Source: GRDC Grain Zone

Research summary

• Previous recommendations for minimising blackleg infection on newly established canola crops advocated that the crops should not be grown in the same paddock more frequently than once in four years to allow sufficient time for all infected canola stubble to breakdown.
  
   
• Research undertaken in three contrasting environments of Victoria has provided new guidelines for minimising infection of canola crops by blackleg fungus (Leptosphaeria maculans).
  
  The survey was conducted at Birchip (low annual rainfall, 350mm), Wonwondah (medium annual rainfall, 450mm), and Lake Bolac (high annual rainfall, 600mm).
  
   
• An extensive survey undertaken in 109 canola paddocks with varying aged canola stubbles (6, 18, 30 and 42 months old) clearly showed that 90% of canola stubble broke down or was buried by 18 months after the canola crop was harvested. The remaining 10% of stubble consisted mainly of the hard, woody roots although small quantities of stubble survived for 42 months.
  
   
• Nearly all blackleg inoculum (95 to 9 %) originated from 6-month old canola stubble and not from older stubble. Thus, the level of blackleg disease in new canola crops was not influenced by the presence of, or proximity to, canola stubble aged 18 to 42 months.
  
   
• Within a canola-growing region, all canola crops will be affected by the pathogen - even crops grown over 1km from 6-month old stubble had moderate levels of infection - and hence some yield loss from blackleg is likely.
  
   
• Ascospore release was found to be very variable, ranging from 300,000 to 900 million ascospores per hectare from stubble of the same age and environment.
  
   
• Extended rotation length between canola crops did not reduce disease severity.
  
   
• Stubble burning can reduce the amount of surviving stubble and therefore the inoculum load by up to 50%.
  
   
• Importantly, distances as small as 100m between the current canola crop and a 6-month old stubble can significantly reduce yield loss from blackleg. Ideally, farmers should leave 500m between a new canola crop and last season's canola stubble.
  
   
• Plants can tolerate considerable infection before yield loss occurs. If infection does not occur before the third leaf stage, plants will not be severely infected by the pathogen. Most yield loss was associated with cankered plants or plants with over 70% of the cross section of the stem infected by the pathogen.

Project aims

The overall aim of this project was to determine how to grow canola crops under lower disease pressure and to then produce a management package that growers could use to determine how they can best produce canola with limited yield loss caused by blackleg.

Selected results

• At the low rainfall environment (Birchip region) more canola stubble existed six months after the canola crop compared to the wetter environments, but this difference diminished with time and was probably due to the wider use of direct tillage practice at Birchip.
  
   
• It is not just the physical amount of stubble remaining that determines subsequent disease pressure. It is also the number of ascospores released from the stubble.
  
   
• Generally, the number of ascospore released from the stubble declined as the stubble aged. However, environmental conditions also had a large effect. If rainfall was high, nearly all the ascospores were discharged from the 6-month old stubble leaving very few to be released in subsequent years. If rainfall was low, fewer ascospores were released, leaving ascospores to be released at a later date when rainfall increased.
  
   
• At both the low and high rainfall environments more than 99% of all blackleg ascospores released originated from 6-month old stubble. At Wonwondah, the percentage was 95%.
  
   
• Short rotation paddocks (canola / wheat / canola) surveyed at Junee had the same level of disease as paddocks that had no canola crops for at least the previous three years. This supported the findings reported above that most inoculum originates from 6-month old stubble, and confirmed growers' experiences.
  
   
• At Wonwondah, the entire district was surveyed to determine both ascospore release from each paddock containing canola stubble (regardless of age) and the level of disease in each new canola crop. The results showed a strong correlation between disease severity and distance to 6-month old canola stubble. There was no correlation between disease severity and distance to older stubbles.
   
• Transects heading away from 6-month old stubble showed that disease severity decreased markedly in the first 100m and then generally declined up to 500m away from these younger stubbles. Disease severity at 500 to 1000 metres was similar to 500m from 6-month old canola stubble.
  
   
• Similar levels of ascospores were released from all stubbles collected (ranging from plants with no visual cankers to plants being completely severed). It appears that all stubbles were equally colonised after the plants had died, regardless of the severity of infection when the plant was living.
  
   
• The stubbles of canola varieties and other Brassica species with a range of blackleg resistance were screened for ascospore release. It was shown that ascospore discharge was influenced by the genetic source of blackleg resistance.
  
  This trait may be useful in reducing the amount of inoculum from stubble, but may also be linked to an enhanced likelihood of resistance breakdown, as seen with the B. sylvestris resistance during 2003. Future research will determine how to best deploy this trait.
  
   
• Canola plants infected from the cotyledon to the third leaf growth stage suffered very damaging levels of blackleg infection. Conversely, plants infected after the three-leaf growth stage did not develop severe internal infection and did not develop stem cankers (even in susceptible cultivars).
  
  These findings highlight that genetic blackleg resistance at the seedling stage is crucial; that fungicide seed dressings only have to protect the plant for a limited period; and that in some situations it may be possible to manipulate sowing time so that plants are at the third leaf growth stage before ascospore release commences.

Recommendations

• After selecting for maturity and herbicide requirements, always choose the canola cultivar with the highest blackleg resistance.
  
   
• Plan canola plantings spatially across the farm, leaving at least 100m, but preferably 500m, between the current crop and last year's canola stubble.
  
   
• Consider block-planting canola on the farm to ensure that it is possible to get large distances between crops and stubble.
  
   
• Burning stubble is a good disease management tool, but is not essential.
  
   
• Assess the levels of infection in crops to determine if current practices are sufficient.
  
   
• If disease severity is high in present crops and good management practices are already in place, use a fungicide to protect seedlings in future years.
  
   
• If the months prior to seeding are very dry, ascospore release may commence too late to cause severe blackleg symptoms. If possible, sow canola crops before ascospore release commences.
  
   
• If growing canola and then a cereal, there is no advantage in burning the cereal stubble to destroy the remaining canola stubble - the remaining canola stubble is only responsible for a small proportion of blackleg inoculum.
  
   
• Be aware that occasionally disease pressure can be extreme and no management practices will be sufficient to avoid considerable yield loss.

Other information

Marcroft SJ, Sprague SJ, Pymer SJ, Salisbury PA, Howlett BJ (2003). Factors affecting production of inoculum of the blackleg fungus Leptosphaeria maculans in south-eastern Australia. Australian Journal of Experimental Agriculture 43, 1231-1236.

Marcroft SJ, Sprague SJ, Pymer SJ, Salisbury PA, B. J. Howlett BJ (2004). Crop isolation, not extended rotation length reduces blackleg (Leptosphaeria maculans) severity of canola (Brassica napus) in south-eastern Australia. Australian Journal of Experimental Agriculture 44, 1-6.

Marcroft SJ, Sprague SJ, Salisbury PA, Howlett BJ (2004). Potential for using host resistance to reduce pseudothecia and ascospores of Leptosphaeria maculans the blackleg pathogen of Brassica napus. Plant Pathology 53, In press.

Marcroft SJ, Sosnowski MR, Scott ES, Ramsey MD, Salisbury PA and Howlett BJ (2003). Leptosphaeria maculans does not cause severe stem lesions or cankers on canola (Brassica napus) plants in south eastern Australia infected after the seedling growth stages. European Journal of Plant Pathology, (submitted).

Eureka files (summaries of GRDC research projects) are produced by Allan Mayfield and Jon Lamb Communications on behalf of GRDC.

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