Ground Cover Supplement : GC Supplement - Foliar fungal diseases of pulses
6 Canola Rotate resistant varieties to outflank blackleg Growing canola varieties with different blackleg resistance genes will slow the development of disease resistance By Dr Steve Marcroft THE FUNGAL DISEASE blackleg can be minimised in canola by: ¢ sowing varieties with high blackleg resistance; ¢ sowing canola at least 500 metres from the previous season’s canola stubble (and never into the previous year’s canola paddocks); and ¢ applying fungicides in crops at high risk of developing blackleg. Blackleg severity is influenced by environment and canola intensity. Regions with more than 20 per cent of the cropping area sown to canola and greater than 500 millimetres of annual rainfall (with more than 100mm falling before sowing in March to May) are at highest risk of blackleg infection. Canola is most vulnerable to blackleg as a seedling. If crops are sown early into warmer conditions and get through the seedling growth stage quickly before blackleg spores are released, they may escape high blackleg severity but may also be more prone to aphid damage and to blackleg stem canker. ROTATE RESISTANCE GENES The blackleg rating for canola varieties, published every year in the Blackleg Management Guide (www.grdc.com.au/ GRDC-FS-BlacklegManagementGuide), is the most important blackleg management tool. The guide classifies Australian canola varieties according to their blackleg resistance groups. If varieties based on the same resistance gene are grown each year, the blackleg fungus can overcome varietal resistance. It is important to switch to another resistance grouping if the same variety has been grown for three years and blackleg infection is increasing. Changing to a canola variety with different resistance genes will slow the development of resistance breakdown. Each year, representative varieties from each of the resistance groups are sown at 40 National Variety Trials (NVT) sites across Australia and monitored for levels of blackleg infection. The monitoring sites are sown without any fungicide protection on seed or fertiliser and do not receive any foliar fungicide applications. Stem canker (rot) incidence is recorded by pulling plants out of the ground, cutting the crown and scoring the percentage of the crown infected (0 to 100 per cent). The monitoring information indicates which resistance groups have higher levels of disease compared with the national average and provide an indication of whether the blackleg population is able to attack one or more particular resistance groups. The annual blackleg monitoring information is released each year via NVT Online (www.nvtonline.com.au). If the previous canola crop had a high level of disease, choosing a variety with a higher blackleg rating will help combat the disease. If the same resistance genes are used each year it is inevitable that the blackleg pathogen will overcome the varietal resistance. Rotating varieties with different resistance genes and avoiding a fungicide-only management approach will reduce the chance of the pathogen overcoming resistance. STUBBLE AND BLACKLEG Blackleg spores come from the previous year’s canola stubble. To minimise blackleg infection, canola should be planted at least 500m from the previous season’s canola stubble. Stubble destruction is not effective in reducing blackleg infection because enough stubble usually remains to harbour blackleg spores. Stubble older than two years produces fewer blackleg spores and will normally have minimal impact on blackleg severity. However, inter-row sowing of canola into two-year-old canola stubble may result in higher levels of blackleg infection if germinating seedlings are immediately next to standing stubble. DETERMINING BLACKLEG RISK A combination of high canola intensity and adequate rainfall increases the probability of severe blackleg infection (Table 1). Determining the extent of blackleg damage each season helps guide canola management decisions the following season (Table 2). Canola crops can be assessed for blackleg damage from the end of flowering through to windrowing (swathing). Pull 60 randomly chosen stalks out of the ground, cut off the roots with a pair of secateurs and, using the reference photos in Table 2, estimate the amount of disease in the stem cross-section. Canola yield loss occurs when more than half the cross-section is discoloured with blackleg. If blackleg monitoring identifies yield loss and the same variety has been grown for three years or more, choose a variety from a different resistance group for the following season. □ FUNGICIDE-TOLERANT BLACKLEG RAISES CONCERNS Blackleg isolates with tolerance to the fungicide fluquinconazole (Jockey®) were identified in a small 2014 survey. Further survey work in 2015 will determine the distribution of the fungicide- tolerant fungal strains, potential yield losses and how the tolerance may affect other fungicides. It is still recommended that growers use fluquinconazole in 2015. However, other blackleg management practices such as strategic use of variety blackleg ratings, separating crops from the previous year’s stubble and rotating variety resistance groups should be given a higher priority. Blackleg fungus reproduces sexually, which means it can quickly ‘mix and match’ its genes to produce strains that can overcome the disease resistance bred into canola varieties.
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