| Venturia inaequalis | |
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| The reproductive conidia of Venturia inaequalis erupting through the cuticle of a crabapple leaf | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Class: | Dothideomycetes |
| Order: | Pleosporales |
| Family: | Venturiaceae |
| Genus: | Venturia |
| Species: | V. inaequalis |
| Binomial name | |
| Venturia inaequalis | |
| Synonyms | |
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Venturia inaequalis is an ascomycete fungus that causes the apple scab disease. [1]
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 of debate for a long time. Recent genetic studies have revealed a considerable uniformity of the species. [2] In addition, the fungus Spilocaea pyracanthae , a parasite of Pyracantha appeared not to genetically differ from V. inaequalis, being thus a special form of the latter.
The fruiting bodies, ascocarps appear in the form of pseudothecia. They are solitary and embedded into the host plant tissue. A pseudothecium has small dark hairs around its opening, and contains pseudoparaphyses along with asci. The asci contain eight haploid ascospores. The haploid chromosome number of V. inaequalis is seven. [3] [4] [5] [6]
The infection cycle begins in the springtime, when suitable temperatures and moisture promote the release of V. inaequalis ascospores.
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, developing asexually the conidia, that 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.
V. inaequalis overwinters mostly as immature perithecia, where sexual reproduction takes place, 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 ineffective conidial spores in the spring.
Effectors are proteins encoded by pathogens, which act to effect a response from a host cell – often modulating the host immune response. Where a host variety is able to recognise and mount a resistance response to the presence of an effector, the effector is referred to as an Avirulence protein.
Presently, only one effector gene, AvrVg, eliciting a resistance response in Apple has been identified in V. inaequalis. [7]
The Venturia inaequalis pathogen is a fungal organism that produces similar symptoms across a range of woody hosts. These include the common pear (Pyrus spp.), firethorn (Pyracantha spp.), mountain ash (Sorbus spp.), and most notably both commercial apples along with ornamental crabapples (Malus spp.). [8] Symptoms of the infection occur on leaves, fruit, flowers, and young green shoots. Foliar symptoms begin to occur in the early spring around budbreak and mainly present as light green lesions that progress to an olive-brown color with a velvety texture due to conidia formation as time passes. These large scab-like lesions can warp the leaf's shape and can eventually lead to defoliation. Lesions formed by primary infection via ascospores tend to have more distinct borders when compared to lesions as a result of a secondary infection cycle via conidia. [9] Young fruit, often infected by foliar conidia, can also display similar symptoms to infected leaves. In this case the lesions progress to bare, brown and corky spots. The apple skin and flesh can split open as the fruit enlarges, though young fruits often prematurely drop. Mature fruits are more resistant to infection and only form small, black 'pin-head scabs' which might not even be noticeable until after storage. [9]
Economic loses due to apple scab over an extended period of time far outweigh the impact of any other apple pathogen. [9] Historically one can find examples the symptoms of V. inaequalis in paintings as far back as the fifteenth century. [10] Fruit production can be limited due to defoliation while limiting fruit bud creation in the following year. [11] While the disease can cause total crop loss in optimal conditions without management, the main economic impact is due to the reduction in both size and marketable quality of the fruit. This issue is further compounded by the fact that apple cultivars with a high market share, Pink Lady for example, are susceptible to the pathogen while more resistant varieties are less well known or desired by consumers. [12] At an industrial production level these threats are only reliably mitigated by expensive, labor-intensive and repeated spraying of pesticides. [11] In regions of apple production where the year's weather is conducive to infection up to 70% of the pesticides applied are used to control for the effects of apple scab. [9] While the main host of economic interest are various apple cultivars the other host species are still affected by the pathogen. Considering most of these are commercially used as ornamental species, flowering crab apples for example, the importance of the pathogen shifts to be a more aesthetic nuisance. In the academic sense V. inaequalis has proved invaluable in the realm of genetic research of pathogenicity. It is one of the first ascomycete fungi to undergo genetic analysis and continues to be useful in that field. The fungi's mechanistic similarity to obligate parasites while still being able to be cultured in media has led to its repeated use in the study of the genes related to pathogenicity. [10]
Protection from initial inoculation, either via sexual ascospores or asexual conidia, with fungicide is the main form of pathogen management. Spray schedules should be created with plant and pathogen development timings in mind and thus should begin with an initial spray at budswell and repeated in 10 to 14 day intervals. Specifics of the spray intervals should also be determined by considering what spray is used, weather patterns (mainly rain and moisture), host species, host growth patterns, and the amount of fungal inoculum present. [13] Cultural practices can also be used to limit or prevent apple scab infection. Orchard design and planting patterns focusing on increased aeration are important in ensuring that susceptible tissues dry prior to initial infection. Proper pruning can further aid this effect. [9] Another form of cultural management is the implementation of proper sanitation. Fallen infected leaves in the fall should be collected and destroyed to reduce the total inoculum able to overwinter and infect the following year. [14] Primary infection is mainly caused by the ascospores that overwinter in the fallen debris and the density of these spores in the spring is directly related to the speed and intensity of an apple scab outbreak. [11] In addition to direct removal of fallen infected tissues, mulching of fallen leaves into the soil to destroy the ascospores is advisable. Application of nitrogenous fertilizer to speed up this process can be done immediately prior to leaf fall or as a ground application. [11] More proactive methods of management exist in the form of genetically resistant cultivars and species. Apple cultivars such as Enterprise, Goldrush, Liberty, Jonafree, Macfree, Prima, Pristine, Redfree, and Sir Prize are resistant to apple scab infection along with multiple varieties of ornamental crab apples. [14] Breeding of resistant lines has seen minimal acceptance in the Americas but more widespread success in Europe, though the durability of this resistance is always a concern. [12] More recently, biofungicidal methods of control have emerged in studies with some promise being shown by the organism Microsphaeropsis ochracea in reduction of initial ascospore inoculum of up to 70–80 percent. While still requiring a fungicide application, the use of the proposed mycoparasite allows for a delayed and lower volume use of chemicals. [12]
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.
Apple scab is a common disease of plants in the rose family (Rosaceae) that is caused by the ascomycete fungus Venturia inaequalis. 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. 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. 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.
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.
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.
Leptosphaeria coniothyrium is a plant pathogen. It can be found around the world.
Monilinia laxa is a plant pathogen that is the causal agent of brown rot of stone fruits.
Podosphaera leucotricha is a plant pathogen that can cause powdery mildew of apples and pears.
Podosphaera macularis is a plant pathogen infecting several hosts including chamomile, caneberrie, strawberries, hop, hemp and Cineraria. It causes powdery mildew of hops.
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.
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.
Glomerella cingulata is a fungal plant pathogen, being the name of the sexual stage (teleomorph) while the more commonly referred to asexual stage (anamorph) is called Colletotrichum gloeosporioides. For most of this article the pathogen will be referred to as C. gloeosporioides. This pathogen is a significant problem worldwide, causing anthracnose and fruit rotting diseases on hundreds of economically important hosts.
Elsinoë ampelina is a plant pathogen, which is the causal agent of anthracnose on grape.
Dibotryon morbosum or Apiosporina morbosa is a plant pathogen, which is the causal agent of black knot. It affects members of the Prunus genus such as; cherry, plum, apricot, and chokecherry trees in North America. The disease produces rough, black growths that encircle and kill the infested parts, and provide habitat for insects.
Ascochyta blights occur throughout the world and can be of significant economic importance. Three fungi contribute to the ascochyta blight disease complex of pea. Ascochyta pinodes causes Mycosphaerella blight. Ascochyta pinodella causes Ascochyta foot rot, and Ascochyta pisi causes Ascochyta blight and pod spot. Of the three fungi, Ascochyta pinodes is of the most importance. These diseases are conducive under wet and humid conditions and can cause a yield loss of up to fifty percent if left uncontrolled. The best method to control ascochyta blights of pea is to reduce the amount of primary inoculum through sanitation, crop-rotation, and altering the sowing date. Other methods—chemical control, biological control, and development of resistant varieties—may also be used to effectively control ascochyta diseases.
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.
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.
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.
Spilocaea oleaginea is a deuteromycete fungal plant pathogen, the cause of the disease olive peacock spot, also known as olive leaf spot and bird's eye spot. This plant disease commonly affects the leaves of olive trees worldwide. The disease affects trees throughout the growing season and can cause significant losses in yield. The disease causes blemishes on the fruit, delays ripening, and reduces the yield of oil. Defoliation and in severe cases, twig death, can occur, and the disease can have long-term health effects on the trees.
Botrytis squamosa is a fungus that causes leaf blight on onion that is distinctly characterized by the two stages – leaf spotting followed by blighting. The pathogen is an ascomycete that belongs to the family Sclerotiniaceae in the order Helotiales. The lesions start out as whitish streaks and take on a yellow tinge as they mature. They cause yield losses up to 30%. This fungus is endemic to the USA and has also been reported in Europe, Asia, and Australia. Typical management of this disease includes chemical fungicides with significant efforts being made to establish a means of biological control.
| Authority control databases: National |
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