Apple Scab Disease – Symptoms

Apple scab

Source: Gauthier, Nicole. 2018. Apple scab. The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-1005-01
Updated 2018. Previous version by Vaillancourt, L.J. and J.R. Hartman.

PATHOGEN: Venturia inaequalis

HOSTS: Apples and flowering crabapples (Malus spp.), hawthorn (Crataegus spp.), mountain ash (Sorbus spp.), firethorn (Pyracantha spp.), and loquat (Eriobotrya japonica). Pear (Pyrus spp.) is infected by a related fungus, Venturia pirina, which causes similar symptoms. However, the apple scab pathogen will not infect the pear, and the pear scab pathogen will not infect the apple.

Apple scab occurs everywhere in the world where apples are grown and results in more losses than any other apple disease. It is most serious in areas that have cool, wet spring weather and may not be economically important in warm and/or dry climates.

Symptoms and Signs

Apple scab results in symptoms on most upper plant parts, most notably leaves and fruit.  Petioles, flowers, sepals, pedicels, young shoots, and bud scales can also become infected.

Apple leaf and fruit symptoms

Apple scab infections are initiated in early spring on emerging and young leaves.  Early lesions appear 10 days later as lighter green areas compared to the surrounding leaf tissue.  Lesions increase in size and become olive-colored and velvety as a result of asexual spore production (conidia). Scab lesions that form on young leaves may expand to more than 1 cm in diameter. Ontogenetic resistance of older leaves, however, usually results in smaller lesions or no visible symptoms. Affected tissues eventually may become distorted and puckered, and leaf lesions become cracked and torn. Severely infected leaves drop from trees.  Two to three consecutive defoliation events can weaken trees, resulting in a greater susceptibility to other stresses such as freeze damage, insect injury, and other diseases.

Fruit lesions are generally blistered and “scabby” in appearance, with a distinct margin. The earliest noticeable symptom on fruit is water-soaked areas that rapidly develop into velvety, green to olive-brown lesions. Infections of young fruit cause fruit distortion as healthy tissue continue to grow. Severely infected fruit often drop prematurely from trees.

Apple Scab resistant variety: Tramin