Apple scab and pear scab are two fungal diseases that causes dark, scabby marks on the fruit and leaves of apples, pears and some other ornamental fruits. They are so similar that they are dealt with in the same way.
Common name Apple scab
Scientific name Venturia inaequalis, Venturia pirina
Main symptoms Dark, scabby marks on fruit and leaves
Plants affected Apples, pears, some other trees and shrubs
Caused by Fungus
Timing Mid-spring onwards
What are apple scab and pear 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, Pyracanthaand Sorbus. A closely related fungus, Venturia pirina, 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 young shoots: Infections cause blistering and cracking that can then provide entry for theapple 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
Pruning out young stems which are blistered, and disposing of 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:
‘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’Arcy Spice’, ‘Discovery’, ‘Duke of Devonshire’, ‘Edward the Seventh’, ‘Ellison’s Orange’, ‘Emneth Early’, ‘Encore’, Epicure’, ‘Exeter Cross’
‘Golden Reinette’, ‘Grenadier’, ‘Ingrid Marie’, ‘John Standish’, ‘King of the Pippins’, ‘King Russet’, ‘Lane’s Prince Albert’, ‘Lord Derby’, ‘Lord Hindlip’
‘Melba’, ‘Merton Russet’, ‘Millers Seedling’, ‘Monarch’, ‘Mother’, ‘Newton Wonder’, ‘Orleans Reinette’, ‘Park Farm Pippin’, ‘Pinova’, ‘Ponsford’
‘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 containing myclobutanil (Bayer Garden Systhane Fungus Fighter*) are specifically labelled against apple and pear scab and can give good control when used according to the schedule recommended by the manufacturers. Fungicides labelled for use on ornamental plants to control other diseases can be used on non-edible Malus and Pyrus trees 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. Unfortunately, most gardeners will not have sprayers capable of treating large, old trees.
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.