Growers are urged to stay alert for signs of septoria as soil temperatures rise.
Although it has been a decade since the last sclerotinia epidemic, the damage it can do – with yield loss of more than 50 per cent in severe, uncontrolled instances – means oilseed rape growers must stay vigilant against the disease, warns Caroline Young, research scientist at ADAS.
She says: “Even though growers currently do a good job in controlling sclerotinia, it is important to note that sclerotinia pressure is still out there. Last year in our trial sites we had up to 30 per cent infection in untreated plots. The potential is there for yield loss.”
Sclerotinia infection becomes likely if temperatures rise above 7degC, if humidity exceeds 80 per cent for more than 23 continuous hours, and when spore inoculum is present.
Dr Young says: “In spring, when the soil warms up, the sclerotia in the soil are a bit like seeds. They germinate and produce mushroom-like apothecia that release spores, and spores land on petals. If the weather is damp when the petals fall off, they sometimes stick to leaves on the way down. If there are sclerotinia spores on these fallen petals, the spores germinate and use the petals as a food base from which to infect and grow into the leaf, down the leaf stem, manifesting as lesions on the main stem. That cuts off everything above it, which is why you get the whiteheads – the infection stops translocation and the top of the plant dies.”
She notes the whiteheads can make disease incidence look worse than the reality if sclerotinia has not been controlled. A well-timed fungicide spray is estimated to give a 0.2t/ha (0.08t/acre) increase in yield response for every 10 per cent increase in sclerotinia in an untreated crop.
“But even low levels of infection below 5 per cent will still be contributing to build up of sclerotia in the soil. Sclerotia can get ploughed back into the soil, and there have been reports that they can last up to ten years there, though obviously fewer and fewer will survive each year. It means that if the conditions are right, sclerotia will germinate to produce apothecia which will release airborne spores, so growers have to be on the alert for infection resurfacing.”
Steve Cook, partner at Hampshire Arable Systems, says by lengthening rotations and limiting potential host crops, he has managed to mitigate against the sclerotinia threat on the 40,469ha (100,000 acres) of land he advises on.
He says: “We are farming on lighter land here in the south, with longer rotations. We have got a growing oilseed rape area, but the crop is no more than one in four, usually one in six. There has been a general move towards longer rotations recently among growers, and I think that certainly plays a part in reducing the threat.
“We have not got many alternate host crops like potatoes or broadleaf crops, and our herbicide programme keeps down weeds like shepherd’s purse, which can also host the disease.”
Even so, he employs flowering sprays to ensure continued protection against the disease, and to also control two other significant threats to his oilseed rape: light leaf spot and alternaria.
By tackling the various disease pressures, Mr Cook helps give his growers the best chance of getting a good yield come harvest – but has encountered instances when the consequences of not spraying have been severe.
“The yield comes almost entirely from seed size, which happens post-flowering. It is so important to do everything you can to have a clean crop. I remember a few years ago, a contractor missed the sprays on one of the oilseed rape fields, and at harvest that field yielded half a tonne per hectare less than the other fields. Getting the sprays right makes a huge difference to yield,” says Mr Cook.