What is apple scab?
Apple scab – the disease can also infect pear trees, hawthorn and mountain ash – is a fungal infection that causes dark marks on apple leaves and fruit. It can be a dramatic problem for commercial fruit growers and for gardeners. The fungus, Venturia inaequalis and Venturia pyrina, arrives as ascospores from dead leaves on the ground which infect newly-developing foliage in spring. The marks can be seen on the leaves right through until leaf fall. The scabby patches on the leaves and fruit release another type of spore, conidia, which can be spread by the wind or by rain splash, and in this way, the disease spreads yet further.
In addition to leaves and fruit, scab can also affect twigs, causing cracks that can become an entry point for the serious disease apple canker. The damage to fruit caused by scab is superficial and the apple remains edible, though not commercially viable. Apple scab can be particularly serious in conditions of high rainfall and humidity during the growing season.
Apple scab symptoms
When your apples and pears show blotches or spots on the green leaves, you are likely to be troubled by scab. These patches are yellow, appearing in spring, and they gradually turn dark green and brown. The rest of the leaf starts turning yellow. Scab infection can cause premature leaf drop in July and August which will of course mar the tree’s appearance and reduce its fruiting capacity. Affected fruits have brown-grey areas on the surface that don’t grow and so tends to split, opening a route for other infections. Though the flavour of the fruit is not affected, its storage quality will be greatly diminished.
Black spots on apple caused by scab should not be confused with bitter pit. The spots on fruit caused by bitter pit stay small, while those caused by scab gradually increase in size.
How do you control scab
If your trees have been affected by scab before, or there are other trees in the immediate area that show the disease, it is a good idea to choose scab-resistant varieties. The ones that I have experience with are as follows:
Scab-resistant apple varieties:
Claygate Pearmain
Charles Ross
Lord Derby
Lane’s Prince Albert
Discovery
Ellison’s Orange
Grenadier
Ingrid Marie
Pinova
Reverend W. Wilks
Rosemary Russet
Suntan
Pear varieties with a degree of scab resistance:
Beurre Hardy
Catillac
Josephine de Malines
Winter Nelis
How do you treat fruit trees with scab?
In the UK, copper-based fungicides – the traditional treatment for fungal diseases – are no longer available. So the best way to reduce the effects of scab in the garden is orchard hygiene. Complete organic control is not easy to achieve, but based on my own experience, it is possible to significantly reduce scab damage to fruit and trees. This is how I do it: it seems to work satisfactorily for me.
How do you treat apple scab naturally:
- In November, wait until the leaves have fallen. The fungus over-winters on the leaves. So, sweep up all the fallen leaves, and then eliminate them by burning or removal from the garden. Don’t compost them. This helps reduce the chances of re-infection in early spring.
- A further reduction can be achieved by pruning out the affected woody twigs which may also carry the fungus. This scabby wood should be burned.
- Then cover the entire area on the ground where the leaves fell with a 2-inch thick layer of mulch, consisting of old, decomposing hay or straw. The purpose of this is to stop the winter spores that may have fallen to the ground from germinating again and causing renewed infections on the newly-forming leaves in spring.
- Only as a last result, I may apply some sulphur on the young leaves in spring. I prefer not to use a copper spray on the ground area underneath the trees, as the copper will kill the worms. This may not work for everygody. It does work for me.
- Scab thrives in stagnant air and high humidity. Therefore in the winter months, prune the tree in such a way that it is open-structured, and wood and leaves dry out quickly.
- Lastly, when picking fruit, try to do this when the fruit is dry.
- Avoid the use of overhead irrigation.
The scab life cycle
Considering a new infection, this generally occurs in summer, though it is unlikely to cause visible symptoms initially. In the first autumn, the leaves fall to the ground where the fungus overwinters and then produces ascospores in spring, when the buds are forming. The spores infect the new leaves, transported by wind and water splash. These infection areas release conidia, secondary fungal spores, that produce further infection, particularly in warm, humid conditions. In summer, the damage to leaves and fruit is very visible. The cycle continues year after year and gradually weakens the tree, to the point that it may even die.