Gardening Advice

Apple scab

Spring has barely started, but it’s already time to think about the late summer and autumn harvests. To ensure we can enjoy apples and pears picked fresh from the tree, we have to keep a watchful eye open for fungal attacks on our fruit trees.
The most common disease, and the one that requires the most treatment from tree growers, is known as apple scab.
This fungus, whose scientific name is Venturia inaequalis, puts in an appearance every year and causes olive-coloured circular marks on the leaves. The disease also affects the fruit, creating marks similar to those on the leaves, which then evolve to form crevasses. If conditions are favourable, the microscopic fungus reproduces quickly. Its mycelium penetrates the leaf and spreads, producing spores that are carried to other leaves by the rain and wind.

The first step to take when buying an apple tree is to check with the nursery that the variety you choose is as resistant as possible to apple scab. For example, the Melrose, Reinette du Mans and Belle de Boskoop varieties have a low risk of developing the disease. The risk also depends on your orchard and on the weather conditions. Avoid spray watering to limit the dissemination of spores, and preserve the flow of air around the tree to help the leaves dry.
Do not use too much fertiliser, and in autumn make sure you sweep up the dead leaves and burn them to limit future contamination in spring.

Treatments exist, but require careful attention when you use them. Copper-based mixtures, for example, limit the spread of the disease. Many brands offer these treatments under generic names: fruit tree treatment, winter treatment etc. The copper should be applied in autumn, when the leaves fall. It works by inhibiting the ability of the spores to germinate the following spring. Then, once the buds begin to swell, an application of Bordeaux mixture (20 grams/litre) will help prevent the disease. This operation should be repeated once the fruits have formed. Take care not to treat fruit trees during flowering.

Unfortunately, sensitive varieties will still need further treatment. Apply wettable sulphur (6 grams/10 litres) every two to three weeks until summer. Adapt the treatment to the weather – suspend application on rainy days, for example, or when the temperature rises over 25°C. New products for amateur gardeners are also appearing, based on algae or oligoelements. These products fortify the plant by stimulating its natural defences.

Apple scab rarely leads to the death of a tree, but it may weaken it through repeated attacks.
Always make sure the products you use are authorised for organic agriculture. We prefer to eat irregular, marked fruit – it is healthier and tastes incomparably better than perfectly uniform fruit.

Laurent Portuguez
head gardener 

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