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Apple scab is a disease of apple trees (genus Malus) caused by the ascomycete fungus Venturia inaequalis. The disease manifests as dull black or grey-brown lesions on the surface of tree leaves, buds or fruits. Lesions may also appear less frequently on the woody tissues of the tree. Fruits and the undersides of leaves are especially susceptible. The disease rarely kills its host, but can significantly reduce fruit yields and fruit quality. Affected fruits are less marketable due to the presence of the black fungal lesions.

Apple scab on a crabapple, lesions are visible on the leaves.

The infection cycle begins in the springtime, when suitable temperatures and moisture promote the release of V. inaequalis ascospores from leaf litter around the base of previously infected trees. These spores rise into the air and land on the surface of a susceptible tree, where they germinate and form a germ tube that can directly penetrate the plant's waxy cuticle. A fungal mycelium forms between the cuticle and underlying epidermal tissue, starting as a yellow spot that grows and ruptures to reveal a black lesion bearing the asexually as the conidia are released and germinate on fresh areas of the host tree, which in turn produce another generation of conidial spores. This cycle of secondary infections continues throughout the summer, until the leaves and fruit fall from the tree at the onset of winter.

The reproductive conidia of Venturia inaequalis erupting through the cuticle of a crabapple leaf.

Over the winter, V. inaequalis undergoes sexual reproduction in the leaf litter around the base of the tree, producing a new generation of ascospores that are released the following spring. Scab lesions located on the woody tissues may also overwinter in place, but will not undergo a sexual reproduction cycle; these lesions can still produce infective conidial spores in the spring.

In affected orchards, new infections can be reduced by removing leaf litter and trimmings containing infected tissue from the orchard and incinerating them. This will reduce the amount of new ascospores released in the spring. Scab lesions on woody tissue should be excised from the tree if possible and similarly destroyed.

Chemical controls can include a variety of compounds. Benzimidazole fungicides (e.g. Benlate) work well but resistance can arise quickly; Captan or Nova, among others, are currently (as of 2004) viable choices. Copper or Bordeaux mixture are traditional controls but are less effective than chemical fungicides and can cause russeting of the fruit. Wettable sulfur also provides some control. The timing of application and concentration varies between compounds. Consult your local regulations regarding the application of fungicides, and be aware of their toxicity and safe handling procedures.