Published in Agron J 100:145-153 (2008)
DOI: 10.2134/agrojnl2007.0092
© 2008 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
CROPPING SYSTEMS
Management Practices to Minimize Tan Spot in a Continuous Wheat Rotation
M. Carignano,
S. A. Staggenborg* and
J. P. Shroyer
Dep. of Agronomy, Kansas State Univ., 2004 Throckmorton Plant Sci. Ctr. Manhattan, KS 66506. Contribution no. 07-189-J from the Kansas Agric. Exp. Stn
* Corresponding author (sstaggen{at}ksu.edu).
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ABSTRACT
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In the central United States, practices that maintain residue in wheat (Tritricum aestivum L.) often lead to yield losses from tan spot (Pyrenophora tritici-repentis) infections. Tillage, fungicides, N fertility, and resistant varieties may reduce tan spot severity. Studies were conducted over five location-years to determine wheat yields and tan spot severity across three residue levels (no-till, reduced till, and burned) and cultivar susceptibility to tan spot. Nitrogen fertilizer and fungicide treatments were also evaluated for their influence on tan spot severity and wheat yields. Tan spot severity ranged from 12 to 64%, based on leaf ratings at anthesis across four environments when second year wheat was no-till planted. Removing previous crop residue by burning or tillage, host plant resistance and fungicide applications reduced tan spot severity across four environments 49, 30, 58, and 93%, respectively. Severities were higher with the susceptible cultivar when planted no-till and with no fungicides applied. Fungicide applications on the susceptible cultivar improved yields 14%, compared with only a 3% in the resistant cultivar. Under a severe tan spot infection, fungicide applications improved yields 34% for the susceptible cultivar and 10% resistant cultivar. Fungicide applications had no effect in three of the five environments. Burning and tillage increased yields 11 and 2% compared with no-till, respectively. Kernel weight was the most commonly affected yield component because tan spot infections affect flag leaf health during grain fill. These results suggest that it may be possible to develop no-till rotations with wheat following wheat without significant risk to producers.
Management Practices to Minimize Tan Spot in a Continuous Wheat Rotation
M. Carignano,
S. A. Staggenborg* and
J. P. Shroyer
Dep. of Agronomy, Kansas State Univ., 2004 Throckmorton Plant Sci. Ctr. Manhattan, KS 66506. Contribution no. 07-189-J from the Kansas Agric. Exp. Stn
* Corresponding author (sstaggen{at}ksu.edu).
Received for publication March 13, 2007.
In the central United States, practices that maintain residue in wheat (Tritricum aestivum L.) often lead to yield losses from tan spot (Pyrenophora tritici-repentis) infections. Tillage, fungicides, N fertility, and resistant varieties may reduce tan spot severity. Studies were conducted over five location-years to determine wheat yields and tan spot severity across three residue levels (no-till, reduced till, and burned) and cultivar susceptibility to tan spot. Nitrogen fertilizer and fungicide treatments were also evaluated for their influence on tan spot severity and wheat yields. Tan spot severity ranged from 12 to 64%, based on leaf ratings at anthesis across four environments when second year wheat was no-till planted. Removing previous crop residue by burning or tillage, host plant resistance and fungicide applications reduced tan spot severity across four environments 49, 30, 58, and 93%, respectively. Severities were higher with the susceptible cultivar when planted no-till and with no fungicides applied. Fungicide applications on the susceptible cultivar improved yields 14%, compared with only a 3% in the resistant cultivar. Under a severe tan spot infection, fungicide applications improved yields 34% for the susceptible cultivar and 10% resistant cultivar. Fungicide applications had no effect in three of the five environments. Burning and tillage increased yields 11 and 2% compared with no-till, respectively. Kernel weight was the most commonly affected yield component because tan spot infections affect flag leaf health during grain fill. These results suggest that it may be possible to develop no-till rotations with wheat following wheat without significant risk to producers.
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INTRODUCTION
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THE INCORPORATION OF CONTINUOUS WHEAT into reduced or no-tillage systems presents a disease-management challenge which is less severe when tillage is used. Large amounts of residue favor higher levels of leaf and spike diseases. Tan spot is an important disease in winter wheat around the world and is caused by the fungus Pyrenophora tritici-repentis. In the sexual stage, the pathogen develops pseudothecia in wheat residue. Pseudothecia mature during the fall and winter, creating ascospores. Ascospores are the primary inoculum discharged during wet periods in the fall or spring (De Wolf et al., 1998). The amount of primary inoculum is an important determinant of the disease severity during the growing season (Bockus et al., 1992). The secondary, or asexual cycle of tan spot involves conidiation on the primary lesions, liberation and dispersal of conidia that infect the upper leaves in the wheat canopy, thereby reducing their photosynthetic capacity during grain fill. Under severe epidemics, tan spot has been reported to reduce wheat yields 20 to 50% (Sharp et al., 1976; Rees and Platz, 1983; Shabeer and Bockus, 1988). Kernel weight and kernels spike–1 were the yield components most affected by the pathogen. Since continuous wheat production is common in the central United States wheat-growing region that encompasses Texas, Colorado, Oklahoma, Kansas, Nebraska, South Dakota, and North Dakota, residue management strategies, such as moldboard plowing and burning, are often used to lower residue-borne diseases like tan spot (Rees and Platz, 1979).
The need to reduce soil erosion and maximize soil water conservation for optimum wheat production has led to the development of reduced and no-till systems, where more crop residue remains on the soil surface and the soil receives minimal or no disturbance during noncrop periods. Research has been conducted comparing conventional and conservation tillage systems for water storage capacity, organic matter content and soil temperature. However, this information is limited, especially studies where burned wheat stubble management is compared with conventional or conservation tillage systems.
It is well known and documented that no-till practices maximize water conservation and precipitation use efficiency. Maintaining crop residue on the surface reduces evaporation by insulating the soil, reflecting incident solar radiation, and decreasing surface wind velocity (Greb, 1983). As a result, more soil water is available for crop growth. An additional 60 mm of available soil water resulted for each Mg ha–1 of crop residue maintained on the surface in the northern Great Plains (Black, 1973). Maintaining crop residue on the soil surface resulted in an additional 26 mm (Greb, 1983) and 20 mm (Wilhelm et al., 1989) of available soil water compared with tilled fallow in the central Great Plains. Wilhelm et al. (1989) demonstrated a linear relationship existed between the amount of residue on the soil surface and water stored in the soil in the Central Great Plains.
A decrease in tillage intensity also contributes to building soil quality by conserving or increasing the amount of soil organic matter (Havlin et al., 1992; Alvear et al., 2005). Studies in Kansas have quantified the relationship between topsoil erosion and wheat productivity. In a Ulysses soil, for every 1 cm of topsoil eroded, organic matter decreased 0.04% More importantly, wheat yields decreased 44 kg ha–1 per 1 cm of topsoil lost (Havlin et al., 1992). In a 12-yr continuous wheat system, organic matter measured as a percentage of the native soil decreased 2%, whereas organic matter decreased 8 and 15% with stubble mulch and conventional tillage, respectively (Lamb et al., 1985). Therefore, less soil disturbance by tillage practices will maintain or increase soil organic matter, producing a direct effect in soil productivity (Diaz-Zorita et al., 1999).
Wheat management practices other than tillage have been reported to affect tan spot and other leaf diseases and reduce their impact on wheat yields. These practices include proper N management, selecting wheat cultivars that are resistant to leaf diseases, and the use of foliar fungicides. Increasing N fertilizer rates have been observed to reduce the development of tan spot lesions in susceptible cultivars (Huber et al., 1987). Bockus and Davis (1993) suggested N fertilizer applications do not have a direct effect on tan spot severity, but rather appear to reduce disease impacts through delayed leaf senescence. However, high N rates have been reported to increase tan spot severity due to high biomass production which created a micro-environment conducive to fungal development in the humid environments of New York (Cox et al., 1989) and Tennessee (Roberts et al., 2004).
In the central wheat growing region, sources of resistance have been identified and incorporated into adapted cultivars to improve disease resistance (Bockus, 1998; Cox et al., 1989). This form of control is relatively inexpensive and environmentally sound. Bockus et al. (1997) observed that resistant cultivars sustained 1 to 9% yield losses from foliar diseases when compared with disease-free plots.
Fungicide applications are not routinely used in Kansas because disease infections do not reach economic levels annually because of a dry climate. Bockus (1998) suggested that fungicides should be applied between boot (Feekes 10) and full spike stages (Feekes 10.5) and Cox et al. (1989) reported that applications should not be made when disease severity at stem elongation is <8%.
As reduced and no-tillage systems are adopted, it will be necessary to determine how other management practices can be used to minimize or prevent yield losses from residue-borne diseases. Our hypothesis was that resistant cultivars, optimizing N rates and fungicide applications would mitigate yield losses from tan spot in no-till wheat following wheat. Therefore, the objectives of this study were to: (i) determine yield differences in second-year wheat under burned, tilled, and no-tilled conditions and (ii) evaluate management practices such as resistant cultivars, increased N rates, and fungicides to minimize tan spot severity and yield loss in continuous wheat.
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MATERIALS AND METHODS
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Field studies were conducted during the 2004–2005 and 2005–2006 winter-wheat growing seasons. During the 2004–2005 growing season, studies were conducted at two locations. The first was located in Saline County, KS (38°55'8'' N, 97°39'0'' W) on a Hord silt loam (fine-silty, mixed, superactive, mesic Cumulic Haplustolls). The second site was located in Marshall County, KS (39°46'12'' N, 96°37'48'' W) on a Wymore silty clay loam (fine, smectitic, mesic Aquertic Argiudolls). In 2005–2006, studies were located in Saline and Marshall Counties, but the sites were at different locations (38°10'24'' N, 97°52'12'' W) and (39°44'5'' N, 96°37'20'' W), respectively but on similar soils. In 2005–2006, an additional site at the North Agronomy Farm in Riley County, KS (39°13'12'' N, 96°36'0'' W) was on a Reading silty loam (fine-silty, mixed, superactive, mesic, Pachic Argiudoll).
The previous crop at all locations was first-year winter wheat in either a sorghum [Sorghum bicolor (L.) Moench]–soybean [Glycine max (L.) Merr.]–winter wheat–winter wheat rotation or a corn (Zea mays L.)–soybean–winter wheat–winter wheat rotation. At all locations, tillage history should be considered incomplete no-till. In these situations, tillage occurs at certain stages of the rotation. Soybean after sorghum and wheat after soybean are no-till planted, but tillage is used between wheat crops. It is not likely that tillage history affected our results as they were not long-term no-till fields. Locations were selected that had been planted to either ‘2145’ (Fritz et al., 2002) or AgriPro ‘Jagalene’ to maximize tan spot inoculum in the second-year wheat planted in this study. These two wheat varieties are rated as being susceptible to tan spot (Jardine and Sloderbeck, 2006). Neither residue levels nor inoculum potential were measured at planting in this study.
The experimental design was a randomized complete block design with split-split-split plot arrangement. Main plots were residue management practices, subplots were cultivars, sub-subplots were N rates, and sub-sub-subplots were a foliar fungicide treatment and an unsprayed control. Each treatment was replicated four times. Residue-management treatments included conventional tillage, burn, and no-tillage. The dimensions of each residue management treatment were 7 m wide by 27 m long. Tilled plots were disked twice and field cultivated once before sowing. In the burned plots, wheat stubble was burned after harvest and not disturbed before planting. No-tillage and burned plots received one herbicide application using glyphosate (N-(phosphonomethyl), glycine) at the rate of 350 g a.i. ha–1 to control weeds and volunteer wheat before sowing. These applications were made using an ATV sprayer at 90 L ha–1 at a pressure of 200 kPa.
Two wheat cultivars were used in this study: ‘2145’ (Fritz et al., 2002), susceptible to tan spot, and ‘Overley’ (Fritz et al., 2004), moderately resistant to tan spot. Both cultivars were sown with a 1.8 m wide no-till drill (Model 3P605NT, Great Plains Mfg., Salina, KS) in 0.19 m rows with a targeted seeding rate of 3300 plants m–2. Due to differences in seed size, the seeding rate for 2145 was 100 kg ha–1 and 120 kg ha–1 for Overley. Kernel weight for Overley was 36.8 g 1000–1 and 31.2 g 1000–1 for 2145 (Roozeboom, 2006). Each cultivar subplot was 1.8 m wide by 27 m long.
Each cultivar subplot was divided into three sub-subplots where each received either 50, 100, or 135 kg N ha–1 as urea (46–0–0). The fertilizer was broadcast by hand in the fall 25 d after sowing. To evaluate the efficacy of fungicide applications on disease severity, a fungicide treatment was imposed in the spring. Fungicide applications at boot (Feekes 10) and anthesis (Feekes 10.5) stages were made to create a "healthy check." The fungicide (Quilt) used was a mixture of azoxystrobin and propiconazole at 240 g a.i. ha–1 and 330 g a.i. ha–1, respectively in 120 L of water ha–1. This fungicide mixture provided broad spectrum control of foliar diseases.
At approximately 25 d after planting, plant counts were taken at two sampling sites within each plot. At each of these sampling sites, all plants were counted in a 0.6 m section of row in two rows to determine plants m–2. Each count area was staked to identify its location within the plot so spike counts could be taken after heading in the spring from the same sampling area that the stand counts were taken.
Tan spot disease severity was determined at two locations within each plot at the milk-soft dough stage (Feekes 11.1). Flag leaves from 25 tillers and main culms were randomly selected and rated on tan spot severity based on the percentage of leaf area exhibiting chlorosis and/or necrosis (Raymond et al., 1985). After heading (Feekes 10.5), spike counts were taken in the two staked areas described previously.
All locations, except Marshall County both years, received a postemergence application of 19.5 g a.i ha–1 of propoxycarbazone-sodium, 10.2 g a.i. ha–1 of Chlorsulfuron (2-Chloro-N-[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)aminocarbonyl]-benzenesulfonamide 62.5% and metsulfuron (Methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]-carbonyl]sulfonyl]benzoate 12.5%), and 100 mL of surfactant ha–1 to control Cheat (Bromus secalinus, L) and Henbit (Lamium amplexicaule, L). These applications were made using an ATV sprayer at 90 L ha–1 at a pressure of 200 kPa. Post-emergence herbicide treatments were not needed during either growing season at the Marshall County sites due to low weed densities.
Grain yield was measured by harvesting each plot with a plot combine (Model 25, Hege Mfg, Colwich, KS). The harvested area was 1.15 m wide by 7.2 m long. Grain was bagged from each plot and weighed with an electronic scale to calculate yield. Grain yield was adjusted to 130 g kg–1 water content using grain moisture readings from a grain moisture tester (Dickey John GAC 2000, Auburn, IL). Subsamples for each plot were dried at 65°C for 72 h to determine 1000-kernel weights. This was accomplished by obtaining 1000 seeds using an electronic seed counter and electronic scale to determine the weight. The number of kernels spike–1 was determined using the relationship [(kg grain ha–1) x 100]/[spikes m–2 x 1000 seed weight]. The timing of treatments and management practices for all locations and both years are summarized in Table 1
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Table 1. Timing of field operations (month/day) for winter wheat grown at three Kansas locations in two growing seasons.
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Variances for grain yields and tan spot severity ratings for location-years were evaluated for homogeneity using Hartley's test for equal variances (Ott, 1988). Homogeneous variances were only observed for the three locations for 2005–2006 growing season. Therefore, the two locations for the 2004–2005 growing season were analyzed separately.
The analysis of variances for yields, yield components, and disease severity were performed using PROC GLIMMIX (SAS Institute, Cary, NC). Replications and locations were considered random. Means were separated using pairwise t tests with a probability level of 0.05 and groupings developed with the "lines" option for LSMEANS.
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RESULTS
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The 2004–2005 growing season began with unfavorable soil moisture conditions at both locations, causing planting to be delayed. Winter precipitation erased most of the soil water deficit at both locations as accumulated winter rainfall was near the 30-yr average (Fig. 1a and 1b
). Despite subzero temperatures, most of the winter had favorable growing conditions (Fig. 2a and 2b
). Spring air temperatures were near normal until the end of the season. Harvest at each location was not delayed despite June precipitation being above normal. The October–June total precipitation was above normal for both locations.
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Fig. 1. Cumulative (dotted line), daily (solid line), and 30-yr normal cumulative (dashed line) precipitation during the 2004–2005 wheat growing season at (a) Marshall and (b) Saline.
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Fig. 2. Daily maximum and minimum air temperatures
during the wheat growing season (2004–2005) at (a) Marshall and (b) Saline.
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The 2005–2006 season started with above-average rainfall at the Riley and Marshall locations (Fig. 3a and 3b
). Wheat planting was delayed at Saline because of very dry soil conditions (Fig. 3c). The winter was drier and warmer than normal at all three locations, which resulted in the driest December to February period on record. Snowfall in March provided some moisture. Uneven rainfall patterns continued through May and June. At Riley, 167 mm of precipitation was received from 1 Nov. 2005 through 30 Mar. 2006 to recharge the soil profile. Precipitation at Saline and Marshall was below normal and affected wheat growth. Dry and warm conditions resulted in an early harvest (Fig. 4a, 4b, and 4c
).
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Fig. 3. Cumulative (dotted line), daily (solid line), and 30-yr normal cumulative (dashed line) precipitation during the 2005–2006 wheat growing season at (a) Marshall, (b) Riley, and (c) Saline.
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Fig. 4. Daily maximum and minimum air temperatures at (a) Marshall, (b) Riley, and (c) Saline during the (2005–2006) winter wheat growing season.
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Marshall 2004–2005
At Marshall in spring 2005, powdery mildew (Blumeria graminis tritici) and stripe rust (Puccinia striiformis) infections occurred in addition to tan spot. A fungicide x tillage x cultivar interaction occurred for disease severity (P = 0.0114, Table 2
). Fungicide applications reduced tan spot severity in all three residue management treatments and both cultivars compared to the nonsprayed control (Table 3
). When fungicide was not applied, the amount of residue left on the soil surface dramatically affected the disease severity in the susceptible cv. 2145. Removing residue by burning decreased tan spot severity by half compared with the no-till treatment. Overley, the resistant cultivar, had the lowest disease severity ratings across the three residue management treatments when fungicides were not applied. No differences were observed between the burn and tilled treatments with Overley.
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Table 2. Analysis of variance results for yield, tan spot severity, plant stand, spike ha–1, kernels head–1, and kernel weight in 2004–2005 season in Marshall County, KS.
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Table 3. Effects of residue management practices, cultivars, and fungicide applications on tan spot severity in 2004–2005 in Marshall County, KS.
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Grain yield was affected by tillage, fungicides, and cultivars as indicted by the significant tillage main effect (P = 0.03) and fungicide x cultivar interaction (P < 0.0001) (Table 2). Yields were reduced 24% (from 6.6 to 4.9 Mg ha–1) when comparing yields from the fungicide and no fungicide treatments in the susceptible cv. 2145. There was no yield difference between the two cultivars when fungicides were applied and no yield difference between the treated and untreated Overley (data not shown). Nitrogen applications also affected yields (P < 0.0001). As N rates increased from 50 to 135 kg N ha–1, tan spot severity decreased linearly (tan spot severity (%) = 17.9 – 0.0058 kg N ha–1, data not shown).
There were no treatment effects for plants ha–1 or kernels spike–1 (Table 2). Cultivars affected spike ha–1 as indicated by a significant main effect (P = 0.0147). Kernel weights were affected by the fungicide x tillage x N (P = 0.0032) and fungicide x cultivar (P = 0.0008) interactions (Table 2). Kernel weight did not respond to N applications in the fungicide treated and in the burned-no fungicide plots (Fig. 5
). When residue was either tilled or left undisturbed before planting, increasing N rates increased kernel weights. Kernel weights were similar for both treatments at 50 kg N ha–1. In the plots where no fungicide was applied and the residue was tilled, kernel weights increased as N rates increased to 100 kg N ha–1 and remained unchanged at 135 kg N ha–1. When no fungicide was applied and wheat planted no-till, kernel weights were similar at 50 and 100 kg N ha–1 and increased as N rates increased to 135 kg N ha–1.
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Fig. 5. Effects of residue management, N rates, and fungicide applications on kernel weight in 2004–2005 season in Marshall County, Kansas.
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Overley had higher kernel weights than 2145 without fungicides (36.2 and 27.7 g 1000 kernels–1, respectively) and when fungicides were applied (35.7 and 31.2 g 1000 kernels–1, respectively). Fungicide applications did not affect Overley kernel weights, but fungicide applications increased kernel weights in 2145 by 13%.
Saline County 2004–2005
In Saline County in 2004–2005, powdery mildew and stripe rust occurred in addition to tan spot. Disease severity ratings were not measured due to an armyworm (Pseudaletia unipuncta Haworth) infestation at heading that damaged flag leaves from the entire plot area. Fungicide applications, tillage, and cultivar influenced yields as indicated by the significant three-way interaction (P = 0.0059, Table 4
). Due to the complexity of discussing a three-way interaction, the two-way interaction of interest (fungicide x tillage) at the level of least interest (cultivars). Residue management had no effect on yields within each fungicide by cultivar treatment (Table 5
). Fungicide applications increased 2145 yields at the three different residue management levels. The largest differences (0.8 Mg ha–1) were observed in the tilled and no-till treatments. For Overley, a yield difference of 0.5 Mg ha–1 was observed in the no-till treatment between the two fungicide treatments (Table 5). Nitrogen rates affected yields (P < 0.0001). As N rates increased from 50 to 135 kg N ha–1, yields increased linearly (yield (kg ha–1) = 37.3 + 0.056 kg N ha–1, data not shown).
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Table 4. Analysis of variance results for yield, plant stand, spike ha–1, kernels head–1, and kernel weight in 2004–2005 season at a site in Saline County, Kansas.
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Table 5. Effects of residue managements, cultivars, and fungicide application on winter wheat yield, kernels spike–1, and kernel weight in 2004–2005 season at Saline County, KS.
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No significant effects for plants ha–1 and spikes ha–1 were observed for any treatments (Table 4). Kernels spike–1 were affected by the fungicide, residue management, and cultivar treatments as indicated by the significant three-way interaction (P = 0.0326, Table 4). Residue management practices of burning and no-till combined with fungicide treatments showed no differences within and between cultivars (Table 5). Differences did occur in kernels spike–1 between no-fungicide and fungicide treatments for 2145 in the tilled treatment. The highest number of kernels spike–1 was observed in the fungicide treated 2145 plots in the tilled treatments (21.5 kernels spike–1) and the lowest (14.14 kernels spike–1) occurred in the no-fungicide Overley in the no-till treatment (Table 5). A N main effect occurred for kernels spike–1 (P = 0.0133, Table 4). As N rates increased from 50 to 135 kg N ha–1, kernels spike–1 increased linearly (kernels spike–1 = 15.0 + 0.019 kg N ha–1, data not shown).
Kernel weights were affected by fungicides and cultivars as indicated by the two-way interaction (P = 0.0178, Table 4). Fungicide applications did not affect kernel weights of Overley, but increased kernel weights of 2145 (Table 5).
Locations 2005–2006
Tan spot developed at all three locations in spring 2006. As a result, fungicides and cultivars influenced tan spot severity as indicated by the significant location x fungicide x cultivar interaction (P < 0.0001, Table 6
). Applying fungicides reduced tan spot severity in all cultivar-location combinations except Overley at Saline (Table 7
). Tan spot severity ratings were also influenced by fungicide, residue management, and cultivar treatments as indicated by the significant three-way interaction (P = 0.0015). Applying fungicides reduced tan spot severity across both cultivars and residue management treatments (Table 7) when compared with the untreated controls. In 2145, when no fungicide was applied, tan spot severity increased as residue levels increased, with no-till having the highest severity ratings (30%) followed by tillage (22%) and then burn (14%). In Overley, when no fungicide was applied, the no-till and tilled plots had similar tan spot severity ratings which were higher than those in the burn treatments.
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Table 6. Analysis of variance results for yield, tan spot severity, plant ha–1, spikes ha–1, kernels spike–1, and kernel weight in 2005–2006 growing season at sites in Marshall, Riley, and Saline Counties in Kansas.
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Table 7. Effects of cultivar and fungicide applications on tan spot severity in 2005–2006 growing season at sites in Marshall, Riley and Saline Counties in Kansas.
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Grain yield was influenced by all treatments, as indicated by the significant location x fungicide x residue management interaction (P = 0.0349) and residue management x cultivar x N interaction (P = 0.0300, Table 6). The highest yield for all treatment combinations was measured at Riley, followed by Marshall and Saline (Table 8
). No differences were observed between fungicide treatments within each residue management treatment across all locations. At Marshall regardless of fungicide treatment, burning resulted in higher yields than either the tilled or no-till system. At Riley, burning and applying fungicides resulted in yields that were greater than those measured without fungicides in the tilled and no-till treatments. When fungicide was applied at Saline, no yield differences were measured among the residue management practices. When no fungicide was applied, burning resulted in higher yields compared with the tilled and no-till treatments.
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Table 8. Effects of tillage treatments, cultivars and fungicide applications on wheat yield, kernels spike–1, and kernel weight in the 2005–2006 growing season at sites in Marshall, Riley and Saline Counties in Kansas.
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Wheat yields were affected by N rates (P = 0.0001, Table 6). Wheat yield was not different between the 50 and 100 kg N ha–1 rates, but declined by about 2.8 kg ha–1 when the N rate was increased from 100 to 135 kg N ha–1 (data not shown). Yield for Overley averaged across all three locations was 3.9 Mg ha–1 compared with 2145 at 3.4 Mg ha–1 (P < 0.0001).
Plants ha–1 and spikes ha–1 were not affected by any treatments (Table 6). Kernels spike–1 were affected differently at each location by fungicides and cultivars as indicated by the significant location x fungicide x cultivar interaction (P = 0.0464, Table 6). The highest number of kernels spike–1 for all treatment combinations was measured at the Saline location, followed by the Marshall and Riley locations (Table 8). No differences in kernels spike–1 were observed between fungicide treatments and between cultivars at Marshall and Riley. At Saline, kernels spike–1 increased in 2145 when fungicides were applied. With Overley, the fungicide treatment did not affect kernels spike–1. Kernels spike–1 were affected by N rates (P = 0.0013, Table 6). Kernels spike–1 were not different between the 50 and 100 kg N ha–1, but decreased 1.75 kernels spike–1 as N rates increased from 100 to 135 kg N ha–1.
Kernel weights were influenced differently at each location by fungicide applications and cultivars as indicated by the significant location x fungicide x cultivar interaction (P = 0.0453, Table 6). At Marshall and Riley, fungicide applications increased kernel weights of 2145. At Saline, fungicide applications did not affect kernel weights of 2145 (Table 8). Fungicide applications did not affect kernel weights of Overley at any location.
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DISCUSSION
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Disease Severity
Tan spot infection levels were affected by tillage practices, cultivar resistance, and fungicide applications in both years. The use of no-till practices for wheat on wheat stubble increased residue levels at planting and increased tan spot severity compared with the burn treatments.. The observed relationships of tan spot infection levels and wheat residue at planting agree with other studies in which severity of tan spot epidemics were associated with residue management (Sutton and Vyn, 1990; Schuh, 1990; Bockus and Claassen, 1992; Fernandez et al., 1999). Tan spot severity was heavier in the susceptible cultivar (2145) compared with the more resistant cultivar (Overley) when fungicides were not applied. In the 2004–2005 season at Marshall under heavy foliar disease pressure, cultivar resistance reduced tan spot infection levels by 39, 25, and 16% under no-till, tilled, and burned treatments, respectively. In the 2005–2006 growing season the foliar disease pressure was at more moderate levels. Cultivar effects accounted for decreases in tan spot severity of 20, 15, and 8% at Marshall, Riley, and Saline sites, respectively when fungicides were not applied. These results suggest that the level of tan spot resistance available to farmers is adequate to produce acceptable control under moderate epidemics. However, this resistance is limited to only a few varieties suited to this region and needs to be expanded if second-year no-till wheat is to be successful.
Ratings observed in the 2004–2005 season were higher than those reported in other studies (Rees and Platz, 1979; Summerell et al., 1988) due to a heavy disease pressure observed that year at Marshall where inoculum may have been dispersed by wind from the no-till treatments into the burn treatments (Platt and Morrall, 1980; Khasanov et al., 1990; Sone et al., 1994). During the 2005–2006 growing season, differences in severity levels measured at each location were the result of a warm winter with little snowfall that may have reduced inoculum levels. Summerell and Burgess (1989) found that the overwintering saprophytic pathogen survives better in cold, dry conditions. Uneven rainfall with high temperatures throughout the growing season may have also affected disease severity. Wolf and Hoffmann (1993) reported that ascospore liberation is favored by high relative humidity and temperatures >10°C.
Grain Yield
During 2004–2005 at Marshall, wheat yields were related to fungicide treatments and cultivar resistance to tan spot. Average yield losses of 8 and 25% were measured for the resistant and susceptible cultivars, respectively, when the two fungicide treatments were compared within a cultivar. These results are similar to those of Bockus et al. (1992). When no fungicides were applied, the resistant cultivar (Overley) produced yields that were 18% greater (1.11 Mg ha–1) than the more susceptible cultivar (2145).
During the 2004–2005 season at Saline, favorable weather conditions for disease and wheat development prevailed from April to June, so no yield differences were measured between residue management treatments. This lack of response may be the result of wind-dispersed inoculum from the no-till plots into the tilled and burned plots (Platt and Morrall, 1980; Khasanov et al., 1990; Sone et al., 1994). Bockus and Claassen (1992) found yield differences between residue management treatments and attributed these results to weather conditions unfavorable to disease development. The effectiveness of cultivar resistance to sustain yields through different residue management practices was greater for Overley, as lower yield responses occurred between the no-fungicide and fungicide treatments compared with 2145. These results corroborate other reports (Bockus et al., 1992) that observed the highest response to fungicide applications and yield losses were usually achieved in susceptible cultivars.
Weather conditions varied across locations in 2005–2006 and influenced yield responses to residue management practices and fungicide treatments. Adequate early-season precipitation occurred at Riley (97 mm) and Marshall Counties (55 mm), followed by a dry winter with above-normal temperatures. In the spring, below-average but well distributed rainfall from April through June and normal temperatures produced above-average yields. At Saline, low yields were caused by low precipitation before sowing (22 mm), followed by a dry and warm winter. Even though rainfall received during the spring was higher than at Riley, this came late in the growing season. Therefore, these conditions produced an unfavorable environment for tan spot development, resulting in no fungicide effect within each residue management practice at the three locations. Similar results have been reported in Kansas by Bockus and Claassen (1992). However, yield losses were directly related to surface-residue levels at planting at Marshall due to higher disease pressure. The relationships between surface-residue levels, disease pressure, and yield loss were less consistent at Riley compared with Marshall. Similar results have been reported in other studies (Sutton and Vyn, 1990; Schuh, 1990; Bockus and Claassen, 1992; Fernandez et al., 1999). At the Saline location, when fungicides were not applied, yield differences among the residue management treatments were related more to water stress than to disease severity because wheat yields under no-till had higher yields than the tilled treatment, despite the fungicide treatment.
Plants per Hectare and Spike per Hectare
Plants ha–1 and spike ha–1 were unaffected by tan spot in both growing seasons at all locations because these components were already determined before the fungicide was applied. Similar results were reported in other studies (Shabeer and Bockus, 1988; Kelley, 1993).
Kernels per Spike
During the 2004–2005 season at Marshall, kernels spike–1 were unaffected by tan spot. Kelley (2001) observed that foliar fungicide treatments did not influence kernels spike–1 within each residue-management treatment because this yield component was already determined before fungicides were applied. However, Rees and Platz, (1983) and Shabeer and Bockus (1988) reported that fungicide applications increased kernels spike–1.
Kernels spike–1 in the 2004–2005 season at Saline County was affected by the tillage x cultivar x fungicide treatment interaction. Both cultivars had the highest kernels spike–1 in the tilled treatment where the response to the fungicide applications was only observed in the susceptible cultivar (2145).
In the 2005–2006 season at all locations, kernels spike–1 was affected by the location x cultivar x fungicide interaction. In our study, kernels spike–1 increased when fungicides were applied to 2145. At Riley and Marshall, lower kernels spike–1 were observed compared with Saline County due to favorable fall weather conditions, which increased tillers plant–1 (data not presented) and decreased kernels spike–1 at these locations.
Kernel Weight
In 2004–2005 in Marshall, kernel weight was affected by the fungicide x tillage x N rate interaction. Fungicide applications increased kernel weight, but increases were not correlated to increased N rate. The effectiveness of applied fungicide on increasing kernel weight was greater at the lower N rates independent of each residue management treatment. These observations disagree with those of Howard et al. (1994) in that they suggested that excessive N applications may subject the wheat to greater disease infection, thus increasing the need for fungicide applications.
In the 2004–2005 season at Saline, the magnitude of kernel weight increase also differed based on cultivar resistance to tan spot and fungicide applications. Overley had heavier kernel weights and did not respond to fungicide applications compared with 2145. A similar interaction occurred in the 2005–2006 season, but the magnitude of kernel weight increase differed among locations. Kernel weight was higher at Riley and Marshall than at Saline due to the compensation for the number of kernels as discussed above. Results from another study (Carr et al., 2003) also demonstrated that yield components compensate when another yield component is reduced because of other factors. Overley had heavier kernel weights than 2145 under both fungicide treatments at all three locations. This appears to be a genetic response because both years at planting, Overley had greater seed weight, requiring a higher seeding rate to achieve a similar plant density as 2145. Low disease pressure at all three locations resulted in no response to fungicide treatment in the resistant cultivar (Overley). However, the susceptible cultivar (2145) responded to fungicide applications at Marshall and Riley Counties, but not at Saline because of lower tan spot severity and drought conditions.
Fungicide responses were observed by Kelley (1993 and 2001), who reported that kernel weights were affected by fungicide treatments. Rees and Platz (1983) reported that kernel weight was more important than kernels spike–1 in determining wheat yields. Cultivar responses to fungicide treatments in the 2005–2006 growing season and in 2004–2005 at Saline County agree with those reported by Bockus et al. (1997), who observed that the increase in kernel weight from fungicide applications was related to cultivar resistance to tan spot and disease severity levels.
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CONCLUSIONS
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Residue management practices, cultivar resistance, and fungicide applications affected tan spot severity and wheat yield in both growing seasons. When fungicides were not applied and tan spot infection levels were high, lower severity ratings and higher yields were observed in the tilled and burned treatments compared with those measured under no-till. Fungicide applications under these conditions controlled tan spot and improved no-till yields to levels similar to those in the tilled treatments. When infections were less severe, the combination of a resistant cultivar and no-till resulted in yields similar to treatments that reduce residue on the soil surface. These results suggest that second year, no-till planted wheat has a high likelihood of producing similar yields as tillage or burning of residue. This study demonstrated that the use of a resistant cultivar under high residue conditions is a producer's most important decision. It also found that fungicide applications can improve wheat yields when tan spot infections are severe to very severe, as they often can be in second-year wheat grown under no-till.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
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ABSTRACT
INTRODUCTION
