What is apple scab?
Apple scab is a fungal infection that causes dark marks on apple leaves and fruit. The fungus, Venturia inaequalis, 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, it can also affect twigs, causing cracks that can become an entry point for the serious disease apple canker. Fruit damage 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.
How to deal with apple scab
Orchard hygiene is important for this type of disease, and for the home gardener, it is the only feasible method of combating infection. Prune out any affected twigs, and remove fallen leaves and infected fruit, disposing of them definitively (don’t compost or recycle). Some varieties are naturally resistant to scab infection, such as Adam’s Pearmain, Ashmead Kernel, Beauty of Bath, Charles Ross, Cheddar Cross, Court Pendu Plat, D’Arcy Spice, Discovery, Edward the Seventh, Ellison’s Orange, Epicure, Golden Reinette, Grenadier, Ingrid Marie, Jonathan, Lane’s Prince Albert, Lord Derby, Melba, Merton Russet, Newton Wonder, Orleans Reinette, Pinova, Reverend W. Wilks, Suntan, Winston. Avoid the use of overhead irrigation.