Apple scab is a common disease of plants in the rose family (Rosaceae) that is caused by the ascomycete fungus Venturia inaequalis.

[6][5] The reduction of fruit quality and yield may result in crop losses of up to 70%, posing a significant threat to the profitability of apple producers.

[7] The disease cycle begins in early spring, when cool temperatures and abundant moisture promote the release of sexual spores (ascospores) from overwintering structures (pseudothecia) found in the debris at the base of previously-infected trees.

[4] At the end of the season, heavily-infected fruit and foliage fall from the canopy, allowing for the development of pseudothecia, which serve as a source of primary inoculum for the next spring.

The most notable revision was made in 1989 by plant pathologists William MacHardy and David Gadoury, who determined that ascospores required 3 hours less than originally calculated in order to establish a new infection.

These practices include cleaning leaf litter from the base of previously-infected trees, as well as removing infected woody material from the canopy when performing annual pruning.

Furthermore, regular pruning will improve air flow and light penetration in the canopy, which ultimately inhibits the development and spread of disease.

[4] The management of apple scab using chemical controls is primarily concerned with preventing the initiation of primary infection cycles by reducing the germination of ascospores.

Although these sprays were among the earliest methods of preventing the development of apple scab, they do little to manage pre-existing infections and application may significantly damage the foliage of treated trees.

One of the most widely-recognized products is Serenade® ASO, a microbial biofungicide which uses Bacillus subtilis as its active ingredient and may be used to control foliar diseases caused by bacteria or fungi.

[19] While the development of transgenic resistant cultivars may reduce management costs in orchards, limited market acceptance pose a barrier to early adoption by commercial growers.