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Published online 1 January 2007
Published in Agron J 99:113-121 (2007)
DOI: 10.2134/agronj2006.0171
© 2007 American Society of Agronomy
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Disease Management

Planting Date and Nitrogen Effects on Fusarium Head Blight and Leaf Spotting Diseases in Spring Wheat

K. D. Subedi*, B. L. Ma and A. G. Xue

Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Research Center (ECORC), Central Experimental Farm, K.W. Neatby Bldg., 960 Carling Ave., Ottawa, ON, Canada, K1A 0C6

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

Received for publication June 9, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Cultural management practices play important roles in minimizing losses caused by Fusarium head blight (FHB) and leaf spotting diseases (leaf-spot) in wheat (Triticum aestivum L.). A field experiment was conducted in Ottawa, Canada, for 3 yr (2003–2005) in two sites representing clay loam and sandy loam soils to examine the effects of planting date and N management on the incidence and severity of FHB and leaf-spot {tan spot, Septoria leaf blotch and Stagonospora nodorum blotch) in spring wheat. The cultivar AC Brio was seeded at three dates at approximately 10-d intervals starting in the last week of April. The five N treatments included 0, 60, and 100 kg N ha–1 applied as starter; 60 + 40 kg N ha–1 (starter + top-dress at boot stage); and 60 + 40 kg N ha–1 (starter + foliar spray at boot stage). Generally, incidence of FHB and severity of leaf-spot were greater in the heavy-textured soil than in the light-textured soil. Across site–year, the incidence of FHB ranged from 13 to 45%, and severity varied from 14 and 52%. On both soils, planting date had a consistently greater effect on the incidence and severity of FHB: the later the planting date beyond 9 May, the greater the incidence of FHB. The late-planted wheat also had the greatest severity of leaf-spot. Planting after 9 May also significantly reduced grain yield by 15 to 45%. Wheat grown without added N had the greatest incidence of FHB and leaf-spot severities than that supplied with adequate N, especially in the heavy-textured soil. The effect of N on FHB was not consistent over site–years, but leaf-spot was significantly reduced with a starter application of N at 100 kg ha–1. Our data indicate that environmental variation induced by different planting dates or by different growing seasons and site appeared to have greater effects on FHB and leaf-spot in wheat than N application. The results suggest that the incidence of FHB and severity of leaf-spot in spring wheat can be minimized through early planting combined with adequate starter N application.

Abbreviations: DON, dexynivalenol • FHB, Fusarium head blight • GS, growth stage • LAS, leaf area symptoms • RH, relative humidity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
FUSARIUM HEAD BLIGHT (FHB) or scab, caused by Fusarium graminearum Schwabe (teleomorph: Gibberella zeae) is one of the most devastating diseases of wheat, barley (Hordeum vulgare L.) and other cereals in North America (Bai and Shaner, 1994; McMullen et al., 1997; Markell and Francl, 2003; Xue et al., 2004). Historically, the disease was more prevalent in eastern Canada than in the West (Sutton, 1982; Gilbert and Tekauz, 2000). In a survey of 28 random fields in Ontario, Xue et al. (2005) observed FHB in 27 fields and the incidence of disease ranged from 10 to 100% with a mean of 51%.

The disease reduces grain yield through in a number of ways including floret sterility, poor seed filling, and reduced seed size (Argyris et al., 2003; Hatcher et al., 2003). In addition to the significant loss in yield, production of the mycotoxin dexynivalenol (DON) in the infected spikes can make the grain unsuitable for food and/or feed (Humphreys et al., 2001; Hatcher et al., 2003; Liu and Anderson, 2003; Miedaner et al., 2003). Fusarium-infested seeds also reduce seedling emergence and number of tillers per unit area (Gilbert et al., 2003). Crop debris, particularly cereal stubble, hosts the fungal inoculum (Dill-Macky and Jones, 2000; Fernandez et al., 2001). Severity of the disease increases when heading or flowering periods of susceptible crops coincide with wet and warm weather (Bai and Shaner, 1994; Fernandez et al., 2001; Gilbert et al., 2003).

Host resistance has long been considered the most practical and effective means of controlling FHB (Sutton, 1982; Liu and Anderson, 2003). Several studies have reported differences between susceptible and moderately resistant cultivars in both the infection severity and the mode of resistance (Ribichich et al., 2000; Miedaner et al., 2003). There is a consensus that resistant cultivars will provide the most stable and durable solution to the problem (Gilbert and Tekauz, 2000). However, no cultivars are yet available with complete resistance (Chongo et al., 2001). Even with resistant cultivars, growing environment may influence the occurrence of disease. Hall and van Sanford (2003) reported a highly significant genotype-by-environment interaction for FHB resistance.

Until cultivars with high levels of resistance to FHB are developed, integrated crop management practices including foliar fungicide application, crop rotation and tillage practices may be the best means to keep the disease under control. Parry et al. (1995) reviewed FHB in small cereals and concluded that there were surprisingly few reports of successful fungicidal or biological control treatments for FHB in the field. Dion et al. (2001) reported some benefits of fungicide spray application on controlling FHB, but did not detect significant differences in DON content. Similarly, Martin (2001) concluded that foliar fungicides were inconsistent in their effectiveness to control FHB in cereals.

Environment has a major role on growth, survival, and dissemination of the pathogen and hence the incidence and disease severity of FHB (Parry et al., 1995; Gilbert and Tekauz, 2000). Frequent rainfall, high humidity, and heavy dew that coincide with flowering and early kernel-fill period of the crop favor infection and development of disease (McMullen et al., 1997). Most species of Fusarium produce inocula, grow best, and are pathogenic to cereal heads at warm temperature and under moist conditions (Bai and Shaner, 1994; Doohan et al., 2003; Xu, 2003; Brennan et al., 2005). Similarly, Bai and Shaner (1994) and Brennan et al. (2005) also observed that the incidence of FHB increased under wet and hot conditions. Brennan et al. (2005) reported that F. graminearum inoculation caused greater visual disease symptoms and greater yield loss of wheat at a higher (20°C) temperature than at lower (16°C) temperatures. The time of wheat planting therefore, can influence the incidence of disease by inducing temperature and moisture variability in both soil and air. Xue et al. (2004) reported that the incidence of Fusarium spp. in wheat increased by twofold when harvesting was delayed compared to early harvest. However, no study has investigated the effect of early or late planting on the incidence and severity of FHB in spring wheat.

Some researches have examined how certain agronomic and nutrient management practices affect the incidence of FHB and leaf-spot. Martin et al. (1991) and Lemmens et al. (2004) observed a significant increase in FHB intensity with increasing N. In contrast, Teich and Nelson (1984), while surveying FHB under several cultural practice regimes showed that fields with moderate to low N had significantly higher FHB than with high N. The effects of N application methods, timing and rates on FHB and leaf-spot are not consistent, and is a subject of further investigation.

Tan spot [Pyrenophora tritici-repentis (Died). Shoemaker] of wheat is a destructive disease found in wheat-growing regions worldwide that can lead to serious yield loss (Ciuffetti and Touri, 1999). In addition to tan spot, Septoria leaf blotch (Septoria tritici Roberge in Desmaz.) and Stagonospora nodorum blotch (Stagonospora nodorum Berk.) are also predominant, having significant impact on wheat yield in the northern Great Plains (Krupinsky et al., 1994). Nitrogen may play an important role in the development of such diseases (Krupinisky and Tanaka, 2001). Generally, increases in N application favors leaf-spot disease. However, Fernandez et al. (1998) found that the severity of leaf-spot in wheat was greater when N was deficient. Krupinsky and Tanaka (2001) observed that the severity of leaf-spot disease was consistently lower in the N applied treatment. In contrast, Olesen et al. (2003) reported an increased severity of Septoria leaf blotch and powdery mildew [Blumeria graminis (DC) E.O. Speer f. sp. tritici Ém. Marchal] in winter wheat, with increased N application.

Planting date affects leaf-spot by avoiding favorable weather conditions for disease development. Simon et al. (2004) observed that late heading date was positively associated with more necrosis because of favorable weather conditions. To what extent planting date and N management affect leaf-spot in the humid temperate environment of eastern Canada is not studied.

The objective of this study was to determine the effects of planting date and N fertilization methods, rates and timing on the incidence and severity of FHB and leaf-spot in spring wheat.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Field experiments were conducted at two sites in Ottawa, Canada (45°17' N, 75°43'' W and 45°17' N, 75°45'' W) for 3 yr (2003–2005). Site 1 had a light (sandy loam) soil while Site 2 represented a heavy-textured (loam to silty-loam) soil. At both sites, wheat was the preceding crop so as to increase the opportunity for natural infestation of FHB and leaf-spot. The two sites were separated by a distance of approximately 15 km, but had similar climate. Soil samples from 0- to 30-cm depth were analyzed for available NO3–N, P, K, and pH. The available NO3–N was 3.5 to 4.1 mg kg–1 at Site 1, and from 5.4 to 9.5 mg kg–1 at Site 2. Soil pH ranged from 6.1 to 6.9.

In each site–year, a 3 x 5 factorial experiment was laid out in a split-plot design with four replications. Three planting dates at approximately 10-d intervals (Table 1) were the main-plot and five N treatments [N1, 0 kg N ha–1; N2, 60 kg N ha–1 all starter; N3, 100 kg N ha–1 all starter; N4, 60 kg N ha–1 starter + 40 kg N ha–1 top-dress at boot stage; and N5, 60 kg N ha–1 basal + 40 kg N ha–1 foliar spray GS 45] were randomly arranged within each main plot. Each subplot consisted of 16 rows spaced 20 cm within 9 m row length (28.8 m2). A commercial cultivar of spring wheat (AC Brio) was planted at 300 seeds m–2 with a grain drill. Seeds were treated with fungicide Vitaflo-280 (carbathiin + thiram) before sowing. Fertilizers P and K were broadcast applied before sowing according to soil test recommendations. Nitrogen fertilizers were applied with ammonium nitrate (NH4NO3, 33.5% N) according to the treatment specification. For the N4 treatments, 60 kg N ha–1 as NH4NO3 was applied at planting, and 40 kg N ha–1 as NH4NO3 was broadcast evenly at growth stage (GS) 45 (Zadoks et al., 1974) using a hand-held spreader. For the N5 treatment, liquid urea ammonium nitrate (28% N) was diluted in water and sprayed over the crop at boot stage through a boom fitted with six nozzles connected to a tractor driven sprayer, with a spray width that covered each plot in one pass. The spray volume was 300 L ha–1, at a pressure of 207 kPa.


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  		Table 1. Planting dates, days (± standard deviation) to different phenological events and timing of Fusarium graminearum inoculum application at two experimental sites in 2003, 2004, and 2005.

 
Postemergence herbicide Buctril M (bromoxynil/MCPA) was applied at 0.56 kg (a.i.) ha–1 in 200 L water to control weeds. To supplement natural infection of Fusarium, plots were inoculated with artificially infested barley and corn (Zea mays L.) kernels that had been autoclaved, inoculated with three strains of F. graminearum (FGAC Taho, FG178148, and FG Orono), separately in three trays and were incubated at 20°C for 4 to 6 wk and air-dried. After incubation, the inocula were thoroughly mixed and evenly broadcast over the crop twice, when at the flag leaf ligule was visible (GS 39) and at the beginning of heading (GS 49), separately for each planting date (Table 1). At each application, about 1.7 kg of inoculums was applied in each plot. Precautions were taken to avoid cross-infection such that windy and rainy periods were avoided during inoculation.

Daily mean temperatures and relative humidity (RH) were recorded at the Automated Weather Station of the Central Experimental Farm from the first planting date to the harvest of last planting. These weather data were presented in average of every 10 d (Fig. 1 ). The time taken from planting to seedling emergence, booting, completion of heading (GS 59), anthesis (GS 69) and physiological maturity (GS 93) were recorded. Detailed measurements of grain yield and its components are presented by Subedi et al. (2006). At around milk-dough stage (GS 83–85), field assessment of the foliar disease complex—mainly tan spot, Septoria leaf blotch and Stagonospora nodorum blotch—ratings were made on the flag and penultimate leaves for all planting dates. A total of 30 culms with a spike per plot were randomly selected and the severity of foliar disease on each leaf was scored using a 0 to 7 scale (8 values at 12.5% each class). This scale was then converted to leaf area with symptoms (LAS) on a 0 to 100% scale. At milk-dough stage (GS 85), the incidence and severity of FHB was assessed in 200 random heads plot–1. The incidence of FHB was determined as percentage of heads with necrotic spikelets infected with Fusarium, and FHB severity was estimated as the percentage of infected spikelets on each spike. The FHB Index was then calculated as (% FHB incidence x % FHB severity)/100. Grain yield was measured by harvesting the eight central rows. Grain samples were oven-dried at 80°C until a constant weight and yields were corrected at 135 g kg–1 moisture.


Figure 1
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  		Fig. 1. Daily mean temperature (°C) and relative humidity (RH %) averaged over 10-d intervals from planting to maturity of wheat at the Central Experimental Farm, Ottawa, in 2003, 2004, and 2005.

 
Experimental data from each year were subjected to analysis of variance using a General Linear Models procedure (PROC GLM, SAS Institute, 1996). Treatment mean differences were compared using the least significant difference (LSD0.05) test if the F tests were significant (P ≤ 0.05). Correlation analysis of grain yield was performed with FHB incidence, FHB Index, and severity of leaf-spot on flag leaf.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Growing Environment
Weather remained wet throughout the 2003 growing season with regular rainfalls. Accumulated precipitation during the growing season (planting to physiological maturity) was about 300 mm for all the planting dates at both sites except the last planting date at Site 2, which received only 250 mm of total precipitation. In 2004, all planting dates received more or less the same amounts of rain (200 mm). Although, the total rainfall was greater in 2003, distribution of rainfall was more uniform during the 2004 growing season. In 2005, total rainfall were 274 mm for the first planting, 245 mm for the second planting and 235 mm for the third planting, while the total seasonal rain from April to September was 350 mm.

Both temperature and RH fluctuated more in 2003 and 2005 than in 2004 (Fig. 1). In 2005, the mean temperature at the first planting date was around 7°C as compared to around 10°C in other years. Mean RH remained at {approx}80% from heading (later part of June to Mid-July) to maturity in 2003 and 2004, but was {approx}75% in 2005 for the same period.

Phenology
Crop maturity was reached within 106 d for all planting dates and N treatments in all years (Table 1). There was no planting date-by-N interaction on the number of days taken to complete different phenological stages. The experiment at Site 1 in 2005 was abandoned for final observations because of a severe yellowing with a patchy appearance of the crop. Crops in the heavy-textured soil (Site 2) took longer (2–5 d) to reach GS 45, GS 59 and GS 93 than those in the light-textured soil, except in the third planting at Site 2, which took overall shorter duration due to late planting (Table 1). Date of planting had significant effects (P < 0.001) on the number of days to reach different physiological stages and total crop duration under both soil types in both years. The later the crop was planted, the fewer number of days it took to reach heading and maturity. Nitrogen treatment had a significant effect only on time taken to reach GS 93. In all cases, the zero N treatment matured significantly earlier, by 2 to 4 d, than the other N treatments, which took a similar duration (data not shown), possibly due to early senescence. There was no significant alteration in the duration of grain filling (i.e., period from GS 69 to GS 93) due to rate, timing, and method of N application (data not presented).

Effect on Leaf-Spot Diseases
A separate account was not kept for each disease, and the severity of the leaf-spot complex was expressed as percentage area with symptoms. Tan spot was the predominant leaf spotting disease followed by Septoria and Stagonospora leaf blotches in all the site–years. There was a greater severity at Site 2 than at Site 1 in all years (Fig. 2 and 3 ). Irrespective of site and year, the severity was greater on the penultimate leaf (25–50%) than on the flag leaf (10–40%). Wheat plants within the last planting date had greater severity on the penultimate leaf than plants sown at the two earlier dates (Fig. 2A, 2C). On the flag leaf, planting date had no effect on the incidence of leaf-spot in 2003 at Site 1, and in 2004 at Site 2 (Fig. 2B, 2D). In the other cases, the severity in the flag leaf was greater in the last planting date and the first planting date tended always to have lower severity compared to later dates.


Figure 2
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  		Fig. 2. Effect of planting date on the incidence of leaf-spot in the penultimate leaf (A and C) and flag leaf (B and D) of wheat expressed as leaf area with symptoms (%) in 2003 and 2004 at Site 1; and in 2003, 2004, and 2005 at Site 2. The D1, D2, and D3 are the first, second, and third planting dates, respectively, in each site–year. Bars labeled with different letters within each year–site are significantly different at P ≤ 0.05.

 

Figure 3
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  		Fig. 3. Effect of N treatment on the severity of foliar diseases in the penultimate leaf (A and C), and flag leaf (B and D) of wheat expressed as leaf area with symptoms (%) at Site 1 in 2003 and 2004; and at Site 2 in 2003, 2004, and 2005 growing seasons. Bars labeled with different letters within each site–year are significantly different at P ≤ 0.05. The N treatments are: N1, 0 kg N ha–1; N2, 60 kg N ha–1 all starter; N3, 100 kg N ha–1 all starter; N4, 60 kg N ha–1 starter + 40 kg N ha–1 top-dress at boot stage; and N5, 60 kg N ha–1 basal + 40 kg N ha–1 foliar spray GS-45.

 
Nitrogen treatment had significant effects on leaf-spot especially on the penultimate leaf in all site–years (Fig. 3A, 3C). However, the pattern of response to N differed markedly between the two sites and between the 3 yr. At Site 1, wheat plants in the 0 N treatment, disease was less severe on the penultimate leaf, than in the N4 and N5 treatments (Fig. 3A). The reverse was observed at Site 2, where the 0 N treatment (N1) had significantly greater severity than the other treatments except in 2005 (Fig. 3C). Generally, disease was less severe in the N3 treatment (i.e., 100 kg N ha–1 at planting) than in the other N treatments. On the flag leaf, there was no clear N treatment trend at Site 1 (Fig. 3B). Severity was greater in N1 at Site 2 in all years (Fig. 3D). At Site 2, the severity on penultimate leaf responded to N similarly to the flag leaf.

A planting date-by-N interaction was significant (P < 0.05) for leaf-spot severity at Site 2 in 2003 and 2005 (Table 2). In 2003, there was no difference among N treatments within the first and third planting dates, but treatment differences were observed in the second (i.e., normal planting) date. The N1 treatment had greater severity than the other N treatments. In 2005, severity was significantly different only in the first planting date; the N1 treatment had the highest leaf spot diseases followed by the N5 treatment, and the N4 treatment had the lowest incidence. It was observed that the lower the amount of N supplied, the greater was the severity of leaf-spot.


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  		Table 2. Interactions between planting date and N treatment on the severity of leaf-spot in the flag leaf of wheat expressed as leaf area with symptoms (%) at Site 2 in 2003 and 2005. The least significant difference (LSD0.05) for means separation is 19.8 for 2003 and 9.3 for 2005.

 
Planting Date Effects on the Incidence and Severity of Fusarium Head Blight
No interactions were observed between planting date and N treatment for the incidence of FHB in any site–years. The incidence of FHB was very high especially in 2003 at Site 2 (Fig. 4 ). Planting date had an inconsistent though pronounced effect on FHB incidence. At Site 1 in both years, the later the planting date, the higher the incidence of FHB; the last planting date had the greatest incidence of disease in both years. At Site 2, there was no difference in the three planting dates in 2004, but the wheat sown at the second planting date (i.e., the optimum time) had the lowest incidence of FHB than the other two dates in 2003. In 2005, the FHB incidence progressed from the first to the last date of planting. With the exception of Site 2 in 2004, the general trend was that the later plantings resulted in higher FHB incidence. There was a significant linear relationship between planting date and FHB incidence, and about 32 to 54% variability were explained (Fig. 5 ).


Figure 4
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  		Fig. 4. Effect of planting date on the incidence (%), severity (%), and disease index of Fusarium head blight in 2003 and 2004 at Site 1; and in 2003, 2004, and 2005 at Site 2. The points labeled with different letters within each site–year are significantly different at P ≤ 0.05.

 

Figure 5
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  		Fig. 5. Relationships between planting date and incidence of Fusarium head blight in 2003 and 2004 at Site 1; and 2003, 2004, and 2005 at Site 2.

 
In contrast to the incidence, the severity of FHB was higher in 2004 than in 2003 and 2005 (Fig. 4). The effect of planting date was inconsistent over site–years. At Site 1, the third planting had the greatest FHB severity followed by the first and second planting dates in 2003. In contrast, the severity of FHB in the third planting date was lower than in the earlier planting dates in 2004. At Site 2, the first planting had greater disease severity followed by the second and third plantings in 2003, but there was no difference among the planting dates in 2004. In 2005 also, wheat sown at the first planting date had higher FHB severity than the other two dates. Therefore, unlike FHB incidence, there was no clear trend of FHB severity due to planting dates. However, it was observed that the first planting date had generally greater severity of FHB than the later planting dates.

There were differences in the FHB Index in all site–years except Site 1 in 2004 (Fig. 4). In 2003, at Site 1, the greatest FHB Index occurred in the third planting followed by the second and first plantings. At Site 2, there were not consistent patterns in 3 yr. In 2003, the FHB index was very high and the first planting date had the highest Index followed by the third planting and second planting. In 2004, the second planting date had the lowest FHB Index while the first and third dates were similar. The results in 2005 were different than in 2004 such that first and second planting dates had similar and significantly lower FHB Index than the third planting date. In four out of five site–years where planting date effects were significant, the optimum planting date (i.e., second planting) had lower FHB index than the last planting date.

Nitrogen Effects on Incidence and Severity of Fusarium Head Blight
Although N treatment had some significant effects on the incidence and severity of FHB in both years at Site 1 and only in 2003 at Site 2, the differences were practically small and inconsistent among N treatments, over years and sites (data not shown). Therefore, the effects of N on the incidence and severity of FHB were inclusive and not of practical significance.

Grain Yield
Effect of planting date on grain yield is summarized in Table 3. Grain yields ranged from 1.5 to 3.8 Mg ha–1, and Site 2 had slightly greater grain yields in all years than the Site 1. Year 2004 had the lowest grain yields at both sites than other years. Planting date had significant effects on grain yield in all site–years except at Site 2, in 2004. In all of the site–years where effects of planting date were significant, the third planting date had the lowest grain yields, while the first and second planting dates had similar grain yields. Grain yields were reduced by 15% (2003) to 26% (2004) at Site 1, and by as much as 45% in 2003 at Site 2 due to late planting (i.e., the third planting date) as compared to the normal planting date (i.e., the second planting). In all site–years, there was no difference in grain yield between the first and second plantings. Nitrogen treatments affected grain yield in all site–years, but the only difference occurred between the 0 N treatments and the rest of the N treatments (data not shown). Correlation analysis showed that grain yield was more affected by leaf-spot in flag leaf (r = –0.28 to –0.58, N = 60) than with the incidence of FHB (r = –0.24 to –0.41) or FHB Index (r = –0.16 to –0.27).


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  		Table 3. Effect of planting date on the grain yield of wheat at two sites for 3 yr, averaged over five N treatments.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This study examined the possibility of managing FHB disease through adjusting planting date and N amendments. The severity of FHB differed considerably between years and sites, reflecting climatic effect is predominant. Planting dates had larger effects on disease incidence and grain yield of spring wheat than N treatments. Irrespective of N treatment, delayed planting beyond 9 May resulted in a significantly higher incidence of FHB in four out of five site–years. Late-planted wheat also tended to have more foliar diseases in three out of five site–years (Fig. 2). Planting date had also a significant effect on the incidence of FHB, but the patterns differed between the two sites, most likely associated with variations in soil and weather conditions. When the FHB incidence data were plotted against the planting dates in 3 yr, there was a positive linear relationship (Fig. 5) that also indicated that the later the crop was planted the more it suffered with FHB. However, not all of the variability was explained. The smaller R2 values were as a result of inconsistent results over site–year. The inconsistent results were partly because of the variation in planting dates and weather patterns over sites and years. For example, the third planting in 2003 at Site 2 was exceptionally late while the first planting in 2003 at the same site was earlier than other years.

High temperatures combined with high humidity might have increased the incidence of FHB and other foliar diseases in late-planted plots. Xue et al. (2004) observed that variation in the incidence of FHB for location and year were likely related to the levels of inoculum and weather conditions before and during harvesting time. They also noted that environmental conditions before and during harvest time had a significant impact on the occurrence and frequency of the various FHB species in spring wheat. Fernandez et al. (2001) reported that when flowering periods of susceptible crop coincided with the favorable weather conditions (particularly wet and warm), the severity of FHB increases. When the weather parameters during the flowering period are examined, postheading period of the crops planted in the third dates in all site–years coincides with the periods having humidity >80% (Fig. 1). For example, the crop planted on 6 June at Site 2 in 2003 started heading 44 d after planting. This period was around 20 July; during this period, the relative humidity was at its peak. Similarly, in 2004, the wheat planted on the third date (19 May) reached heading after 55 d, which was around July 13. During this period, both the mean temperature (>20°C) and relative humidity (>80%) peaked. The higher temperature combined with higher humidity increased the incidence of FHB and other foliar diseases in the late-planted wheat. Brennan et al. (2005) also observed greater visual FHB symptoms and yield loss at higher temperature than at lower temperatures. The favorable weather factors and build-up of inoculum in the field increased the incidence of FHB in the later planted wheat.

There was an interesting observation that the severity of leaf-spot on the flag leaf was higher in N1 treatment than the other N treatments, and lowest with the high N (i.e., N5, 100 kg N ha–1) treatment, but only at Site 2 (Fig. 3). This might have been because the crop with high N remained greener and healthy for longer than the one with limited N, which dried quickly in which severity of leaf-spot progressed rapidly by the time of sampling (GS 85).

Nitrogen treatments showed some effect on FHB severity in a sandy soil (i.e., Site 1) but not at Site 2. The inherent N availability was greater in the heavy-textured soils of Site 2; therefore, the effects of N treatment on FHB severity were smaller, although the overall disease severity (26–37%) was comparable to Site 1 (24–48%).

Unlike the observations of Stack et al. (1986) and Martin et al. (1991), the rate and timing of N application had minimal effects on FHB incidence and severity. Results of earlier studies suggested that high N rates, especially when applied as foliar spray, would increase the incidence of FHB, but our study showed no such indication even in the foliar N applied treatment. Interestingly, there was a greater incidence of FHB in the unfertilized (0 N) treatment in two out of five site–years, which also agrees with the findings of Lemmens et al. (2004). The variations in weather occurring at different planting dates were a greater factor contributing to the elevated FHB incidence than the rates, timing and method of N application. Moreover, the inconsistent N treatment effects across site–years indicate that the growing environment plays a more important role for the infection and spreading of FHB than the methods and rates of N application.

Response of grain yield to N application rates, timing and method was smaller than the effect of planting date. There was no difference in grain yield due to N application from 60 to 100 kg ha–1. Similarly, split application of N at boot stage as top-dress or as foliar spray resulted in no yield advantage over the starter application of the same amount of N. Grain yield was substantially reduced when planting was delayed, and especially so at Site 2 in 2003 where planting was exceptionally delayed. There was an indication that the significantly lower grain yield in the late-planted wheat was partly due to greater severity of leaf-spot and incidence of FHB. For example, the incidence of FHB was significantly greater in the third planting dates in both years at Site 1 and in 2003 at Site 2, and grain yields were also significantly reduced as compared to the other planting dates in these site–years. The negative correlations of grain yield with leaf-spot and FHB incidence also support this observation.

This study demonstrates that planting as early as possible (by the 1st week of May) would help to minimize the incidence and severity of FHB in spring wheat under eastern Ontario conditions. Delaying planting will likely result in higher incidences of leaf-spot and FHB, and eventually yield penalty. Nitrogen application methods and timing had no direct and consistent effects on FHB incidence and severity of leaf-spot, but there were indications that wheat with no added N were affected most by FHB and leaf-spot than the wheat crop that received sufficient N. No advantage of split-application or foliar application of N was realized in terms of reduced diseases in question or grain yield. The inconsistent effects of N on FHB and leaf-spot diseases suggests that weather variable as created by different planting dates over years and soil differences have greater roles in the incidence and severity of these diseases than N management.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge the excellent technical assistance of L. Evenson, Y. Chen, and D. Balchin. ECORC contribution no. 05-537.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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