Although many oilseed rape crops were drilled earlier than usual due to the good conditions at, and following, harvest, this can lead to further challenges. Heather Briggs seeks expert advice on disease control this autumn.
Light leaf spot could be problematic particularly in susceptible varieties if growers fail to spray this autumn, warns ADAS plant pathologist Faye Ritchie.
The disease affects all regions of the UK, making it one of the most important disease threats to oilseed rape. It is a difficult disease to control with fungicides alone therefore, particularly in high-risk areas, there are benefits to using varieties with high resistance ratings.
Conditions this year meant some growers drilled early, which is a good strategy to mitigate against phoma, however, light leaf spot epidemics are often more severe in earlier sown crops.
For light leaf spot control it is, therefore, important to have varieties with good resistance coupled with well-timed fungicide applications.
Fungicides applied for phoma control in September or early October are often not effective for good light leaf spot control, notes Dr Ritchie. Fungicide applications from late October/November onwards are known to be more effective and can be used to provide early protection.
Although many farms still have dry soils, as the autumn progresses there may be mild, damp conditions favouring light leaf spot and which also cause difficulties in getting on the land to spray, making control challenging.
Dr Ritchie says: “Monitoring crops in January and February will also be key to good control.
“There is no threshold for light leaf spot treatment pre-stem extension so if disease is found it should be treated as soon as possible if it is seen.
“Light leaf spot is easier to manage with fungicides when levels are low and difficult to control if fungicides are applied at stem extension to heavily infected crops.
“Both susceptible and resistant varieties will benefit from a protectant approach to fungicide applications,” she adds.
At the moment both azole and non-azole chemistry is effective against phoma and light leaf spot.
Several years ago a non-azole product, Refinzar (penthiopyrad + picoxystrobin), came to the market but will no longer be available to use after 30th November this year.
It will mean, going forward, growers will be reliant on azole and azole/non-azole co-formulations for the control of autumn oilseed diseases, points out Dr Ritchie.
“The risk of pathogens developing resistance to fungicides is always a threat, particularly if we have to rely more heavily on a single mode of action for disease control.
“It will be important to think about where we use different modes of action within the fungicide programme and ensure that we use mixtures of products with different modes of action or alternate fungicide products with different modes of action, from autumn applications through to early spring and flowering sprays.”
Wet conditions during August and September are key for spore development on stubbles, with backward crops and small plants at greater risk from stem canker development. This is due to the fungus having a shorter distance to travel from the leaf to petiole and to the stem.
She remarks that phoma is still an important OSR disease across much of England with untreated autumn infections leading to spring and summer stem cankers that can reduce yields by as much as 25 per cent in severe years.
“I would advise combining high performance with good all round disease resistance that includes stem canker and light leaf spot where crops are at high risk from these diseases.
“Aim to spray as soon as the phoma threshold of 10–20 per cent of infected plants is reached, and consider a second application when re-infection is seen which can be between 3–6 weeks later,” she continues.
“Growing a more resistant variety helps to slow down the time it takes for the fungus to reach the stem following initial infection which can help to take some of the pressure off.
“By the time phoma leaf spot occurs this year, early-sown crops are likely to be large enough for it to be less of an issue but later sown crops will be more at risk.
“It is possible that we could be well in to October before phoma leaf spotting is seen if the dry weather continues and a one-spray approach may be sufficient.
“Crops should be monitored after the first application and follow up spray planned if phoma leaf spot re-infection is seen.”