Apple scab
Encyclopedia
Apple scab is a disease to Malus
trees, such as apple trees, caused by the ascomycete fungus
Venturia inaequalis
. The disease manifests as dull black or grey-brown lesions on the surface of tree leaves, buds or fruit
s. Lesions may also appear less frequently on the woody tissues of the tree. Fruits and the undersides of leaves
are especially susceptible. The disease rarely kills its host, but can significantly reduce fruit yields and fruit quality. Affected fruits are less marketable due to the presence of the black fungal lesions.
s from leaf litter around the base of previously infected trees. These spores rise into the air and land on the surface of a susceptible tree, where they germinate and form a germ tube
that can directly penetrate the plant's waxy cuticle. A fungal mycelium
forms between the cuticle and underlying epidermal tissue, starting as a yellow spot that grows and ruptures to reveal a black lesion bearing asexually
as the conidia are released and germinate on fresh areas of the host tree, which in turn produce another generation of conidial spores. This cycle of secondary infections continues throughout the summer, until the leaves and fruit fall from the tree at the onset of winter.
Over the winter, V. inaequalis undergoes sexual reproduction
in the leaf litter around the base of the tree, producing a new generation of ascospores that are released the following spring. Scab lesions located on the woody tissues may also overwinter in place, but will not undergo a sexual reproduction cycle; these lesions can still produce infective conidial spores in the spring.
s, new infections can be reduced by removing leaf litter and trimmings containing infected tissue from the orchard and incinerating them. This will reduce the amount of new ascospores released in the spring. Additionally, scab lesions on woody tissue can be excised from the tree if possible and similarly destroyed.
Chemical controls can include a variety of compounds. Benzimidazole
fungicide
s (e.g., Benlate [now banned in many countries due to its containing the harmful chemical Benzene]) work well but resistance can arise quickly;
Similarly, a number of chemical classes including sterol inhibitors such as Nova 40, and strobilurins such as Flint among others, were used extensively but are slowly being phased out because of resistance problems.
Contact fungicides not prone to resistance such as Captan
are viable choices. Copper or 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. The timing of application and concentration varies between compounds.
15 gene
s have been found in apple cultivar
s that confer resistance against apple scab. Researchers hope to use cisgenic
techniques to introduce these genes into commercial cultivars and therefore create new resistant cultivars. This can be done through conventional breeding but would take over 50 years to achieve.
Malus
Malus , the apples, are a genus of about 30–35 species of small deciduous trees or shrubs in the family Rosaceae. Other studies go as far as 55 species including the domesticated Orchard Apple, or Table apple as it was formerly called...
trees, such as apple trees, caused by the ascomycete fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
Venturia inaequalis
Venturia inaequalis
Venturia inaequalis is an ascomycete fungus that causes the Apple scab disease.-Systematics:Venturia inaequalis anamorphs have been described under the names Fusicladium dendriticum and Spilocaea pomi. Whether V. inaequalis is a single species or contains several cryptic species has been a matter...
. The disease manifests as dull black or grey-brown lesions on the surface of tree leaves, buds or fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
s. Lesions may also appear less frequently on the woody tissues of the tree. Fruits and the undersides of leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
are especially susceptible. The disease rarely kills its host, but can significantly reduce fruit yields and fruit quality. Affected fruits are less marketable due to the presence of the black fungal lesions.
Life cycle
The infection cycle begins in the springtime, when suitable temperatures and moisture promote the release of V. inaequalis ascosporeAscospore
An ascospore is a spore contained in an ascus or that was produced inside an ascus. This kind of spore is specific to fungi classified as ascomycetes ....
s from leaf litter around the base of previously infected trees. These spores rise into the air and land on the surface of a susceptible tree, where they germinate and form a germ tube
Germ tube
A germ tube is an outgrowth produced by spores of spore-releasing fungi during germination.The germ tube differentiates, grows, and develops by mitosis to create somatic hyphae....
that can directly penetrate the plant's waxy cuticle. A fungal mycelium
Mycelium
thumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other...
forms between the cuticle and underlying epidermal tissue, starting as a yellow spot that grows and ruptures to reveal a black lesion bearing asexually
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
as the conidia are released and germinate on fresh areas of the host tree, which in turn produce another generation of conidial spores. This cycle of secondary infections continues throughout the summer, until the leaves and fruit fall from the tree at the onset of winter.
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
in the leaf litter around the base of the tree, producing a new generation of ascospores that are released the following spring. Scab lesions located on the woody tissues may also overwinter in place, but will not undergo a sexual reproduction cycle; these lesions can still produce infective conidial spores in the spring.
Control
In affected orchardOrchard
An orchard is an intentional planting of trees or shrubs that is maintained for food production. Orchards comprise fruit or nut-producing trees which are grown for commercial production. Orchards are also sometimes a feature of large gardens, where they serve an aesthetic as well as a productive...
s, new infections can be reduced by removing leaf litter and trimmings containing infected tissue from the orchard and incinerating them. This will reduce the amount of new ascospores released in the spring. Additionally, scab lesions on woody tissue can be excised from the tree if possible and similarly destroyed.
Chemical controls can include a variety of compounds. Benzimidazole
Benzimidazole
Benzimidazole is a heterocyclic aromatic organic compound. This bicyclic compound consists of the fusion of benzene and imidazole. The most prominent benzimidazole compound in nature is N-ribosyl-dimethylbenzimidazole, which serves as an axial ligand for cobalt in vitamin B12. Benzimidazole, in...
fungicide
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
s (e.g., Benlate [now banned in many countries due to its containing the harmful chemical Benzene]) work well but resistance can arise quickly;
Similarly, a number of chemical classes including sterol inhibitors such as Nova 40, and strobilurins such as Flint among others, were used extensively but are slowly being phased out because of resistance problems.
Contact fungicides not prone to resistance such as Captan
Captan
Captan is the name of a general use pesticide that belongs to the phthalimide class of fungicides. Though it can be applied on its own, Captan is often added as a component of other pesticide mixtures. It is used to control diseases on a number of fruits and vegetables as well as ornamental plants...
are viable choices. Copper or Bordeaux mixture
Bordeaux mixture
Bordeaux mixture is a mixture of copper sulfate and slaked lime used as a fungicide in vineyards. It is used mainly to control garden, vineyard, nursery and farm infestations of fungi, primarily downy mildew which can result from infections of Plasmopara viticola. It was invented in the Bordeaux...
are traditional controls but are less effective than chemical fungicides and can cause russeting
Russet apple
Russeting on apples is a particular type of skin, slightly rough, usually with a greenish-brown to yellowish-brown colour. Many apple cultivars have some natural russeting, but some are almost entirely covered in it, notably the Egremont Russet. Russet apples often exhibit a scent and flavour...
of the fruit. Wettable sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
also provides some control. The timing of application and concentration varies between compounds.
15 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s have been found in apple cultivar
Cultivar
A cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...
s that confer resistance against apple scab. Researchers hope to use cisgenic
Cisgenesis
Cisgenesis, sometimes also called Intragenesis, is a product designation for a category of genetically engineered plants. A variety of classification schemes have been proposed , that order genetically modified organisms based on the nature of introduced genotypical changes rather than the process...
techniques to introduce these genes into commercial cultivars and therefore create new resistant cultivars. This can be done through conventional breeding but would take over 50 years to achieve.