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Keywords:

  • blackleg;
  • canola;
  • grazing;
  • Phoma lingam ;
  • phoma stem canker;
  • sheep

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Brassica napus (canola, oilseed rape), an important break crop for cereals across the Australian wheat belt, is being rapidly adopted as a dual-purpose (forage and grain) crop in mixed farming systems. Stem canker caused by the fungus Leptosphaeria maculans is the most important disease of B. napus in Australia. The primary source of inoculum is airborne ascospores released during autumn/winter which coincides with the grazing of dual-purpose crops. Field experiments were defoliated by sheep to determine the effect of grazing on blackleg stem canker severity at plant maturity in B. napus cultivars differing in their resistance level and grazed at different times. One cultivar was sown on different dates to investigate the impact of grazing at the same time, but at different growth stages. Defoliation by mowing was compared to defoliation by livestock. Similar amounts of dry matter remained after defoliation by machinery (0·66 t ha−1) or livestock (0·52 t ha−1). However, stem canker severity was higher in the grazed (40% of crown cross-section diseased) compared with the mown (25%) treatment, which was higher than the ungrazed control (9%). Stem canker severity generally increased with grazing, but the increase was eliminated or reduced in cultivars with good resistance. Grazing during vegetative plant growth minimized the increase in stem canker severity compared with grazing during reproductive growth. Currently, cultivars with good L. maculans resistance are recommended in high disease situations. To avoid excessive yield loss in dual-purpose B. napus crops due to L. maculans it is recommended that such cultivars are grown even in low-moderate disease situations.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

In Australia, dual-purpose crops describe those crops that provide both forage for livestock during the late-autumn/winter months and grain production in the spring/summer after the removal of livestock. Dual-purpose wheat production has increased substantially after the release of long-season wheats that provide a broad early sowing window with anthesis and flowering at similar times to spring wheat (Kelman & Dove, 2007). Brassica napus (canola, oilseed rape) is grown throughout the Australian wheat belt as a break crop for cereals, resulting in increased production in the subsequent cereal crop through reduced cereal disease levels and improved grass-weed control (Kirkegaard et al., 2008a). The potential for traditional grain-only cultivars of canola to be managed as a dual-purpose crop has been previously demonstrated (Kirkegaard et al., 2008a) with successful adoption by growers across southeastern Australia. Brassica napus has potential to produce good biomass with early sowing, has little reduction in grain yield when grazed with appropriate livestock management, and increases farm profitability as well as providing flexibility and risk mitigation (Kirkegaard et al., 2008a, 2010). Dual-purpose B. napus crops allow farmers with mixed crop–livestock farming enterprises to maintain livestock production across seasons by maintaining grazing area, while gaining the advantages of a break crop for subsequent cereal crops.

Leptosphaeria maculans is the causal pathogen of blackleg stem canker (phoma stem canker), the most significant threat to B. napus production in Australia and worldwide (Fitt et al., 2006). Oilseed brassicas have been grown in Australia since the late 1960s, initially with cultivars introduced from Canada (Colton & Potter, 1999). These cultivars were highly susceptible to L. maculans and were decimated in the early 1970s, which prompted the establishment of breeding programmes across Australia to develop locally adapted cultivars with high levels of genetic resistance. Leptosphaeria maculans releases airborne ascospores from infected B. napus residue during autumn and winter which coincides with the period when crops are grazed (McGee, 1977; Salam et al., 2003). Previous studies indicate an increase in L. maculans severity in grazed spring B. napus, although these studies were conducted under low disease pressure (Kirkegaard et al., 2008a). Sprague et al. (2010) conducted field experiments with a wide range of Brassica lines in three regions (two with high disease pressure) using mechanical defoliation to simulate grazing. Mechanical defoliation increased stem canker severity at all sites but the increase was less in B. napus lines with moderate to high levels of stem canker resistance. Plants defoliated during vegetative growth tended to develop less disease than those defoliated after the initiation of stem elongation. No similar studies are currently available for B. napus grazed by livestock.

Leptosphaeria maculans can infect all plant tissues, with hyphae from germinating spores infecting directly through stomata or other openings without the production of penetration structures such as appressoria (Hammond & Lewis, 1987; Chen & Howlett, 1996; Sprague et al., 2007). Damage or disruption to the physical barriers of the plant surface can increase susceptibility to infection by L. maculans (Newman & Plumridge, 1983; Khangura & Barbetti, 2001), which suggests that defoliation (either mechanical or by livestock) will render plants more susceptible to infection. Although Sprague et al. (2010) demonstrated increased stem canker severity after mechanical defoliation, there are key differences between simulated grazing and grazing with livestock under field conditions: (i) grazing generally occurs over a period of days or weeks; (ii) individual plants are grazed intermittently throughout the grazing period, generating ongoing wounding; (iii) damage to the plant can be from removal of plant tissue or by trampling; (iv) herbivores are often selective in the plant tissues they remove, usually favouring leaves; and (v) the release of L. maculans inoculum and infection can occur throughout the grazing period. Given these differences, the increase in stem canker severity may be greater with grazing by livestock than mechanical defoliation, although the effect of potential removal of infected tissue by grazing is unknown. Stem canker severity rather than leaf infection is the principal cause of yield loss associated with infection by L. maculans, as the fungus damages the vascular tissue, thereby reducing water and nutrient uptake (Hammond et al., 1985). Ultimately, the impact of grazing on stem canker development is therefore of most interest. Yield loss is evident in plants with >80% internal infection of the stem at physiological maturity (Marcroft et al., 2004; Sprague et al., 2010).

Field experiments were conducted over two seasons to assess the effect of grazing B. napus crops with sheep on stem canker severity caused by L. maculans at plant maturity. Mechanical defoliation (simulated grazing) by mowing was compared to defoliation by sheep at one grazing time. Cultivars differing in their genetic resistance to L. maculans or the same cultivar at different growth stages were grazed at 14-day intervals throughout the winter and stem canker severity compared with an ungrazed control.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Experimental sites, design and management

Field experiments were conducted in 2007 and 2008 to investigate the effect of grazing by sheep in a range of B. napus cultivars at different plant growth stages on severity of disease caused by L. maculans. Experiments were located near Young, New South Wales (NSW), Australia (34° 23′ 7″S, 148° 19′ 54″E), in adjacent fields on a commercial property, with B. napus residue from the previous season’s crop within 300 m of the experiment each year. The experimental site was in a region of agricultural production where B. napus is an important component of the rotation and L. maculans is known to cause significant yield loss (Kirkegaard et al., 2006). This paper reports the effect of grazing on blackleg stem canker; however, biomass and yield data from the same experiments are reported in Kirkegaard et al. (2012, in press). Australian spring B. napus cultivars ATR-Beacon, 46Y78CL, Surpass501TT and Skipton were sown in 2007 and 2008 (Table 1). Herbicide tolerance is present in cultivars ATR-Beacon and Surpass501TT (triazine) and 46Y78CL (Clearfield). Cultivar 46Y78CL is a hybrid. In 2008, 46Y78CL was sown on three dates approximately 2 weeks earlier (sowing 1) and 2 weeks later (sowing 3) than the main experiment (sowing 2). In addition, winter B. napus cv. Columbus, which has a high level of resistance to L. maculans (Marcroft et al., 2002; Light et al., 2011), was sown in 2007. Columbus was sown earlier than the spring cultivars due to its winter habit. The cultivars were selected to represent a range of resistance to L. maculans based on the Australian Blackleg Rating (ABR). The Australian Blackleg Ratings are produced annually and are used to rank cultivars for blackleg resistance based on the percentage of plants that survive to maturity. In 2008, an alpha VS (very susceptible) to R (resistant) scale replaced the previous numerical 1·0 (susceptible) to 9·0 (resistant) scale.

Table 1. Summary of growing season rainfall (GSR), sowing dates, Brassica napus cultivars and timing of grazing treatments for field experiments conducted in 2007 and 2008 at Young, New South Wales, Australia. Long-term average GSR is 458 mm
2007 GSR (Apr–Oct) 330 mm2008 GSR (Apr–Oct) 300 mm
Sowing dateCultivarABRaSowing dateCultivarABR
  1. aAustralian Blackleg Rating. In 2008, an alpha VS (very susceptible), M (moderate) to R (resistant) scale replaced the previous numerical 1·0 (susceptible) to 9·0 (resistant) scale.

  2. bThe major gene resistance in cv. Surpass501TT was overcome in some regions of Australia in 2003 and the cultivar was subsequently withdrawn from the market (Sprague et al., 2006).

20 MarchColumbusn/a17 April46Y78CLMR
27 AprilATR-Beacon5·530 AprilATR-BeaconMS-S
 Skipton6·5 SkiptonMS
 46Y78CL8·0 46Y78CLMR
 Surpass501TT9·0b Surpass501TTRb
   12 May46Y78CLMR
Grazing treatmentsDates Grazing treatmentsDates 
G121 June–6 July G116–30 June 
G26–20 July G230 June–7 July 
G320 July–3 Aug G314–28 July 
G43–17 Aug Mechanical defoliation (mow)28 July 
G517 Aug–5 Sept G428 July–11 Aug 
G65–20 Sept G511–25 Aug 
G720–30 Sept G625 Aug–1 Sept 
   G78–13 Sept 

Experiments were similar in both years with strips of each cultivar (4·4 × 110 m) randomized within three blocks. Grazing treatments (c. 12 m wide) were imposed as subplots across these strips at approximately fortnightly intervals from June through to September (Table 1), with an ungrazed control area. Experiments were essentially a strip-plot design. However, grazing treatments were not randomized within blocks due to the impracticalities of managing animals on small plots and moving sheep between plots. Due to the prevalence of canola residue in the region and the airborne dispersal of ascospores, disease pressure was considered to be uniform across the site. Weeds were controlled using recommended herbicides and crop nutrition (N, P, S) was managed according to pre-sowing soil tests using fertilizer banded with seed and N top-dressed as urea, post-grazing.

Grazing and mechanical defoliation

Seven grazing treatments were imposed by sheep and were conducted at fortnightly intervals from approximately mid-June to mid-September (Table 1). Sheep numbers were closely managed for each grazing treatment to achieve even removal of plant material between cultivars. Plant growth stages (Harper & Berkenkamp, 1975) were recorded at the beginning of each grazing period.

In 2008, a mechanical defoliation treatment was imposed to compare disease levels in plants grazed by sheep or defoliated mechanically, as has been imposed in previous experiments (Sprague et al., 2010). Plants were defoliated to 10–15 cm above ground level with a self-propelled mower and all cut plant material was removed from the plots. A grazing treatment (G4; Table 1) was initiated on the same day as the mechanical defoliation was performed. Dry matter cuts were taken on ungrazed control and defoliated treatments (28 July), and the grazing treatment (11 August) to determine the amount of aboveground dry matter. Plants were removed at ground level from two quadrats (c. 0·8 m2), washed to remove any adhering soil (mainly in the grazed treatment due to trampling) and total dry matter calculated from oven-dried material.

Trap plants and ascospore release

Plants of a highly susceptible spring canola cultivar grown in pots were placed at the experimental site for the duration of each grazing period to provide an indication of disease pressure. Seeds of B. napus cv. Q2 (ABR 2·0/VS) were sown in pots (26 cm diameter × 24 cm high) of pasteurized potting medium (compost containing recycled soil, leaf mulch, vermiculite, peat moss, river loam, perlite and river sand with additions of lime and blood and bone; steam-pasteurized at 70°C for 45 min) and thinned to three plants/pot after emergence. Pots were sown periodically so that plants were at growth stage 2.2 when transferred to the field. Prior to field placement, plants were grown in a temperature-controlled glasshouse (20°C day/18°C night) before hardening off in partially enclosed growth frames. At the onset of each grazing period, two pots were placed at five locations approximately 10 m apart (total 30 plants) immediately adjacent to the area to be grazed. Trap plants in these pots were not grazed. Pots were removed on the last day of each grazing treatment and grown in an uncovered area at CSIRO in Canberra where there is no natural source of L. maculans inoculum, to represent normal growing conditions for B. napus in Australia. Pots were watered and fertilized as required during growth. Stem canker severity was assessed once lower pods had begun to turn from green to yellow, by cutting at the crown transversely with a pair of secateurs as described below. In this way the level of inoculum occurring at the site during each grazing period could be monitored, as inoculum prevalence is likely to be a factor contributing to disease development.

The disease model sporacle, developed to simulate timing of ascospore maturity and seasonal release, was used to predict the onset and timing of ascospore release for the experimental sites in 2007 and 2008 (Salam et al., 2003). Weather data from the closest weather station maintained by the Bureau of Meteorology at Young Airport (34° 15′ 0·14″S, 148° 14′ 45·69″E) and detailed daily rainfall records from the farmer’s house (c. 1 km from the field experiments) were used for the simulation.

Disease assessments

Plants were assessed for stem canker severity at the windrowing stage (c. 30% seed colour change). Regular visits were conducted to the sites so that each line was assessed at the appropriate time, as significant variation in maturity existed due to differences in maturity of the lines, sowing time and developmental delays in some lines caused by grazing. Ten consecutive plants in a row from four places within each plot (total 40 plants/plot) were assessed. Plants were cut at the crown transversely with a pair of secateurs and the area of internal stem surface that was blackened was visually quantified (0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100%) (Marcroft et al., 2004).

Statistical analysis

Although grazing treatments were not randomized within the three blocks due to the impracticalities of managing small numbers of sheep on plots and moving animals between plots, disease pressure was considered to be uniform across the site due to the prevalence of canola residue in the region and the airborne dispersal of ascospores. As such, experiments were regarded as a strip-plot design with three replicates for analysis. Experiments were analysed separately by year due to missing data in 2008 using appropriate anova models in GenStat to assess the main effects (principally grazing and cultivar) and interactions (Payne et al., 2009). All disease data were arc-sin transformed prior to analysis to achieve a normal distribution. However, data presented in figures and tables are untransformed data to facilitate interpretation. Treatments were considered to differ when < 0·05. All values presented are mean ± SE.

In 2007, cv. Columbus was excluded from the analysis as stem canker severity was consistently low (average 2·1%) in all treatments. In 2008, the cultivar resistance experiment was analysed three ways due to the loss of 46Y78CL plots in grazing times G5 to G7 when incorrect herbicides were applied. The comparisons analysed were: UG (ungrazed), G1–G4 (all cultivars); UG, G5–G7 (ATR-Beacon, Skipton and Surpass501TT); and UG, G1–G7 (ATR-Beacon, Skipton and Surpass501TT). There were consistent significant main effects and interactions in all analyses and the most conservative LSD was used to determine the differences presented in Figure 3(a). Also in 2008, the mechanical defoliation treatment (mow) was compared to the ungrazed control treatment and the grazing treatment (G4) that began on the same date on which mowing occurred.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Grazing and mechanical defoliation

All cultivars were at the bud-visible stage (GS 3.1) when the mowing and grazing treatments (G4) were imposed (Table 2). Similar amounts of dry matter remained after grazing (0·52 t ha−1) and mowing (0·66 t ha−1) compared to the ungrazed treatment (1·49 t ha−1; Fig. 1). The hybrid cultivar 46Y78CL produced the most dry matter followed by conventional (Skipton) and triazine-tolerant (ATR-Beacon, Surpass501TT) cultivars. There was a significant cultivar × treatment interaction (< 0·05). Although similar amounts of dry matter remained after the mowing and grazing treatments, relatively less biomass was removed in Surpass501TT (36% of dry matter removed) compared to the other cultivars (average 64% removed).

Table 2. Growth stage of Brassica napus cultivars at the beginning of each grazing treatment in 2007 and 2008 at Young, New South Wales, Australia. Growth stage 2 is vegetative, growth stages 3 to 5 are reproductive (Harper & Berkenkamp, 1975). There were three sowing times for 46Y78CL in 2008. A mechanical defoliation treatment (mow) was imposed on the day that grazing commenced in treatment G4 in 2008. The growth stages for mechanical defoliation treatments are in italics
Grazing treatmentATR-BeaconSkipton46Y78CLSurpass501TTColumbus
200720082007200820072008 17/42008 30/42008 12/5200720082007
G122.522.522.92.52.222.52
G222.722.723.13.12.522.72
G323.13.13.123.13.12.723.12
G4 (mow)3.1 3.1 3.4 3.1 3.23.2 3.1 3.13.4 3.1 2
G53.23.24.13.24.1n/an/an/a4.23.22
G64.43.24.43.24.4n/an/an/a4.43.33.1
G75.14.15.14.15.1n/an/an/a5.14.23.2
image

Figure 1.  Effect of grazing and mechanical defoliation (mowing) on dry matter in 2008 at Young, NSW, Australia. Dry matter was measured on 28 July in the ungrazed and mown treatments, and on 11 August when the sheep were removed from the grazing treatment which commenced on 28 July. An asterisk (*) indicates treatment is significantly different to ungrazed, within the same cultivar.

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There was more severe stem canker in the grazed treatment (40·2%) compared to the mown treatment (24·6%), while the ungrazed treatment had the least severe disease (9·3%; < 0·001) (data not shown). There was no treatment × cultivar interaction, although there were significant (< 0·001) differences between each of the cultivars, with disease least severe in Surpass501TT (8·3%), intermediate in 46Y78CL (22·7%) and ATR-Beacon (29·4%), and most severe in Skipton (38·4%).

Cultivar resistance

2007

The average disease level in cv. Columbus across all grazing times was very low (average 2·1% internal stem canker) and was excluded from the analyses (Fig. 2a). Across all other cultivars, grazing significantly increased stem canker severity (32·1%) compared with the ungrazed treatment (20·7%). There was a significant effect of cultivar and a grazing × cultivar interaction (< 0·001). As expected, disease severity was lowest in the resistant cultivar Surpass501TT (12·2%) and highest in the susceptible cultivars Skipton (43·9%) and ATR-Beacon (39·1%), while 46Y78CL (24·5%) was intermediate. Cultivars reacted differently to the grazing treatments, with no increase in stem canker severity due to grazing in the resistant cultivar Surpass501TT and significant increases in the susceptible cultivar ATR-Beacon. Grazing increased stem canker severity at some grazing times in 46Y78CL and Skipton, and there was an unexpectedly high level of stem canker in the ungrazed Skipton. The grazing period of the experiment lasted for 106 days, during which the model predicted ascospores were released on 51 days. The first spores released from stubble of the previous season occurred in mid-June (coinciding with G1) with the majority of spores released during July and August (G2–G5). Stem canker was most severe in the trap plants at the earliest grazing times (G1 and G2) and fluctuated at the later grazing times (Fig. 2b). Grazing periods where stem canker severity was low in the ungrazed trap plants did not coincide with lower stem canker severity in grazed treatments.

image

Figure 2.  Severity of stem canker caused by Leptosphaeria maculans in Brassica napus cultivars grazed by sheep (a) and in ungrazed plants of the highly susceptible B. napus cv. Q2 grown in pots (b) in 2007 at Young, NSW, Australia. (a) Cultivars were grazed on different dates (G1–G7) and compared to an ungrazed (UG) control treatment. Cultivars are ranked left to right in order of increasing resistance to L. maculans. Columbus was excluded from the analysis as disease severity was very low. An asterisk (*) indicates treatment is significantly different to the UG within the same cultivar. (b) Pots of Q2 trap plants were located in the experiment adjacent to the grazing treatment for the duration of each grazing period.

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2008

Overall, stem canker severity was higher in 2008 than in 2007 (Figs 2 & 3). As in 2007, disease severity was lowest in Surpass501TT (16·9%), highest in Skipton (51·9%) and ATR-Beacon (41·8%), and intermediate in 46Y78CL (25·8%; UG, G1–G4 only; Fig. 3a). In contrast to 2007, the severity of stem canker recorded in the ungrazed treatment for Skipton was similar to that of ATR-Beacon. Stem canker severity was increased at all grazing times in cultivars ATR-Beacon, Skipton and 46Y78CL compared to the ungrazed control. Grazing did not increase the severity of stem canker in Surpass501TT when grazing occurred during vegetative growth (G1–G3), but increased significantly when plants were reproductive (G4–G7; Table 2). Stem canker severity increased more with later grazing mainly due to the high disease levels in Surpass501TT at later grazing times (Fig. 2b). Grazing occurred over 99 days, during which the model predicted ascospores were released on 54 days. As in 2007, the first spores released from stubble of the previous season occurred in mid-June (G1) and the majority of spores were released during July and August (G2–G6). However, the timing of spore release was more evenly distributed than in 2007. Stem canker severity was higher in trap plants in 2008 than in 2007, with disease severity between 55 and 75%, except at grazing times G3 and G5 which were less than 40% (Fig. 3b). As in 2007, grazing periods where stem canker severity was low in the trap plants did not coincide with lower stem canker severity in grazed treatments.

image

Figure 3.  Severity of stem canker caused by Leptosphaeria maculans in Brassica napus cultivars grazed by sheep (a) and in ungrazed plants of the highly susceptible B. napus cv. Q2 grown in pots (b) in 2008 at Young, NSW, Australia. (a) Cultivars were grazed on different dates (G1–G7) and compared to an ungrazed (UG) control treatment. Cultivars are ranked left to right in order of increasing resistance to L. maculans. Data are not available (n/a) for 46Y78CL for treatments G5–G7 in 2008. An asterisk (*) indicates treatment is significantly different to the UG within the same cultivar. (b) Pots of Q2 trap plants were located in the experiment adjacent to each grazing treatment for the duration of each grazing period.

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Growth stage at grazing

Compared to the ungrazed control (10·7%), disease increased in G1 and G2 (mean 19·6%), and increased further in G3 and G4 (mean 35·8%; Fig. 4). Although there were no differences between the sowing times, stem canker was more severe in the latest sowing time in the ungrazed control treatment. There was a significant sowing time × treatment interaction (< 0·001) whereby stem canker was more severe in the first two sowings than in the last sowing. Indeed, stem canker severity was not increased in the last sowing time at G1 and was significantly lower than the ungrazed treatment at G2 (Fig. 4). Stem canker severity increased as plants were grazed later in their development, particularly once bud elongation had commenced (Table 2).

image

Figure 4.  Stem canker severity caused by Leptosphaeria maculans in Brassica napus cv. 46Y78CL sown on three different dates but grazed at the same times in 2008 at Young, NSW, Australia. ^ indicates treatment is significantly less than ungrazed (UG), * indicates treatment is significantly greater than UG within each sowing date.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Grazing of B. napus crops occurs in the late-autumn to winter months when ascospores of L. maculans are released. These experiments found an interaction between grazing (by sheep) and stem canker severity at plant maturity, which is consistent with previous findings by Kirkegaard et al. (2008a) and Sprague et al. (2010). Stem canker severity was generally increased by grazing but more so in crops with low resistance to L. maculans. There was only one case in which grazing reduced disease severity. Interestingly, late grazing appeared to significantly increase infection in the otherwise highly resistant Surpass501TT in 2008. Higher levels of stem canker were recorded in 2008 than 2007 in both the susceptible trap plants and in the field experiments but disease severity in trap plants was not coincident with that in the field. The timing of ascospore release predicted by sporacle was consistent between 2007 and 2008. Growing season rainfall (330 mm and 300 mm; long term average 358 mm) was also similar between the seasons, although there were more extended periods of daily rainfall in 2008 conducive to ascospore germination and infection. However, the difference in infection levels across the years seems unlikely to be related to the seasonal conditions alone and other possible reasons for the higher infection in 2008 are discussed below.

Mechanical defoliation removed similar amounts of biomass compared with grazing, but stem canker severity was >60% higher in the grazed compared with the mown treatment. There are key differences between grazing and mechanical defoliation in B. napus crops which interact with, and influence disease development. Grazing generally occurs over a period of days or weeks and individual plants are grazed intermittently. Damaged plants with increased susceptibility to infection are exposed to L. maculans ascospores intermittently throughout the grazing period. In contrast, the period of increased susceptibility to infection in mechanically slashed plants is only a few days as tissues are repaired, thereby providing less opportunity for passive pathogen entry. Mechanical slashing removes plant material at a pre-determined height, while grazing animals can be selective in the plant tissues they remove and damage to the plant is often caused by trampling. Leptosphaeria maculans does not produce penetrative structures such as appressoria but directly infects plants, and therefore damage or disruption to the physical barriers of the plant surface can increase susceptibility to infection by L. maculans. Yield loss due to L. maculans is principally due to stem cankers which restrict the uptake of water and nutrients. Damage to the plant which provides entry close to the stem and crown increases the likelihood of stem canker formation, as the distance needed to travel by infecting fungal hyphae is reduced. Sprague et al. (2010) found that in defoliated and inoculated plants, stem canker severity was increased by 20% in plants that were defoliated and inoculated at the leaf petiole c. 2 cm from the stem compared with those defoliated at c. 7–10 cm from the stem. While mechanical defoliation has adequately simulated impacts of grazing on crop phenology in some studies on canola (Kirkegaard et al., 2008b, 2010) and cereals (e.g. Dann, 1968; Dann et al., 1977; Francia et al., 2006), the results here indicate that grazing treatments should be imposed by livestock under realistic conditions in experiments investigating disease development in dual-purpose B. napus crops.

Although stem canker severity was generally increased by grazing, cultivars with a high level of genetic resistance were more robust than those with a low or moderate level of resistance. Grazing increased stem canker severity by an average of 64% (2007) and 74% (2008) compared to the ungrazed treatment in the susceptible cv. ATR-Beacon (ABR 5·5/MS-S). In comparison, stem canker severity increased by 8% in both 2007 and 2008 (G1–G4 only) in the resistant cv. Surpass501TT (ABR 9·0/R). In 2008, there was an average 94% increase in stem canker severity in the last three grazing treatments (G5–G7) in Surpass501TT, with the reasons for this discussed below. Sprague et al. (2010) reported a similar pattern of disease increase in susceptible and resistant cultivars when 13 B. napus lines ranging in resistance from ABR 5·0 to 9·0 were defoliated by mechanical slashing. Stem canker severity increased by almost 50% in susceptible cultivars compared to approximately 30% in resistant cultivars. Breeding efforts in Australia have focussed on selection of B. napus cultivars with resistance to stem canker development, with most cultivars susceptible to leaf infection (Salisbury & Wratten, 1999). The resistance in cultivars with a high ABR appears to be effective even when plants are rendered more susceptible to infection by L. maculans due to the damage caused by grazing or defoliation, at least during the vegetative stages of plant growth when crops would most likely be grazed. Grazing later than bud elongation not only increases susceptibility to infection by L. maculans but delays flowering, so that significant yield penalties result from pod fill occurring in hot, dry conditions (Kirkegaard et al., 2008a).

The ranking of stem canker severity under grazing was generally consistent with the ABR of cultivars. However, some cultivars reacted differently to grazing in the two seasons even though these experiments were located in close proximity. In 2007, stem canker severity was consistently low in the resistant cultivar Surpass501TT in all grazing treatments, but stem canker severity increased significantly at later grazing times (G5–G7) in 2008. Grazing also increased stem canker severity in the moderately resistant cultivar 46Y78CL in some grazing treatments in 2007 and in all treatments in 2008. In addition to seasonal conditions, the disease response of cultivars to L. maculans is dependent on the composition of the local pathogen population at the site. Leptosphaeria maculans is a sexually out-crossing species with the ability to rapidly evolve in response to exposure to host resistance (McDonald & Linde, 2002; Sprague et al., 2006). It is possible that the increased susceptibility of 46Y78CL in all grazing treatments in 2008 is in response to selection of L. maculans isolates able to overcome the resistance in this cultivar, as 46Y78CL was widely grown commercially in the local area over a number of seasons. However, the large increases in stem canker severity in later grazing treatments for Surpass501TT cannot be explained by a breakdown of resistance. A possible explanation for the apparent breakdown in resistance with later grazing is that the damage caused by grazing in elongated plants allowed direct entry of L. maculans into the stem tissue, thereby bypassing any of the usual mechanisms of resistance in Surpass 501TT that may operate at the leaf, petiole or petiole/stem junction. Studies using GFP-labelled isolates of L. maculans have shown that the hyphae grow rapidly within xylem vessels in the stem and root which act like a ‘plant highway’ (Sprague et al., 2007). The observation of increased disease severity when plants were grazed later is further supported by the results from the cultivar 46Y78CL sown on three dates and grazed at the same time (but at different growth stages) in 2008. Stem canker severity increased more in the plants that were at a later stage when grazed, particularly those grazed after the stems started to elongate.

Grazing of B. napus crops rarely reduced stem canker severity. This is consistent with findings by Sprague et al. (2010) who defoliated 36 brassica lines at three sites and found no reduction in stem canker severity. There are few studies available that adequately investigate the interactions of defoliation and fungal disease development in dual-purpose crops. In wheat, the effects of septoria leaf spot (Septoria tritici), leaf rust (Puccinia triticina) and stem rust (P. graminis) were reduced by grazing, which the authors attributed to removal of disease inoculum (Dann et al., 1977; Williams & Littlefield, 2007). However, disease levels were not specifically measured. The growing apex of cereals is protected from browsing livestock under the soil surface prior to elongation which allows the removal by grazing of up to 90% of aboveground biomass (Freer et al., 1997), thereby significantly retarding inoculum development. Foliar fungal pathogens of wheat do not invade crown or root tissues and removal of infected leaf tissue effectively removes disease propagules. In comparison to cereals, up to 70% of aboveground biomass in B. napus is available for removal by livestock as the presence of the apex above ground prohibits heavier grazing without yield penalty (Kirkegaard et al., 2008a). Because L. maculans hyphae grow from infected leaves to the stem base via the petiole (Hammond et al., 1985), infected stem tissue may remain after grazing despite the removal of diseased leaf and petiole tissue. The results here certainly provide no evidence that defoliation by grazing will reduce disease, on the contrary care must be taken to avoid using cultivars with low resistance and/or grazing after stem elongation has commenced under conditions of high disease risk.

In Australia, recommended strategies to control L. maculans include the use of cultivars with genetic resistance, isolation of current B. napus crops from stubble of previous crops and fungicides applied to seed (fluquinconazole) or fertilizer (flutriafol) at sowing. All of these same approaches apply to grazed crops although disease control from fungicide applications to seed or fertilizer may be limited in dual-purpose crops by chemical with-holding periods (e.g. domestic with holding period for fluquinconazole applied to seed in Australia is 8 weeks from sowing). Growers intending to graze B. napus crops should be advised to sow cultivars with a high ABR even in low disease situations and to graze during vegetative growth to avoid excessive yield loss due to L. maculans.

The successful management of B. napus as a dual-purpose crop has prompted the investigation of these crops in areas where canola has not previously been grown, especially in higher rainfall zones where grazing enterprises predominate and cropping is relatively new. Fodder brassicas (B. napus, B. campestris and various hybrids) are already grown as spring fodder crops in these areas and are not routinely screened for L. maculans resistance. In these long-season environments, winter canola (B. napus) cultivars have shown significant potential for dual-purpose use and may provide new sources of material highly resistant to L. maculans (Kirkegaard et al., 2008a, 2010). The development of farming systems incorporating grazed canola and fodder brassicas in higher rainfall environments will generate new dynamics of inoculum sources and infection potential, and vigilance will be required to capitalize on these opportunities without threatening the preservation of strong disease resistance.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This project was funded by the Australian Grains Research and Development Corporation (Project CSP00085) and supported by CSIRO. The authors acknowledge technical assistance provided by Paul Hely (CSIRO), Moin Salam (Western Australian Department of Agriculture) for generation of ascospore release data, Agritech Research NSW and staff at the CSIRO Ginninderra Experiment Station for sowing and managing the experiments.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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