Oilseed rape will start to pull away rapidly now that spring finally seems to have arrived and, in Scotland’s often tight oilseed rape rotations, sclerotinia risk will be important when crops come in to flower. Decisions to treat to protect against sclerotinia should be based on the risk of disease and this assessment should include the presence of other susceptible crops in the rotation. The host range for sclerotinia is very broad and includes many valuable cash crops such as peas, beans, potatoes, carrots and vegetable crops like lettuces. Oilseed rape is therefor a key crop in a field rotation to break the cycle of sclerotinia and reduce the risk to these other crops.
Sclerotinia risk is made up of the risk from previous crops in the form of dropped sclerotinia (resting bodies), the risk that these germinate and spread airborne spores at a time when the crop is flowering, the risk that these can infect the petals and the risk that these petals then fall and stick to the stem. That is a lot of steps in a chain to come together which is why severity of infection is so erratic between seasons. Your previous experience of the disease is a key driver – if you have past experience of outbreaks you will have a reservoir of infection and a higher need to treat. If you have other susceptible and high value crops in the rotation that will be another key reason to protect flowering rape crops.
At the minute our risk is low because crops are yet to flower and temperatures have also been too cool to allow infection. It’s likely now though that warmer temperatures will coincide with flowering and risk will be furthered increased if flowering is prolonged and by wet weather over that period. One spray for sclerotinia at mid-flowering is the basic choice in low risk crops. This will coat as many petals as possible before they fall and help to prevent sclerotinia from infecting the stem where petals stick. Two sprays are now fairly common where the risk is higher and these can be spaced 2 weeks apart over the flowering period. Fungicide options include SDHI, strobilurin and azole options and doses of products need to be half rate or above to give the necessary persistence.
If you do have to use several applications because risk is high or flowering is prolonged, then try and alternate products with differing modes of action and also consider what you have already used earlier in the season to manage foliar diseases and try to ring the changes from that. Stewardship guidance on the use of strobilurins has changed so that 2 sprays would now be the maximum on oilseed rape. There have been reports of SDHI resistance in sclerotinia in Europe of and on for a few years and one possible case in the UK so mixing and alternating from SDHIs if you intend using more then one flowering spray is important. It’s good practice not to use more than two SDHI sprays in the whole oilseed rape programme (including what was applied to manage light leaf spot) and it is also important not to over use azoles. They carry no specific limitation in terms of number of applications although there may be limits in total dose. Prothioconazole has a maximum application limit over the season of 1.26 l/ha which may preclude its use as a flowering spray if that total dose has already been applied to the crop.
You can check sclerotinia infection risk at the AHDB link here.
Fiona Burnett (SRUC) for the Farm Advisory Service
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