Editing Apple scab

{{Short description|Plant disease caused by fungus}}
{{distinguish|Applecrab}}
{{Infobox plant disease
| name = Apple scab
| image = Apple fruits scab.jpg
| caption = The main symptom of apple scab in infected trees is the development of brown lesions on the foliage and fruits.
| common_names = 
| hosts = Apple
| causal_agents = ''[[Venturia inaequalis]]''
| vectors = 
| EPPO_codes = VENTIN
| distribution = 
| treatment = 
}}

'''Apple scab''' is a common disease of plants in the rose family ([[Rosaceae]]) that is caused by the [[Ascomycota|ascomycete]] [[fungus]] ''[[Venturia inaequalis]]''.<ref name=":02">{{Cite web|url=https://extension.psu.edu/apple-disease-apple-scab|title=Apple Disease - Apple Scab|website=Penn State Extension|language=en|access-date=2020-02-18}}</ref> While this disease affects several plant genera, including ''[[Sorbus]], [[Cotoneaster]],'' and ''[[Pyrus communis|Pyrus]]'', it is most commonly associated with the infection of ''[[Malus]]'' trees, including species of flowering crabapple, as well as cultivated [[apple]].<ref>{{Cite web|url=https://extension.umn.edu/plant-diseases/apple-scab|title=Apple scab of apples and crabapples|website=extension.umn.edu|language=en|access-date=2020-03-09}}</ref><ref name=":2">{{Cite web|url=https://ag.umass.edu/landscape/fact-sheets/apple-scab|title=Apple Scab|date=2015-03-06|website=Center for Agriculture, Food and the Environment|language=en|access-date=2020-03-09}}</ref> 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.<ref name=":1">{{Cite web|url=https://www2.gov.bc.ca/gov/content/industry/agriculture-seafood/animals-and-crops/plant-health/insects-and-plant-diseases/tree-fruits/apple-scab#cycle|title=Apple Scab Management in British Columbia - Province of British Columbia|last=Agriculture|first=Ministry of|website=www2.gov.bc.ca|access-date=2020-02-02}}</ref><ref name=":0">{{Cite web|url=https://www.apsnet.org/edcenter/disandpath/fungalasco/pdlessons/Pages/AppleScab.aspx|title=Apple scab|last=Gauthier|first=Nicole|date=2018|website=American Phytopathological Society|access-date=2020-02-02}}</ref> 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.<ref name=":3">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</ref><ref name=":0" /> 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.<ref name=":3" /> 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.<ref name=":8">{{Cite journal|last1=Bowen|first1=Joanna K.|last2=Mesarich|first2=Carl H.|last3=Bus|first3=Vincent G. M.|last4=Beresford|first4=Robert M.|last5=Plummer|first5=Kim M.|last6=Templeton|first6=Matthew D.|date=2011|title=Venturia inaequalis: the causal agent of apple scab|journal=Molecular Plant Pathology|language=en|volume=12|issue=2|pages=105–122|doi=10.1111/j.1364-3703.2010.00656.x|issn=1364-3703|pmc=6640350|pmid=21199562}}</ref>
[[Image:Apple scab.jpg|right|thumb|Apple scab on [[Malus|crabapple]], lesions are visible on the leaves.]]

== History and distribution ==
The earliest official reports of apple scab were made in 1819 by Swedish botanist, [[Elias Fries]].<ref name=":3" /> However, genetic studies have indicated that apple scab likely emerged in Central Asia.<ref name=":4">{{Cite journal|last=Gladieux|first=Pierre|date=2008|title=On the Origin and Spread of the Scab Disease of Apple: Out of Central Asia|journal=PLOS ONE|volume=3|issue=1|pages=e1455|doi=10.1371/journal.pone.0001455|pmid=18197265|pmc=2186383|bibcode=2008PLoSO...3.1455G|doi-access=free}}</ref> 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.<ref name=":4" /><ref name=":8" />  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.<ref name=":8" />

== Disease cycle ==
The disease cycle begins in early spring, when cool temperatures and abundant moisture promote the release of sexual spores ([[ascospore]]s) from overwintering structures ([[pseudothecia]]) found in the debris at the base of previously-infected trees.<ref name=":0" /> 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.<ref name=":3" /> Following their dissemination, [[ascospores]] are transported to the surfaces of newly-emerged leaves and blossoms by wind and splashing water.<ref name=":02" /> The tissue is then penetrated either directly with a [[germ tube]] or indirectly using an [[appressorium]], thus initiating a new infection.<ref name=":8" /> Shortly after penetration, light-green, irregularly shaped [[lesions]] develop on infected foliar tissue and gradually darken, expand, and pucker as the infection progresses.<ref name=":1" /><ref name=":0" /> 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.<ref name=":1" /> Within 10 days of infection, asexual [[Conidium|conidia]] 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.<ref name=":1" /> At the end of the season, heavily-infected fruit and foliage fall from the canopy, allowing for the development of [[pseudothecia]], which serve as a source of primary inoculum for the next spring.<ref name=":0" />
[[Image:Apple scab SEM.jpg|thumb|The reproductive conidia of ''Venturia inaequalis'' erupting through the cuticle of a crabapple leaf.]]

== Predicting infection ==
First developed in 1944 by American plant pathologist, W.D. Mills, a [[Mills Table]] 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.<ref>{{Cite book|last=Schumann|first=Gail|title=Plant diseases: Their biology and social impact|publisher=The American Phytopathological Society|year=1991|location=St. Paul, Minnesota, USA|pages=173–177}}</ref> 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.<ref name=":7">{{Cite journal|last=Singh|first=Krishna P.|date=September 2019|title=Aerobiology, epidemiology and management strategies in apple scab: science and its applications|journal=Indian Phytopathology|volume=72|issue=3|pages=381–408|doi=10.1007/s42360-019-00162-5| s2cid=204148966 |issn=0367-973X}}</ref> 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.<ref name=":7" /> 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.<ref name=":8" />

== Management of apple scab ==

=== Cultural controls ===
[[Cultural control]]s may be used as a first step when seeking to reduce the incidence of new [[infection]]s. 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.<ref name=":2" /> 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.<ref name=":2" /><ref name=":0" /> 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.<ref name=":1" />

=== 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.<ref name=":0" /> 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.<ref>{{Cite journal|last1=Li|first1=B.|last2=Xu|first2=X.|date=2002|title=Infection and Development of Apple Scab (Venturia inaequalis) on Old Leaves|journal=Journal of Phytopathology|language=en|volume=150|issue=11–12|pages=687–691|doi=10.1046/j.1439-0434.2002.00824.x|issn=1439-0434}}</ref> [[Benzimidazole]] [[fungicide]]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 [[Fungicide Resistance|resistance]] to this class of fungicides, along with several others, including demethylation inhibitors and [[quinone outside inhibitor]]s.<ref>{{Cite journal|last1=Köller|first1=Wolfram|last2=Parker|first2=D. M.|last3=Turechek|first3=W. W.|last4=Avila-Adame|first4=Cruz|last5=Cronshaw|first5=Keith|date=May 2004|title=A Two-Phase Resistance Response of Venturia inaequalis Populations to the QoI Fungicides Kresoxim-Methyl and Trifloxystrobin|journal=Plant Disease|language=en|volume=88|issue=5|pages=537–544|doi=10.1094/PDIS.2004.88.5.537|pmid=30812659|issn=0191-2917|doi-access=}}</ref> 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.<ref name=":8" />

In [[Organic farming|organic]] 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.<ref name=":0" /> Moreover, research has indicated that applications of copper-based fungicides may result in changes in the structure and functionality of [[Soil microbiology|soil microbiota]], thus having a negative effect on [[soil health]].<ref>{{Cite journal|last1=Wightwick|first1=Adam M.|last2=Salzman|first2=Scott A.|last3=Reichman|first3=Suzanne M.|last4=Allinson|first4=Graeme|last5=Menzies|first5=Neal W.|date=2012-08-14|title=Effects of copper fungicide residues on the microbial function of vineyard soils|journal=Environmental Science and Pollution Research|volume=20|issue=3|pages=1574–1585|doi=10.1007/s11356-012-1114-7|pmid=22890509|s2cid=5745969|issn=0944-1344|hdl=11343/283109|hdl-access=free}}</ref> As such, alternative management strategies appropriate for organic production systems are currently being developed.{{cn|date=February 2023}}

=== Biological Control ===
Biological control refers to the use of a population of one organism (a [[Biological pest control|biological control agent]]) to suppress the population of another.<ref>{{Cite web|url=https://www.iobc-global.org/download/IOBC_InternetBookBiCoVersion6Spring2012.pdf|title=IOBC Internet Book of Biological Control, version 6|last=van Lenteren|first=J.C.|date=2012|website=International Organisation for Biological Control}}</ref> 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.<ref>{{Cite web|url=https://www.cropscience.bayer.us/products/fungicides/serenade-aso|title=Serenade ASO Fungicide {{!}} Crop Science US|website=www.cropscience.bayer.us|access-date=2020-04-06}}</ref><ref>{{Cite web|url=https://pnwhandbooks.org/plantdisease/host-disease/apple-malus-spp-scab|title=Apple (Malus spp.)-Scab|date=2015-09-11|website=Pacific Northwest Pest Management Handbooks|language=en|access-date=2020-04-06}}</ref> Furthermore, several fungal antagonists have been isolated and identified as potential biocontrols. One of such antagonists is ''[[Cladosporium cladosporioides]]'' (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.<ref name=":62">{{Cite journal|last1=Köhl|first1=Jürgen|last2=Scheer|first2=Christian|last3=Holb|first3=Imre J.|last4=Masny|first4=Sylwester|last5=Molhoek|first5=Wilma|date=2014-10-15|title=Toward an Integrated Use of Biological Control by Cladosporium cladosporioides H39 in Apple Scab (Venturia inaequalis) Management|journal=Plant Disease|volume=99|issue=4|pages=535–543|doi=10.1094/PDIS-08-14-0836-RE|pmid=30699552|issn=0191-2917|doi-access=free}}</ref>

== Resistance breeding programs ==
The first formal resistance breeding programs for apple scab began in the early 20th century with the development of the [[PRI disease resistant apple breeding program|PRI Apple Breeding Program]] by [[Purdue University]], [[Rutgers University]], and the [[University of Illinois]]. 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.<ref>{{Cite journal|last=Janick|first=Jules|date=2006|title=The PRI Apple Breeding Program|url=https://hort.purdue.edu/newcrop/historyPRI.pdf|journal=HortScience|volume=41|pages=8–10|doi=10.21273/HORTSCI.41.1.8|doi-access=free}}</ref>  Modern genetic work has found that a total of fifteen [[gene]]s may confer resistance to apple scab.<ref name=":3" /> Many of these genes have been isolated from wild ''Malus'' spp. populations in East Asia, where a high level of species diversity still remains.<ref name=":8" /> 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 [[Genetically modified food|transgenic technology]].<ref name=":3" />  Another resistance gene is Va from Antonovka.<ref>Dunemann F, Tree Genetics and Genomes 2010:6(5) 627-635</ref> 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.<ref name=":8" /> Moreover, researchers have observed a breakdown of resistance genes by ''Venturia'' populations, posing another significant barrier to the success of this technology.<ref name=":62" />

==See also==
*''[[Venturia inaequalis]]''

== References ==
{{reflist}}

{{Apples}}

[[Category:Apple tree diseases]]
[[Category:Fungal tree pathogens and diseases]]