Apple scab

Last updated
Apple scab
Apple fruits scab.jpg
The main symptom of apple scab in infected trees is the development of brown lesions on the foliage and fruits.
Causal agents Venturia inaequalis
HostsApple
EPPO Code VENTIN

Apple scab is a common disease of plants in the rose family (Rosaceae) that is caused by the ascomycete fungus Venturia inaequalis . [1] While this disease affects several plant genera, including Sorbus, Cotoneaster, and Pyrus , it is most commonly associated with the infection of Malus trees, including species of flowering crabapple, as well as cultivated apple. [2] [3] 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. [4] [5] 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. [6] [5] 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. [6] 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. [7]

Contents

Apple scab on crabapple, lesions are visible on the leaves. Apple scab.jpg
Apple scab on 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. [6] However, genetic studies have indicated that apple scab likely emerged in Central Asia. [8] 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. [8] [7] 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. [7]

Disease cycle

The disease cycle begins in early spring, when cool temperatures and abundant moisture promote the release of sexual spores (ascospores) from overwintering structures (pseudothecia) found in the debris at the base of previously-infected trees. [5] 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. [6] Following their dissemination, ascospores are transported to the surfaces of newly-emerged leaves and blossoms by wind and splashing water. [1] The tissue is then penetrated either directly with a germ tube or indirectly using an appressorium, thus initiating a new infection. [7] Shortly after penetration, light-green, irregularly shaped lesions develop on infected foliar tissue and gradually darken, expand, and pucker as the infection progresses. [4] [5] 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. [4] Within 10 days of infection, asexual 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. [4] 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. [5]

The reproductive conidia of Venturia inaequalis erupting through the cuticle of a crabapple leaf. Apple scab SEM.jpg
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. [9] 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. [10] 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. [10] 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. [7]

Management of apple scab

Cultural controls

Cultural controls 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. [3] 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. [3] [5] 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. [4]

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. [5] 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. [11] Benzimidazole fungicides 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 inhibitors. [12] 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. [7]

In 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. [5] 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. [13] As such, alternative management strategies appropriate for organic production systems are currently being developed.[ citation needed ]

Biological Control

Biological control refers to the use of a population of one organism (a biological control agent) to suppress the population of another. [14] 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. [15] [16] 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. [17]

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, 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. [18] Modern genetic work has found that a total of fifteen genes may confer resistance to apple scab. [6] Many of these genes have been isolated from wild Malus spp. populations in East Asia, where a high level of species diversity still remains. [7] 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. [6] 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. [7] Moreover, researchers have observed a breakdown of resistance genes by Venturia populations, posing another significant barrier to the success of this technology. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Black sigatoka</span> Pathogenic fungus

Black sigatoka is a leaf-spot disease of banana plants caused by the ascomycete fungus Mycosphaerella fijiensis (Morelet), also known as black leaf streak. It was discovered in 1963 and named for its similarities with yellow Sigatoka, which is caused by Mycosphaerella musicola (Mulder), which was itself named after the Sigatoka Valley in Fiji. In the same valley an outbreak of this disease reached epidemic proportions from 1912 to 1923.

<i>Uncinula necator</i> Species of fungus

Uncinula necator is a fungus that causes powdery mildew of grape. It is a common pathogen of Vitis species, including the wine grape, Vitis vinifera. The fungus is believed to have originated in North America. European varieties of Vitis vinifera are more or less susceptible to this fungus. Uncinula necator infects all green tissue on the grapevine, including leaves and young berries. It can cause crop loss and poor wine quality if untreated. The sexual stage of this pathogen requires free moisture to release ascospores from its cleistothecia in the spring. However, free moisture is not needed for secondary spread via conidia; high atmospheric humidity is sufficient. Its anamorph is called Oidium tuckeri.

<span class="mw-page-title-main">Black rot (grape disease)</span> Species of fungus

Grape black rot is a fungal disease caused by an ascomycetous fungus, Guignardia bidwellii, that attacks grape vines during hot and humid weather. “Grape black rot originated in eastern North America, but now occurs in portions of Europe, South America, and Asia. It can cause complete crop loss in warm, humid climates, but is virtually unknown in regions with arid summers.” The name comes from the black fringe that borders growing brown patches on the leaves. The disease also attacks other parts of the plant, “all green parts of the vine: the shoots, leaf and fruit stems, tendrils, and fruit. The most damaging effect is to the fruit”.

Pyrenophora teres is a necrotrophic fungal pathogen of some plant species, the most significant of which are economically important agricultural crops such as barley. Toxins include aspergillomarasmine A and related compounds.

<i>Venturia inaequalis</i> Species of fungus

Venturia inaequalis is an ascomycete fungus that causes the apple scab disease.

<i>Podosphaera leucotricha</i> Species of fungus

Podosphaera leucotricha is a plant pathogen that can cause powdery mildew of apples and pears.

<i>Diaporthe helianthi</i> Species of fungus

Diaporthe helianthi is a fungal pathogen that causes Phomopsis stem canker of sunflowers. In sunflowers, Phomopsis helianthi is the causative agent behind stem canker. Its primary symptom is the production of large canker lesions on the stems of sunflower plants. These lesions can eventually lead to lodging and plant death. This disease has been shown to be particularly devastating in southern and eastern regions of Europe, although it can also be found in the United States and Australia. While cultural control practices are the primary method of controlling for Stem Canker, there have been a few resistant cultivars developed in regions of Europe where the disease is most severe.

<i>Ascochyta</i> Genus of fungi

Ascochyta is a genus of ascomycete fungi, containing several species that are pathogenic to plants, particularly cereal crops. The taxonomy of this genus is still incomplete. The genus was first described in 1830 by Marie-Anne Libert, who regarded the spores as minute asci and the cell contents as spherical spores. Numerous revisions to the members of the genus and its description were made for the next several years. Species that are plant pathogenic on cereals include, A. hordei, A. graminea, A. sorghi, A. tritici. Symptoms are usually elliptical spots that are initially chlorotic and later become a necrotic brown. Management includes fungicide applications and sanitation of diseased plant tissue debris.

<i>Phomopsis obscurans</i> Species of fungus

Phomopsis obscurans is a common fungus found in strawberry plants, which causes the disease of leaf blight. Common symptoms caused by the pathogen begin as small circular reddish-purple spots and enlarge to form V-shaped lesions that follow the vasculature of the plant's leaves. Although the fungus infects leaves early in the growing season when the plants are beginning to develop, leaf blight symptoms are most apparent on older plants towards the end of the growing season. The disease can weaken strawberry plants through the destruction of foliage, which results in reduced yields. In years highly favorable for disease development, leaf blight can ultimately lead to the death of the strawberry plants. A favorable environment for the growth and development of the Phomopsis obscurans pathogen is that of high temperature, high inoculum density, a long period of exposure to moisture, and immature host tissue. In the case of disease management, a conjunction of cultural practices is the most effective way of reducing the infection.

<i>Stegophora ulmea</i> Species of fungus

Stegophora ulmea is a foliar disease of elms commonly known as black spot of elm, twig blight, and elm leaf scab. It is characterized by yellow spots that become black spots on the leaves. The pathogen is an ascomycete fungus native to North America. Stegophora ulmea is its teleomorph name. It has two anamorph names, Gloeosporium ulmicolom referring to the macroconidia stage and Cylindrosporella ulmea referring to the microconidia stage. This pathogen was formerly known as Gnomonia ulmea.

Elsinoë mangiferae, common name "mango scab", is also known Denticularia mangiferae or Sphaceloma mangiferae (anamorph). It is an ascomycete plant pathogen native to tropical regions and specific for survival on only one host, the mango. Originally described in 1943 from Florida and Cuba specimens, this pathogen has since spread worldwide and is becoming a pathogen of great concern for the mango industries in Australia and India. The species was first described formally in 1946.

Erysiphe heraclei is a plant pathogen that causes powdery mildew on several species including dill, carrot and parsley.

Pseudopezicula tetraspora is a species of fungus in the family Helotiaceae. Found in North America, it is a plant pathogen that infects grape, causing a disease known as "angular leaf scorch".

<i>Elsinoë ampelina</i> Species of fungus

Elsinoë ampelina is a plant pathogen, which is the causal agent of anthracnose on grape.

This article summarizes different crops, what common fungal problems they have, and how fungicide should be used in order to mitigate damage and crop loss. This page also covers how specific fungal infections affect crops present in the United States.

Peach scab, also known as peach freckles, is a disease of stone fruits caused by the fungi Cladosporium carpophilum. The disease is most prevalent in wet and warm areas especially southern part of the U.S. as the fungi require rain and wind for dispersal. The fungus causes scabbing, lesions, and defoliating on twig, fruit, and leaf resulting in downgrade of peach quality or loss of fruits due to rotting in severe cases.

<span class="mw-page-title-main">Common spot of strawberry</span> Plant fungal disease

Common spot of strawberry is one of the most common and widespread diseases afflicting the strawberry. Common spot of strawberry is caused by the fungus Mycosphaerella fragariae. Symptoms of this disease first appear as circular, dark purple spots on the leaf surface. Mycosphaerella fragariae is very host-specific and only infects strawberry.

<span class="mw-page-title-main">Cherry leaf spot</span> Plant fungal disease

Cherry leaf spot is a fungal disease which infects cherries and plums. Sweet, sour, and ornamental cherries are susceptible to the disease, being most prevalent in sour cherries. The variety of sour cherries that is the most susceptible are the English morello cherries. This is considered a serious disease in the Midwest, New England states, and Canada. It has also been estimated to infect 80 percent of orchards in the Eastern states. It must be controlled yearly to avoid a significant loss of the crop. If not controlled properly, the disease can dramatically reduce yields by nearly 100 percent. The disease is also known as yellow leaf or shothole disease to cherry growers due to the characteristic yellowing leaves and shot holes present in the leaves upon severe infection.

<span class="mw-page-title-main">Pecan scab</span> Fungal disease of pecan trees

Pecan scab is the most economically significant disease of pecan trees in the southeastern United States. Venturia effusa is a fungal plant pathogen that causes pecan scab. The fungus causes lesions and tissue death on pecan twigs, petioles, leaves, nuts and shucks beginning in early spring, with multiple cycles of infection repeating until late summer. Wind and rain spread the fungus to a susceptible host. Control of the disease is achieved by fungicide, sanitation and, in some cases, quarantine.

References

  1. 1 2 "Apple Disease - Apple Scab". Penn State Extension. Retrieved 2020-02-18.
  2. ↑ "Apple scab of apples and crabapples". extension.umn.edu. Retrieved 2020-03-09.
  3. 1 2 3 "Apple Scab". Center for Agriculture, Food and the Environment. 2015-03-06. Retrieved 2020-03-09.
  4. 1 2 3 4 5 Agriculture, Ministry of. "Apple Scab Management in British Columbia - Province of British Columbia". www2.gov.bc.ca. Retrieved 2020-02-02.
  5. 1 2 3 4 5 6 7 8 Gauthier, Nicole (2018). "Apple scab". American Phytopathological Society. Retrieved 2020-02-02.
  6. 1 2 3 4 5 6 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
  7. 1 2 3 4 5 6 7 8 Bowen, Joanna K.; Mesarich, Carl H.; Bus, Vincent G. M.; Beresford, Robert M.; Plummer, Kim M.; Templeton, Matthew D. (2011). "Venturia inaequalis: the causal agent of apple scab". Molecular Plant Pathology. 12 (2): 105–122. doi:10.1111/j.1364-3703.2010.00656.x. ISSN   1364-3703. PMC   6640350 . PMID   21199562.
  8. 1 2 Gladieux, Pierre (2008). "On the Origin and Spread of the Scab Disease of Apple: Out of Central Asia". PLOS ONE. 3 (1): e1455. Bibcode:2008PLoSO...3.1455G. doi: 10.1371/journal.pone.0001455 . PMC   2186383 . PMID   18197265.
  9. ↑ Schumann, Gail (1991). Plant diseases: Their biology and social impact. St. Paul, Minnesota, USA: The American Phytopathological Society. pp. 173–177.
  10. 1 2 Singh, Krishna P. (September 2019). "Aerobiology, epidemiology and management strategies in apple scab: science and its applications". Indian Phytopathology. 72 (3): 381–408. doi:10.1007/s42360-019-00162-5. ISSN   0367-973X. S2CID   204148966.
  11. ↑ Li, B.; Xu, X. (2002). "Infection and Development of Apple Scab (Venturia inaequalis) on Old Leaves". Journal of Phytopathology. 150 (11–12): 687–691. doi:10.1046/j.1439-0434.2002.00824.x. ISSN   1439-0434.
  12. ↑ Köller, Wolfram; Parker, D. M.; Turechek, W. W.; Avila-Adame, Cruz; Cronshaw, Keith (May 2004). "A Two-Phase Resistance Response of Venturia inaequalis Populations to the QoI Fungicides Kresoxim-Methyl and Trifloxystrobin". Plant Disease. 88 (5): 537–544. doi: 10.1094/PDIS.2004.88.5.537 . ISSN   0191-2917. PMID   30812659.
  13. ↑ Wightwick, Adam M.; Salzman, Scott A.; Reichman, Suzanne M.; Allinson, Graeme; Menzies, Neal W. (2012-08-14). "Effects of copper fungicide residues on the microbial function of vineyard soils". Environmental Science and Pollution Research. 20 (3): 1574–1585. doi:10.1007/s11356-012-1114-7. hdl: 11343/283109 . ISSN   0944-1344. PMID   22890509. S2CID   5745969.
  14. ↑ van Lenteren, J.C. (2012). "IOBC Internet Book of Biological Control, version 6" (PDF). International Organisation for Biological Control.
  15. ↑ "Serenade ASO Fungicide | Crop Science US". www.cropscience.bayer.us. Retrieved 2020-04-06.
  16. ↑ "Apple (Malus spp.)-Scab". Pacific Northwest Pest Management Handbooks. 2015-09-11. Retrieved 2020-04-06.
  17. 1 2 Köhl, Jürgen; Scheer, Christian; Holb, Imre J.; Masny, Sylwester; Molhoek, Wilma (2014-10-15). "Toward an Integrated Use of Biological Control by Cladosporium cladosporioides H39 in Apple Scab (Venturia inaequalis) Management". Plant Disease. 99 (4): 535–543. doi: 10.1094/PDIS-08-14-0836-RE . ISSN   0191-2917. PMID   30699552.
  18. ↑ Janick, Jules (2006). "The PRI Apple Breeding Program" (PDF). HortScience. 41: 8–10. doi: 10.21273/HORTSCI.41.1.8 .
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.