Apple Scab in Apple Trees

Apple scab is a common disease that is found in plants that include the rose family, Rosaceae. It is caused by the ascomycete fungus called Venturia inaequalis. While this disease can affect several plant genera that include Sorbus, Cotoneaster, and Pyrus. This disease is most commonly associated with the infection of the Malus trees, even the 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. It can develop infections that are dark, irregularly-shaped lesions. 

Although apple tree scab rarely kills its host, infection leads to deformation of fruit and premature growth of the leaf, ultimately resulting in fruit drop. Due to this reason, the susceptibility of the host plant can be enhanced to abiotic stress and secondary infection. The reduction of quality of the fruit and yield may result in the loss of crop up to 70%, a significant threat was imposed to the profitability of apple producers. To reduce scab-related yield losses, growers combine preventive practices often that include sanitation and resistance breeding. By taking reactive measures, such as targeted fungicide or biocontrol treatments, the prevention of incidence and spread of apple scab fungus in their crops.

The earliest official reports of apple scab were made by Swedish botanist Elias Fries in 1819. However, genetic studies related to apple scab indicated that it has emerged in Central Asia. As neither the conidia nor spores of this disease are capable of travelling to greater distances. The apple scabs can spread by migrating humans through the movement of the domesticated apple trees. By the end of the 19th century, the disease had spread to North America and Oceania alongside the importation of host plants. In all regions where apples are cultivated today apple scab is present in nearly all of these regions. The most significant infections that are occurring in the temperate areas, where it is cool and moist in the spring.

Apple Scab Fungicide Disease Cycle

The disease cycle of the apple scab begins in the early spring where the cool temperatures and abundant moisture promotes the release of the sexual spores or ascospores from overwintering structures or pseudothecia. These structures are found in the debris at the base of previously infected trees. Moisture is a critical factor that is involved in the development of the disease as rainfall. It not only triggers the release of ascospores but also involves the causing of infection of new hosts by helping the spores that adhere and germinate on the healthy tissue of new hosts. 

Following their dissemination, ascospores are transported to the newly-emerged leaves surfaces and blossoms by splashing water and wind. The tissues are then penetrated either directly with the help of a germ tube or indirectly through the appressorium; this initiates a new infection. After penetration, irregularly shaped lesions that are light-green in colour develop on the infected foliar tissue and gradually they get dark and can expand, and pucker as the infection progresses. Lesions found on fruit are black or brown and they are irregularly shaped. The older fruit lesions can cause the underlying tissue to become dry, corky, and eventually get disfigured by splitting. 

Within ten days of infection, the asexual conidia can develop on the darkened lesions. These allow the establishment of secondary infections on healthy leaf and fruit tissues. This cycle may repeat every one to two weeks during the growing season under optimal conditions. At the end of the season of that fruit, the heavily infected fruit and foliage fall from the canopy. This allows for the development of pseudothecia, which serves as a source of primary inoculum for the next spring.

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Apple Scab Symptoms

First developed in the year 1944 by an American plant pathologist named W.D. Mills created a table called the Mills Table. This table predicts the likelihood of an apple scab infection that can be developed based on the average temperature it is exposed to and the number of hours of leaf wetness or moisture content the host plant gets exposed to. This prediction system has rapidly been adopted in both Europe and North America regions, where the apple growers utilize it as an early warning system for new infections. This allows them to apply preventive fungicides at the right time. Several revisions have also been made to the Mills Table since it was created. 

Among these the most notable revision that was made by plant pathologists William MacHardy and David Gadoury in the year 1989. They determined that ascospores require three hours less than the originally calculated time in order to establish a new infection. While some other methods of prediction include ascospore maturation models and leaf orchard leaf canopy models. These are combined with electronic weather monitoring and used as the most widely used tool for predicting the apple scab infection periods.

The apple scab symptoms that are found on fruit are similar to those found on leaves. Scabby spots are found that are sunken and tanned. They may have velvety spores that are present in the centre. As these spots get mature, they become larger in size and turn brown and become corky. Infected fruit becomes distorted and may crack thus allowing the entry of other secondary organisms. Severely affected fruit may drop, this happens especially when it is young.

Apple Scab Treatment

Choosing the resistant varieties when possible is necessary, in order to provide apple scab treatment some steps has to be followed that include:

• Gather under the trees and destroy the infected leaves in order to reduce the number of fungal spores that are available or ready to start the disease cycle over again next spring.

• Watering the plants in the evening or early morning hours and avoiding over-irrigation give the leaves less time to dry out before infection can occur.

• Spreading a three to a six-inch layer of compost under trees by keeping it away from the trunk and by covering the soil can prevent the splash dispersal of the fungal spores.

• For best control, apple scab spray that is liquid copper soap early can be used two weeks before symptoms appear normally. Alternatively, begin the applications when the disease first appears, and repeat it at least seven to ten days' interval till the blossom drops.

• A finely ground wettable powder can be used in pre-blossom applications and must be used before the rainy or spore discharge periods. 

• There is an earth-friendly systemic fungicide that works the way through the entire plant in order to combat a large number of diseases that are found on the ornamentals, turf, fruit, and many more.

• Some of the sprays are safe, they can concentrate on insect attacks and fungal problems. For the best results, a protective spray can be applied in the early season. Thoroughly spray all parts of the plant, especially the new shoots.

Apple Scab Prevention

Apple scab control is done in the below-mentioned ways:

Cultural Controls: Cultural controls may be used as a first step to reduce the incidence of new infections. These practices can include the cleaning of leaf litter from the base of previously infected trees, and also the removal of the infected woody material from the canopy region while performing the annual pruning. Doing these things will reduce the amount of primary inoculum in the stage of spring and subsequently it delays the establishment of the disease. Furthermore, regular pruning will improve the flow of air and the penetration of light into the canopy. This ultimately inhibits to a certain extent the development and spread of disease. Another aspect found in cultural control is water management. As water triggers, the release of ascospore happens and promotes the germination of vulnerable tissue. During this time the growers are advised to monitor the watering periods and to avoid using the overhead watering systems. Doing so may ultimately aid in the reduction of infection periods that are caused by natural precipitation.

Chemical Controls: The management of apple scabs using chemical controls is primarily concerned with preventing the initiation of the primary infection cycles by reducing the germination of ascospores. The fungicides are typically applied in the early season when ascospores are first released. However, the fungicide applications may also be made later in the season in order to prevent infection of old leaves, which can help reduce the amount of primary inoculum for the following season. The most commonly-used classes of fungicide are Benzimidazole fungicides for managing apple scab in conventional orchards. There is some evidence that the disease is developing resistance to the respective class of fungicides. Along with this, several other fungicides include 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.

Apple scab treatment organically can be done by the production systems, where growers commonly use copper or sulfur-based protectant sprays in order to reduce the efficacy of the primary inoculum. Although these sprays were among one of the earliest methods of preventing the development of apple scabs, they do little to manage pre-existing infections and application may significantly damage the foliage of treated trees. Moreover, some research has indicated that the applications of copper-based fungicides may result in changes that occur in the structure and functionality of soil microbiota, thus having a negative effect on soil health. 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 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 acts as a microbial fungicide that uses the Bacillus subtilis as its active ingredient and may be used to control foliar diseases that are caused by bacteria or fungi. Furthermore, several fungal antagonists have been isolated and identified as potential biocontrols. One of such antagonists is Cladosporium cladosporioides. This antagonistic fungus has demonstrated significant bioactivity against apple scab. This antagonist was indicated in the year 2015 study, it was found that applications of C. cladosporioides. This could reduce the leaf scab incidence by 42-98% and the apple scab incidence by 41-94% in both conventionally and organically managed orchards.

Conclusion

Apple scab is the most common disease that is found in apple and crabapple trees in Minnesota. Scab is caused by a fungus that infects both the leaves and fruit. Scabby fruit is often found unfit for eating. Infected leaves have olive-green coloured to brown coloured spots. Leaves with many leaf spots can turn yellow and fall off early as young. Leaf loss can weaken the tree when it occurs many years in a row. Planting disease-resistant varieties are the best way to manage the scab. Fungicides can also be used to manage the apple scabs. Proper timing of the sprays is required for the fungicides to control the disease.