Apple scab is a common disease of plants in the rose family (Rosaceae) that is caused by the ascomycete fungus Venturia inaequalis. While this disease affects several plant genera, including Sorbus, Cotoneaster, and Pyrus, it is most commonly associated with the infection of Malus trees, including species of flowering crabapple, as well as cultivated apple. The first symptoms of this disease are found in the foliage, blossoms, and developing fruits of affected trees, which develop dark, irregularly shaped lesions upon infection. Although apple scab rarely kills its host, infection typically leads to fruit deformation and premature leaf and fruit drop, which enhance the susceptibility of the host plant to abiotic stress and secondary infection.
right|thumb|Apple scab on [[Malus|crabapple, lesions are visible on the leaves.]]
History and distribution
The earliest official reports of apple scab were made in 1819 by Swedish botanist, Elias Fries. As neither the spores nor conidia of this disease are capable of travelling great distances, it is likely that apple scab spread through the movement of domesticated apple trees by migrating humans. This prediction system has been rapidly adopted in both Europe and North America, where apple growers use it as an early warning system for new infections, allowing them to apply preventive fungicides when appropriate. Several revisions have been made to the Mills Table since its creation. 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. Benzimidazole fungicides are among the most commonly used classes of fungicide for managing apple scab in conventional orchards; however, there is some evidence that the disease is developing resistance to this class of fungicides, along with several others, including demethylation inhibitors and quinone outside inhibitors. To manage the development of fungicide resistance, growers can reduce the number of applications made throughout the season and alternate between different classes of fungicide. As such, alternative management strategies appropriate for organic production systems are currently being developed.
Biological control
Biological control refers to the use of a population of one organism (a biological control agent) to suppress the population of another. There are very few biological control agents registered for the control of apple scab. 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. Furthermore, several fungal antagonists have been isolated and identified as potential biocontrols. One of such antagonists is Cladosporium cladosporioides (strain H39). This antagonistic fungus has demonstrated significant bioactivity against apple scab. This was indicated in a 2015 study, which found that applications of C. cladosporioides could reduce leaf scab incidence by 42–98% and apple scab incidence by 41–94% in both conventionally and organically managed orchards.
Resistance breeding programs
The first formal resistance breeding programs for apple scab began in the early 20th century with the development of the PRI Apple Breeding Program by Purdue University, Rutgers University, and the University of Illinois. Since its inception in 1945, the PRI Apple Breeding Program has used controlled crosses between cultivated apples and wild Malus species to develop 1500 resistant cultivars, 16 of which (including 'Prima,' 'Jonafree,' and 'Goldrush') have been named released into market. Modern genetic work has found that a total of fifteen genes may confer resistance to apple scab. 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.