Light Leaf Spot
Pyrenopeziza brassicae (Cylindrosporium concentricum)
Pathogen
Fungus
Hosts
Oilseed Rape, brassica vegetable and forage crops.
Symptoms
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.
Development
The disease develops in early spring from infected seed and stubble. Spores from the leaf spots are spread to other leaves, flower buds, pods and plants by rain splash. Infection can occur with as little as 6 hours of leaf wetness at 12 – 20°C. After infection LLS symptoms become visible after 14 days at 15°C or 28 days at 5°C.
Favourable Factors
A mild winter followed by a cool, wet spring.
Importance
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.
Untreated crops can suffer yield losses of 50% or more.
Control
Varietal resistance as documented in the Recommended List.
Burying stubble from previous crops in the vicinity of where new crops are being established reduces the risk of spores being blown onto the new crop.
Foliar fungicides. Fungicides work best as protectants so early action of application at the first sign of disease in the autumn is essential and early detection and treatment, before disease is well established, tends to be more effective than later applications at stem extension.
At early stem extension the threshold for applying fungicides is 15 % plants infected with LLS.
