Sclerotinia
Sclerotinia stem rot was evident in many canola crops prior to windrowing this year. Management of the disease in future will become increasingly important, due to the intensity of canola being grown across southern NSW. The availability of a fungicide to control the disease, should make management easier, however in reality, it presents another situation where growers are faced with a significant cost to prevent a problem, which is unpredictable and sporadic. Harvest results from this year will help quantify the commercial benefit of fungicide application. However, the widespread occurrence of frost and drought stress, has also demonstrated that expenditure on such products can actually increase the risk profile of the business.
It is important to remember with Sclerotinia, that for the disease to develop and produce yield loss, a number of related factors need to occur in sequence:
Sufficient inoculum needs to be present in the paddock, on the farm or in the district
Prolonged moist soil conditions in late winter early spring, combined with moderate temperatures of 150C to 250C, must prevail for fruiting bodies to form and release ascosprores
Canola crops must be actively flowering with moist leaves for the ascosprore to survive on after release
The crop canopy requires moisture when the petals fall and lodge on the branch junctions along the stem, for the fungus to grow, invade the stem and form sclerotia
If one of these factors does not occur, the incidence of the disease will be reduced or eliminated. This is why Sclerotinia is such a hard disease to predict and manage.
Given the large area of canola grown in the last few years, it is understandable that the inoculum level was high in 2013. However, it is surprising that the other factors occurred as well, given the poor rainfall for the second half of the growing season.
Current canola varieties have no varietal resistance to Sclerotinia. Therefore the three main management strategies are:
Geographic separation of canola crops from canola stubbles
Varietal selection to reduce vegetative growth and spread flowering times
Strategic application of fungicide
The first two options should be a part of planning crop rotations, whereas the decision to apply fungicide is more complicated.
In higher rainfall areas, fungicide application may become more common, where it is likely that all of the disease factors will occur more regularly.
In the medium rainfall areas, the decision is more difficult, due to the unpredictability of all the disease factors occurring in any one year. An assessment needs to be made of the potential Sclerotinia risk in each season, prior to initiating a fungicide program. If a high likelihood of an economic loss cannot be identified, a fungicide should not be applied. Incurring control costs on an annual basis, for a loss event that occurs infrequently, simply increases the cost structure of the business.
Sclerotinia is a threat to canola production and fungicides such as Prosaro can reduce the impact of the disease. However, careful consideration needs to be given to the incidence of Sclerotinia, plus realistic yield loss likely to be incurred, before committing to a spray program.