Keeping Light Leaf Spot under control
24 Aug 2017
Light leaf spot is no longer a disease confined to the northern areas of the UK, with it now being very widespread, and commonly found down south. Early control and an integrated approach are key in keeping on top of this disease.
According to Ms Smith, the increased incidence in light leaf spot is due to a combination of factors, “More oilseed rape is grown now than a decade ago and rotations have got a lot tighter, with rotations of one in three years not uncommon. LLS relies on inoculum from the previous year to initiate infection and most crops are now established using minimum tillage techniques, so infected stubble is not buried.”
Early control of light leaf spot
“Get on top of LLS early,” is the key message from BASF Business Development Manager, Clare Tucker, “In addition to applying an autumn fungicide, it’s then important to monitor the crops from late winter onwards. It is very difficult to see LLS in the early stages so collect plants and incubate the leaves.”
This involves placing leaves in a polythene bag for three to four days at room temperature to exacerbate any symptoms. Ms Tucker added, “Little white spore masses appear on the leaf and it just gives you a heads up that your crop is infected. Aim to treat the crop as soon as you see the symptoms either from the incubation test or in the field – this may well be in late January or February.”
Ms Smith said, “All of the products are much more effective if applied in a protectant rather than a curative situation. If you apply your fungicides in a timely manner, you are still able to achieve good control.
“We have found that you can get good control of LLS if the first application is as soon as the disease is spotted in the autumn/winter and then another in spring before stem extension; 0.75 t/ha yield responses under severe epidemic pressure from two applications, if they are well timed. However, fields can be too wet to travel over winter and often LLS is not spotted early enough.”
Ms Tucker said, “It is crucial that growers start to get on top of LLS before stem extension begins because after this LLS starts to cause yield loss.
“Towards the end of February, just before stem extension, growers will be measuring the Green Area Index (GAI) of the crop to judge if the use of a PGR will be economically justified.
“Caryx, a BASF PGR, gained registration on the strength of its PGR ability alone and can be applied from the start of stem extension due to the broad temperature range it will work in. If a LLS fungicide has not already been applied, then there is an opportunity to add this with Caryx.”
Flowering spray and resistance concerns
If LLS is very prevalent, a top up application of fungicide at flowering may be necessary.
Ms Smith said, “The problem with LLS is that you don’t eradicate it, you are just trying to get on top of it and keep it down in the bottom of the canopy. LLS is polycyclic so re-infection can occur throughout the season and the pathogen can cycle in as little as four weeks if conditions are conducive. The spores are transferred by rain splash so cool, showery weather is ideal for disease spread. Although the fungus can be inhibited by high summer temperatures, the epidemic was severe last year in many parts of the country, with disease progressing right up the canopy to reach the pods.
“At this point, it is likely that two to three azole fungicides have already been used in the programme and so from a resistance management point of view, it is essential to use a product with a different mode of action.
“In the control of LLS a lot of emphasis is placed on azole products and there are concerns about fungicide insensitivity.
“We are urging people not to rely solely on azoles and to realise that there are other products out there with different modes of action like Pictor. If you have LLS and Sclerotinia, then Pictor is the ideal product at this timing.”
Ms Tucker added, “You may not think you have LLS at flowering but from a resistance management perspective, you’re still going to have spores there and so if you put another azole on you are increasing the selection pressure.”
Ms Smith said, “We are urging growers to have a joined-up approach to fungicide use in oilseed rape as at the moment, it can be a bit fragmented. We need a proper strategy, like we have in wheat, where we have a programme for the season and mix up the actives.
“We also need a more integrated approach, using varietal resistance alongside fungicide application. Growing a resistant variety will give you that bit of insurance if you can’t get your spray on when planned.
“Until fairly recently, a lot of the popular varieties grown have been susceptible to LLS. That’s changing; there are varieties on the East /West Recommended List with resistance ratings of 7” and according to Dr Smith, “Some promising candidate varieties are starting to come through as well.”
Real Results Circle
Ms Smith is part of our OSR Profitability expert panel and will be a key contributor to our OSR Profitability Live, as well as our resource centre.
She said, “It is an exciting initiative to be involved with. We are hoping to engage growers, to take them through the season and show them what we are finding, not only in terms of disease but for a wider range of topics. I think it will be educational and provide a heads up and a call to action for growers, helping them to make timely, informed decisions which may affect the overall profitability of their crop.”