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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
* Corresponding author (fernandezm{at}agr.gc.ca)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CD, crop district • CPF, crop production factors • CPS, Canada Prairie Spring • CWAD, Canada Western Amber Durum • CWRS, Canada Western Red Spring • DON, deoxynivalenol • FDK, Fusarium-damaged kernels • FHB, Fusarium head blight • GF, glyphosate formulations
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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There are several Fusarium species that can cause FHB. The most important FHB pathogen in North America is F. graminearum Schwabe [teleomorph Gibberella zeae (Schwein.) Petch]. This pathogen produces mycotoxins harmful to humans and livestock. The most commonly found mycotoxin in infected grain is deoxynivalenol (DON). Because of processing problems and potential food safety concerns, tolerance levels for Fusarium-damaged kernels are very low. For the Canada Western Red Spring (CWRS) wheat class, FDK greater than 0.25% by weight will cause downgrading to CWRS #2, over 1% to CWRS #3, and over 2% to CWRS #4 (Canadian Grain Commission, 2003). These low tolerance levels represent substantial economic losses to producers in affected areas.
To date, no wheat cultivars with good resistance to FHB have been registered in the Canadian Prairies. The best resistance available in currently registered cultivars is intermediate, and is described as "fair" in the Saskatchewan Varieties of Grain Crops publication (Saskatchewan Agriculture. Food and Rural Revitalization, 2003). Further, relatively few studies have attempted to identify agronomic factors that may be associated with the spread and development of FHB.
Cromey et al. (2002), Obst et al. (1997), Teich and Hamilton (1985), and Teich and Nelson (1984) have determined the effect of crop sequence on FHB severity, the presence of F. graminearum in heads or grain, and/or levels of the mycotoxin DON in a corn (Zea mays L.)–wheat rotation. However, corn is not a common crop grown in rotation with wheat or barley in the Canadian Prairies. Comparisons of cereal, other than corn, and noncereal crops in rotation with wheat have not shown consistent differences in disease levels. Obst et al. (1997) did not find differences in mycotoxin levels between a wheat crop grown after canola (Brassica sp.) and wheat grown after wheat or barley, whereas wheat crops grown after soybean [Glycine max (L.) Merr.] had the lowest FHB and DON levels (Dill-Macky and Jones, 2000; Schaafsma et al., 2001).
Several studies have also examined the effect of tillage practice on FHB or FDK, including DON levels (Champeil et al., 2004; Dill-Macky and Jones, 2000; Krebs et al., 2000; Miller et al., 1998; Schaafsma et al., 2001; Yi et al., 2001), but the findings vary with respect to the impact that tillage and amount of crop residue had on disease levels, and often no difference among tillage systems was observed (Miller et al., 1998; Teich and Nelson, 1984).
For the most part, the effect of herbicides on FHB development in wheat has not been examined. Teich and Hamilton (1985) and Teich and Nelson (1984) reported no significant difference in disease between wheat fields with or without herbicides applied, but they did not identify the herbicide types used. However, previous studies have reported direct and indirect effects of glyphosate on plant diseases and pathogens (Levesque and Rahe, 1992), which suggest that glyphosate could cause an increase in fungal populations. Although no previous studies examined the effect of glyphosate on FHB or F. graminearum in cereals, a number have examined interactions of glyphosate with Fusarium spp., including F. avenaceum (Fr.:Fr.) Sacc. and F. culmorum W.G. Smith Sacc. (Brown and Sharma, 1984; Levesque et al., 1987). Kawate et al. (1997) reported that Fusarium populations were greater in the rhizosphere soil from glyphosate-treated, than from untreated, henbit (Lamium amplexicaule L.), whereas Lynch and Penn (1980) reported that glyphosate-treated quackgrass (Agropyron repens Beauv.) was rapidly colonized by F. culmorum, which subsequently caused damage to the following barley crop. More recently, it has been shown that glyphosate application to glyphosate-tolerant soybean caused a significant increase in the isolation frequency of the causal agent of sudden death syndrome, F. solani (Mart.) Sacc. f. sp. glycines form. nov. (Sanogo et al., 2001), and Fusarium populations on the roots and rhizosphere of the plants (Kremer, 2003).
It is not known how agronomic practices might affect FHB levels in wheat in areas of the Canadian Prairies where this disease has increased in recent years. A comprehensive strategy for control of this disease requires identification of the crop production factors that might be increasing the risk of FHB infection in this region. The objective of this study was to identify agronomic practices, or crop production factors, such as crop susceptibility, tillage system, herbicide use, previously grown crop, N fertilizer use, and seeding rate and date, that might be associated with the development of FHB in spring wheat in eastern Saskatchewan.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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At the end of the growing season, each producer was asked directly to supply information regarding the agronomic practices used on the crop(s) sampled. The information included cultivar, seeding rate and date, crop history, N fertilizer use, herbicide and fungicide use, and method of tillage management (questionnaire available on request). Only wheat crops that had not been treated with a fungicide at flowering were included in this study.
The number of wheat crops sampled was 89 in 1999, 128 in 2000, 189 in 2001, and 253 in 2002. Most of the cultivars sampled belonged to three market classes: 80% were CWRS, 7% Canada Prairie Spring (CPS), and about 13% were Canada Western Amber Durum (CWAD) (T. turgidum L. var. durum) wheat. Similar proportion of cultivars in each of the wheat market classes were sampled each year. The most common wheat cultivar sampled was AC Barrie (36% of total number of crops); followed by CDC Teal (9%), Kyle (6%), and AC Avonlea, AC Cadillac and AC Intrepid (5% each). The remainder of the wheat cultivars constituted less than 5% each of all the crops sampled.
The bulk of the agricultural soils of these CDs 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 70 g kg–1 organic matter. Spring wheat is the dominant crop occupying between 55 and 60% of the total cereal area. Corn is seldom grown in this area, but oilseed (canola, and flax, Linum usitatissimum L.) and pulse (primarily field pea, Pisum sativum L.) crops are becoming increasingly important components of producers' crop rotations.
Growing season (May to August) weather data for the study area from 1999 to 2002 were obtained from Environment Canada (2003). Seeding was done in the second to third week of May, and harvest was done from early to late September. Canola was the most common noncereal crop grown before wheat (43% of fields), followed by pea at 11%, flax or a cereal crop (mostly wheat) at 9% each, and lentil (Lens culinaris L.) at 5%. Summerfallow was the least common land use practice in the year before sampling (10%). Two years previous to the sampling, the most common crop was a cereal (62%), with wheat being the dominant crop (35%). This was followed by oilseed crops (19%), summerfallow (13%), and pulse crops (5%).
The proportion of sampled fields that received mechanical tillage operations was higher in 1999 and 2000 (60 and 56%, respectively) than in 2001 and 2002 (30 and 39%, respectively). The main tillage implement used in 1999 to 2001 was a cultivator (58% of fields that received one tillage pass), followed by a harrow (29%). In 2002, tillage operations were performed for the most part with a harrow (57%).
Most fields (average of 61% for the 4 yr) did not receive any herbicide in the spring before planting, while the rest generally received a glyphosate formulation (GF). There was considerably less use of GF in 2002 than in the previous 3 yr, which reflects the dry conditions in the spring of that year. An average of 92% of the fields received a post-emergent herbicide, the most common belonged to Group 1 (52%), followed by Groups 4 (42%) and 2 (28%) (Saskatchewan Agriculture, Food and Rural Revitalization, 2004). The majority of producers did not apply a pre-harvest herbicide, with only 8% applying a GF. In addition, 85% of producers did not apply a post-harvest herbicide, while 12% applied a GF. All herbicides, including GF, were applied at recommended rates (Saskatchewan Agriculture, Food and Rural Revitalization, 2004).
Head and Grain Sampling
At the mid-milk to early-dough stage of crop development (growth stage 75–83, Zadoks et al., 1974), 100 wheat heads from each field were taken at random, following a large circular pattern. Sampling started at about 40 m from the edge of the field. Samples were placed in paper bags, and transported to the Semiarid Prairie Agricultural Research Centre near Swift Current, Saskatchewan where they were 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 heads with FHB symptoms (incidence) was based on 50 heads taken randomly from the 100 collected heads. Disease severity was estimated visually on the basis of the percentage of spikelets discolored on each head. To confirm infection by Fusarium spp. and for species identification, individual glumes showing discoloration were carefully removed, surface-sterilized for 1 min in 0.6% (v/v) NaOCl, and rinsed twice in sterile distilled water. They were then plated on modified potato dextrose agar [12 g Bacto agar (Difco, Detroit, MI) and 5 g potato dextrose agar L–1 distilled water] amended with streptomycin sulfate (160 mg L–1), chlortetracycline (60 mg L–1), and dichloran (2,6–dichloro-4-nitroaniline) (6.5 mg L–1) and incubated for 7 d under fluorescent and near-UV lights at 15°C night/22°C day, 16-h photoperiod. Fungi were identified on the basis of colony and spore morphology and asexual and/or sexual reproductive structures. A FHB index [(% of heads infected x mean severity of infection)/100] was calculated for each of the samples on the basis of the isolation of Fusarium spp.
From 2000 to 2002, grain samples from most of the wheat crops sampled were also obtained from cooperating producers. For 2000 and 2001, kernels with FDK-like symptoms were visually identified in a 50-g subsample, removed, and weighed. The percent FDK-like symptoms was determined on the basis of total weight of the sample. A subsample of about 30 to 40 kernels with FDK symptoms was then plated on nutrient agar as above, and fungi growing out of kernels were identified after 7 to 10 d of incubation. A percent FDK was then calculated on the basis of the percent isolation of Fusarium spp. Because of the unfavorable harvest conditions in 2002, which led to considerable deterioration of the grain, the samples from 2002 were not analyzed for percent FDK.
Categorization of Crops–Fields into Crop Production Factors (CPFs)
The information obtained directly from producers was used to categorize the crops–fields according to CPF, which was then used for further analysis to determine the association of each CPF with FHB and FDK levels. For cultivar susceptibility to FHB, wheat crops were categorized into "susceptible" and "intermediate" cultivars. Susceptible cultivars included all the CWAD and CPS cultivars, and the CWRS cultivars rated as "poor" in the Saskatchewan Varieties of Grain Crops publication (Saskatchewan Agriculture. Food and Rural Revitalization, 2003), whereas intermediate cultivars were the CWRS rated as "fair" in the same publication. 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" had a total of seven or more operations, and those under "minimum-till" had a total of one to six operations in the previous 3 yr (i.e., up to two tillage passes per year). There were no tillage operations in fields under "zero-till" management during the same period of time. Residue cover was not estimated for any field. For previously grown crop, fields were categorized according to the crop, if any, grown the previous year: "cereal," "oilseed," "pulse," or "summerfallow." Applications of herbicide Groups 1, 2, 4, and 9 (GF), were categorized according to whether the fields had received any one of these herbicide groups in the previous 18 mo or 3 yr. On average, fields that were treated with GF received 1.9 applications in the previous 18 mo and 2.6 applications in the previous 3 yr. In regards to N fertilizer use, crops that had received more than 20 kg N ha–1 for summerfallow, or more than 35 kg N ha–1 for stubble, were considered to have received "adequate N", whereas those that received less were considered to have received "inadequate N" fertilizer (Saskatchewan Agriculture, 1987). No soil analysis was performed on any of the fields. For seeding rate, crops were considered to have been seeded at "recommended" rates if they were seeded at 95 to 106 kg ha–1 for CWRS and CPS, and 101 to 135 kg ha–1 for CWAD (Saskatchewan Agriculture, 1987). Seeding rates above these were categorized as "high," while those below these rates were categorized as "low." For seeding date, actual dates were converted to day of the year.
Statistical Analysis
To assess the relative contribution of each CPF to the total variance of the FHB index or percent FDK, we compared the ratios of the sum of squares of each CPF to the total corrected sum of squares of an analysis of variance (ANOVA) after determining the homogeneity of variances (Gomez and Gomez, 1984). The model included cultivar susceptibility, tillage system, and previously grown crop. Because the use of GF is highly dependent on tillage system, we defined GF as a factor nested within tillage system.
The mean FHB index and percent FDK data for each year were transformed by arcsine (Gomez and Gomez, 1984) before testing for normality by the Shapiro-Wilk test (Shapiro and Wilk, 1965). In addition, for the variables categorized into CPFs, we conducted a test of homogeneity of variances (Gomez and Gomez, 1984) for the arcsine-transformed data. If this test determined that the variances of the variables were homogeneous, the data were subjected to an ANOVA. Mean separations with one degree of freedom contrasts were calculated on the transformed data.
Additionally, FHB index and percent FDK values were grouped into high and low classes by the median as class separator, except for 2002 when, because of a very low proportion of wheat crops with FHB index above zero, we used the mean as class separator. The effect of each CPF on the proportion of observations in the high and low FHB and FDK classes was tested by a chi-square test, under the assumption that when a CPF had no effect, the distribution of the observations into high and low FHB/FDK classes would remain unchanged.
Because of the interdependence between tillage system and GF application, the chi-square test used to evaluate the effects of previous GF application on the distribution of observations into FHB index or percent FDK classes was conducted for each tillage class separately. However, this interdependence produced a nonuniform distribution of observations in the various categories, resulting in some of the cells having few observations (less than five), which could compromise the integrity of the chi-square test. As a result, the effect of previous GF application on the proportion of fields in the low or high FHB index or percent FDK classes was analyzed only for those categories that had more than five observations in each cell.
All statistical analyses were conducted by JMP 5.0.1.2 (SAS Institute, 2002). Throughout the analysis, statistical significance was indicated by +, *, and **(P 
0.10, P 0.05, and P 0.01, respectively); ns indicates not significant at P > 0.10.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Overall, the proportion of wheat crops with FHB symptoms was highest in 2000 and 2001 and lowest in 2002, whereas the mean FHB index was highest in 2001 and lowest in 1999 and 2002 (Table 2). The highest FHB index in an individual crop was observed in 2001, followed by 2000. Despite differences in the mean FHB index between 2000 and 2001, the mean percent FDK was similar in both years. This probably resulted from the loss of the light, shriveled FDK during harvest and/or postharvest cleaning. Nevertheless, the average percent FDK in the harvested grain samples was high enough in both years to cause downgrading of the grain on the basis of threshold levels established by the Canadian Grain Commission (2003).
Regardless of the transformation used, the FHB index and percent FDK data were not normally distributed. This was attributed to the data having a large proportion of observations with values close to zero and few observations with larger values. However, the test of homogeneity of variances using a model with crop susceptibility, previously grown crop, tillage system, and previous GF application nested within tillage system indicated that the variances were homogeneous within each year.
Crop Production Factors and Disease Development
Of all CPFs examined, only cultivar susceptibility, previously grown crop, previous GF application nested within tillage system, and tillage system had significant (P 0.10–0.01) effects on either the proportion of fields with high FHB index, or the mean FHB index in one or more years from 1999 to 2002. In addition, the first three CPFs had significant (P 0.10–0.01) effects on either the proportion of fields with high percent FDK, or the mean percent FDK in 2000 and/or 2001. Neither the use of herbicide Groups 1, 2, or 4, N fertilizer applied in the spring, nor seeding rate or date significantly affected disease levels in any year of the study (data not shown).
On a yearly basis from 1999 to 2001, the four CPFs (crop susceptibility, previously grown crop, previous GF application nested within tillage system and tillage system) explained from 17 to 20% of the FHB variance (Table 3), whereas in 2002, a year with conditions not conducive to high levels of infection, they explained just under 10% of the variance. The four CPFs explained 32 and 22% of the FDK variance in 2000 and 2001, respectively. The proportion of the FHB and FDK variance explained by each individual CPF was highly variable among years, suggesting that weather conditions during the growing season was the most important factor determining disease levels and the relative importance of each CPF. Indeed, when we pooled all the data and introduced a term in the model to represent the effect of growing season conditions (year term), the model explained 54% of the FHB variance, with the year term alone explaining 40% of the variance. The lower contribution of the year factor to the variance of percent FDK (1.2%) than to that of the FHB index reflects the lower variability in environmental conditions between 2000 and 2001 than among the 4 yr of the study (Table 1). The observation that the environment was the most important factor determining disease development and fungal populations agrees with previous studies in Minnesota (Dill-Macky and Jones, 2000), New Zealand (Cromey et al., 2002), and Ontario (Miller et al., 1998; Schaafsma et al., 2001), where disease levels were higher than those observed in our study area.
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0.01), the coefficient of determination was only 0.21. The observation that cultivar susceptibility accounted for a larger proportion of the variance of FDK than of FHB suggests that the intermediate resistance available in some common wheat cultivars in western Canada plays a more important role in infection and development of visual symptoms on the seeds than on the heads. It is not known, however, if any symptomless seeds harbored Fusarium infections.
Cultivar Susceptibility
In 2000 and 2001, susceptible cultivars had a significantly (P 0.10–0.01) higher mean FHB index than those with intermediate resistance (3.6% for susceptible, 1.8% for intermediate in 2000; 9.5% for susceptible, 7.6% for intermediate in 2001), whereas in 2000, susceptible cultivars had a higher (P 0.01) proportion of crops in the high FHB index class (66%) than cultivars with an intermediate level of resistance (37%). In both 2000 and 2001, susceptible cultivars also had a higher (P 0.01) mean percent FDK (0.9% for both 2000 and 2001) than cultivars with intermediate resistance (0.2% in 2000, 0.4% in 2001), and there was also a higher (P 0.01) proportion of grain samples from susceptible cultivars in the high FDK class than from cultivars with intermediate resistance (75% for susceptible, 27% for intermediate for 2000; 62% for susceptible, 39% for intermediate for 2001). As expected, differences in susceptibility to FHB among wheat cultivars did not play an important role under the low disease pressure experienced in 1999 and 2002. In a FHB survey conducted in Ontario, Schaafsma et al. (2001) reported that although wheat cultivar was the most important agronomic variable to influence DON level across the years of their study, no cultivar effect was observed in dry years.
Previously Grown Crop
There were significant effects of previously grown crop on the FHB index only in 2000 and 2002, and on percent FDK in 2001, although these were not consistent. In 2000, wheat crops with a significantly (P 0.10) higher proportion in the high FHB index class had been preceded by a cereal crop (71% compared with 33–49% for other crops or summerfallow), whereas in 2002, there were more wheat crops in the high FHB index class that were preceded by an oilseed crop (mostly canola) than by another cereal or a pulse crop (32% for oilseed crop, 16–17% for other crops, P 0.05). In 2001, compared with crops preceded by an oilseed or pulse crop, crops preceded by a cereal crop had more samples in the high percent FDK class (67% for cereal crops, 41–43% for other crops, P 0.10) and a higher mean percent FDK (0.9% for cereal crops, 0.5% for other crops, P 0.05).
Overall, our results agree with those of Obst et al. (1997) who found no rotation effect on disease development. The lack of significant or consistent effects of the previously grown crop on FHB or FDK levels in our study could be explained, at least partly, by the colonization by F. graminearum and F. avenaceum, the most important FHB pathogens in our study area, of residues of noncereal crops, including canola, flax, pea, and lentil (Fernandez, 1991; Fernandez et al., 2003). These pathogens were also isolated from roots of live noncereal crops grown in rotation with cereal crops in the same FHB-affected area of eastern Saskatchewan (Fernandez, 2003).
Previous GF Application and Tillage Management
The distribution of fields into high and low FHB index classes was affected by tillage system only in 2001, when over 50% of the fields under conventional-till and minimum-till were in the high FHB index class, compared with 39% of the fields under zero-till (Table 4). In no other year was there a significant effect (P > 0.10) of tillage system on the proportion of fields in the high FHB index class. However, the mean FHB index was significantly (P 0.10) affected by tillage system in 3 of the 4 yr (Table 5), suggesting that this CPF affected the severity of the infection to a greater extent than the spread of the disease. In general, the mean FHB index was highest under minimum-till and lowest under zero-till. Tillage system did not affect the proportion of fields in the high percent FDK class or the mean percent FDK in either 2000 or 2001 (P > 0.10) (data not shown).
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Dill-Macky and Jones (2000) attributed the lack of difference in FHB levels in wheat between chisel plow and zero-till treatments to the density and layering of residue in the latter which reduced residue to soil contact and might have affected the sporulation potential of the pathogen. The layering of the crop residues, poor fungal colonization of the upper fresh residues, leaching of antifungal compounds in initial stages of residue decomposition, and/or other factors related to the microenvironment might explain the lower disease levels under zero-till than minimum-till systems observed in our study. In a previous study, Fernandez et al. (1999b) observed a lower density of pseudothecia of Pyrenophora tritici-repentis (Died.) Drechs. (tan spot pathogen) under zero-till than conventional-till.
In any case, the significant effect of previous GF application on disease development suggests that the lower disease levels observed under zero-till compared with minimum-till management were not related to previous GF application, but to factors intrinsic to zero-till management and the lack of disturbance of residues which appears to have affected inoculum levels and/or its availability for head infection.
As indicated earlier, because GF applications depended on tillage system, we analyzed this factor as an effect nested within tillage system. Classifying fields into those that did not receive a GF application in the previous 18 mo and those that received at least one application in the same period of time, allowed us to verify that previous GF application produced in most cases no significant (P > 0.10) change in the mean FHB index of wheat grown in fields under conventional-till or zero-till management (Table 6). However, in most cases, wheat crops under conventional-till or zero-till grown in fields previously treated with GF had a higher mean FHB index, although this was only significant in 1999 for conventional-till (P 0.01). Under minimum-till, application of GF at least once in the previous 18 mo produced a significant (P 0.10–0.01) increase in the mean FHB index and in the proportion of fields in the high FHB index class (P 0.05) in every year of the study (Tables 6 and 7). The same observation was made for mean percent FDK (Table 8) and the proportion of fields in the high percent FDK class (Table 9). The higher percent FDK levels in wheat crops grown in GF-treated fields would have resulted in further loss of market value. For 2000 and 2001, the years with the greatest disease development, the magnitude of the increase in disease levels observed in wheat crops grown in GF-treated fields under minimum-till management was an average of 128% for the mean FHB index and 147% for mean percent FDK. Furthermore, in 2001, analysis of crops according to the number of GF applications received in the previous 3 yr showed that it took more than two GF applications to observe a significant increase in the FHB index (data not shown).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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On the basis of the observations made across years, we concluded that in eastern Saskatchewan, growing spring wheat in fields where GF had been previously applied, and growing susceptible wheat cultivars under minimum-till management, resulted in the most damage due to FHB in years conducive to disease development. Factors other than previous GF application appeared to have resulted in lower FHB levels in wheat crops grown under zero-till than minimum-till management.
Previous GF application was the only CPF that was significantly associated with higher FHB levels every year of the study and one of only two CPFs associated with higher percent FDK in 2000 and 2001. Its effect on the FHB index did not appear to be influenced by environmental conditions as much as for other CPFs whose effects on disease levels were not consistent from year to year. However, it is not known if a similar association of GF application and FHB would occur in environments less or more conducive to disease development.
On the basis of the statistically significant and consistent association between previous GF application and FHB development in spring wheat throughout our 4-yr study conducted in producers' fields, further research is needed to elucidate the underlying mechanisms determining these effects.
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ACKNOWLEDGMENTS |
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Received for publication March 26, 2004.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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