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Apple Scab
Apple scab is a common disease of both wild and cultivated apple and flowering crabapple (Malus spp.) trees that is caused by the ascomycete fungus Venturia inaequalis. The first symptoms of this disease are found in the foliage, blossoms, and developing fruits of affected trees which, when infected, develop dark, irregularly-shaped lesions.

Although apple scab rarely kills its host, infection typically leads to fruit deformation, premature leaf and fruit drop, and enhances susceptibility of tree abiotic stress and secondary infection. As a result of reduction in fruit quality and yield, apple scab may reduce in crop losses of up to 70%, posing a significant threat to the economies of apple-producing regions.

History and Distribution
Studies have indicated that apple scab emerged in Central Asia. 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 infected plant material by humans. The earliest reports of apple scab in Europe were made in 1819 by Swedish botanist, Elias Fries. By the end of the 19th century, the disease had spread to North America and Oceania, largely through human migration. Established populations of apple scab have since been recorded in every apple-producing region in the world.

Disease Cycle
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. Moisture is a critical factor in the development of the disease as rainfall not only triggers the release of ascospores in the spring, but also facilitates the infection of new hosts by helping the spores adhere to and germinate on the healthy tissue of new hosts. Following their dissemination, ascospores are transported via wind and land on the surface of newly-emerged leaves and blossoms and penetrate the underlying tissue using a germ tube. Shortly after penetration, light green lesions develop on the infected tissue and gradually darken, expand, and pucker as the infection progresses. Older foliar lesions are typically brown-green in colouration and irregularly shaped. Lesions on fruit are black or brown and irregularly shaped. Older fruit lesions cause the underlying tissue to become dry, corky, and eventually disfigured by splitting. Within 10 days of infection, asexual conidia will develop on the darkened lesions and allow for the establishment of secondary infections in healthy leaf and fruit tissue. Under optimal conditions, this cycle may repeat every 1-2 weeks during the growing season. At the end of the season, heavily-infected fruit and foliage will fall from the canopy, allowing for the development of pseudothecia, which serve as a source of primary inoculum for the next spring.

Management of Apple Scab
A variety of cultural controls may be used to reduce the incidence of new infections. 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. Doing so will reduce the amount of primary inoculum in the spring and subsequently delay the establishment the disease. Furthermore, regular pruning will improve air flow and light penetration in the canopy, which ultimately inhibits the development and spread of disease.

While good cultural practices are the predominant method of controlling apple scab, many growers also employ applications of fungicides to further reduce infection in susceptible crops. Benzimidazole fungicides, e.g., Benlate (now banned in many countries due to its containing the harmful chemical benzene) work well but resistance can arise quickly. A number of other chemical classes including sterol inhibitors such as Nova 40, and strobilurins such as Sovran are used extensively; however, some of these are slowly being phased out because of resistance problems.

Contact fungicides not prone to resistance, such as Captan, are viable choices. Potassium bicarbonate is an effective fungicide against apple scab, as well as powdery mildew, and is allowed for use in organic farming. Copper and 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. Timing of application and concentration varies between compounds.

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. It may be noted that the introduction of resistant cultivars has been of somewhat limited success due to low fruit quality.

Modern genetic work has found that a total of fifteen genes may confer resistance to apple scab. Many of these genes have been isolated from wild Malus spp. populations in East Asia, where a high level of species diversity still remains. Of these resistance genes, the Vf (Rvi6) gene is the most well-studied. As such, researchers are currently working to develop resistant cultivars using transgenic technology.

Epidemiological Modelling
Understanding infection periods is another critical aspect in preventing the incidence and spread of apple scab in apple crops. First developed by American plant pathologist, W.D. Mills, a Mills Table predicts the likelihood of infection developing based on the average temperature and the number of hours of leaf wetness. Growers may use this table as an early warning system for new infections, allowing them to apply fungicides in a timely matter..