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MANAGING BLACKLEG IN THE DISEASE-PRONE ENVIRONMENT OF WESTERN AUSTRALIA

Martin J Barbetti1,2 and Ravjit K Khangura1

1Agriculture Western Australia, Locked Bag No. 4, Bentley Delivery Centre 6983, Western Australia
2
Faculty of Agriculture, The University of Western Australia, Mounts Bay Rd, Nedlands 6907, Western Australia

ABSTRACT

In Western Australia (WA), the canola industry has expanded from 35,000 ha in 1993 to 510,000 ha in 1998. However, blackleg, caused by Leptosphaeria maculans, is a major constraint to its sustainability of a large-scale canola industry. There has been an increasing incidence and severity of blackleg in WA crops since 1993, such that severe crown cankering now occurs in most crops, and sporadic incidences of extensive seedling deaths occur in worst affected areas. This disease causes significant production losses, despite adult plant resistance present in commercial varieties and the widespread use of the fungicide flutriafol (Impact«) applied to fertiliser. The Mediterranean-type environment of WA favours disease carryover on residues, encourages severe epidemics of residue borne pathogens, aligns ascospore showers to emergence and maximises the disease impact on yield as the moisture supply dwindles at the end of the season. There are now 1 million ha of infected residues in WA canola production areas and it is no longer feasible to isolate new plantings from canola residues. There is an urgent need for improved management of blackleg if the canola industry is to remain a viable and sustainable component of long term rotations and if a contraction as occurred in early 1970’s is to be avoided. Improvements should include development and use of varieties with improved seedling and adult plant resistance, refining fungicide use for seedling protection and reducing inoculum pressure on the crop. Reduced pressure may be achieved by avoiding proximity to residues and by restricting rotations to 1 year in 4 or greater, which goes against the current industry trend. Reduced pressure may also result from improved crop residue management, including residue removal and treatments to enhance residue decomposition and reduce spore load.

KEYWORDS Leptosphaeria, rotations, flutriafol, resistance, canker, losses

BLACKLEG DISEASE IN WESTERN AUSTRALIA

Blackleg, caused by the fungus Leptosphaeria maculans, is the most serious disease of canola. While the fungus can infect any part of the plant at all growth stages, invasion of the crown is the most damaging phase. Crown invasion in seedlings can cause their death. In adult plants, badly affected plants are girdled at ground level and lodge, while less severely affected plants remain standing but seed is pinched because pods fail to fill properly due to restricted sap flow. Blackleg previously thwarted an attempt in the early 1970’s to develop a rapeseed industry in WA (Bokor et al. 1975). Only the development of varieties with adult plant resistance (Wratten and Murray 1982; Salisbury et al. 1995) has allowed a canola industry to remerge.

There has been a rapid increase in the area sown to canola in WA and a corresponding increase in the levels of blackleg. In 1993 there were 35,000 ha of the crop, but blackleg levels were generally insignificant and there were no apparent economic losses. However, even though disease levels were low, they were sufficient to allow rapid colonisation of senescing plants, making the residues an important source of disease for subsequent crops. In 1994 there were 110,000 ha with abundant blackleg leaf lesions in many crops. In the high rainfall southern areas there were a few crops with up to 20% of plants with crown cankers (Table 1). The worst yield loss reached 5%. In 1995 there were 120,000 ha of canola with abundant blackleg leaf lesions on most crops in all areas. In the high rainfall southern wheatbelt there were some crops with 40% of plants with cankers. Occasional crops of the variety Siren showed 5-10% lodging from cankers. The worst yield loss was about 10%. In 1996 there were 95,000 ha of canola. Some crops had 5-30% seedling mortality (Table 2) and 10-40% lodging of flowering plants from blackleg. Yield losses were common, with the worst yield loss estimated to be 50%. In 1997 there were 275,000 ha, with some crops with as much as 70% seedling mortality. Numerous crops had significant yield losses and total yield loss from blackleg across the whole of the 1997 crop area was estimated at 11% and represents $9.1mAUS at current prices of approximately $300/tonneAUS. In 1998 the area cropped to canola increased to 510,000ha and represented a repeat of the 1997 scenario but with slightly less severe blackleg. Total loss from blackleg was estimated to be $13.4mAUS. Blackleg is now a severe threat to a viable and sustainable canola industry in WA.

Table 1. The worst and mean incidences of blackleg crown cankers in commercial canola crops in WA during the period 1993 to 1998. * In 1993-1995 insufficient commercial crops were surveyed for an accurate indication of overall mean severity to be made.

Year Crown cankers(%) Mean severity(%)

1993 5 *

1994 20 *

1995 40 *

1996 90 48

1997 94 42

1998 70 35

Table 2. The worst blackleg seedling mortality in commercial canola crops in WA over the period 1993 to 1998.

Year Seedling Mortality(%)

1993 0

1994 2

1995 10

1996 30

1997 70

1998 35

WHY BETTER BLACKLEG MANAGEMENT IS NEEDED IN WA

In the south-west of WA, canola is grown in a Mediterranean environment that is a major determinant of the behaviour of the disease. Firstly, the dry summers favour survival of the fungus because of slow decomposition of crop residues. All residues remain a source of inoculum for up to 4 years. Rapidly increasing amounts of infected residues have resulted in more inoculum and more intense and frequent periods of spore dispersal. Secondly, the Mediterranean environment aligns major ascospore showers to seedling emergence as both are dictated by the early seasonal rains. Thirdly, relatively mild winters favour infection and canker development by the fungus (Barbetti 1975a), encouraging the development of severe epidemics. Fourthly, the moisture stress conditions at the end of the growing season in medium to low rainfall areas coincide with maximum blackleg damage to plant crowns, increasing the impact on yield. Fifthly, the dry summers associated with sandy soils result in sand blasting that makes plants more prone to blackleg infection. Finally, the physical damage caused to seedlings from strong winds makes plants more prone to this disease.

In WA, blackleg is exacerbated by the widespread use of varieties with relatively low resistance to blackleg, with more than 80% of the area sown in 1998 to the variety Karoo, a variety which has only intermediate resistance (score 5) under WA conditions. Stubble retention practices and slow decomposition of residues allow the blackleg fungus to survive for up to 4 years. The widespread use of minimum tillage practices encourages residues on the soil surface rather than incorporated into soil, and may lengthen the period of survival in the residues to 5 or 6 years. There is currently in excess of 1,000,000 ha of infected canola residues and this is expected to increase further next season. These increased areas of infected residues have made the practice of sowing canola well away from residues (eg. 1-2 kilometres) impractical, and in most areas it is no longer feasible to avoid proximity to them when planting canola. Use of tighter rotations resulting from a shortage of suitable land for canola, especially in the high rainfall areas, makes it even more difficult to avoid planting near infected residues. The relatively high frequency of sowing canola varieties with low blackleg resistance in the vicinity of infected residues indicates that many canola growers have been unable or unwilling to make a proper assessment of the disease risk. Available host resistance is now inadequate to avoid significant yield losses. Means of disposing of residues, enhancing residue breakdown and of suppressing sporulation in the residues urgently need to be investigated.

ASSESSING THE RISKS AND LOSSES FROM BLACKLEG

Estimating potential yield losses

Canola growers in WA can now estimate their yield losses in different situations and for different scenarios on their individual farms, using the actual recorded losses from different disease pressures shown in Table 3.

Table 3. Losses in canola varieties with different levels of adult resistance growing directly on canola residues of different ages to produce different blackleg disease pressures.

Variety

WA adult
resistance
score

Residue age

4
mild

age (years)

3
moderate

2
severe

1
very severe

           

Hyola 42

4

10-15%

25-35%

60-70%

100%

Karoo

5

5-10%

20-25%

50-60%

80-90%

Oscar

6

0-5%

10-15%

40-50%

70-90%

Dunkeld

7

0%

5-10%

30-40%

75-85%

Relative risks from blackleg in different areas

Highest risk areas are those with > 450 mm annual rainfall. In these areas there are frequent ascospore showers and the conditions are favourable for maximum disease development. It is recommended that growers only sow varieties with a resistance score of 6 or more. Medium risk areas are those with 450-325 mm rainfall. In these areas there are less frequent and less intense ascospore showers as rainfall events are less frequent and there is often less residues. Here, it is recommended that growers only sow varieties with a resistance score of 6 or more. Low risk areas are those with < 325 mm annual rainfall. In these areas there are evev less frequent and less intense ascospore showers as rainfall events are less frequent and there is often less residues. Here, it is recommended that growers only sow varieties with a resistance score of 5 or more. In all risk areas, varieties of lower resistance than recommended can be sown at sites that are well isolated (>2km) from any canola residues.

OPTIONS FOR MANAGEMENT OF BLACKLEG

Options for management of blackleg in WA include the use of resistant varieties; application of various cultural measures such as avoiding inoculum (residues), destruction of residues, long duration rotations and distancing from residues; and use of fungicides.

Use of host resistance

Host resistance is largely adult plant resistance. However, such varieties with good adult plant resistance can suffer serious yield losses if attacked in the early seedling stage. Commercial varieties with intermediate adult plant resistance to blackleg suffer serious economic losses even if infected after the more susceptible seedling period has passed. The frequency of infection and the level of disease on partially resistant plants is determined by inoculum level, and may also be affected by shifts in virulence in pathogen populations. The resistance of current varieties (Table 4) may not be sufficient under the increased inoculum pressure resulting from increased areas of infected residues and use of rotations tighter than 1 in 4 years. The diverse conditions in WA make it difficult to gauge what level of resistance will be adequate to protect the crop in each situation. It is essential that improved levels of both seedling and adult plant resistance are identified for WA conditions. In the interim, planting canola should be avoided in highest risk situations.

Table 4. Adult plant canker resistance ratings for canola varieties in WA. Varieties are rated on a 1-9 scale, where 1 = extremely susceptible, 2 = very susceptible, 3 = susceptible, 4 = moderately susceptible, 5 = intermediate, 6 = moderately resistant, 7 = resistant, 8 = highly resistant, and 9 = immune.

Variety Adult resistance Variety Adult resistance

score (WA) score (WA

Conventional Triazine

varieties tolerant

varieties

Hyola 42 4 Karoo 5

Charlton 6 Clancy 6

Oscar 6 Drum 6

Monty 6 Pinnacle 6

Mystic 6

Dunkeld 7

Scoop 7

Rainbow 7

Grouse 7

Use of cultural practices

As a minimum requirement, canola should not be grown on the same land more frequently than 1 in every 4 years. Growers need to avoid canola residues by not sowing into or alongside them, nor sowing downwind of residues close by. While a minimum separation of 500m from remaining residues is critical, ideally this separation should be greater than 1.5 km. Growers should also attempt to reduce canola residues by means such as raking and burning, burying using cultivation, or grazing.

Use of fungicides

While use of fungicides have been partially evaluated (Ballinger et al. 1988a,b; Barbetti 1975b; Brown et al. 1976, Khangura and Barbetti 1997), they need further investigation for their potential to protect seedlings from seedling mortality under high inoculum levels. Previously untried fungicides need to be tested. Use of flutriafol (Impact«) as an in-furrow fertiliser treatment is registered for use in WA. While its use has allowed increased flexibility in disease management, significant losses can and do occur even with its use. It should be viewed as an interim management tool until varieties with improved resistance are available. The situations in which flutriafol use is likely to be economic is where there is a combination of moderate host resistance (5-6 on the WA adult resistance scale), good yield potential, and moderate to severe blackleg disease pressure.

ALTERATIONS NEEDED FOR IMPROVED MANAGEMENT OF BLACKLEG

Major alterations in crop management are required if canola is to become a sustainable part of long term rotations in WA. Improvements should include development and use of varieties with improved seedling and adult plant resistance, refining fungicide use for seedling protection and reducing inoculum pressure on the crop. Reduced pressure may be achieved by avoiding, where possible, proximity to residues and by restricting rotations to 1 year in 4 or greater, which goes against the current industry trend. Reduced pressure may also result from improved crop residue management, including residue removal and treatments to enhance residue decomposition and reduce spore load. Recommendations to WA canola growers for managing blackleg in 1999 included selecting the most resistant variety, avoiding paddocks with residues, reducing existing residues, assessing the level of risk, and using flutriafol on the fertiliser where appropriate.

ACKNOWLEDGEMENTS

We thank the Grains research and Development Corporation for financial support.

REFERENCES

1. Ballinger, D.J., Salisbury, P.A., Kollmorgen, J.F. and Potter, T.D. (1988a). Evaluation of fungicides, applied at sowing, for control of blackleg of rapeseed. Australian Journal of Experimental Agriculture 28, 511-5.

2. Ballinger, D.J., Salisbury, P.A., Kollmorgen, J.F., Potter, T.D. and Coventry, D.R. (1988b). Evaluation of rates of flutriafol for control of blackleg of rapeseed. Australian Journal of Experimental Agriculture 28, 517-9.

3. Barbetti, M.J. (1975a). Effects of temperature on development and progression in rape of crown canker caused by Leptosphaeria maculans. Australian Journal of Experimental Agriculture and Animal Husbandry 15, 705-8.

4. Barbetti, M.J. (1975b). Benomyl and carbendazim fail to provide effective control of blackleg in rape. Australian Plant Pathology Society Newsletter 4, 11.

5. Bokor, A., Barbetti, M.J., Brown, A.G.P., MacNish, G.C., Poole, M.L. and WoodáP. McR. (1975). Blackleg - a major hazard to the rapeseed industry. Journal of Agriculture, Western Australia 16, 7-10.

6. Brown, A.G.P., Barbetti, M.J. and Wood, P. McR. (1976). Effect of benomyl on "blackleg" disease of rape in WA. Australian Journal of Experimental Agriculture and Animal Husbandry 16, 276-9.

7. Khangura, R. and Barbetti, M.J. (1997). Blackleg monitoring and the role of Impact« in-furrow for its management in Western Australia. Proc. ‘Eleventh Australian Research Assembly on Brassicas’, Perth, Western Australia, Oct. 6-10, 1997, pp. 16-18.

8. Salisbury, P.A., Ballinger, D.J., Wratten, N., Plummer, K.M. and Howlett, B.J. (1995). Blackleg disease on oilseed Brassica in Australia: a review. Australian Journal of Experimental Agriculture 35, 665-72.

9. Wratten, N. and Murray, G. M. (1982). A population improvement approach for developing resistance to blackleg in rapeseed. Cruciferae Newsletter 7, 62.

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