Stubble management for disease control in irrigated crops varies little from the surrounding dryland practices. Crop biomass and stubble loads may be higher, and more moisture is available to the crop, but the overall disease environment mirrors that of dryland. The growth in the use of overhead irrigation may increase the risk slightly, but little work has been done to quantify the risk and most irrigation in this region is still flood.
Table 1: Diseases carried by stubble relevant to the irrigation districts listed in declining order of importance.
|Wheat||Crown rot||Yellow leaf Spot||(Septoria Tritici Blotch*)|
|Barley||Spot Form of Net Blotch||Scald|
|Faba bean||Ascochyta||Chocolate Spot||Cercospera|
*Septoria Tritici Blotch (STB) is currently not a common disease in irrigation. Sclerotinia is not technically a stubble borne disease as it is carried over external to the stubble.
Remember that for disease to flourish, there needs to be a susceptible host, disease inoculum and environmental conditions that favour the development of the disease. Many seasons have seen only two factors present and little disease occurs.
Strategies for avoiding stubble borne diseases in irrigation
Rotation is the number one strategy to avoid disease. A faba bean – canola – wheat – barley rotation minimises the risk of stubbles being present as a source of infection. This rotation also has the benefits of a good N nutrition plan, as well as minimising stubble interfering in crop establishment, i.e. sowing the next crop into stubble that presents minimal issues in terms of biomass that the emerging crop has to contend with (except barley into wheat).
Often the risk associated with stubble borne diseases can be reduced due to the rapid breakdown of stubble in the irrigated system. With stubble gone, the source of the disease is removed. This was demonstrated in crown rot in wheat where a one year break crop (canola or faba beans) during 2012 reduced the incidence of white heads compared to the wheat on wheat situation (Table 2).
Table 2: Crown rot incidence in selected rotations at the Kerang irrigated trials 2010-13.
|2010||2011||2012||2013||Incidence of white heads|
|Wheat||Fabas or Canola||Wheat||Wheat||20-25%|
|Wheat||Wheat||Faba bean or canola||Wheat||< 5%|
While barley rarely shows any signs of crown rot infection, it is a host and a following wheat crop can be infected (hence the light green in the Table 3).
If sclerotinia is present on your farm, both canola and faba bean are hosts and can be infected from soil borne sclerotia if environmental conditions are favourable.
Keep in mind that you may have the perfect rotation and separation between this year’s crop and last year’s stubble, but what is your neighbour planting and where?
2. Resistant variety choiceA current disease guide should be consulted to understand the risk from disease associated with each variety, based on the risk in the paddock.
Varying levels of resistance are available in crop varieties to the stubble borne diseases.
In wheat, useful resistance to YLS is present in some varieties, but little resistance is available to STB and Crown Rot. Similarly there is little resistance to stubble borne diseases available in barley varieties.
Faba bean have yet to have resilient disease resistance in high pressure environments.
Varying levels of blackleg resistance is available in canola varieties (see the GRDC guide), as well as classification of the resistance genetics or group that imparts this resistance.
But keep in mind, given the environment that the crop is growing in, a MS rating may be sufficient to keep the disease at bay.
3. Fungicide Programs
Fungicides as seed treatments or foliar sprays are available to enhance protection. If disease is likely, seed treatments can offer insurance for some diseases.
Be aware that just because a disease is detected, control may not be warranted. As an example, yellow leaf spot is regularly detected across Northern Victoria during the winter months, but as conditions warm up, the disease rarely progresses further up the plants to the leaves that actually drive yield and the economic benefit of using a fungicide may be poor. Regular crop monitoring is required to detect the presence of disease and then to make an assessment on whether control is necessary given the level of infection, variety resistance and environmental conditions.
Similarly, barley crops often have spot form of net blotch during winter but the disease fails to persist once conditions begin to warm up.
It is a given that faba beans will have a preventative fungicide program in place for chocolate spot (Botrytis spp.) each season to prevent yield loss at flowering and pod development with an earlier program to control Cercospora (Cercospora zonata) and Ascochyta (Ascochyta fabae) in the vegetative phase if conditions are favourable. Coverage is essential irrespective of which fungicide is used.
4. Stubble Management
To reduce the risk of stubble borne disease, managing the stubble to remove the inoculum source is an option. Fully irrigated crops, through dense canopies and moisture availability, offer the opportunity for stubble to be removed through natural breakdown during the season. ICC trials have shown success with incorporation of cereal stubbles in the autumn with additional N fertiliser and then pre-irrigation that rapidly decomposes the stubble with little impact on sowing operations and/or nitrogen tie-up in-crop.
Even without incorporation, the guaranteed moisture supply and dense crop canopies in spring can result in significant stubble decomposition in one season, decreasing the risk of disease carryover through to the next year.
This research is being conducted by ICC as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 Maintaining profitable farming systems with retained stubble in Victoria and Tasmania).