Apple scab is a common fungal disease of apple trees caused by the organism Venturia inaequalis. It can affect nearly all parts of the plant, including foliage, developing fruit, and woody tissues. The symptoms progress through distinct stages, changing in color and texture as the infection matures on different plant structures. Accurate identification involves examining the characteristic appearance of the lesions, which varies significantly depending on the host tissue.
Identifying Scab on Leaves
The first visual evidence of apple scab often appears on the leaves as small, slightly lighter green or yellowish spots. These initial lesions are typically circular and can be seen on either the upper or lower surface of the leaf tissue. As the fungus develops, these areas transform, taking on a characteristic velvety texture and an olive-green coloration due to spore production.
These spots gradually enlarge, darkening to brownish-black or black, and can reach up to a half-inch in diameter. Older lesions lose their velvety appearance, becoming dry and slightly raised, sometimes developing a fringed or feathery border. Multiple lesions may merge, covering large portions of the leaf surface, frequently concentrating along the leaf veins.
In cases of severe infection, the surrounding leaf tissue may turn yellow, and the entire leaf can become distorted, curled, or puckered. Heavy infection leads to premature leaf drop, which can weaken the tree if it occurs repeatedly over several years.
Identifying Scab on Fruit
Symptoms on the fruit begin as small, dark, slightly raised circular spots that are initially olive-green and velvety. These spots can develop at any stage of the fruit’s development, but early infection causes the most severe deformities. As the fruit matures, the tissue beneath the lesion dies, and the spots take on a distinctly rough, dark brown to black appearance.
The mature lesions are described as having a “corky” or scabby texture, which is the origin of the disease’s common name. This hardened tissue forms because the fungus prevents skin cells from expanding, causing the surrounding healthy tissue to grow unevenly. When lesions occur early, this restricted development leads to fruit becoming stunted, misshapen, or severely deformed.
Heavily infected fruit may develop deep cracks extending through the corky lesions, which makes the apples unusable. Late-season infections, occurring just before or at harvest, result in “pinpoint scab.” These manifest as numerous, tiny, dark spots that may not be noticeable at harvest but develop and darken while the fruit is in storage.
Identifying Scab on Twigs and Flowers
While less common than leaf or fruit symptoms, the disease can be visually identified on the woody parts of the tree, especially on young shoots and twigs. On these tissues, the infection appears as small, reddish-brown blisters or cankers. These lesions are typically small, but they may eventually cause the outer bark to flake off.
The characteristic of these twig lesions is slight blistering, which can lead to cracking of the bark on the infected shoot. These symptoms are most visible on the current season’s growth. In the spring, the fungal pathogen can also infect delicate flower parts, including the sepals and petals.
Infected blossoms may show dark, velvety lesions that resemble the early leaf spots. Severe infection of the flower stem (pedicel) or the flower itself often results in the entire blossom or newly formed fruitlet dropping prematurely.