Light Leaf Spot
What is Light Leaf Spot?
Light leaf spot is the most damaging disease to crops in Scotland and in recent years is increasingly being identified as a serious problem in England. Fungicides work best as protectants so early action of 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.
How to spot Light Leaf Spot
Large numbers of very small white spots (spore masses) are visible on green leaf tissue. 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.
How to control Light Leaf Spot
Varietal resistance to light leaf spot is the first line of defence. If there is history of the disease in your area, 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 ontocurrent cropping area the new crop.
Proline275 (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 Proline275. Two applications are recommended; one at the first sign of infection in the autumn in(often mid-late November) autumn and another in winter or early spring at around 50 to 100. Use a minimum of 75% of full labelle rate (0.46 L/ha) in the autumn and 50-75% (0.32L – 0.46L/ha) as soon as further symptoms are seen from January to pre stem extension or once 15% plants are affected at early stem extension. 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.