Blackleg continues to pose a significant threat to canola production in southern Australia. Three main reasons are:
1. The increased area of canola sown.
2. Blackleg reproduces sexually enabling it to quickly overcome varietal resistance.
3. Extensive use of Triazole fungicides (Group 3 DMI inhibitors) to reduce the affect of Blackleg.
A breakdown in genetic or varietal resistance and or the development of resistance to Triazole fungicides, would be devastating to canola production and in turn wheat production.
Sound sustainable management involves more than just fungicide application. An integrated approach to management is required in order to ensure sustainable cost effective results.
Managing Blackleg involves first assessing the risk, followed by application of appropriate management strategies to reduce the risk to an acceptable level.
Following is a summary of the factors that are likely to influence the Blackleg risk in any given season:
1. Regional Canola Intensity
Increasing the area of canola grown in one locality increases the prevalence of Blackleg spores, which in turn increases the risk of Blackleg infection. Where 20% or more of the total crop sown in a district is canola, the risk is classified as high. Less than 5% is classified low risk.
2. Annual Rainfall
Blackleg risk is higher in higher rainfall areas. Where the annual rainfall exceeds 600mm, the risk is classified as high. Where annual rainfall is below 250mm, the risk is classified as low.
3. Pre Sowing Rainfall
Where rainfall from March to May exceeds 100 mm, the Blackleg risk is classified as high. March to May rainfall below 60 mm, results in a lower risk.
4. Blackleg severity in the previous year
A high incidence of Blackleg infection in any given year, will lead to an increased disease presence in nearby crops the following year.
The level of disease in a crop can be determined immediately post windrowing. To do so, 50 randomly selected plant stems should be pulled out and have their roots cut off at ground level using a pair of secateurs. Dark colouring in the inside of the stem is a sign of Blackleg infection. By scoring 50 plants it is possible to gauge the level of infection within the crop. A high level of infection results in a high risk to the following year’s crop grown in the immediate area.
Details of how to score Blackleg in the field can be found in the GRDC Facts Sheet titled Blackleg Management Guide. This guide is located under the Fact Sheets section of the GRDC website.
Understanding the level of risk will help to prioritise the need for management practices to reduce the impact of Blackleg on crop yield.
Following is a list of management practices which will impact on the incidence of Blackleg in a canola crop:
• Varieties with S-VS rating are high risk
• Varieties with R-MR rating are lower risk
Where possible, a variety with the highest Blackleg resistance rating should be chosen.
Distance from last year’s canola stubble
• Growing canola within 100 – 200m of a previous year’s canola stubble is high risk
• Growing canola greater than 500m from the previous year’s stubble is lower risk
Where possible, canola crops should be rotated around the property to obtain 500 metres or more separation from the previous year’s canola stubble.
Seed or fertiliser applied fungicides will reduce the level of Blackleg infection, but fungicides are not a cure. If severe disease infection is likely, fungicides will reduce, but not avoid, yield loss.
Canola stubble conservation
• Destruction of the previous year’s stubble does not reduce Blackleg infection.
• Stubble older than two years generally has less affect on Blackleg severity.
• However, interow sowing canola into two year old canola stubble, where seedlings may come into contact with old standing stubble, can result in higher levels of Blackleg infection.
Canola is most vulnerable to Blackleg as a seedling. Sowing early into warm soil conditions will help the plant grow quickly through the seedling stage. This can reduce the level of Blackleg infection.
Years of growing the same variety
The Blackleg pathogen will overcome varietal resistance if the same genetics are used each year. Sowing the same variety for 3 or more years consecutively in close proximity, can create a high risk situation.
By sowing a variety with different resistant genes, the ability of the Blackleg pathogen to overcome the genetic resistance will be reduced.
Rotate Resistance Groups
Many canola varieties have been classified into groups, based on the type of genetic resistance they have to Blackleg. Sowing varieties from the same group for 3 years or more in the same location will significantly increase the Blackleg risk.
The following table outlines the current resistance ratings and resistance groups for some of the common canola varieties.
Varietal selection must also take into account proven local performance. Varieties such as Crusher TT and ATR Cobbler have poor Blackleg ratings, but have performed well on many farms. In this case, the varieties should be scored post windrowing and rated based on the actual level of Blackleg present. Where there is a relatively low incidence of Blackleg and the varieties have performed well, they can be grown again utilising the management strategies outlined above to minimise the risk in the following year.
Successfully minimising the impact of Blackleg involves sound planning and management of all factors outlined above. Many of these factors do not involve additional costs, but are likely to lead to increased yields and lower risk.