Apple scab

What is apple scab?

Apple scab is a disease caused by the fungus, Venturia inaequalis, that 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 other trees and shrubs including Cotoneaster, Pyracantha and Sorbus.  A closely related fungus, Venturia pirina, causes a similar disease on fruiting and ornamental pears only.

Symptoms

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 young shoots: Infections cause blistering and cracking that can then provide entry for the apple canker pathogen
• On fruit: Black scabby blotches develop and, as the fruit matures, these restrict expansion of the skin, leading to distortion and cracking. Light attacks only damage the skin and eating quality is hardly affected (though the disease is commercially very serious, because growers cannot sell scabby fruit). However, if the fruits crack as a result of scab they become prone to fruit rots and will not store well

Control

Non-chemical control

Pruning out infected stems or shoots, and burning or composting fallen leaves and infected fruit will reduce the amount of fungus available to start infections in the next season. Unfortunately, this is generally of little 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’.

Chemical control

Fungicides containing difenoconazole (Westland Plant Rescue Fungus Control) and myclobutanil (Bayer Garden Systhane Fungus Fighter) can give good control when used according to the schedule recommended by the manufacturers.

For the most effective control, you need to cover the whole tree. Unfortunately, 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)

Biology

The fungus spends the winter on fallen leaves and also infected shoots if these are not pruned out. In spring airborne spores are released from the infected 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.