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CROP ECOLOGY, MANAGEMENT & QUALITY

Impacts of Crop Production Factors on Fusarium Head Blight in Barley in Eastern Saskatchewan

^a Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1030, Swift Current, SK, Canada S9H 3X2
^b Indian Head Research Farm, Agriculture and Agri-Food Canada, P.O. Box 760, Indian Head, SK, Canada S0G 2K0
^c 142 Rogers Rd., Saskatoon, SK, Canada S7N 3T6

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

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Fusarium head blight (FHB) in barley (Hordeum vulgare L.) is well established in the eastern Canadian Prairies and appears to be moving westward. A survey of 192 barley crops in eastern Saskatchewan was conducted to determine the impact of agronomic practices on FHB (1999–2002) and Fusarium-damaged kernels (FDK) (2000–2001). The most common species isolated from spikes and kernels were F. sporotrichioides, F. avenaceum, and F. graminearum, followed by F. poae and F. culmorum. Disease tended to be higher under minimum-till compared with conventional- or zero-till. Fusarium sporotrichioides was favored by a previous cereal crop, whereas F. avenaceum was higher after a pulse crop, and F. graminearum decreased after a pulse but not an oilseed crop. The latter two pathogens were also more prevalent after diversified cropping sequences than after two cereal crops. Summer fallow, or summer fallow alternated with cereals, decreased FDK. Previous glyphosate (Group 9 herbicides) use was associated with increased infection by all Fusarium spp., whereas Group 1 herbicides were associated with increased infection by F. poae and F. sporotrichioides. Effects of both herbicide groups depended on tillage system. Number of previous glyphosate applications was also correlated with FHB caused by F. avenaceum and F. graminearum. We concluded that in eastern Saskatchewan, barley grown under minimum-till where glyphosate had been sprayed and following diversified cropping sequences would sustain the greatest damage due to FHB and FDK caused by F. avenaceum and F. graminearum.

Abbreviations: C, cereal • CT, conventional-till • DON, deoxynivalenol • F, summer fallow • Fav, Fusarium avenaceum • FDK, Fusarium-damaged kernels • Fg, Fusarium graminearum • FHB, Fusarium head blight • Fp, Fusarium poae • Fspo, Fusarium sporotrichioides • MT, minimum-till • NC, noncereal • ZT, zero-till

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
FUSARIUM HEAD BLIGHT (FHB), also known as scab or tombstone, became an important barley (Hordeum vulgare L.) disease in the eastern Canadian Prairies about 1997 (Tekauz et al., 2000) and has since spread westward, with negative impacts on grain yield and quality in years conducive to disease development. Fusarium head blight surveys conducted throughout the Canadian Prairies detected head and kernel infections of barley crops in eastern Saskatchewan (Clear et al., 2000; Fernandez et al., 2002) and Alberta (Turkington et al., 2002), although in fewer fields and at lower levels than in Manitoba (Tekauz et al., 2000). In the last few years, due to unfavorable weather for disease development at flowering, FHB has occurred at low levels in Saskatchewan (Pearse et al., 2006); however, there is still the potential for this disease to continue spreading westward and adversely impact the production and marketing opportunities for barley in these regions.

Barley is second only to spring wheat (Triticum aestivum L.) in terms of the land area devoted to cereal production on the Canadian Prairies. Of the 4.1 million ha of barley grown annually, mainly in the subhumid Parkland region (Campbell et al., 2002), about 75% is planted to cultivars developed for the malting industry and 25% is planted to feed cultivars for use in animal rations. Both two-rowed and six-rowed malt barley cultivars are grown in an approximate ratio of two to one.

Several Fusarium species can cause FHB in barley. The most important FHB pathogen in Manitoba and in the midwestern USA is F. graminearum Schwabe [teleomorph Gibberella zeae (Schwein.) Petch], followed by other species, the most common of which are F. avenaceum (Fr.:Fr.) Sacc. (teleomorph G. avenacea Cook), F. poae (Peck) Wollenweb., and F. sporotrichioides Sherb. (Salas et al., 1999; Tekauz et al., 2000, 2006). Sturz and Johnston (1985) reported that the most common species isolated from barley spikes in Prince Edward Island in the early 1980s were F. graminearum and F. poae, followed mainly by F. avenaceum and F. culmorum (W.G. Smith) Sacc. In Saskatchewan and Alberta, F. graminearum has been less commonly isolated from infected barley spikes and kernels than in regions where the disease is more prevalent. Fusarium avenaceum was reported as the most or one of the most common species found in infected spikes and kernels of barley (Clear et al., 2000; Pearse et al., 2006; Turkington et al., 2002).

Because F. graminearum is the most important FHB pathogen in the most affected barley-producing areas, deoxynivalenol (DON) has been reported as the most important mycotoxin in infected crops. Nonetheless, other mycotoxins have also been associated with infection of barley by other Fusarium spp. (Abramson et al., 2002; Campbell et al., 2000; Salas et al., 1999).

The best resistance in cultivars registered in western Canada was described as "Fair" or "Fair+" by Saskatchewan Agriculture, Food and Rural Revitalization (2005). For malting barley (Special Select), the tolerance for Fusarium-damaged kernels (FDK) is Nil, and for Select and Standard Select, it is 0.2% (Canadian Grain Commission, 2005).

Reducing FHB levels and preventing continued damage to the barley industry will help Canadian producers remain competitive and protect market opportunities. Although the most effective way of controlling FHB is by developing barley cultivars with improved levels of resistance, knowing which agronomic practices contribute to reduced disease and inoculum levels should form part of a comprehensive strategy for disease control. There are few reported studies on the impact of agronomic practices on FHB in barley. In studies conducted in Quebec, Rioux et al. (2005) found that DON content was greater in barley grown under minimum-till rather than conventional-till management and that barley preceded by a mixed forage crop of orchardgrass (Dactylis glomerata L.) and red clover (Trifolium pratense L.) had a higher DON content than barley grown in monoculture. There are no consistent crop rotation or tillage system effects on disease development among studies conducted on wheat (Dill-Macky and Jones, 2000; Fernandez et al., 2005; Miller et al., 1998; Schaafsma et al., 2001). In all the latter studies, F. graminearum was the predominant pathogen in spikes or kernels.

The objective of this study was to determine how FHB and FDK development, and relative prevalence of Fusarium pathogens, in barley crops grown in eastern Saskatchewan is affected by crop production systems, in particular, tillage method and cropping sequence. This information would help to identify agronomic practices that may reduce the further spread of damage to barley from FHB on the Canadian Prairies.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Description of Fields Sampled
Commercial fields (experimental units) were selected at random within Crop Districts 1B and 5A in southeast and east-central Saskatchewan to represent the most common cropping practices in the area. The bulk of the agricultural soils of these regions are Black Chernozems, derived from glacial deposits with loam to clay loam texture (Padbury et al., 2002). The depth of the surface horizon averages 20 to 25 cm, and the soils contain about 7% organic matter. Further details about the study area were reported by Fernandez et al. (2005).

A total of 192 barley crops were sampled from 1999 to 2002 (30 in 1999, 63 in 2000, 50 in 2001, and 49 in 2002). The most common barley cultivars sampled were Excel (19% of all crops sampled in all 4 yr), Metcalfe and Robust (14% each), Harrington (8%), CDC Stratus (7%), and CDC Dolly (6%). The remaining barley cultivars constituted less than 5% each of all the crops sampled. The average seeding dates for the barley crops sampled were 3 June 1999, 16 May 2000, 21 May 2001, and 20 May 2002. The average harvest dates were 23 Sept. 1999, 10 Sept. 2000, 2 Sept. 2001, and 13 Sept. 2002.

Spike and Grain Sampling
At the mid-milk to early-dough stage of crop development (growth stages 75–83; Zadoks et al., 1974), 100 spikes from each field were taken at random, following a large circular pattern. Sampling started about 40 m from the edge of the field. Samples were placed in paper bags, transported to the Semiarid Prairie Agricultural Research Centre near Swift Current, Saskatchewan, dried at 40°C for 48 h, and stored in a cold (4°C) room until analysis in late summer/early fall. An estimate of percentage of spikes with FHB-like symptoms (incidence) was based on 50 spikes taken randomly from the 100 collected spikes. Disease severity was estimated visually based on the percentage of spikelets discolored on each spike. To confirm infection by Fusarium spp. and for species identification, the individual lemma showing discoloration were carefully removed, surface-sterilized for 1 min in 0.6% NaOCl, and rinsed twice in sterile distilled water. They were then plated on modified potato dextrose agar (Burgess et al., 1988; Fernandez and Chen, 2005) and incubated for 7 d under fluorescent and near-UV lights at 22°C day/15°C night, 16 h photoperiod (light intensity of 110 µmol s^–1 m^–2). Fusarium spp. were identified on the basis of colony and spore morphology and reproductive structures using descriptions and keys in Samson et al. (2002) and Watanabe (2002). A FHB index [(% of spikes infected x mean severity of infection)/100] was calculated for each of the barley crops sampled based on the presence of Fusarium isolates in the discolored lemma tissue plated. From 2000 to 2002, FHB indices were also calculated for each crop based on the percentage isolation of the most common Fusarium spp.: F. avenaceum (FHB-Fav), F. graminearum (FHB-Fg), F. poae (FHB-Fp), and F. sporotrichioides (FHB-Fspo).

In 2001 and 2002, grain samples from most of the barley crops sampled were also obtained from cooperating producers. Kernels with FDK-like symptoms were visually identified in a 50-g subsample, removed, and weighed. The percentage of FDK-like symptoms was determined based on total weight of the sample. A subsample of up to 30 to 40 kernels with FDK symptoms was then plated on modified potato dextrose agar as above, and fungi growing out of kernels were identified after 7 to 10 d of incubation. A percentage "total FDK" was then calculated based on the percentage isolation of Fusarium spp. In addition, percentage FDKs was also calculated based on the percentage isolation of the most common species (FDK-Fav, FDK-Fg, FDK-Fp, and FDK-Fspo).

Categorization of Barley Crops/Fields into Crop Production Factors
At the end of the growing season, each producer provided information regarding the agronomic practices used on the crop(s) sampled. The information included cultivar, crop history, method of tillage management, pesticide use, and fertilizer rates. The information obtained from producers was used to categorize the crops/fields according to crop production factor, which was then used for further analysis to determine the association of each production factor with the various disease parameters.

For cultivar susceptibility to FHB, barley crops were categorized into "susceptible" and "intermediate" cultivars. Susceptible cultivars were those rated as "Poor," and intermediate cultivars were those rated as "Fair" or "Fair+" by Saskatchewan Agriculture, Food and Rural Revitalization (2005).

For tillage system, fields were categorized according to the total number of tillage operations they received 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 a total of 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 time period. The number of tillage operations in the previous 3 yr for fields under MT averaged 3.2, whereas for fields under CT the average was 8.5. Residue cover was not estimated for any field. Overall, more crops were grown under MT (61%) than under CT (18%) or ZT (22%). Herbicide applications within each tillage system were categorized according to whether the fields had received any of the herbicide Groups 1, 2, 4, and 9 (Saskatchewan Agriculture and Food, 2006) in the previous 18 months.

For previously grown crop(s), fields were categorized according to the crop, if any, grown the previous year: cereal, oilseed, pulse, or summer fallow. The most common crop planted the year before the barley crop sampled was an oilseed (44% of all crops sampled in all 4 yr), with canola (Brassica spp.) constituting 76% of all oilseed crops. The second most common crop planted before barley was a cereal (39%), whereas the least common was a pulse (7%). Summer fallow was practiced on an average of 10% of fields the year before barley. 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 combination of a cereal and a noncereal crop (C–NC), or summer fallow (F) and a crop (C–F or NC–F). Most barley crops grown after a cereal or oilseed crop were under MT (60–63%), followed by ZT (20–26%); most grown after a pulse crop were under either MT or ZT (76%); and most of those grown after summer fallow were under CT (35%) or MT (65%). When fields were categorized according to the two previous crops, most barley crops grown after C–C or C–NC (74–92%) or NC–NC (100%) were under MT or ZT, whereas most crops grown after C–F or NC–F were under MT (60–65%), followed by CT (30–40%).

Based on fertilizer input in the spring and/or previous fall, barley crops on average preceded by a cereal or oilseed crop had received the most N (mean of 62–63 kg ha^–1), followed by those grown after a pulse crop (51 kg ha^–1), with crops grown after summer fallow having received the least N (27 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 65 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 (34 kg ha^–1). No soil N testing was performed on any of the fields sampled.

Weather data for May to August for three locations in each of the crop districts from 1999 to 2001 were obtained from Environment Canada (2003) and reported by Fernandez et al. (2005). Spring 1999 was cooler and wetter than the following 3 yr, especially 2000 and 2001, and the long-term mean. The month of July, when most barley crops would have flowered, had the highest mean maximum temperature and lowest precipitation in 2002, and highest precipitation in 2001 (mean maximum temperature for 1999: 23.8°C; 2000: 25.8°C; 2001: 25.0°C; and 2002: 26.7°C; precipitation for 1999: 75 mm; 2000: 84 mm; 2001: 60 mm; and 2002: 55 mm).

Statistical Analyses
Disease- and fungal-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 susceptibility classification cross classified with cropping sequence or tillage system. 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; verified 14 June 2007). Effects were declared significant at P 0.10. Contrasts were performed among cropping sequences and tillage systems for total FHB index or percentage FDK, and for those attributed to the most commonly isolated fungi. Only the contrasts that showed significant effects for at least one of these parameters are presented in the tables. Fungi for which there were no significant effects of previous crop/cropping sequence or tillage system were not included in the tables. Pearson correlations were also performed between the FHB indices and percentage FDK attributed to the various fungi.

The effect of herbicide application was further examined for disease-related variables for each of the tillage systems. Differences among the different herbicide group 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). Pearson correlations were performed between the total number of Group 9 herbicide applications in the previous 18 mo and the FHB indices. Effects were again declared significant at P 0.10.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
FHB, FDK, and Fusarium Species in Infected Spikes and Kernels
Percentage of barley fields affected and mean FHB levels were higher in 2000 and 2001 than in 1999 or 2002 (Table 1). Percentage FDK in 2000 and 2001 was high enough to cause downgrading of the grain from Special Select to Select/Standard Select or feed (Canadian Grain Commission, 2006).

The relative prevalence of Fusarium spp. on kernels after harvest was similar to that observed on spikes during kernel development. Simple correlations performed between FHB and FDK attributed to the most common fungi showed significant (P 0.01–0.05) moderate correlations for F. avenaceum (r = 0.26 and 0.31, for 2000 and 2001, respectively), F. graminearum (r = 0.59 and 0.64), and F. sporotrichioides (r = 0.34 and 0.42).

FHB and Crop Production Factors
When FHB data were analyzed based on cropping sequence, total FHB index, and that attributed to F. avenaceum and F. graminearum were significantly higher for susceptible cultivars than for those with intermediate resistance (Table 2). However, there were no significant effects of cropping sequence on the total FHB index but only on the FHB indices attributed to F. avenaceum and F. graminearum. For the most part, there were no interactions of cropping sequence with cultivar susceptibility. Barley tended to have higher mean levels of FHB-Fav when grown after a pulse crop, although this varied, with cultivar susceptibility being the highest for susceptible cultivars (1.0%) (data by cultivar susceptibility not presented). Levels of FHB-Fav were also lower after C–C than after the other cropping sequences with two consecutive crops. In contrast, FHB-Fg was present at the lowest mean levels in barley grown after a pulse crop but tended to be higher after an oilseed than a cereal crop. Similar to FHB-Fav, FHB-Fg was also lower after C–C than after continuous cropping sequences with at least one noncereal crop in the previous 2 yr. Instead, FHB-Fp was lower after NC–NC than after sequences that included cereal crops (C–C or C–NC) (P = 0.043–0.058). Barley grown after a cereal had a higher FHB-Fspo than when grown after an oilseed crop, and although nonsignificant for all crops combined, levels of FHB-Fspo were among the highest after C–C and the lowest after NC–NC for both susceptible and intermediate cultivars (data by cultivar susceptibility not presented).

Because of the small sample size for tillage systems other than MT in most of the cropping sequences, tillage analysis was performed on all barley crops, regardless of the cropping sequence. Overall, there were no significant tillage effects, but interactions with cultivar susceptibility were significant in most cases (Table 2). For most of the fungi, differences between FHB indices in barley grown under CT and reduced tillage varied with cultivar susceptibility. For total FHB index, FHB-Fg, and FHB-Fspo, susceptible cultivars had the lowest disease levels under CT, whereas cultivars with intermediate resistance had the lowest levels under ZT; barley grown under MT had similar or higher disease levels than that grown under the other tillage systems.

FDK and Crop Production Factors
There were no significant interactions between previous crop or cropping sequences with cultivar susceptibility. Percentage FDK-total and FDK-Fav in 2000 and 2001 tended to be higher in barley grown after a pulse crop than after the other crops, but not significantly so at P < 0.10 (Table 3). In contrast, FDK-Fg and FDK-Fp were lower in barley grown after a pulse than other crops, although significantly so only for the former, whereas FDK-Fspo was significantly lower in barley grown after an oilseed.

Analysis of the FDK data by tillage system for all cropping sequences combined showed that for the most part, there were no significant interactions of tillage system with cultivar susceptibility. Barley grown under MT management had significantly higher percentage total FDK, FDK-Fg, and FDK-Fp than barley in the other tillage systems combined (Table 3). Lowest levels of FDK-Fg and FDK-Fp were observed under ZT, whereas lowest levels of FDK-Fspo were observed under CT, resulting in the latter being the only FHB index that was significantly different for CT than for reduced tillage (MT and ZT) (P = 0.007).

Herbicide Effects on FHB
Analysis of herbicide use in the previous 18 mo according to previous crop or summer fallow in fields under MT management showed that for Group 1 herbicides, barley fields preceded by summer fallow constituted a greater proportion of the unsprayed fields than for the other herbicide groups (Table 4). Compared to the other herbicide groups, for glyphosate (Group 9 herbicides) there was a greater percentage of barley crops preceded by a cereal crop in fields that had not been sprayed in the previous 18 mo, and a greater percentage of barley crops preceded by an oilseed crop (mostly canola) in fields that had been sprayed. Of all the Group 9–treated barley fields under MT management preceded by an oilseed crop, more than 40% of them had received in-crop applications of glyphosate, suggesting that many of these were herbicide-tolerant canola cultivars.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

The frequency of the fungi isolated from affected spikes and grain was also similar to what was reported from the province-wide Saskatchewan surveys of barley crops (Fernandez et al., 2000, 2001, 2002; Pearse et al., 2003). Although the relative frequency of fungi isolated from infected barley spikes in this study differed from those isolated in a parallel study of common and durum (T. turgidum L. var. durum) wheat (Fernandez et al., 2005), F. avenaceum was among the most commonly isolated fungi for both wheat and barley. In Manitoba, although the most widespread and commonly isolated species from FHB-affected barley has been F. graminearum, followed by F. poae, F. sporotrichioides, and F. avenaceum (Tekauz et al., 2003), F. avenaceum in 2005 was the second most commonly isolated species from FHB-affected barley crops sampled in that province (Tekauz et al., 2006).

In our study, the Fusarium species associated with FDK in the barley crops sampled have also been reported elsewhere as the most commonly isolated species from infected barley grain in the Canadian Prairies. Clear et al. (2000) reported that F. graminearum, followed by F. poae, F. sporotrichioides, and F. avenaceum were the most commonly isolated species from infected barley grain in Manitoba in 1995 to 1997, whereas F. poae and F. avenaceum prevailed in Saskatchewan. However, in the two previous years (1993–1994), F. avenaceum was the second most common species after F. graminearum in infected barley grain in Saskatchewan (Clear et al., 1996). Turkington et al. (2002) and Clear et al. (2000) reported F. avenaceum as the most commonly isolated species from barley FDK in 1995 to 1997 in Alberta, followed by F. poae. Relative to the other species, F. sporotrichioides was isolated at lower levels from infected barley kernels than spikes in our study, especially in 2000, suggesting that spike infection by this fungus did not always result in detectable kernel infections after harvest. However, there was a significant correlation between the FHB index and percentage FDK attributed to this fungus.

Analysis of the data by tillage system suggested that MT management favored disease development, especially percentage FDK. Rioux et al. (2005) reported that barley grown under MT had higher DON content than when grown under CT. The observation that overall barley grown under MT management had higher disease levels than barley grown under the other tillage systems agrees with the report by Fernandez et al. (2005) for common and durum wheat crops, which had a similar preponderance of oilseed as the previously grown crop.

A previous year of summer fallow affected FDK in 2000 and 2001 more than the overall mean FHB levels, reducing kernel infection by most fungi. Similar to the analysis of the data from the 4 yr of this study, analysis of the 2000 and 2001 data only also showed that there were no significant differences in FHB levels between barley grown after summer fallow and when grown after a crop (separate analysis of 2000–2001 data not presented). Observations by Sturz and Johnston (1985) that overall Fusarium isolations from barley spikes were higher in barley grown on stubble than on summer fallow agree with results from our FDK analysis but not with the FHB data. On average, barley crops preceded by summer fallow, or by a year of summer fallow and a cereal crop, received lower N input than barley grown after the other sequences. While Martin et al. (1991) reported no consistent effects of added N on seed infection in barley caused by the same fungi as in our study, Lemmens et al. (2004) found significant increases in FHB and DON in wheat as a result of N fertilization, although higher rates of applied N (up to 160 kg N ha^–1) were used in their study compared to ours. However, because no soil N analysis was performed in our study, differences among cropping sequences for total N available are not known.

Fusarium avenaceum and F. graminearum on spikes were less prevalent in continuous cereal systems mostly under MT (C–C) than in continuous diversified systems (C–NC, NC–NC) under MT or ZT, or with a noncereal alternated with summer fallow (NC–F), mostly under CT or MT. However, levels of FHB-Fav and FHB-Fg in barley after C–C were similar to those in barley grown after a cereal alternated with summer fallow (C–F), which were also mostly under CT or MT management. These results suggest that cropping sequence had a greater impact on infection of barley by F. avenaceum and F. graminearum than tillage system and that noncereal crops appear to have played a more important role in disease development attributed to these fungi in succeeding barley crops than the presence of host cereal crops grown continuously in the previous 2 yr (C–C).

There was also a differential effect of the previous noncereal crop on F. avenaceum and F. graminearum on spikes and kernels. Compared to other crops, a previous pulse crop favored an increase in FHB and FDK caused by F. avenaceum, but it resulted in a decrease in that associated with F. graminearum. Dill-Macky and Jones (2000) also reported lower FHB and DON levels, attributed mostly to F. graminearum, in spring wheat grown after soybean [Glycine max (L.) Merr.] than after a wheat crop. In contrast, Rioux et al. (2005) reported that a previous forage crop of orchardgrass and red clover was more conducive to DON production in barley than when the preceding crop was another barley crop. Changes in the prevalence of FHB pathogens associated with the preceding crop have been reported before. Cromey et al. (2002) found that while F. graminearum was the predominant grain pathogen in spring wheat planted after corn (Zea mays L.), the percentage of grain infected by this pathogen decreased, and that of F. avenaceum and F. poae increased, in wheat planted after other crops.

The increase in disease levels caused by F. avenaceum after pulse crops could be attributed to the susceptibility of pulses to this pathogen (Hwang et al., 2000). In the same area that this study was conducted, F. avenaceum was found at higher levels in pulse than in cereal or canola roots and residues (Fernandez, 2007; Fernandez et al., 2003a).

Fusarium avenaceum and F. graminearum tended to be present at similar or higher levels on barley spikes when grown after an oilseed than a cereal crop. Canola and flax (Linum usitatissimum L.) stem residues from the cereal fields sampled in this area were also shown to have a higher percentage isolation of F. avenaceum than cereal residues (Fernandez et al., 2003a), suggesting that oilseed residues could be an important source of inoculum for this pathogen. Correlations between FHB-Fav and FDK-Fav in susceptible barley cultivars with percentage F. avenaceum isolation from crop residues collected in the same fields at the time of sampling of the barley spikes were significant for oilseed (r = 0.496 for FHB-Fa, r = 0.457 for FDK-Fa, P < 0.05) but not for cereal residues (P > 0.10) (Fernandez, unpublished data). The similar or higher F. graminearum levels in barley grown after an oilseed crop (mostly canola) or after diversified sequences involving mostly canola crops partly agrees with Obst et al. (1997), who did not find any differences in DON levels between winter wheat grown after canola than after another cereal crop; however, Rioux et al. (2005) reported that barley grown in rotation with canola had lower DON levels. The lack of an effect of a previous oilseed crop on FHB-Fg levels in barley could be partly explained by the colonization by F. graminearum of stem residues (Fernandez et al., 2003a) and roots (Fernandez, 2007) of these crops; however, isolation of this pathogen from oilseed tissue was low. Most previous oilseed crops were preceded by a cereal crop 2 yr previous to the barley crop sampled, and these older residues may have also been an inoculum source. However, it is not known why F. graminearum levels in barley were lowest when preceded by two cereal crops, given that inoculum levels would have been expected to be higher than when a noncereal crop was included in the sequence. The significantly higher grain yields of the same barley crops when they followed an oilseed versus a cereal crop (Fernandez et al., 2007) suggests that a previous oilseed crop may have resulted in a higher N status in the subsequent barley crop. In addition, barley fields preceded by two cereal crops had also received on average less N input (55 kg ha^–1) than when a noncereal crop was included in the sequence (65 kg ha^–1), and also had significantly lower grain yield than when there was at least one noncereal crop in the previous 2 yr, as reported by Fernandez et al. (2007). The higher N availability when a noncereal crop was included in the sequence may have affected FHB development (Lemmens et al., 2004). Other unknown factors related to the presence of oilseed crops may have also contributed to FHB development in the barley crops grown afterward in those fields.

However, based on the previous herbicide use pattern in barley fields analyzed according to the previously grown crop, and on the association of herbicide use with disease levels, it appears that the use of Group 9 (glyphosate) herbicides, in fields preceded mostly by an oilseed crop, were also associated with increased levels of all Fusarium pathogens, although these effects varied with tillage system. Some of these Group 9 herbicide applications had been done in-crop, indicating that they were done on glyphosate-tolerant canola. However, due to the nature of this study and the small sample size for unsprayed barley fields preceded by an oilseed crop, it was not possible to separate the impact of previously grown oilseed crops from that of previous Group 9 herbicide applications on disease levels. The other herbicides associated with significant increases in FHB levels attributed to F. poae and F. sporotrichioides belonged to Group 1, although this again depended on tillage system. According to the herbicide-use pattern in fields under MT, there were as many fields preceded by a cereal as by an oilseed crop sprayed with Group 1 herbicides, and more previous cereal crops than for Group 9. Although according to the analysis of the effect of previous crop on disease levels previous cereal crops resulted in significantly higher levels of FHB-Fspo on the succeeding barley crops, as for Group 9, it was not possible to separate the effect of previous crop from that of Group 1 herbicide use on Fusarium infections.

The association of previous Group 9 herbicide applications with FHB levels is similar to the observations made for spring wheat regarding total FHB index, FHB-Fg, and FHB-Fav (Fernandez et al., 2003b; Fernandez et al., 2005). The wheat study did not find any significant effect of the other herbicide groups on disease levels. As indicated for wheat, the mechanism(s) responsible for the increase in disease levels in barley associated with previous Group 9 herbicide use is not known. However, based on the correlations between the total number of Group 9 herbicide applications in the previous 18 mo and FHB-Fg and FHB-Fav levels in barley crops, it is apparent that the impact of this herbicide on disease levels was greater for cultivars with intermediate resistance than for susceptible cultivars, suggesting that cultivar susceptibility may override the apparent impact of Group 9 herbicides on disease levels. Barley crops with intermediate resistance grown under MT management in fields that had received two glyphosate applications in the previous 18 mo had similar or slightly lower mean percentage FHB-Fav (0.4%) and FHB-Fg (0.5%) than the mean for all susceptible barley crops grown under MT (0.4 and 0.7%, respectively). In a parallel study of common root rot of the same barley crops sampled in this study (Fernandez et al., 2007), glyphosate was found to be the only herbicide associated with significant increases in Fusarium levels in subcrown internodes in fields under MT management.

According to Fernandez et al. (2007), the other crop production factors that affected Fusarium infections on spikes in this study were also similar to those that affected the percentage isolation of Fusarium spp. from subcrown internodes of the same barley crops sampled from 1999 to 2001. The similar impact of production factors on FHB and common root rot points to the importance of agronomic practices vis-à-vis the environment in the development of these barley diseases in eastern Saskatchewan.

Based on our observations, we conclude that growing barley under MT management where glyphosate had been applied, and in continuous diversified rotations, would result in the most damage due to FHB caused by two of the most important pathogens in this and other affected regions, F. graminearum and F. avenaceum. It is not known if barley grown in areas with traditionally higher FHB levels or where F. graminearum is the predominant pathogen would be more or less impacted by the same crop production factors. In any case, determining the relative contribution of cropping sequence, tillage method, and herbicide applications to FHB development in barley can assist in devising the most appropriate agronomic recommendations for its control.

Considering that currently popular production practices appear to be associated with FHB development in this region, and based on the importance of F. avenaceum, a wide-host range pathogen, relative to the other Fusarium pathogens, breeding for resistance to FHB seems to be the most practical way of controlling this important cereal disease. Furthermore, incorporating resistance to Fusarium infections in roots and crowns may also be important for controlling the development and spread of FHB in barley on the western Canadian Prairies. However, determining the mechanism responsible for the association of previous glyphosate applications with spike infections caused by F. graminearum and F. avenaceum would help in disease control and possibly in maintaining the resistance of barley to this important disease.

	ACKNOWLEDGMENTS

 
This work was conducted with financial support from the Saskatchewan Agriculture Development Fund. We thank S. Stolhandske-Dale, Y. Chen, M. Boire, and D. Kern for technical assistance.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
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Received for publication September 21, 2006.

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