Apple scab is a common disease of plants in the rose family (
Rosaceae
Rosaceae (), the rose family, is a medium-sized family of flowering plants that includes 4,828 known species in 91 genera.
The name is derived from the type genus ''Rosa''. Among the most species-rich genera are '' Alchemilla'' (270), ''Sorbus ...
) that is caused by the
ascomycete
Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...
fungus ''
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'' ...
''.
While this disease affects several plant genera, including ''
Sorbus
''Sorbus'' is a genus of over 100 species of trees and shrubs in the rose family, Rosaceae. Species of ''Sorbus'' (''s.l.'') are commonly known as whitebeam, rowan ( mountain-ash) and service tree. The exact number of species is disputed depe ...
,
Cotoneaster
''Cotoneaster'' is a genus of flowering plants in the rose family, Rosaceae, native to the Palaearctic region (temperate Asia, Europe, north Africa), with a strong concentration of diversity in the genus in the mountains of southwestern China an ...
,'' and ''
Pyrus
Pears are fruits produced and consumed around the world, growing on a tree and harvested in the Northern Hemisphere in late summer into October. The pear tree and shrub are a species of genus ''Pyrus'' , in the family Rosaceae, bearing the po ...
'', it is most commonly associated with the infection of ''
Malus
''Malus'' ( or ) is a genus of about 30–55 species of small deciduous trees or shrubs in the family Rosaceae, including the domesticated orchard apple, crab apples, wild apples, and rainberries.
The genus is native to the temperate zone of th ...
'' trees, including 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, which develop dark, irregularly-shaped lesions upon infection.
Although apple scab rarely kills its host, infection typically leads to fruit deformation and premature leaf and fruit drop, which enhance the susceptibility of the host plant to abiotic stress and secondary infection.
[Jha, G., Thakur, K., & Thakur, P. (2009). The ''Venturia'' Apple Pathosystem: Pathogenicity Mechanisms and Plant Defense Responses. ''Journal of Biomedicine and Biotechnology'', 2009. doi:10.1155/2009/680160] The reduction of fruit quality and yield may result in crop losses of up to 70%, posing a significant threat to the profitability of apple producers.
To reduce scab-related yield losses, growers often combine preventive practices, including sanitation and resistance breeding, with reactive measures, such as targeted fungicide or biocontrol treatments, to prevent the incidence and spread of apple scab in their crops.
History and distribution
The earliest official reports of apple scab were made in 1819 by Swedish botanist,
Elias Fries
Elias Magnus Fries (15 August 1794 – 8 February 1878) was a Swedish mycologist and botanist.
Career
Fries was born at Femsjö (Hylte Municipality), Småland, the son of the pastor there. He attended school in Växjö.
He acquired an ...
.
However, genetic studies have indicated that apple scab likely 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 domesticated apple trees by migrating humans.
By the end of the 19th century, the disease had spread to
North America and
Oceania alongside the importation of host plants. Today apple scab is present in nearly all regions where apples are cultivated, with the most significant infections occurring in temperate areas, where it is cool and moist in the spring.
Disease cycle
The disease cycle begins in early spring, when cool temperatures and abundant moisture promote the release of sexual spores (
ascospore
An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or s ...
s) from overwintering structures (
pseudothecia
An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mo ...
) 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, 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
An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or s ...
are transported to the surfaces of newly-emerged leaves and blossoms by wind and splashing water.
The tissue is then penetrated either directly with a
germ tube or indirectly using an
appressorium
An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal "pressing" organ, from which a minute infection peg grows and enters the host, using turgor pressure capable ...
, thus initiating a new infection.
Shortly after penetration, light-green, irregularly shaped
lesions
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classifi ...
develop on infected foliar tissue and gradually darken, expand, and pucker as the infection progresses.
Lesions on fruit are black or brown and irregularly shaped, with older fruit lesions causing the underlying tissue to become dry, corky, and eventually disfigured by splitting.
Within 10 days of infection, asexual
conidia
A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non- motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to t ...
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 fall from the canopy, allowing for the development of
pseudothecia
An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mo ...
, which serve as a source of primary inoculum for the next spring.
Predicting infection
First developed in 1944 by American plant pathologist, W.D. Mills, a
Mills Table
Mills is the plural form of mill, but may also refer to:
As a name
* Mills (surname), a common family name of English or Gaelic origin
* Mills (given name)
*Mills, a fictional British secret agent in a trilogy by writer Manning O'Brine
Places U ...
predicts the likelihood of an apple scab infection developing based on the average temperature and the number of hours of leaf wetness that the host plant is exposed to. This prediction system has been rapidly adopted in both Europe and North America, where apple growers use it as an early warning system for new infections, allowing them to apply preventive fungicides when appropriate.
Several revisions have been made to the Mills Table since its creation. The most notable revision was made in 1989 by plant pathologists William MacHardy and David Gadoury, who determined that ascospores required 3 hours less than originally calculated in order to establish a new infection.
While other methods of prediction include ascospore maturation models and leaf orchard leaf canopy models, the Mills Table, combined with electronic weather monitoring, remains the most-widely used tool for predicting apple scab infection periods.
Management of apple scab
Cultural controls
Cultural control
In agriculture cultural control is the practice of modifying the growing environment to reduce the prevalence of unwanted pests. Examples include changing soil pH or fertility levels, irrigation
Irrigation (also referred to as watering) is t ...
s may be used as a first step when seeking 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 of the disease. Furthermore, regular pruning will improve air flow and light penetration in the canopy, which ultimately inhibits the development and spread of disease.
Another aspect of cultural control is water management. As water triggers ascospore release and promotes germination on vulnerable tissue, growers are advised to monitor watering periods and avoid using overhead watering systems. Doing so may ultimately aid in reducing infection periods caused by natural precipitation.
Chemical controls
The management of apple scab using chemical controls is primarily concerned with preventing the initiation of primary infection cycles by reducing the germination of ascospores. As such, fungicides are typically applied early in the season, when ascospores are first released.
However, fungicide applications may also be made later in the season to prevent infection of old leaves, which can help reduce the amount of primary inoculum for the following season.
Benzimidazole
Benzimidazole is a heterocyclic aromatic organic compound. This bicyclic compound may be viewed as fused rings of the aromatic compounds benzene and imidazole. It is a colorless solid.
Preparation
Benzimidazole is produced by condensation of o- ...
fungicide
Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...
s are among the most commonly-used classes of fungicide for managing apple scab in conventional orchards; however, there is some evidence that the disease is developing
resistance to this class of fungicides, along with several others, including demethylation inhibitors and
quinone outside inhibitor
Qo inhibitors (QoI), or quinone outside inhibitors, are a group of fungicides used in agriculture. Some of these fungicides are among the most popular in the world. QoI are chemical compounds which act at the quinol ''outer'' binding site of th ...
s. 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.
In
organic
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
production systems, growers commonly use copper- or sulfur-based protectant sprays to reduce the efficacy of primary inoculum. Although these sprays were among the earliest methods of preventing the development of apple scab, they do little to manage pre-existing infections and application may significantly damage the foliage of treated trees.
Moreover, research has indicated that applications of copper-based fungicides may result in changes 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 (a
biological control agent
Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also invo ...
) 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 is Serenade® ASO, a microbial biofungicide which uses ''Bacillus subtilis'' as its active ingredient and may be used to control foliar diseases caused by bacteria or fungi. Furthermore, several fungal antagonists have been isolated and identified as potential biocontrols. One of such antagonists is ''
Cladosporium cladosporioides
''Cladosporium cladosporioides'' is a darkly pigmented mold that occurs world-wide on a wide range of materials both outdoors and indoors. It is one of the most common fungi in outdoor air where its spores are important in seasonal allergic dise ...
'' (strain H39). This antagonistic fungus has demonstrated significant bioactivity against apple scab. This was indicated in a 2015 study, which found that applications of ''C. cladosporioides'' could reduce leaf scab incidence by 42-98% and apple scab incidence by 41-94% in both conventionally and organically managed orchards.
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
Purdue University is a public land-grant research university in West Lafayette, Indiana, and the flagship campus of the Purdue University system. The university was founded in 1869 after Lafayette businessman John Purdue donated land and mon ...
,
Rutgers University
Rutgers University (; RU), officially Rutgers, The State University of New Jersey, is a Public university, public land-grant research university consisting of four campuses in New Jersey. Chartered in 1766, Rutgers was originally called Queen's ...
, and the
University of Illinois
The University of Illinois Urbana-Champaign (U of I, Illinois, University of Illinois, or UIUC) is a public land-grant research university in Illinois in the twin cities of Champaign and Urbana. It is the flagship institution of the Universi ...
. 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.
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 and is currently being used by researchers seeking to develop resistant cultivars using
transgenic technology.
While the development of transgenic resistant cultivars may reduce management costs in orchards, limited market acceptance pose a barrier to early adoption by commercial growers.
Moreover, researchers have observed a breakdown of resistance genes by ''Venturia'' populations, posing another significant barrier to the success of this technology.
See also
*''
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'' ...
''
References
{{Apples
Apple tree diseases
Fungal tree pathogens and diseases