Apple scab is a disease caused by the fungus, Venturia inaequalis, which spreads by airborne spores and survives the winter on fallen leaves. Expect scab marks to appear on leaves from mid-spring until leaf fall in autumn.

This is a disease specific to apples and a few other other trees and shrubs including Cotoneaster, Pyracantha and Sorbus. A closely related fungus, Venturia pyrina, causes a similar disease called pear scab on fruiting and ornamental pears only.

You may see the following symptoms:

• On leaves: Patches of olive-green spots or blotches appear, which are initially velvety as they release airborne spores, and then darkening. Affected leaves often fall prematurely
• On twigs: Infections cause blistering and cracking that can provide an entry point for the apple canker pathogen
• On fruit: Brown or black scabby blotches develop. As the fruit enlarges, these can restrict expansion of the skin, leading to distortion and cracking. Light attacks only blemish the skin and eating quality is hardly affected (though the disease is commercially very serious, because growers cannot easily sell scabby fruit). However, if the fruits crack as a result of scab they become prone to fruit rots and will not store well

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

Pruning out twigs that are blistered, and disposing of fallen leaves and infected fruit will reduce the amount of the fungus available to start infections in the next growing season. Unfortunately, this can be of limited value unless the trees are isolated, because the spores can be blown for long distances.

Some apple cultivars are resistant to infection, including:

A-C

‘Adam’s Pearmain’, ‘Alfriston’, ‘Ashmead’s Kernal’, ‘Barnack Beauty’, ‘Beauty of Bath’, ‘Brownlees Russet’, ‘Charles Ross’, ‘Cheddar Cross’, ‘Claygate Pearmain’, ‘Cockle Pippin’, ‘Cornish Aromatic’, ‘Cornish Gilliflower’, ‘Court Pendu Plat’, ‘Crawley Beauty’

D-F

‘D’Arcy Spice’,  ‘Discovery’, ‘Duke of Devonshire’, ‘Edward the Seventh’, ‘Ellison’s Orange’, ‘Emneth Early’, ‘Encore’, Epicure’, ‘Exeter Cross’

G-L

‘Golden Reinette’,  ‘Grenadier’, ‘Ingrid Marie’, ‘John Standish’, ‘King of the Pippins’, ‘King Russet’, ‘Lane’s Prince Albert’, ‘Lord Derby’, ‘Lord Hindlip’

M-Q

‘Melba’, ‘Merton Russet’, ‘Millers Seedling’, ‘Monarch’, ‘Mother’, ‘Newton Wonder’, ‘Orleans Reinette’, ‘Park Farm Pippin’, ‘Pinova’, ‘Ponsford’

R-Z

‘Rajka’, ‘Red Devil’, ‘Reinette du Canada’, ‘Resi’, ‘Reverend W. Wilks’, ‘Rosemary Russet’, ‘Ross Nonpareil’, ‘Rubinola’, ‘Santana’ (but is very susceptible to canker), ‘Stirling Castle’, ‘Suntan’, ‘Tom Putt’, ‘Topaz’, ‘Wagener’, ‘Wealthy’, ‘Wheeler’s Russet’, ‘Winston’, ‘Woolbrook Russet’.

Resistant pears include:

‘Beurre Hardy’, ‘Docteur Jules Guyot’, ‘Fondante d' Automne’, ‘Gorham’, ‘Hessle’, ‘Jargonelle’, ‘Josephine de Malines’, ‘Nouveau Poiteau’, ‘Catillac’, ‘Black Worcester’, ‘Souvenir du Congrès’.

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

No fungicides are currently being produced for use by home gardeners on trees from which the fruit will be consumed.

Fungicides labelled for use on ornamental plants to control other diseases can be used on ornamental Malus (crabapple) and Pyrus trees (provided the fruit are not intended for consumption) and may provide some incidental control. They are used at the owner’s risk (test-spray a small area first to ensure that plant damage does not occur), but are safe to the operator when used as directed.

For the most effective control you need to cover the whole tree, but most gardeners will not have sprayers capable of treating large, old trees.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

The fungus spends the winter on fallen leaves and also infected twigs if these are not pruned out. In spring airborne spores are released from the infected fallen leaves, which causes the initial infections on the newly developing foliage. As these develop, they release large quantities of a second type of spore, also airborne or spread by splash from raindrops, which spreads infection throughout the growing season.