Light leaf spot is an ever present concern in Scottish oilseed rape crops and new findings on the disease’s biology, coupled with concerns over fungicide resistance and recent product losses have done nothing to clarify best practice advice for growers.
Managing light leaf spot is challenging because the pathogen adapts so readily to new varieties and fungicides. It is carried on trash and is splash borne but also moves between crops in waves of ascospores throughout the season – spikes of ascospore production over summer and harvest are often noted. Newly emerging crops are therefor exposed early in the season, but it isn’t at all clear why main symptoms don’t appear until nearer Christmas. The disease is spread upwards and outwards through the canopy by rain splash and cause the classic scorched type symptoms that can easily be mistaken for other forms of damage. Incubating a few leaves in a plastic bag for a few days will reveal the classic spores which look like salt grains around the edge of the lesion. The disease will cause puckering of the leaves and distortion of the pods if it gets that high up the canopy. Yield losses in SRUC trials are commonly in the order of 0.5 t/ha in untreated plots.
There is always debate over when to treat for light leaf spot, how often and at what rate. Two sprays, one in the autumn and one in the spring, is the standard approach and sprays need to be applied protectantly i.e. ahead of most infection to be effective. The timing of the epidemic varies between season though so in an early epidemic like last season in all the AHDB fungicide performance trials it was the autumn spray that gave most of the yield response. Two spray programmes added an average of almost 0.4 t/ha to yield in the trials of which only 0.1 t/ha came from the spring spray. In other seasons though it is often the spring spray that is most responsive where the epidemic is later.
In the absence of a crystal ball or accurate prediction scheme that is why crop walking to catch the disease at its early stages and two sprays to cover both bases are sensible. Ultimately your own experience of the disease is important in judging whether you have historically been affected at significant levels. Fungicide treatment has to be applied preventatively to be successful but not so early that sprays don’t persist into late autumn and winter so generally late October / early November before fields become impassable is a good timing. The risk of not treating in the autumn and dropping one of the two spray timings is that the disease is too well established to manage when you can travel over ground again in the early part of next season. Using varieties with decent light leaf spot resistance is crucial in reducing risk, given the endemic nature of the disease in Scotland and all varieties on Recommended List for the north are rated 6 and above.
Historically light leaf spot fungicides have been predominantly azole based so there are on-going concerns that this family of fungicides is not as effective as it once was because the disease has adapted. The introduction of SDHI chemistry for light leaf spot management a couple of seasons ago was welcome as it allowed some mixing and alternation of chemistry and a reduced reliance on a single group which is good fungicide resistance stewardship practice and makes resistance failures less likely. Fungicide options include azoles like Folicur and Proline and also SDHI and strobilurin mixes like Pictor. Tebuconazole (as in Folicur) has some growth regulatory effect which can be useful in forward crops but use carefully if the autumn / winter forecast is hard.
Fiona Burnett, SRUC for the Farm Advisory Service
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