Apple Scab Disease is one of the most destructive diseases that affects the yield, fruit quality, and overall economic viability of apples and crab apples.
It was first distinguished during the 19th century and from that point, it has been a significant danger to apple plantations. This disease is common in temperate climatic zones where the pathogen thrives in cool and moist environmental conditions.
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Causal Organism of Apple Scab Disease
The fungus Venturia inaequalis belonging to the family Venturiaceae, order Pleosporales, class Dothideomycetes under the phylum Ascomycota is the cause of apple scab. It is a hemi-biotrophic pathogen that does not directly kill the host but relies on the host for its nutrition.
Symptoms of Apple Scab Disease
Symptoms on leaves
The first sign of the disease is seen on leaves as small olive-green to dark brown spots which are generally circular and highly raised with a velvety texture. They generally grow on the underside of young leaves, which are more susceptible to infection. As the disease advances, these spots grow larger and may fuse forming large blotches. The color of this lesion becomes more darkened brown or almost black in case of severe infections. Infected leaves may become twisted or deform as the disease advances. This deformation is caused by the unusual growth of fungal infection. Serious infections can prompt defoliation, where leaves drop rashly.
Symptoms on fruits
At first the symptoms appear similar to those of leaves. As the fruit matures, these spots transform into more prominent scabs. The infected regions become harsh and corky leading to a characteristic scabbed appearance. The scabs may be isolated or may enlarge covering a large area on fruit. In severe cases, the fruit can crack as it continues to grow further which can create the entry points for pathogens.
Twigs and blossom symptoms
Apple scab can also affect tree twigs. The lesion on twigs looks similar to leaves and fruits but might be elongated in appearance. Blossoms can also be infected which exhibit small dark lesions similar to leaves. The infection may lead to the withering and dropping of blossoms which leads to the reduction of several fruits.
Morphology of the host: Venturia inaequalis
The mycelium of Venturia inaequalis is septate by cross walls. The hyphae develop underneath the epidermal layer and contain haploid nuclei. It produces both sexual and asexual spores when the condition is favorable. It also develops a dark-brown ring-like structure called the melanised appressorial ring which is formed at the base of appressoria. It produces fruiting bodies during sexual reproduction in the form of pseudothecia.
Reproduction in Venturia inaequalis
Asexual reproduction
Venturia inaequalis reproduces asexually by the formation of conidia. Conidia are unicellular brown to olive-green spores which are borne on conidiophores.
Conidiophores are short-branched or un-branched hyphae that bear conidia and are hyaline to pale brown.
The conidial surface is sticky which helps to adhere to the surfaces of fruits and leaves for secondary infection.
Sexual reproduction
The pathogen reproduces sexually by producing fruiting bodies called pseudothecia. During sexual reproduction, antheridium is formed from the hyphal tip of one individual, and trichogyne is formed from the hyphal tip of another opposite individual.
The nucleus of antheridium is transferred to trichogyne during fertilization where it is incorporated into a cell at the base of the pseudothecial initial. After fertilization, pseudothecial initial develops into pseudothecium inside a cavity known as a stroma.
Pseudothecia are flask-shaped structures that are deeply embedded into the dead leaf tissue. Pseudothecium walls are made up of layers of melanin-rich cells that give them a characteristic black or dark brown appearance.
The diploid nucleus undergoes meiosis to form eight haploid ascospores inside the sac-like structure called asci.
Each ascospore is ellipsoid to fusiform in shape and consists of two cells divided by a central septum. These ascospores are released from the pseudothecia via a small opening called the ostiole.
During moist conditions, the pseudothecia protrude and fall from the leaves, and the ascospores are dispersed by rain and wind.
Ascospores are the inoculum for primary infection.
Apple Scab Disease Cycle and Epidemiology
Primary infection phase
The onset of apple scab disease begins during early spring marked by the release of ascospores from pseudothecia.
Ascospore release– When apple leaves first start to emerge in the early spring, the pseudothecium from the previous season takes up moisture from rain or dew and swells up which leads to the release of spores via ostiole. Because moisture is a key factor both in discharge and infection, synchronization of wet periods is essential.
Dispersal and germination– When ascospores are released, they are carried out by wind to newly emerging leaves and flower buds. These susceptible tissues need to have sufficient moisture on their surface for germination to take place. The ascospore forms the germ tube which penetrates inside the epidermis and forms the fungal mycelium underneath.
Primary lesion formation- The first visible signs of apple scab in the growing season are the primary lesions that develop on young leaves or fruit due to initial infection by ascospores. These infections start as little olive-green spots that progressively turn out into more obscure, smooth scabs as they mature.
Secondary infection phase
Venturia inaequalis enters the secondary infection phase following the development of primary infection. Conidia are responsible for the spread of disease produced and disseminated in this phase and are responsible for secondary infection.
Conidia production and dissemination– Conidia formed on conidiophores as a result of asexual reproduction on primary lesions are released frequently and are transmitted by wind and rain resulting in new infections on other parts of the same tree or adjacent trees.
Infection spread– The conidia are primarily spread by rain splash or wind leading to new infections. The secondary infection can be more extreme than the primary infection since it can happen repeatedly and significantly.
Lesion formation– The secondary infections cause secondary lesions on the leaves and fruits. As these lesions form, they become new sources of conidia, thereby perpetuating the disease cycle and exacerbating its spread.
Advancement of disease– Under favorable environmental conditions, the cumulative effect of primary and secondary infections throughout the growing season can result in several outbreaks.
Leaf senescence and defoliation– A severe infection can cause leaf senescence and defoliation. This lessens the photosynthetic capacity, possibly debilitating the tree and impacting fruit production.
Damage to the fruit– Infected fruits have lesions that become corky, and dark and frequently crack as fruit grows. These damaged fruits are unmarketable resulting in a decrease in yield and quality.
Overwintering phase– As the growing season ends, Venturia inaequalis prepares to overwinter and set the stage for the next infection cycle.
Pseudothecia formation and maturation– During autumn, the infected leaves fall into the ground and provide the substrate for pseudothecia formation. The mycelium undergoes sexual reproduction during winter resulting in mature pseudothecia for early spring and the release of ascospores after the onset of favourable season.
Environmental Factors Impacting Epidemiology
Temperature– The ideal temperature for apple scab lies between the range of 12°C and 24°C (54°F to 75°F). At temperatures below 5°C, the ascospore release is repressed. On the other hand, the pathogen’s ability to infect decreases significantly at temperatures above 25°C.
Moisture– Moisture is critical for both the release of ascospores and the germination of conidia. For successful infection, prolonged leaf wetness caused by rain, dew, or irrigation is ideal. For example, at temperatures somewhere in the range of 12°C and 20°C, even 6 hours of leaf wetness can lead to significant levels of infection.
Humidity– During the growing season, conidia production and its dissemination are aided by high relative humidity, particularly above 90%. When humidity remains high for extended periods, the pressure from the apple scab can increase markedly causing severe outbreaks.
Host susceptibility– One important aspect of the spread of the disease is the vulnerability of the apple cultivars. Cultivars such as McIntosh and Granny Smith are resistant however Liberty and Enterprise are highly susceptible. However, even the resistant cultivars can also succumb to the disease if the environmental conditions are highly favorable for the pathogen or if few virulent traits emerge.
Pathogen variability and virulence– Over time, Venturia inaequalis can mutate and evolve new races that can overcome previously effective resistant genes in apple cultivars. This evolutionary adaptability requires nonstop advancement of new resistant varieties and the implementation of disease management practices to keep the distance in check.
Apple Scab Disease Management
A combination of cultural practices, careful monitoring, resistant cultivars, and the effective application of fungicides are necessary for successful apple scab management.
Cultural practices– One of the most important ways to control apple scab is to implement cultural practices that make it harder for pathogens to survive and spread.
Sanitation– Fallen leaves and other debris on the orchard floor can contain the overwintering structures (pseudothecia) of Venturia inaequalis, making their removal essential. The likelihood of developing infections in the early spring is significantly reduced by eliminating these inoculum sources. Fertilizing the soil or mulching the leaves can decrease the pathogen making it less likely to survive and initiate infection in the following season.
Pruning– Appropriate pruning to further develop airflow inside the tree canopy can decrease the span of leaf wetness, consequently reducing the condition favorable for pathogen growth and infection. Thinning out dense foliage permits leaves and fruits to dry rapidly after rain or dew, which can decrease the chance for Venturia inaequalis to establish it.
Use of Resistant Varieties: Some apple varieties have been bred specifically for resistance to Venturia inaequalis. Strong resistance to pathogens has been shown by cultivars such as ‘Liberty,’ ‘Enterprise,’ and ‘GoldRush’. An orchard’s overall disease pressure and chemical intervention requirements can be significantly reduced by incorporating these varieties. But, it’s important to note that resistance is not complete. Over time, the pathogen can mutate to overcome resistance genes, which makes it vital for breeders to continue developing novel resistant varieties.
Diversification– Creating a combination of safe and susceptible cultivars can help in the management of apple scab. This is because resistant trees serve as a barrier and slow the pathogen’s ability to spread from one tree to another. This method can slow the spread of the disease within an orchard.
Fungicide applications– Fungicides play an important role in controlling apple scab, mainly in orchards with susceptible cultivars or in areas where environmental conditions favor the infection.
When applied preventatively before the pathogen has an opportunity to infect, fungicides are most effective Applications are usually timed around key stages of the apple scab life cycle, such as just before bud break or during early leaf appearance. This timing aims at the pathogen when it is mostly susceptible and helps shield new growth from infection.
Integrated fungicide programs– It is vital to use fungicides with different means of action. Rotating between fungicide classes or using combination products can decrease the likelihood that the pathogen will develop resistance to any single treatment. Moreover, applying fungicides at the lowest effective rate and adhering to recommended application intervals are critical for sustaining their efficiency over time.
Organic Options: For organic orchards, where synthetic fungicides are not permitted, sulfur and copper-based products are generally used to manage apple scabs. Even though these treatments, while less effective than some synthetic fungicides, they can still be used in conjunction with good cultural practices and resistant varieties to provide adequate control.
Monitoring and Forecasting: An integrated apple scab management program must include regular monitoring and the use of disease forecasting models.
Scouting: Successive orchard scouting helps in identifying the early signs of apple scab. Growers can better time fungicide applications more accurately and prevent the disease from spreading by identifying the first lesions. Scouting also provides useful information on the overall health of the orchard and the efficiency of current management practices.
Weather Monitoring: Scab infections are strongly correlated with the conditions of the environment. Weather data can be used by growers to figure out when ascospores or conidia will be released. Fungicide applications can be timed more effectively with this information, reducing the need for unnecessary treatments.
Disease Prediction Models: Several models can use weather data to predict outbreaks of apple scab. These models can assume the risk of contamination and assist cultivators with arriving at informed conclusions about the time to apply fungicides. Using such models can upgrade fungicide use, limit costs, and lessen the natural effect of infectious prevention activities.
Biological Control: To reduce disease pressure, biological control involves the use of natural enemies or antagonists of Venturia inaequalis.
Antagonistic Microorganisms: Beneficial fungi and bacteria that can outcompete or inhibit Venturia inaequalis are currently the subject of ongoing research. These microorganisms can be applied to the orchard to decrease the pathogen’s ability to inhabit leaves and fruit. Though it is still under development, biological control methods for apple scabs offer a promising, sustainable method for management of apple scab.
Compost Teas: A few cultivators use compost teas produced using composted organic matter and used as a foliar spray. These are rich in beneficial microorganisms that can suppress the apple scab pathogen. The compost teas are a less expensive and environment-friendly option for disease management.
Post-Harvest Management: Apple scab can still be managed and its impact on subsequent growing seasons can be reduced after the harvest.
Leaf Litter Management: Overseeing leaf litter by eliminating or treating the fallen leaves is significant for decreasing the overwintering inoculum. At times, cutting and mulching the leaf litter can assist with breaking down the leaves faster depriving the pathogen of its overwintering habitat.
Orchard Floor Management: Venturia inaequalis can be less likely to survive if the orchard floor is kept clean and free of debris. Cover trimming or the utilization of mulch can likewise suppress the microbe by promoting the activity of beneficial microorganisms that decompose the infected leaves.
References
- MacHardy, W. E. (1996). Apple ScAB: Biology, Epidemiology, and Management. American Phytopathological Society.
- apple scab, Venturia inaequalis Pleosporales: Venturiaceae. (n.d.). https://www.forestryimages.org/browse/subinfo.cfm?sub=9478&cat=16
- | Plantwise Knowledge Bank. (n.d.). PlantwisePlus Knowledge Bank. https://plantwiseplusknowledgebank.org/doi/full/10.1079/pwkb.species.56212
- Rancāne, R., Valiuškaitė, A., & Stensvand, A. (2023). Primary inoculum of Venturia inaequalis (Cooke) Wint. in its asexual form in apple – a review. Frontiers in Horticulture, 2. https://doi.org/10.3389/fhort.2023.1175956
- Vedantu. (n.d.). Apple ScAB. VEDANTU. https://www.vedantu.com/biology/apple-scab
- Apple scab. (n.d.). Apple Scab. https://www.apsnet.org/edcenter/disandpath/fungalasco/pdlessons/Pages/AppleScab.aspx
- Gessler, C., & Pertot, I. (2011). Vf scab resistance of Malus. Trees, 26(1), 95–108. https://doi.org/10.1007/s00468-011-0618-y
- Morton, V., & Staub, T. (2008). A short history of fungicides. APSnet Feature Articles. https://doi.org/10.1094/apsnetfeature-2008-0308
- V.K. GUPTA, J. S. and. (2002). Studies on apple scab forecasting in Himachal Pradesh. Indian Phytopathology, 48(3), 325-330. https://epubs.icar.org.in/index.php/IPPJ/article/view/20748
- Verma, S., Gautam, H. R., & Khosla, K. (2022). Apple Scab (Venturia inaequalis Wint) Management Using a Novel Fungicide Combination in the North-Western Himalayas of India. International Journal of Economic Plants, 9(1), 011–017. https://doi.org/10.23910/2/2022.0454a