Fast flowering oilseed rape faces intense Sclerotinia risk
Oilseed rape crops are moving through growth stages incredibly quickly in warm conditions. The forecast is for an early and very intense flowering period, especially across eastern and southern England. Combined with warm soils that will trigger the early release of Sclerotinia spores, crops could be subjected to high risk of infection, warns Gary Jobling of Syngenta.
Gary Jobling in green leaf OSR
“If we have crops with a short flowering duration this season, plants will be exposed to high levels of risk over a short period, so maintaining a high level of protection with two spray treatments is imperative. If the flowering is extended – which may be the case in the wetter north of England and Scotland – careful timing of application through the flowering period can give protection across the full duration,” he advised.
Whilst the incidence of Sclerotinia has been relatively low in oilseed rape for the past three seasons, the disease life-cycle means there will still be a high burden of soil-borne sclerotia following the major outbreaks in 2007 and 2008. He highlights that since Sclerotinia attacks are notoriously difficult to predict, it makes a two-spray strategy essential to cover the key infection risk through the flowering period, with the first Amistar spray at first flower and a second application at first petal fall.
Mr Jobling advocates growers with oilseed rape fields at various stages of flowering should, where possible, treat individual crops accordingly. “Whilst Sclerotinia spores can travel on the wind, those fields that have grown oilseed rape in recent years, or are directly downwind of fields with a history of disease, will be most at risk and a priority for protective programmes.”
And he reminds growers that it doesn’t necessarily have to be rainfall that provides the moisture that is a precursor for infection. Modelling of historic weather patterns and the corresponding levels of Sclerotinia infection has clearly demonstrated that periods of high humidity during flowering can be sufficient to trigger unexpected damaging attacks. This could be important in predicting risk and tailoring future fungicide programmes.
Green leaf retention
Mr Jobling also highlights that green leaf retention, through disease control and physiological effect on plants, remains key to driving the high yields and increased oil content of seed that has been repeatedly seen from Amistar fungicide programmes.
These benefits of maintaining crop vigour and green leaf could be especially important if crops are drought stressed during the growing season. It has been suggested that crops that remained green for longer last year were better able to take up and utilise nitrogen inputs late in the season, which was one of the key factors in some of the very high oilseed rape yields achieved.
Trials and growers’ own experiences have repeatedly shown that Amistar programmes can extend green leaf retention that not only increases overall yield, but also raise the average oil content of seed. With oil bonuses calculated as a percentage of the tonnage price, current oilseed rape values of around £360/t favour varieties and agronomy inputs that will increase oil content – typically worth around £5.40/t per 1% oil content increase over 40%. Last season saw oil contents touching the 50% mark, which would be worth an extra £270/ha to the grower with a five tonne/ha crop.
“For growers looking to achieve consistent high oilseed rape yields, it’s no secret that a two-spray Amistar programme, at the very first signs of flowering and at mid-flowering, will deliver greater yields,” he advised.
Targeting verticillium wilt
Growers and agronomists are also now far more aware of the threat of verticillium wilt, and looking to manage the disease more effectively. Mr Jobling believes increasing infection levels may be one of the reasons why crops that look good right up to harvest, fail to perform when the combines go in.
He urges growers to inspect stubbles soon after harvest for the tell-tale signs of verticillium wilt, which could help them manage future crops in the rotation more effectively. Look out for characteristic grey or black stripes on the stem that extend continuously from soil level. The discolouration lies just under the outer cell layers, so when the stem is cut open the centre cortex will still be white; if the centre is brown it is more likely to be the result of phoma infection.
Where verticillium wilt is a risk, ADAS trials and growers’ experience indicate that Amistar treatments for Sclerotinia and Alternaria may also have a beneficial effect. Growers who have identified fields where the disease is an issue will be best placed to take advantage of future fungicide and varietal options, he added.
