The growers group met in May at Hamish Campbell’s farm, where they were joined by Faye Ritchie, of ADAS, and Syngenta’s James Southgate to address some autumn establishment challenges, and the latest thinking to assure healthy plants.
New research tracking epidemics of light leaf spot (LLS) has revealed first symptoms occurring in crops which will influence optimum application timing, which may be as late as January or February. Trials in Yorkshire this year, for example, indicated mid-December treatment achieved the most reliable control, even though leaf spotting did not take off until mid- to late-January.
James Southgate, Syngenta field technical manager, emphasised that these trials reinforced the need to focus on phoma with the earlier autumn fungicide treatments.
“It is clearly more cost-effective to prioritise phoma with Plover applications in October and November, and preserve alternative modes of action for LLS in December and January, when they will have best effect. We are looking at working with ADAS and the iOSR group to provide all growers with predictions of phoma infection, including the effects of weather and varietal resistance, which will help to achieve best possible results with Plover applications.”
James also said phoma risk was closely linked with the varietal characteristics and the vigour of the crop in autumn. “Strong establishment and fast growth creates a larger leaf size, which means any phoma leaf spot infection will be slower to reach the stem.
“Furthermore, there is evidence of the complex interaction between pests, primarily cabbage stem flea beetle, and disease which has been shown to increase the severity of infections. Hybrid vigour in autumn, along with good agronomic practice, can therefore help crops to outgrow some of the pressures from pests and diseases, minimising the impact in autumn and throughout the growing season.”
Managing light leaf spot has been the biggest oilseed rape agronomy change in recent years for Cambridgeshire grower Russ McKenzie.
ADAS OSR pathologist Faye Ritchie pointed out weather conditions in the UK had been highly favourable to the polycyclic nature of LLS. This meant, once established, it could reproduce and cycle within the crop every few weeks, unlike monocyclic phoma which relied on new spores being produced on crop debris and coming into the crop to reinfect it. LLS even continued to develop within the plant at far lower temperatures than phoma.
Given these multiple infection cycles, Faye urged growers to focus on preventative treatments with their winter and early spring LLS fungicides. Preventative treatment at early onset of disease had proven more effective in controlling the disease.
Research had already revealed reduced sensitivity to azole fungicides in UK LLS populations, with the mechanism similar to Z. tritici resistance in wheat, she warned. While it is unknown how widespread this is, any action which limits selection for such strains will help to delay this.
Growers and agronomists should therefore look to avoid repeated use of products with the same mode of action such as azoles and use alternative modes of action wherever possible in the fungicide programme, including flowering sprays, she advised.
Faye said varietal resistance could play a highly important role in both reducing the effects of disease and giving growers greater flexibility in application timing, plus extra resilience if applications were delayed or missed. She believed the low resistance ratings of some OSR varieties could be a contributing factor to the increasingly costly incidence of the disease.
ADAS has calculated each resistance rating score equates to about £30/hectare in yield, with benefits to using more resistant varieties with ratings of 6 and above, particularly if the timing of fungicide applications were not optimal.
Syngenta’s OSR manager Kat Allen said a variety such as SY Harnas, with a resistance rating of 6, would have an average £60/ha improvement over a variety with a rating of 4, for example. While LLS had been seen as a predominantly UK disease and of lower priority for European OSR variety breeders, recent severe incidences reported in Denmark and other northern European countries are expected to make selecting for resistance more important.
James Southgate reported studies had shown the effects of turnip yellows virus (TuYV) on overall plant vigour. Stunted plants within the crop have been singled out and identified with high levels of TuYV infection, compared to surrounding plants clear of virus which have 80% greater biomass.
He said: “We are familiar with the purpling leaves of virus-stressed crops in autumn, but this has demonstrated the implications extend into spring and beyond, with effects on plant health and vigour. It highlights the need for growers to be vigilant for the aphids which transmit the virus flying into the crop in autumn, and particularly targeting the threat of Myzus persicae aphids when they first occur.”
James revealed the results of new trials at the Syngenta Innovation Centre, in Oxford, which demonstrated Plenum treatment immediately after aphids were found migrating into the crop, reduced the incidence of TuYV by about 60%. Where treatment was delayed for two weeks, infection was still reduced by a third, but a higher level of losses would have been incurred.
The trials also highlighted the importance of using an oil adjuvant with Plenum treatments in autumn to enhance the speed of uptake and rainfastness. When heavy rain fell 24 hours after application, without an oil the control of aphids and reduction in virus was significantly reduced.
OSR crops which die off early from the effects of disease, pest or drought suffer severely from lower oil content, warned Cotswold grower, Hamish Campbell, who hosted the recent iOSR growers’ meeting.
He sees the implication of low oil content first-hand through his successful R-Oil cold-pressed rapeseed oil business, which is operated alongside a 1,600-hectare joint farming venture with Martin Parkinson, farm manager for the Cotswold Farm Park.
He said: “Where we’ve kept our crops green through to desiccation oil content is typically 47-49%. “Seed which comes in from contract growers where crops have died off prematurely can be 5% lower in oil, about 43-44%.”
In addition to the green-leaf fungicide strategy, Hamish believes sulphur is a key component for plant health and oil content, so typically they look to apply 120kg/ha of SO3 (equivalent to about 50kg of S) a year. Martin Parkinson added the best results had been from applying it in liquid fertiliser treatments to the growing crop, with trials of late post-pod set applications providing no advantage.
Clearly desiccation timing can also have an implication on seed oil content, if crops are stopped prematurely. iOSR growers on a study tour to Germany learned how crops there were left for longer to maximise oil content from extended summer days, a view shared by Norfolk grower Chris Eglington, who aims to let crops mature fully before desiccation, which may mean starting to harvest milling wheat before the OSR.
Hamish said desiccation did remain a vital tool to even up seed maturity, which is essential for safe storage and efficient oil extraction. He added: “With some of this season’s patchy crops it is going to be essential, but the key will be getting the timing right to maximise oil premiums and yields.”