Genetics, forecasting, spray timings and nutrition all have a role to play in controlling light leaf spot. Marianne Curtis reports
Light leaf spot (LLS) has been prevalent in oilseed rape crops in many areas of the country this year, following a wet autumn which favoured spread of the disease and interfered with spray timings.
The CropMonitor service, which tracks disease activity in England, found LLS affected 85% of crops and 34% of plants in spring 2015 (the latest data available). This was lower than in the previous spring, when 87% of crops and 44% of plants were affected but higher than the long-term mean from 2004-2013 of 54% of crops and 19% of plants affected.
While there is no treated versus untreated trial data available currently, past data indicates yield losses of 20-30% and possibly higher in more susceptible varieties, according to NIAB plant pathologist Dr Jane Thomas.
As the disease has grown in significance over the past few seasons, growers are inevitably looking to better control it, but opinions vary on the best approach.
ADAS plant pathologist Dr Faye Ritchie says this year LLS has been the worst she has seen for 10 years.
“Levels were high and we saw it very early on in the season. In some crops in the West, the disease was there from mid-November; it is hard to see very early on but there were early symptoms on susceptible varieties.
“It was difficult to get fungicides on in November with wet and windy weather. Once you have a severely diseased crop it is difficult to manage. It is easier to manage when levels are low.”
Getting fungicides on in a timely manner is a key factor in control of the disease, believes Martin Smart, arable manager at 1,600-hectare Ashton Farms, in Wiltshire, who sprays twice in autumn and twice in spring.
He says: “We keep on top of spraying rather than have big gaps and don’t try to stretch the interval. Weather patterns are a dictating factor and herbicide spraying ties you into a lot of things.
“Light leaf spot is not really a problem at the moment. I think we’re lucky more than anything else.”
Growers choosing to reduce fungicide application frequency for phoma control as they come to rely more on genetic resistance to this disease could account for some of the rise in LLS in England, says Dr Neal Evans, European research lead at WeatherINnovations Consulting.
He says: “Some [growers] are only doing one autumn spray. It seems a second spray targeted at phoma control was also taking out light leaf spot.”
In Scotland, where LLS has traditionally been a problem, phoma is less prevalent so there is more scope to get timing better for LLS, says professor Fiona Burnett, head of crop and soil systems at Scotland’s Rural College (SRUC).
“With autumn spraying you need to go as late as you can while it is still possible to travel, so late October, early November. This gives coverage over winter, then a second spray at stem extension time in March.
“If you’re worrying about phoma as well, timing is early autumn but three applications takes a certain mindset. A compromise is a midway timing – slightly too late for phoma and slightly too early for LLS. Work still needs to be done on optimising timing of fungicides.”
However, refining forecasting of the disease, which would help in this endeavour, is tricky, as Dr Evans found out while conducting a recent study looking at spore and weather data, with the aim of developing a date-driven rather than the current risk-based forecast.
“We used to believe LLS ascospores only came in in late September, October and early November but the interesting thing to come out of this project is the pathogen produces ascospores all through the season from May. So the crop is emerging into an environment already full of LLS.”
Fungicides for control of LLS have been evaluated for the last six years by ADAS in a project conducted by AHDB. Trial sites are at ADAS High Mowthorpe, North Yorkshire, SRUC, near Edinburgh or Aberdeenshire, and a NIAB site in Dorset.
Fungicides are applied in autumn (usually November) with a second application at pre- or early stem extension in February/March. Leaf disease assessments and harvested yield are recorded.
Products tested in 2015/16 were Proline 275 (prothioconazole), Orius P (tebuconazole + prochloraz), Orius 20EW (tebuconazole + prochloraz), Refinzar (penthiopyrad + picoxystrobin), Pictor (boscalid + dimoxystrobin) and Cirkon (prochloraz + propiconazole).
No significant difference was found between products when applied at equivalent rates. At Malton, yield improvement ranged from 0.08-0.31 tonnes/ha (untreated, 4.13t/ha). The Scottish site showed significant yield improvements up to 1.06t/ha (untreated, 3.95t/ha). In Dorset no significant differences in yield were found.
A cross site analysis for yield for products included in both 2014 and 2015 trials (all except Cirkon which was new for 2015) showed yield responses to fungicide application of up to 0.59t/ha (untreated, 3.95t/ha), with no significant differences between products and doses.
Results of trials for the 2015/16 season are due to be published by AHDB in October/November.
In the final stage of the project, he hopes to look at teasing out weather patterns conducive to infection events and whether a prediction method similar to the Smith Period used for potato blight can be developed.
Genetic resistance to LLS is receiving increasing attention in the East and West regions as the disease has spread. However, the AHDB Recommended List (RL) for these regions stop short of requiring a minimum score of 6 for LLS which applies for the North region.
Currently there are five varieties on the East/West RL 2016/17 which score 6 or more for both LLS and phoma: Elgar, V316OL, Harper, Trinity and DK Cabernet.
However, disease resistance is being given a higher priority through introduction of the new agronomic merit rating, used when recommending varieties.
“This is to be welcomed – the importance of disease resistance has gone up,” says Prof Burnett.
In Scotland, growers have long favoured varieties with higher LLS resistance scores, she says.
“There has been a higher minimum standard for a good number of years, with 6 the minimum but people persistently going for 8s and 9s. LLS adapts very rapidly, so you keep having to change as varieties decline. Because they see a problem with LLS, they vote with their feet and grow the most resistant varieties.”
More resistant varieties are the way to go for growers who had a LLS problem in crops this year, advises Dr Ritchie.
But on farms relatively untroubled by LLS, selecting for genetic resistance is not top priority. “Do you write a variety off because it has a lower score when its other merits are good? It doesn’t take that high a priority,” says Mr Smart.
The recent high incidence may also simply be a spike, cautions NIAB oilseed and pulse specialist Simon Kightley.
“On a couple of occasions before there have been spikes in light leaf spot in the South. In the 1994/95 season and early 2000s the weather conditions meant there was such a build-up of light leaf spot in the South, which at the time had a devastating effect on some of the popular varieties of the day. The weather pattern shifted again and light leaf spot dropped.”
He puts the current high levels down to two warm, wet winters.
“That is good for allowing build up. I’d love to see a couple of proper cold winters which would solve light leaf spot and flea beetle as well.”