Review Article, J Plant Physiol Pathol Vol: 13 Issue: 1

Sustainable Management Practices of Apple Scab (Venturia inaequalis) Disease–A Review

Amitava Mondal^*

Department of Agriculture, JIS University, Agarpara, Kolkata,India

*Corresponding Author:Amitava Mondal
Department of Agriculture, JIS University, Agarpara, Kolkata,India
E-mail:amitava435.am@gmail.com

Received date: 04 June, 2024, Manuscript No. JPPP-24-138148;
Editor assigned date: 07 June, 2024, PreQC No. JPPP-24-138148(PQ);
Reviewed date: 24 June, 2024, QC No. JPPP-24-138148;
Revised date: 06 March, 2025, Manuscript No. JPPP-24-138148(R);
Published date: 13 March, 2025, DOI: 10.4172/2329-955X.1000374.

Citation: Mondal A (2025) Sustainable Management Practices of Apple Scab (Venturia inaequalis) Disease–A Review. J Plant Physiol Pathol 13:1.

Abstract

Keywords: Apple; Apple scab; Disease resistance; Venturia inaequalis; Management

Introduction

The most serious disease of apple is bacterial scab (Venturia inaequalis) and mostly found in hilly areas where spring and summer season is cool and moist. The most significant horticultural products are cultivated for commercial purposes in temperate regions of the world [1]. It is well known everywhere in the globe, particularly in tropical and sub-tropical regions where growth is more challenging. The crops are often eaten either fresh or after being stored for up to six months or even longer. Recently apple juice quality is heavily affected by scab disease and market values are decrease day to day. Additionally, it may be a crucial raw material for various industries that prepare food, including juice, sauce, slices, vinegar, and cider, as well as for fresh consumption [2]. According to the popular thought that, "an apple a day keeps the doctor away" meaning that apples are a healthful food that can prevent the need for medical care. All the chemicals are found in apples are generally required for their regular growth and development [3]. Numerous pathogens, including fungus, bacteria, viruses, mycoplasmas, and nematodes are can cause a disease in apples. Successful disease management is typically achieved through a well-integrated approach. Unfavorable weather condition is now badly affected the apple fruit pulps due to cause by many fungal and bacterial infection and the commercial productions are more hampered by this affect. More than 70 infectious disorders have been reported to apples, the majority of which are brought on by pathogenic fungus including in fruit rot, fruit spots, canker, postharvest degradation, root rots, leaf spots, leaf blights, and flower blight. Apple scab is the primary fungal disease in commercial apple production in temperate and humid regions of the world, then the major fungal diseases [4]. But during the early spring, the illness is more severe in temperate nations with chilly, humid climates [5]. Understanding the virulence and pathogenicity requirements for fungal infection is crucial because it serves as a target for researchers to locate and harness resistance genes to combat these microbes [6]. It has been demonstrated that, depending on the genetic background, pathogen, and environment, the phenotypic effects of resistance genes against V. inaequalis (ViR) range from total immunity to near-susceptibility [7]. Apple scab diseases can be controlled by various techniques, including using fungicides, selecting resistant or tolerant rootstocks and graft (scion) varieties, environmental improvement, biological disease control, and selective orchard site selection [8].

Literature Review

Economic importance of apple scab

At the moment, apple scab is the disease with the greatest economic impact, particularly in temperate apple-growing countries with heavy summer rainfall and chilly, wet spring weather [9]. An apple crop that is left unchecked may suffer from decreased fruit quality and quantity or nearly total destruction. According to Vaillancourt and Hartman, apple scab is seen on petioles, leaves, sepals, fruits, and seldom on the scales of immature shoots and buds. The primary cause of the direct losses is fruit and pedicel infection.

The reduction in fruit quantity and quality is the main financial loss that scab causes to the producer. In addition to the crop, the disease has several other effects on the tree. A severe illness might stop fruit from setting. According to MacHardy, young apples may fall off if there are infections on the flowers, pedicels, petioles, and young fruits during or right after the flowering phase. In addition to lowering leaf photosynthetic activity, fewer fruit buds and leaves forming, more leaves falling, and a detrimental impact on normal wood growth necessary for good yields, severe scab assaults also negatively affected tree survival in the next year. Significant losses can also result from scab formation during storage.

On sensitive apple varieties, defoliation occurs after severe leaf infection. The low viability of apple trees, higher vulnerability to winter damage, decreased production of leaf buds and fruits, lesser growth, and decreased yields in the following years are further detrimental impacts of scab. Spraying fungicides can reduce losses if the scab is chemically managed, but production costs rise in tandem with growing health and environmental concerns. Due to inadequate vitamin content, scab can form and cause fruit rot during storage, resulting in large losses. Numerous factors, such as cultivar susceptibility, frequency of infection periods, and cleanliness, affect the rate of disease progression as well as the severity and ultimate complications of the illness.

Biology of apple Scab

Every nation where cultivated apples are grown is probably home to apple scab, which is brought on by the fungus V. inaequalis. Only members of the genus Malus are affected by the illness. In temperate countries with chilly, rainy spring weather, it is particularly bad. When scab initially started to occur in orchards is unknown. In temperate countries with chilly, rainy spring weather, it is particularly bad. When scab initially started to occur in orchards is unknown. The first evidence of scab's existence goes back to 1600 and is found in a painting by Michelangelo Caravaggio called "The Supper at Emmaus," which is housed at the National Gallery in London. Fries published the first report on scab in Sweden in 1819. All of the apple tree's green, herbaceous organs are virulently affected by the fungus. With symptoms often seen on the leaves and fruits, it can infect and colonize the tree's sepals, leaves, fruits, petioles, flowers, and even twigs.

The pathogen and its taxonomic classification

A pathogen fungus that includes two distinct states V. inaequalis, which is the perfect (sexual) or saprophytic stage, and Spilocaea pomi Fr., which is the imperfect (asexual) or parasitic form is what causes apple scab.

Venturia is the genus to which V. inaequalis belongs. According to MacHardy, it belongs to the family Venturiaceae, class Loculoascomyctes, subdivision of Ascomycota, and order Pleosporales. According to Lepoivre, Spilocaea pomi Fr. belongs to the Hyphomycètes class under the Moniliales order subdivision of Deuteromycota. In essence, V. inaequalis is limited to Malus species.

For all non-Malus plants, it is not a pathogen. However, according to Cam, et al., the pathogens that cause scab on Malus sp. and Pyracantha sp. are regarded as two formae speciales that belong to V. inaequalis. Although the primary host of V. inaequalis is the genus Malus, not all genotypes of Malus are vulnerable.

As one of the earliest ascomycetes to be researched, V. inaequalis is still used practically in many genetic studies, such as those examining its heritability of pathogenicity and sexual compatibility.

This is due to its capacity to be grown and reproduce in vitro, as well as its resemblance to other parasites that infect young live tissues for extended periods of time without causing evident harm. The great variety of V. inaequalis in nature and its long-term phenotypic and genotype stability are two of the traits that make it a prime candidate for genetic research. The heterothallic fungus V. inaequalis infects living tissues.

Numerous nations have undertaken in-depth studies on the bioecology of V. inaequalis. The spread of infection risks of apple trees is influenced by abiotic factors such as light or relative humidity, air temperature, duration of leaf wetness, apple cultivar susceptibility, pathogen inoculation, fruit maturity, growth phase of the tree, and the release of ascospores (sexual spores). Some commercial genotypes have resistance that varies over time as a result of a specific fungus adapting to the plant host, and resistance can even be broken by new races in some wild Malus species or genotypes. Breeding resistant apple cultivars is hampered greatly by the existence of many races of V. inaequalis, which are found in Japan and the USA. Golden Delicious is an excellent example; in the early 1900’s, it was believed to be rather resistant to scab, but it is currently one of the most vulnerable varieties.

Apple scab life cycle and epidemiology

Members of the Maloideae subfamily are infected by the ascomycete V. inaequalis, which results in the illness apple scab. As a hemibiotrophic fungus, V. inaequalis grows both on and inside live leaves and undergoes a necrotrophic phase. Pseudothecia, or tiny black fruiting structures shaped like flasks, are mostly created during the winter months in fallen leaves. Early spring is when the ascospores inside pseudothecia begin to grow. Under the right circumstances, spores are forcefully released into the air when the rainy leaves on the orchard get wet. The two phases of V. inaequalis' life cycle are the asexual or secondary phase and the sexual or main phase (Figure 1). Winter is the key season for the primary phase, while summer is the primary phase.

Figure 1: Apple scab disease cycle (Source: Aerobiology, epidemiology and management strategies in apple scab: Science and its application).

Discussion

Primary phase (Sexual reproduction)

Primary infection usually results during this period. The fungus mostly overwinters as sexual fruiting structures called pseudothecia, which form in apple leaf litter after a brief period of saprophytic vegetative growth that lasts no more than four weeks after leaf abscission. Like the majority of ascomycetes, V. inaequalis is anisogamous, meaning that the sex organs of the male and female haploid parents develop into antheridia and ascogonia, respectively. Furthermore, it is heterothallic, meaning that the two gametangia's plasmogamy can only occur if their parents are of the opposite mating type, that is, if they have distinct mating type alleles on the Mating Type (MAT) locus. The several genes located on this MAT locus encode distinct gene products, which interact intricately to determine the mating type. The asci create the sexual spores, or ascospores, which are then conveyed by the pseudothecium. According to Turechek, the ideal temperature ranges are 8–12°C for ascogonia production and 16–18°C for ascospore maturation.

Ascospores make up the majority of the initial inoculum, which is released by springtime rains over a period of five to nine weeks. There are two cell walls on these sexual spores. The outermost cell wall is delicate and thin. Ascospores are shielded from winter conditions by their thick and elastic inner cell wall.

A double cell wall is also present in the asci. Ascospores can only be released when the asci's inner and outer cell membranes rupture. Rainfall causes a thin layer of water to accumulate around the pseudothecia, which allows the asci to absorb water and enlarge. The inner cell wall eventually fractures due to the rising pressure, which initially breaches the outer cell wall. Sunlight promotes ascospore discharge, which primarily occurs during the day. Up to 200 meters from the source, the wind disperses the spores.

When conditions are right, an infection may occur after the main inoculum (inoculation) arrives on the plant. For spores to germinate, the leaf surface must have free wetness. As long as the Relative Humidity (RH) remains over 95% once germination has begun, it will continue. The invasion and development of the pathogen is not ensured by spore germination. The temperature, the length of time that the leaves are moist, and the sensitivity of the plant and the infected plant organ (leaves vs. sepals and petals, leaf age) all affect how long the fungus will continue to grow. The Mills' periods refer to the meteorological parameters that Mills and LaPlante initially presented. These parameters define the length of (leaf) wetness necessary for infection at various temperatures. These standards have evolved into a common tool for determining when conducive circumstances for infection arise, together with electronic weather monitoring, allowing for more precise targeting of fungicide treatments. When leaves and fruit are young and at their most vulnerable developmental stage early in the growing season, infection risk is highest.

Through an appressorium, the germ tubes originating from ascospores enter the cuticle (rather than stomata) and develop into subcuticular runner hyphae. These subcuticular hyphae give rise to multilayered, pseudoparenchymatous formations known as stromata at regular intervals. It is assumed that stromata, which are composed of cells that divide laterally, get their nourishment from the subcuticular area. There is no penetration of host (epidermal) cells.

Secondary phase (Asexual reproduction)

Conidia are the first stage of V. inaequalis' asexual reproduction and are in charge of secondary infection. The conidial stage of V. inaequalis is referred to as Spilocaea pomi. The conidia are singlecells, measuring 6–12 μm in width and 12–22 μm in length, and have an olive or brown appearance. They are formed successively at the tips of short hyphae known as conidiophores. The freshly formed scab lesions have a characteristic velvety appearance due to the conidia and conidiophores, which are mass generated on the thick mat of mycelium.

Conidia settle on an apple bloom, fruit, and leaves after being dispersed by the wind and falling rain. They adhere to the surface and begin to germinate. A new infection is created when the cuticle is broken by hyphae germination. V. inaequalis conidia have the ability to stick to and germinate on non-host plants as well. Similar to ascospores, conidia can release their seeds anywhere from a few days to a few weeks following the initial leaf infection. Their release is dependent on temperature, moisture content, and humidity. Wet and chilly days in the spring, summer, and fall are ideal for the establishment of secondary infection by conidia. When the weather is right, many cycles of conidial generation and secondary infection occur throughout a certain growth season. In fall, late infection may go undetected. However, fruits may be impacted during storage.

Symptoms of the disease and host range

Both leaves and fruits might exhibit severe and observable symptoms. On the undersides of leaves, dark green, velvety patches characterize the first lesions of scab. When they are young and expanding, applescab may severely damage leaves and fruits. As they become older, mature leaves and fruits become more resilient. There is more tissue that is vulnerable to infection during the major growth period in early spring, which increases the risk of illness compared to later in the season. While defoliation from apple scab might weaken the tree and affect its ability to survive the winter, it usually does not kill the trees. Young scab lesions are tiny, uneven, and pale on leaves. They have a velvety texture, round shape, and olive hue as they get older. More severe stages of lesions become black and elevate somewhat. Leaves with thick scabs may dry out, shrivel, and fall (Figure 2). Fruit loss and aberrant development (fruit distortion) can result from early infections. The fruits have lesions that resemble those on the leaves, although they will eventually develop cracks as they become older. Black, round, minuscule (0.1–4 mm in diameter) lesions known as "pin-point scab" will develop during storage if the fruit is infected in the latter part of the summer or right before harvest.

Figure 2: Apple scab disease symptoms. a) Apple scab spots appeared when water accumulates on the surface of leaves (primary infection stage); (b-c) Secondary scab infection on leaves; (d-e) Bacterial water soaked lesion on fruit; (f) Small dotted like spot on fruits.

On the branches of certain apple varieties, scab infections manifest as light brown swellings encircled by whitish rings. Twig infections manifest as peeling with a dark green mass beneath it and as tiny reddish-brown spots. Certain apple varieties that are infected may have tiny, reddish-brown lesions on the twigs and dark green lesions on the sepals at the base of the flower during bloom.

The genus Malus is attacked by V. inaequalis. This covers a range of apples and crabapples, including the widely available wild variety. Hawthorn (Crataegus spp.), mountain ash (Sorbus spp.), firethorn (Pyracantha spp.), and loquat (Eriobotrya japonica) have also been observed to harbor scab.

Control measures of the disease

Apple scab control management practices generally target the three reproductive strategies of the disease the establishment of the primary infection, the dispersability of diverse biotypes for increased likelihood of sexual reproduction, and the generation of new genotype combinations that are successful in order to disrupt the disease cycle. We have two basic tactics that we can use to disrupt the three reproductive strategies: the preventive method, which focuses primarily on reducing the ascospore dose during the saprophytic phaseand utilizing the apple tree's natural resistance; and the defensive tactic, which aims to shield the tree from ascospore and conidial infection. Therefore, in addition to choosing cultivars with traits that make them resistant to disease, preventive methods include chemical, biological, and physical approaches for either eliminating or attacking the fungus present in the leaf litter or for breaking it down. Defensive strategies, on the other hand, involve the use of fungicidal treatments from bud break through the summer and occasionally until harvest (Figure 3).

Figure 3: Three reproductive strategies and practices for controlling each reproductive strategy (Jamar, 2011).

Integrated Disease Management (IDM)

The ultimate objective of Integrated Disease Management (IDM), a multidisciplinary approach to disease control, is to minimize environmental disruption, achieve long-term efficacy, and be affordable. IDM strategies include chemical application, biological controls, cultural controls, and genetic resistance. When combined, these approaches have a higher chance of being successful for preventing disease than when used alone, such as calendar-based pesticide spraying. They may also slow the emergence of fungicide resistance. IDM strategies for scab management often concentrate on either lowering the amount of primary inoculum or fungicide applications while utilizing predictive models to estimate when the disease will manifest, or both. There is a lot of pathogenic diversity in V. inaequalis, and new races of the virus are constantly being discovered. IDM is therefore the best strategy to reduce yield losses due to scab. In terms of tree nutrition, plant protection product spraying quality, soil management, restrictions on post-harvest treatments, selection and rate of application of active substances used in disease and pest control, and timing of pest treatments based on an assessment of the actual risk they pose, Malavolta stated that IDM has made tremendous progress. Because pruning enhances spray deposition in the tree canopy, it has been demonstrated that using fungicide in conjunction with pruning greatly reduces leaf scab.

Cultural control

Reducing scab infection involves standard cultural and hygienic measures like shredding leaves, burning or burying them in the ground, and applying 5% urea (about 40 pounds of urea in 100 gallons of water) to leaves that are on the ground. These measures work to prevent the growth of V. inaequalis. Leaf litter and the scab pseudothecia it housed were destroyed by flail mowing or urea fertilizer application in November and April, respectively, resulting in decreases in scab risk of 80–90% and 50–66%. These inoculum reduction techniques, however, may be costly and unfeasible for some commercial operations, and they are unable to completely eliminate all sources of primary inoculum.

Crab apple trees at the margins of the orchard should generally be destroyed to stop the inoculum from spreading to apple trees. In order to prevent the formation of scabs, regular trimming is also necessary to ensure that sunshine reaches the canopy, and that air moves between the trees. According to Beckerman, diseases can be reduced or even completely avoided by choosing locations that get more than six hours of sunlight each day, properly spacing trees, and using appropriate pruning techniques to open the tree canopy.

Biological control methods

According to Pal and Gardener, biological control refers to the process of managing or inhibiting plant disease through the utilization of other microorganisms. In order to manage or control V. inaequalis, many antagonistic agents have been found by various research. Applying Microsphaeropsis ochracea, for instance which naturally grows on dead leaves and can be isolated in August and September, for instance, can effectively counteract V. inaequalis, reducing spring ascospore production by 95 to 99% when compared to untreated treatments. Standard infection models based on the conventional ascospore ripening procedure may be applied because this approach did not change the ejection distribution over time. Stated differently, there is currently no commercial availability for this prospective biological control approach. Apple scab control to commercially acceptable levels would need the application of both cultural and possible biological treatments in conjunction with a fungicide spray program; these approaches alone would not be enough. According to Vincent, et al. applying A. bombacina and M. ochracea in the fall reduced the generation of spring ascospores by around 81 and 85%, respectively. Generally speaking, one useful strategy to prevent apple scab in the future is to use naturally occurring microbial antagonists of V. inaequalis to break down apple leaf litter in the fall.

The primary natural biological agents that clear dead leaves off apple orchard floors in the winter and early spring are earthworms, or Lumbricus terrestris. The quantity and mass of leaves buried in grassed-down orchards are closely correlated with the soil's L. terrestris weight. The earthworms may bury up to 184 kg of leaves ha-1 in certain orchards with between 275 and 367 kg of L. terrestris ha-1, which is 90% of the whole autumn leaf fall. In addition, this makes a significant contribution to the soil's organic matter rotation and redistribution. Two antagonistic fungi, Chaetomium globosum and Athelia bombacina, were also discovered by MacHardy in 1996 and may be helpful as biological control agents for V. inaequalis. Applying C. globosum during the secondary infection season may be advantageous since it decreases the number and size of lesions, conidial density, conidial germ tube germination rate, and conidial elongation. Additional authors demonstrated the inhibitory effects of numerous fungi (Auerobasidium botrytis, Cladosporium spp.) and isolates of epiphytic yeast from the plyllosphere of apple trees on V. inaequalis. up to 80% in the plantlets of apple trees for germination and mycelial development. Certain Trichoderma longibrachiatum strains have also been discovered as averse to V. inaequalis, decreasing the amount of ascospores that naturally infected plants produce.

Host plant resistance

It is crucial to integrate and cultivate apple varieties that are more resistant to apple scab diseases and other pests, in addition to factors like productivity, fruit quality, simplicity of tree maintenance, and commercial requirements. For this reason, in response to V. inaequalis, more than 100 apple cultivars have been produced to date (Table 1).

Table 1: List of apple varieties reaction to apple scab.

Commercial apple lines are now being bred to include disease resistance from wild Malus accessions in order to reduce the requirement for fungicide sprays. According to Gessler and Pertot, the earliest attempts to do this were undertaken a century ago. Since Malus floribunda 821, a crabapple, was the first species to exhibit genetic resistance to the apple scab pathogen, the majority of scab-resistant cultivars in use today rely on a single introduction of scab resistance from Malus floribunda 821, known as Vf. Presently, cultivars that exhibit resistance to scabs encompass many dominant resistance genes, the majority of which are situated at the Vf locus within the apple genome.

Frequently, resistance breeding is the most successful and efficient way to treat the apple scab disease. Nevertheless, this has been made more difficult by the fact that the fungus exists in several forms or races, and that because the scab strains may adapt to a particular host plant, plants that are resistant to one race may be vulnerable to another. Therefore, if new scab races emerge, the immunity of particular wild Malus species or specific cultivated genotypes may be compromised. A notable example is "Golden Delicious," which at the beginning of the 20th century was thought to be very resistant to scabs but is now quite sensitive. Because of this, disease management strategies for cultivars resistant to scabs should account for the potential virulence of newly emerging V. inaequalis strains on cultivars that are resistant to just one strain of the pathogen. Apple cultivars often vary significantly in terms of how resistant they are to scabs and how vulnerable they are. According to apple breeding initiatives, for example, more than 50 scab-resistant cultivars have been made available in Europe and New Zealand; Prima, Redfree, and Liberty are notable examples (Table 1).

Mechanical control

The fungus Venturia inaequalis is the cause of apple scab disease, which is usually managed using a mix of chemical, biological, and cultural techniques. Based on research and review papers, the following is a summary of mechanical control techniques:

Pruning: Apple scab incidence can be decreased by pruning apple trees to increase light penetration and air circulation. As a result, the fungus's growth and dissemination are hindered. Pruning should be done correctly to get rid of diseased plant material and encourage strong development.

Sanitation: Clearing the orchard floor of fallen leaves and other plant debris might help lower the amount of inoculum that could be present for an infection the next season. As a result, the fungus's capacity to overwinter is diminished. The spread of disease should be reduced by adhering to regular cleanliness procedures.

Mulching: Encircling apple trees' bases with a thick layer of mulch can help stop spores from splattering onto leaves and infecting them. Mulch also contributes to the preservation of soil temperature and moisture, which indirectly influences the onset of illness Aldwinckle and Sutton, 1987.

When to prune: If diseased plant parts are removed in the dormant season, they can be removed before the fungus becomes active in the spring.

Barrier films: By covering apple fruits with barrier films or coatings, a physical barrier that keeps fungus spores from infecting the fruit can be created. These movies might also contain antifungal qualities, which would stop diseases from spreading.

Chemical control

Fungicides applied to cultivars resistant to scab can help shield the tree from primary scab and any successful secondary scab by preventing infection by any newly virulent strain of the fungus. To manage apple scab, many fungicides are applied 10–15 times over the season (Table 2). Apple farmers often use a combination of preventative and curative fungicides to stop the disease's growth. Table 3 displays the different sprays that should be applied in J and K at key apple growth stages according to the approved timetable. If a subsequent infection is still anticipated after walnut-sized fruit, spraying with any protector or systemic fungicide should continue.

Table 2: Plant protection schedule for the management of insects and apple scab.

Table 3. Spray schedule for management of apple scab disease.

Structural and biochemical defense mechanisms of apple scab

Apple defensive mechanisms against the causative agent of apple scab are a combination of biochemical (defense proteins, enzymes, phytoalexins, phytoanticipins, hormones, etc.) and structural (cuticle).

Structural defense mechanisms

One of the most significant structural defensive systems found in plants is the cuticle. According to Kolattukudy, it is a protective layer that covers the epidermis of leaves, new shoots, and other aerial plant organs devoid of periderm. It is composed of cuticular waximpregnated three-dimensional hydrocarbon polymer and lipid. Plant critical activities, such as limiting water loss and preventing the growth and development of disease-causing pathogens like bacteria and fungus, are determined by the physical and chemical characteristics of cuticular waxes.

The pathogen, V. inaequalis, depends on a specific quantity of free water on the leaf surface for survival. Its ability to infiltrate the host plant quickly is also determined by the cuticle's composition, thickness, and strength. Generally speaking, when a leaf develops, its hydrophilicity and cuticle characteristics alter. According to Jha, et al., this might contribute to ontogenic resistance.

Biochemical defense mechanisms

Accumulation of Pathogenesis-Related (PR) proteins: It has been established that many PR proteins play a part in apple scab protection. The extracellular matrix and the continuity of neighboring cells' cell walls combine to produce the apoplast, according to a comparison study by Gau, et al., between the apoplastic protein accumulation of the susceptible cultivar "Elstar" and the Rvi6 resistant cultivar "Remo." It is crucial to every way the plant interacts with its surroundings. Mass spectrometry and Two-dimensional gel Electrophoresis (2-DE) were used to identify variations in the concentrations of certain PR proteins between the two cultivars. Following infection, the amount of apoplastic proteins that could be detected more than quadrupled in the susceptible "Elstar." According to Gau, et al., the isoelectric point of the majority of the additional proteins found was between 4 and 5.

Before infection, the quantities of β-1,3-glucanase (36-40 kDa), chitinase (27-28 kDa), and endotoxinase type III (27-28 kDa) in cv. "Remo" were greater than in cv. "Elstar." These are the corresponding PR-2, PR-3, and PR-8 proteins. Following V. inaequalis infection, the concentrations in "Elstar" resemble those in "Remo." This implies that only the resistant cultivar exhibits a constitutive buildup of these proteins. The fungal cell wall can be hydrolyzed by B-1,3-glucanase, chitinase, and endochitinase. Through an as-yet-unidentified mechanism, the chito-oligosaccharides produced by endochitinase activity would trigger defense systems. The apoplast of the resistant cv. Remo has constitutively increased quantities of a thaumatin-like protein (PR-5). Upon infection, the buildup rises in the vulnerable "Elstar." Sweet-tasting protein thaumatin (21 kDa) is thought to be a PR protein prototype. Additionally, osmotinlike proteins and a PR-1 protein (15–16 kDa) were found by these authors. These are not constitutively present in "Elstar," but they are in the resilient "Remo."

Within the first week following V. inaequalis infection, the "Elstar" concentration of a non-specific lipid transfer protein (PR-14; 9 kDa) decreased to an undetectable level. One possibility might be that this protein is involved in pathogen identification and the start of the defensive response. According to Blein, et al., it would interact with the pathogen's effectors and cause a systemic, non-specific resistance. According to Diaz-Perales, et al., the lipid transfers protein aids in the transfer of phospholipids across membranes and is involved in the development of the cuticle and epicuticular wax. Paris et al. verified the decreased accumulation of the Mald3 gene, which codes for this lipid transport protein.

Following infection in HcrVf2-transformed "Gala," there is a rise in the accumulation of many Mald1 proteins of the ribonuclease type (PR-10). Furthermore, upon infection, there is an upregulation in the expression of the genes that code for defensing-like proteins (PR-12). The antifungal action of plant defenses would be mediated by modifications to the permeability of fungal membranes and inhibition of the manufacture of fungal macromolecules.

Phenolic compounds of apple and their relationship to scab resistance

Apple's protection against V. inaequalis would involve phenolic chemicals. For example, the resistant cv. "Sir Prize" becomes vulnerable when Phenyl Alanine Ammonia-Lyase (PAL), a crucial enzyme in the phenol production signal transduction pathway, is eliminated. Since PAL catalyzes the initial step in the phenylpropanoid pathway, it plays a role in the production of phenolic chemicals in plants, including lignin, flavonoids, and phenylpropanoids. It is well recognized that a variety of triggers, including a pathogenic onslaught, can cause PAL activity to increase rapidly. It is found in both sensitive and resistant cultivars that the major phenolic chemicals. The relative and absolute proportions of these substances vary, though. When compared to susceptible cultivars, Rvi6 cultivars typically have higher total phenol contents and higher concentrations of specific phenolic molecules, though these levels vary throughout the season and are influenced by cultural practices. Chlorogenic acid is one example of a phenolic molecule that is found in greater concentrations in older leaves and in resistant apple cultivars. The phenols themselves as well as the byproducts of their breakdown would encourage the emergence of resistance. For instance, the most abundant phenolic glycoside in apples is phenolizin, which inhibits V. inaequalis. Since it is mostly found in the cuticle, it will have an impact on the germination and penetration of the fungus into the subcuticular area, which is the most crucial stage in the fungus' survival following inoculation. Phlorizin is changed into phloretin by V. inaequalis. Additionally, this molecule possesses antifungal properties. Gessler, et al., deduced from the various investigations that resistance is caused by a local accumulation and transformation triggered by an elicitor, rather than the intrinsic existence of phenols.

A buildup of flavanols occurs in the area next to the scab lesions in apple infected by V. inaequalis. When fungi attack, Malus furan and dibenzofuran compounds are generated, which inhibit V. inaequalis germination and growth.

Conclusion

 

In temperate and humid apple-producing regions of the world, apple scab which is caused by the ascomycete fungus V. inaequalis (Cooke) Wint. Is the most economically devastating disease. In order to produce more and sustainable apple production worldwide, this disease must be managed. The most common method of managing apple scabs is to apply fungicides repeatedly, which may be expensive financially because of the fungicide treatments and in time allotted to managing scabs. Therefore, it is obvious that new approaches are required to produce a long-lasting control for apple scab. Future approaches to managing apple scabs will probably include the use of chemicals or fungicides, working through elicitation or priming to boost plant defense, and the creation of new, resilient apple cultivars that are resistant to scabs.

A breeding effort designed to produce apple scab resistance that is long-lasting also requires an understanding of the biology and variability of V. inaequalis. To enhance existing disease management approaches, more research on the ecology and epidemiology of V. inaequalis is needed. The host-pathogen connection between V. inaequalis and Malus X domestica should be investigated using both novel and traditional methods. In order to increase the overall effectiveness of both organic and conventional apple production, significant study should be focused on the control of apple scab employing bio-agents, enhanced cultural practices, and the development of resistant cultivars.

For the management of apple scabs, Integrated Disease Management (IDM), which integrates chemical, biological, cultural, and resistance control measures in a comprehensive manner as opposed to employing a single component strategy, has often shown to be more successful and long-lasting. As a result, each nation will allocate funds for research to: identify and develop commercial varieties with long-lasting resistance characteristics; enhance the efficacy of phytosanitary measures intended to lower the inoculum and learn about the potential of such measures in order to decrease the need for fungicides; discover substitute fungicides for sulphur and copper; improve strategies and protection schemes; (v) optimize treatment timing; enhance plant protection product application techniques; and develop specific cultural practices that are less conducive to the disease's development.