Traditionally associated with Scotland and Northern England, light leaf spot is now widespread throughout the UK. Fungicides work best as protectants so early application at the first sign of disease in the autumn is essential. If crops are inadequately protected light leaf spot can spread through the plants affecting leaves, stems, flowers and pods. Untreated crops can suffer yield losses of 50% or more.
Large numbers of very small white sugar-type spores are visible on green leaf tissue either on the top or underside of the leaf.
These quickly develop into discrete lesions with pinkish centres and many more of the spore-forming spots surrounding them.
In the most severe cases whole leaves can be killed; these often do not abscise and instead remain attached to the plant. When disease pressure is intense, light leaf spot affects the developing leaves and buds causing plant stunting and leaf distortion.
Infection will also progress from leaves onto stems and lateral branches. Elongated, fawn lesions, surrounded by black speckling will be seen. When conditions are humid white spore masses will also form on and around these stem lesions.
If the weather allows the disease to progress, pods are also affected. The whole raceme can become infected resulting in distorted pods that turn brown and may shatter prematurely or produce little yield.
Varietal resistance to light leaf spot is the first line of defence, and so it is vital to consider growing a variety with a good resistance profile.
Cultivation techniques are also very helpful in controlling the disease. Burying stubble from previous crops in the vicinity of where new crops are being established reduces the risk of spores being blown on to current cropping area the new crop.
Proline (prothioconazole) is the strongest product for control of light leaf spot. Timely application is essential for effective control as fungicides need to be applied preventatively, which can be difficult given the weather conditions often prevailing at the right time for treatment.
Without any useful alternatives it is vital to protect the activity of triazoles such as Proline. Two applications are recommended; one at the first sign of infection in the autumn (often mid-late November) and another in winter or early spring. Higher rates should be used when disease pressure is severe, if there is a history of reduced azole sensitivity or in high disease risk areas.
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Nearly 300 people completed our OSR: Big Picture Quiz in autumn 2018. By cross-analysing the findings we’ve been able to pull out some of the differences between those growers averaging over 4.0t/ha compared with those averaging under 4.0t/ha.