How to recognise it
Apple scab can affect various parts of the plant:
Leaves – Rounded, green/brown blotches appear on the underside of leaves, quickly moving to the upper surface, particularly those which are around the blossom (because these are the first to emerge). They patches may be slightly velvety to touch and later start to crinkle and blister. The scab patches may spread to completely cover the leaf which may then fall prematurely.
Stems – On the twiggy parts of the stems blister-like swollen areas may appear which then bust in spring to produce green/brown pustules.
Fruit – Dark spots appear on the fruit which turn very dark green and become corky and cracked (due to the cells in the affected area dying and, therefore, not being able to divide and grow with the rest of the fruit). The spots affect fruits of all ages. More numerous, smaller, sunken spots may appear later in the season (known as pin-point scab), including on fruit in storage (although the fruit will have been infected on the plant as the infection rarely spreads in storage). Scab infections don’t cause rotting directly, but they can provide a point of entry for other infections which may lead to the fruit rotting.
The disease occasionally affects the flowers.
This is a fungal infection which overwinters on fallen leaves or, in severe cases, in stem lesions. In the spring spores are produced which infect young leaves, causing the blotches to appear. The fungus develops on the leaves and spores are blown or splashed onto other leaves and fruit.
Why it’s a problem
This very common disease doesn’t actually do too much direct damage. It may spoil the appearance of the fruits, but they are still edible and the symptoms only affect the skin. If left untreated the early leaf fall can reduce the overall vigour of the tree and, therefore, the crop production. On a commercial scale this is a major problem, but in the domestic garden it is more of a nuisance.
Having said that, if the infection is allowed to spread uncontrolled, the resulting stem lesions can make the tree more prone to more serious infections, such as apple canker.
Where you are likely to find it
Most of the popular apple types are susceptible to scab and many varieties once considered resistant (eg Bramley’s Seedling) can now be affected.
The Venturia inaequalis fungus can also infect crabapples, hawthorns (Crataegus), cotoneasters, mountain ashes (Sorbus) and firethorns (Pyracantha).
It tends to be more prevalent in seasons when there has been wet weather (because more spores are discharged in rain), particularly when the weather has been wet when the tree is blossoming.
How to deter it
Good hygiene around the plant, such as disposing of falling leaves (not into the compost) and only using clean pruning tools can help to reduce the risk of infection.
Regularly prune the tree to ensure that there is plenty of air circulation between the branches.
Select varieties of apple which show a greater resistance to apple scab.
How to get rid of it
Collect and destroy (eg by burning) fallen leaves to remove the overwintering sites for the fungus.
Prune out stems which are affected by the symptoms.
Spray plants with an appropriate fungicide fortnightly from the time the first bud starts to open until the blossom drops. You can then stop spraying unless you find evidence of a scab infection in the stems, in which case you should continue to spray fortnightly until the autumn. With severe infections you may need to continue this longer spraying regime for 3 years, then reverting to just spraying until the blossom falls.
If neighbouring gardens also have apple trees then chat to your neighbours about them implementing preventative measures, as there’s no good you working hard to reduce the infection if they spores can simply be blown over from next door’s tree!
Is it good for anything?!
Other useful information
Other species of the Venturia fungus also cause scab on pears and cherries.