The wet spring has provided ideal conditions for Light Leaf Spot (LLS) to flourish in winter oilseed rape crops as the disease is spread by rain splashes, and as LLS comes into the crop from November onwards, it requires treatment into the spring months.
However this year some growers may have missed their autumn sprays, normally targeted against phoma but have an effect against LLS, or it was so wet that they could not travel.
The implications of these high infection levels for next season’s crops are considerable, as the fungus which causes LLS can continue to cycle in the crop for the whole season providing that weather conditions are conducive.
The fungus survives on debris after harvest and then is ready to infect nearby crops emerging in the autumn.
Growers are urged to consider the role that genetics can play in helping to defend against the disease which is equated to the ’septoria tritici of oilseed rape’ and which experts believe costs growers £140m a year in lost yields.
"Left unchecked LLS can depress yield by more than 1t/ha. Fungicides, of course, can play an important role in controlling the disease but inherent, genetic resistance will give an underlying protection that is invaluable when weather conditions are not favourable to spraying the crop, or the disease is not spotted," says Agrii’s technical seed manager, David Leaper.
"Last year we didn’t see such an impact on yield from LLS in England in spite of seeing significant disease in crops in early spring.
"Last year February and March were cold and dry, so leaves tended to drop off and the levels of infection were reduced. This year with the cold, wet spring, plants have held onto their leaves and it has been easier to see the symptoms develop and get progressively worse."
ADAS’ plant pathology researcher, Julie Smith agrees. "We have seen big yield losses from LLS in recent year’s particularly when fields have been too wet to travel and fungicide timings have slipped.
"Even the strongest products will not perform well in highly curative situations so it’s important not to let the disease become established early on."
"As a rule of thumb, 15% plants affected at stem extension equates to a 5% yield loss and I saw commercial crops in the spring which were close to 50% affected.
"LLS will cycle approximately every 4-6 weeks and the wet weather has helped splash spores around and enabled the fungus to progress up the canopy. “
"In some years we only see disease in the lower – mid third of the plant but I have seen several crops this season with actively sporulating lesions on upper leaves.
"These crops are particularly at risk of LLS reaching the pods, which, once infected are likely to senesce prematurely and shatter, causing significant seed losses."
"I have also had calls from concerned growers reporting LLS infection on the buds which has subsequently affected flowering."
With conditions this winter and spring being so suited to the spread of the disease, Mr Leaper expected to see high infections of LLS across the Agrii WOSR national trials sites but in fact levels are much higher than he expected.
"You would expect levels of infection to be high in the north and they are, and we know that LLS has been spreading further and further south and west, but it’s alarming to see just how high the levels of LLS are at both the Kent site and the Agrifocus site in Swindon."
"We have five key trials sites; Cambridgeshire, East Yorkshire, Perth & Kinross, our Agriifocus site in Wiltshire to Kent, at which we have plots of all of the main commercial and candidate OSR varieties.
"In our trials we have included a wide range of conventional, hybrid and HOLL recommended and candidate varieties, with up to 40 varieties in the ground, which allows us to really pull apart how the resistance ratings are standing up to the current pressure, but also to see if we future varieties are going to offer any more than what we already have," he adds.
"We conduct both untreated and treated comparisons, however, it’s important to note that the fungicide regime that we use on the treated plots is more similar to that of a farm level programme, so we do see different results to the AHDB trials that receive a fully comprehensive fungicide programme."
Mr Leaper explains that at the end of March into early April plots across all of the sites were scored for LLS infection on a rating of 1-9 (with 1 being no infection and 9 being severe) and an average across the sites has then been taken for each variety.
"The results have been alarming as there were only four varieties that showed up to or less than 2.5 % infection; the conventional variety Nikita stands out with one of the lowest infection levels at 2.3%, taking into account both treated and untreated results.
"Looking at this in more detail, at both the Agrifocus and Kent treated sites, only 1% of plants were infected, and this only went up to 3% of plants infected on the untreated plots, clearly demonstrating the robustness of Nikita’s 7 for LLS resistance.
"Interestingly, varieties with a resistance rating just one lower than Nikita’s did not perform nearly as well.
"For example, Campus which is rated a 6 for LLS on the RL showed 6% infection in the untreated and 5% in the treated.
"None of the candidate varieties offered any better results than current recommended varieties, so there is no new shining star coming through in the immediate future that will offer better resistance to what we currently have."
Nikita missed out being recommended for the east/west list last year on the basis of its lower phoma rating.
While this was disappointing we know that in an open autumn a vigorous variety like Nikita will grow quickly.
The likelihood of phoma developing into the yield robbing phase of the disease, stem canker, is much reduced says Mr Leaper.
"With current LLS pressure this disease must take precedence as it is a much more progressive disease and more difficult to control with the sprayer. Having robust LLS resistance is crucial
"It’s certainly worth considering this wherever you are in the country when planning variety choice for next year as infection levels look high and autumn conditions are always uncertain for spraying."
Dr Vasilis Gegas, senior oilseed rape breeder with Limagrain, notes that in years when LLS pressure is high, resistance ratings may come under pressure and therefore there’s a distinct advantage in starting with the highest rating possible.
"There have been considerable advances in varietal defence to LLS in the last five years and one of the reasons for this comes down to the fact that with increasing levels of infection year on year, breeders have been able to make better selections and that is why we are seeing varieties with better LLS resistance."
Data from CropMonitor show that LLS is increasing year on year on a national scale so we are not yet winning the battle adds Miss Smith.
"We need to make smarter variety choices and integrate fungicide programmes with varietal resistance.
"We have had limited options on the east –west RL list because most of the varieties have been susceptible to LLS with resistance ratings of 6 or below but things are changing now.
"There are some excellent new varieties coming through and we must start using them if we are to achieve robust disease control whilst minimising the drive toward fungicide insensitivity."
