isolated when no cereal was present in either of the previous 2 yr (i.e., NC–NC or NC–F). Regarding the individual Fusarium spp., there was also a tendency for isolation of F. avenaceum from SIs to be the lowest in barley grown after a cereal crop than after a noncereal crop, whereas the highest percent isolation of F. culmorum occurred when barley was grown after a pulse crop. The latter fungus was the only Fusarium species present at significantly lower levels when barley was grown after summer fallow than after any crop, especially pulses, and was also lowest after C–F. In contrast, F. equiseti tended to be more commonly isolated after summer fallow than after a crop, and its percentage isolation was lowest for cropping sequences that included a cereal and a noncereal crop (C–NC). Fusarium graminearum was also present at higher levels in barley crops grown after a noncereal than a cereal crop. Furthermore, when the 2-yr cropping sequence C–NC was further classified by the first crop in the sequence (i.e., C*–NC vs. O*–C), total Fusarium spp. and F. graminearum were significantly higher when the previous crop was an oilseed (O*–C) than when it was a cereal (C*–NC). There were few differences for M. bolleyi related to previous cropping sequence; this fungus was present at significantly lower levels only after the NC–F sequence.
Analysis of tillage system effects was done for all barley crops regardless of cropping sequence, and also separately for barley preceded by a cereal or an oilseed crop (Table 4). For the most part, there was no interaction of tillage system with crop type. The CRRI was higher under CT than reduced tillage for all crops combined. In most cases, C. sativus was more common, and Fusarium spp. less common, in SIs of barley grown under CT than reduced tillage. This appears to be attributed mostly to higher levels of F. avenaceum and F. graminearum under reduced tillage. For barley grown after a cereal crop, F. graminearum was also found at the highest levels under MT management. Fusarium culmorum was isolated at lower levels under ZT than MT and/or CT when barley was grown after a cereal or an oilseed crop. Fusarium equiseti was also present at lower levels under CT after a cereal crop, whereas M. bolleyi was lowest under CT when barley was grown after a cereal or an oilseed crop.
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Herbicide analysis (yes/no) was done by tillage system, although sample size was larger for MT- than for CT- or ZT-managed fields (Table 5). For the most part, there was no significant interaction of herbicide group with barley crop type (two- or six-rowed). Further, there was no significant effect of herbicide group on CRRI under MT, with significant effects of Group 4 on CRRI for barley under CT and ZT not being consistent. For all herbicide groups, there were significant negative and positive effects of herbicide applications in the previous 18 mo on the most common fungal isolates. For barley grown under MT, Group 1 herbicides were associated with significantly lower levels of total Fusarium spp. and F. culmorum, whereas Group 9 herbicides were associated with higher levels of total Fusarium spp., F. culmorum and F. graminearum, but lower levels of C. sativus. There was also a tendency for levels of F. avenaceum to be higher in sprayed than unsprayed fields for Group 9 but lower for sprayed than unsprayed fields for Group 1 herbicides. The same effects of Group 9 herbicides on fungal isolations observed under MT were in most cases also observed under CT and/or ZT, although in most cases, these were not significant (P > 0.10), except for F. avenaceum for barley under ZT, which was present at higher levels in sprayed than unsprayed fields. In contrast, for Group 2 and 4 herbicides, there were generally lower levels of Fusarium spp. in sprayed than unsprayed fields under CT and/or ZT management.
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Grain Yield
Barley grain yield was greater for six-rowed than for two-rowed barley (Table 6). Barley preceded by summer fallow, C–F, or C–C had significantly lower yields than after a crop or other 2-yr sequences. Barley preceded by an oilseed crop or by two noncereal crops had the highest yields; these were also higher after O*–C than after C*–NC. There were no significant effects of tillage system on grain yield (data not presented).
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Cochliobolus sativus was the most widespread and commonly isolated fungus from lesioned SIs of barley, followed by Fusarium spp. Among these, F. avenaceum and F. equiseti were the most prevalent and common isolates in the more severe SI lesions. The occurrence of F. equiseti in severely discolored tissue may be partly explained by its association with C. sativus; the former fungus, which is considered weakly to moderately pathogenic on cereals (Fedel-Moen and Harris, 1987; Gonzalez and Trevathan, 2000), likely acted as a secondary invader of tissue previously colonized by C. sativus. Similarly, the association of M. bolleyi, considered a weak pathogen of barley (Murray and Gadd, 1981), with the more severely discolored tissue could be, at least partly, attributed to the frequency with which this species occurred together with C. sativus.
Cropping sequence had less impact on the extent of SI discoloration (CRRI) than tillage system, with only the relative levels of the most commonly isolated fungi being affected by the former crop production factor. In general, tillage effects on SI discoloration or percentage fungal isolations did not seem to depend on the previously grown crop. While CRRI and C. sativus isolations from SI were favored by CT management, colonization by Fusarium spp., especially F. avenaceum and F. graminearum, increased under reduced tillage. Our observations on tillage effects on the relative prevalence of these fungi agree with previous studies. In particular, the higher levels of C. sativus and lower levels of F. avenaceum in more intensive tillage systems are similar to observations by Windels and Wiersma (1992), who reported an increase in F. avenaceum and F. graminearum in barley with a reduction in tillage intensity, but no tillage effect for F. acuminatum or F. culmorum. In other studies conducted in eastern Saskatchewan, tillage operations were also positively associated with the occurrence of C. sativus and negatively associated with that of F. avenaceum in SIs of common wheat (Fernandez et al., 2007a) and roots of lentil (Lens culinaris Medik.), and flax and canola plants grown in rotation with wheat or barley (Fernandez, 2007).
Cochliobolus sativus also occurred at higher levels in barley grown in production systems of cereals alternated with summer fallow under CT or MT management than in most other sequences. However, levels of this fungus in barley grown immediately after summer fallow were not significantly different than when grown after a crop. Piening et al. (1969) found higher levels of CRR in barley caused mostly by C. sativus when grown after summer fallow than stubble, whereas Piening and Orr (1988) found that CRR in barley was lower after summer fallow than after another susceptible crop. For the most part, Fusarium spp. were also not significantly affected by summer fallow versus a crop, except for F. culmorum, which was significantly reduced when barley was grown after summer fallow or after a cereal crop and summer fallow. This contrasts with observations by Fernandez et al. (2007a) and Sturz and Johnston (1985), who found lower levels of F. avenaceum in wheat SIs and crowns, respectively, when grown after summer fallow than stubble.
Although an oilseed crop grown in the previous year was associated with lower levels of C. sativus in barley than a cereal crop, barley grown after 2 yr of noncereal crops had lower levels of this pathogen than when grown after a cereal alternated with a noncereal crop, although its levels were not significantly different than after two cereal crops. These observations only partly agree with those of Conner et al. (1996) and Piening and Orr (1988), who found higher CRR levels in barley when grown on barley stubble than when grown following 2 yr of noncereal crops.
Growing a noncereal crop in the previous 1 or 2 yr was in turn associated with higher levels of Fusarium spp. in the succeeding barley crop compared with a cereal crop or other continuous sequences that included a cereal crop. This could be attributed to higher levels of F. avenaceum, F. culmorum, F. equiseti, and F. graminearum observed after an oilseed and/or pulse crop, or after 2 yr of noncereal crops. Fernandez et al. (2007a) reported higher levels of F. avenaceum in SIs of common wheat grown after a pulse than a cereal crop, or after a 2-yr sequence that included at least one noncereal crop than after two cereal crops. Most of the barley crops grown after a noncereal crop, or after two noncereal crops, were under reduced tillage (MT or ZT), which may have confounded these results, considering the positive effect of reduced tillage on Fusarium isolations. However, barley grown after NC–F (mostly under CT or MT) had similar levels of Fusarium spp. than when grown after NC–NC, suggesting that the previously grown crop had a greater impact on these fungi than the method of tillage management.
The positive relationship of reduced tillage systems and previously grown noncereal crops with Fusarium spp. in SIs and the association of F. graminearum with a previously grown oilseed crop were similar to that observed for spike infections of the same barley crops (Fernandez et al., 2007b). The mechanism(s) by which noncereal crops, most of which were canola, contributed to the higher populations of F. avenaceum and F. graminearum, especially the latter, in SIs of barley is not known. However, in both of these barley studies and a spring wheat study conducted in the same area and during the same years (Fernandez et al., 2005), there was also a positive impact of glyphosate applied mostly on fields where canola had been grown on pathogenic Fusarium spp., including F. avenaceum, F. culmorum, and F. graminearum. Glyphosate was in fact the only herbicide associated with higher levels of Fusarium spp. in SIs of barley in the present study. Although analysis of barley crops grown after a crop other than canola showed similar associations of Fusarium isolations with previous glyphosate applications, because of the nature of these studies, the impact of a previously grown canola crop from that of previous glyphosate applications could not be completely separated.
In addition to a positive association of previous glyphosate use with isolation of Fusarium spp. from barley SIs, this study also showed a significant negative association of previous glyphosate use with C. sativus, suggesting changes in populations of the most common root rot fungi associated with the use of this herbicide. No other herbicide group seemed to consistently affect levels of this cereal pathogen. The observation that similar negative associations of previous glyphosate use with C. sativus were also apparent under CT- and ZT-management systems suggests that changes in levels of this pathogen may be due to direct effect(s) of this herbicide and are not related to tillage management. There are no previous reports of glyphosate effects on infection of barley underground tissue by C. sativus; however, Fernandez et al. (2007a) also found a negative association of previous glyphosate application with levels of C. sativus in SIs of common wheat sampled in the same area. The observation that Fusarium spp. increased in fields previously treated with glyphosate formulations agrees with previous reports on Fusarium colonization of other crops being associated with glyphosate use. For example, Levesque et al. (1987) reported that glyphosate application increased root colonization of various treated weeds by F. avenaceum and F. oxysporum Schlechtend.:Fr., and it also increased the propagule density of these Fusarium spp. in the soil. In addition, Levesque et al. (1993) reported that glyphosate-treated wheat seedlings were colonized to a greater extent than untreated seedlings by Fusarium spp. under warm and dry conditions than under lower temperatures and moist conditions. Further, glyphosate-treated quack grass [Elymus repens (L.) Gould] rapidly colonized by F. culmorum caused damage to a subsequent barley crop (Lynch and Penn, 1980).
From our data, we could not determine if the higher Fusarium levels associated with previous glyphosate use was due to effects on fungal inoculum or host susceptibility or to the absence of competition from C. sativus. Furthermore, how much the observed association with previous glyphosate use contributed to increased relative levels of Fusarium spp. in reduced tillage systems, and how much might be due to other factors such as microenvironment in these systems, could also not be determined. Separating the effects of the various agronomic practices relating to cropping sequence and tillage system would be necessary to understand the role that each of these play in disease levels and the relative frequency of the various pathogens.
In general, six-rowed barley crops yielded more than two-rowed barley crops. Grain yield also appeared to be more affected by previous crop(s) or summer fallow than by tillage system. However, the effects of these agronomic factors could not be completely separated. The higher yields of barley grown after two consecutive crops that included at least one noncereal crop than when grown after two cereal crops, or a cereal alternated with summer fallow, should be attributed, at least partly, to higher N input, and/or the higher soil N available that would be expected in diversified cropping sequences. Piening et al. (1983) showed that the yield of barley increased, and CRR decreased, in barley grown on stubble when fertilizer (N and P) was added. In our study, as indicated above, although barley grown after some of the continuous diversified sequences had lower levels of C. sativus than when grown after C–F, sequences consisting only of noncereal crops had higher levels of Fusarium spp., resulting in similar overall CRR levels among cropping sequences. In addition, the coincidence of higher grain yields with a previous oilseed crop was also confounded by an association of yield with glyphosate use in fields under MT management given that most of the fields that received glyphosate applications in the previous 18 mo had been planted to an oilseed crop the year previous. However, analysis of barley crops preceded by a crop other than an oilseed showed that glyphosate applications in the previous 18 mo also had a similar positive effect on yield as for all crops combined. The greater yield advantage of barley crops grown in fields previously sprayed with glyphosate is likely a reflection of the greater weed control provided by this nonselective herbicide.
Based on the results of this CRR survey of barley crops conducted in eastern Saskatchewan, we conclude that growing barley under reduced tillage systems that include glyphosate applications and with noncereal crops incorporated in the rotation will result in lower levels of C. sativus, the most common CRR pathogen in western Canada. However, these production systems will likely result in an increase in infection by Fusarium spp. Although the latter remained at lower levels than C. sativus, increases in populations of F. avenaceum and F. graminearum, especially in areas with higher disease pressure than where the present study was conducted, may cause greater development not only of crown and root rot but also of spike infections in subsequently grown cereal crops. Because Fusarium infections in crowns and roots are less affected by environmental conditions than spike infections, they may also contribute to the maintenance of inoculum in years not conducive to FHB development and thus to the further spread of this disease in the Canadian Prairies. As suggested by Fernandez et al. (2007b) for a study of FHB and Fusarium-damaged kernels on the same barley crops sampled in this study, the observation that similar crop production factors were associated with some of the most common pathogenic Fusarium spp. in SIs and spikes and kernels of barley suggests that measures aimed at reducing crown and root rot caused by Fusarium spp. may also help reduce FHB development in this cereal crop on the Canadian Prairies.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication September 21, 2006.
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This article has been cited by other articles:
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  M. R. Fernandez, R. P. Zentner, R. M. DePauw, D. Gehl, and F. C. Stevenson Impacts of Crop Production Factors on Fusarium Head Blight in Barley in Eastern Saskatchewan Crop Sci., July 30, 2007; 47(4): 1574 - 1584. [Abstract] [Full Text] [PDF]
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