Apple Scab

Apple Scab is caused by Venturia inaequalis, an ascomycete in the Venturiaceae — the same family as the Black Knot fungus. Rather than forming a mushroom, it lives as a thin sheet of mycelium just beneath the waxy cuticle of apple leaves, where it produces velvety mats of asexual spores that give the characteristic sooty appearance.

The fungus has two very different phases. On the living tree in spring and summer it is an asexual, spot-forming leaf coloniser. On dead leaves lying on the ground over winter it switches to sexual reproduction, forming microscopic black flasks (pseudothecia) that shoot ascospores upward into the air just as buds break the following spring.

It is one of the most widely recognised fungal signatures on cultivated trees worldwide, and its distinctive fringed, olive-black lesions on both leaves and fruit surfaces make it easy to spot without magnification.

Leaf lesions

Spots begin as faint, olive-green, water-soaked areas 2-15 mm across with indistinct, feathered edges. As the spore mat matures the surface becomes velvety and sooty, then dark brown to almost black; heavily spotted leaves turn yellow and drop early, often leaving branches thin by late summer.

Fruit lesions

On developing apples the fungus produces rounded, corky, olive-brown to black scabs with a rough surface. Early infections cause the surrounding tissue to grow unevenly, so the apple becomes lopsided, cracked or deeply fissured around the lesion.

Overwintering structures

In fallen leaves, the fungus forms pseudothecia: black, flask-shaped bodies about a tenth of a millimetre wide, embedded in the leaf and visible only as dark specks with a hand lens on a decaying leaf held to the light.

Spore print

Not applicable. Spores are produced in microscopic conidial mats and flasks and are never shed as a visible print.

The fungus is restricted to apples and their close relatives — domestic apple, wild crabapples and some ornamental Malus cultivars — with related Venturia species on pear, hawthorn, rowan and willow. It colonises unfolding leaves, leaf and flower stalks, young shoots and the fruit surface.

It is found wherever apples grow in temperate climates: gardens, orchards, hedgerows and wild crabapple thickets across Europe, Asia, North America, Australia and New Zealand. Infection is strongly tied to wet weather, so lesions build up during rainy springs and cool, humid summers, and the overwintering stage occupies fallen leaf litter beneath the tree from autumn until the following bud break.

Venturia inaequalis is a hemibiotrophic parasite: it feeds on living leaf tissue while keeping the host cell layer alive beneath its subcuticular mycelium, then survives as a saprotroph in dead leaf litter over winter. This dual lifestyle drives its annual cycle.

Ascospores discharged from litter in spring start primary infections on the first leaves. Those lesions produce enormous numbers of asexual conidia that are splashed by rain to new leaves and fruit, giving repeated secondary cycles all summer. Leaf fall carries the mycelium back to the ground, where mating between opposite strains forms the pseudothecia and completes the loop.

Cedar-apple rust produces bright orange-yellow spots on apple leaves with small tube-like structures on the underside, quite unlike the sooty, olive-black, fringed mats of scab. Frogeye leaf spot forms sharply bordered brown spots with pale centres and purple rims, whereas scab lesions have diffuse, feathered margins.

Sooty blotch and flyspeck also darken apple surfaces but sit as a superficial smudgy film or scattered shiny black dots that can be rubbed or wiped off, while scab lesions are corky, raised and set into the skin. Pear scab, caused by Venturia pyrina, is essentially identical in appearance but occurs on pear rather than Malus.