Apple Scab

Fungal spores that cause apple scab disease (Venturia inaequalis) erupting through the cuticle of a crabapple leaf. Magnified about 1500xPhoto by Charles Krause

Apple scab (Venuria inaequalis) is a fungal disease of malus species specifically, affecting all species and nearly all varieties of apple and crabapple.

Can be managed organically. It is good practice to keep the orchard well pruned to allow for unimpeded airflow. Also important is making sure leaves are decaying quickly on the orchard floor since this is where the overwintering spore load will emanate from.