Aster yellows

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Aster yellows
Aster yellows symptoms carrot.jpg
Symptom range: healthy carrot is on right, seriously damaged carrot on left
Causal agents Phytoplasmas
HostsSeveral, see text
VectorsAster leafhopper ( Macrosteles quadrilineatus )

Aster yellows is a chronic, systemic plant disease caused by several bacteria called phytoplasma. [1] The aster yellows phytoplasma (AYP) affects 300 species in 38 families of broad-leaf herbaceous plants, primarily in the aster family, as well as important cereal crops such as wheat and barley. Symptoms are variable and can include phyllody, virescence, chlorosis, stunting, and sterility of flowers. The aster leafhopper vector, Macrosteles quadrilineatus, moves the aster yellows phytoplasma from plant to plant. [2] Its economic burden is primarily felt in the carrot (Daucus carota ssp. sativus) crop industry, as well as the nursery industry. No cure is known for plants infected with aster yellows. [3] Infected plants should be removed immediately to limit the continued spread of the phytoplasma to other susceptible plants. However, in agricultural settings such as carrot fields, some application of chemical insecticides has proven to minimize the rate of infection by killing the vector. [2]

Contents

Hosts and symptoms

Witches'-broom of an infected carrot Aster yellows symptoms carrot 2.jpg
Witches'-broom of an infected carrot

Aster yellows affects a long list of plant species including native plants, annual flowering plants, ornamentals, weeds, and vegetables crops. The largest family affected is the Asteraceae, and ornamental plants commonly infected are asters, marigolds, coreopsis, sunflowers, [4] and purple coneflower. [2] Regarding vegetable crops, onion, lettuce, celery, and carrot are affected with the latter suffering the greatest losses.[ citation needed ]

The range of characteristic symptoms varies by the phytoplasma strain, timing of infection, plant species, temperature, age, and size of the plant. [5] The symptoms can be mistaken for herbicide damage or virus symptoms. They include vein clearing until the entire leaf becomes chlorotic, stunting, deformation, virescence (greening of flowers), phyllody (development of leaf-like flower petals), reddening of foliage, reduced root system, and sterility. [2] [3] Aster yellows does not typically kill perennial host plants. The symptoms caused by the disease are exacerbated by hot climates while some plants in cooler climates may be asymptomatic. [6]

Characteristic symptoms specific to the carrot include initial vein clearing and chlorosis, followed by production of many adventitious shoots, with the tops looking like a witches’-broom. The internodes of such shoots are short as are the leaf petioles. Young leaves are smaller and dry up while the petioles of older leaves twist and break off. Any remaining older leaves turn bronze or red late in the season. Floral parts are deformed and roots are smaller, abnormally shaped, and have woolly secondary roots. The carrot roots are predisposed to soft rots in the field and storage and taste unpleasant to the consumer. [7]

Disease cycle

Aster leafhopper Aster leafhopper.jpg
Aster leafhopper

The aster yellows disease is caused by the aster yellows phytoplasma (AYP) which is a phloem-limited, bacterium-like organism and is vectored by the aster leafhopper, Macrosteles quadrilineatus, a phloem-feeding insect of the order Hemiptera.[ citation needed ]

Phytoplasmas are small (0.5-1 μm in diameter) prokaryotes that reproduce by division or budding in the phloem sieve cells of the host plants, as well as the bodies of their leafhopper vectors. [2] Currently, AYP cannot be cultured in cell-free media, making detailed study somewhat more challenging. AYP has the ability to increase the fecundity and lifespan of their insect vector, thus enhancing the ability of the host to transfer AYP from plant to plant. [8] The pathogen will overwinter in either perennial weeds, ornamentals, or vegetables or within the leafhopper vector. Some examples of weed host plants are thistle, wild carrot, dandelion, field daisy, black-eyed Susan, and wide-leafed plaintain. [7]

The vector leafhopper feeds on the phloem of aster yellows-infected plants by inserting its straw-like mouthpart, a stylet, into the cell and extracting it. Once the phytoplasma is acquired, an incubation period follows in which it multiplies within the leafhopper and then moves to the salivary glands. The pathogen cannot be transferred by the vector until 10–12 days of incubation have passed. [4] After this point, the phytoplasma can be transmitted to a new host through the saliva as the leafhopper feeds. Within 8–24 hours after inoculation, the phytoplasma moves out of the leaf into the host plant phloem. Cells adjacent to the phloem enlarge and die while surviving cells begin to divide, but soon die, too. Surrounding cells in the region of the necrotic area begin to divide and enlarge, producing abnormal sieve elements, while the phloem elements within the necrotic areas degenerate and collapse. Infected plants usually show symptoms after 8–9 days at 25 °C and 18 days at 20 °C, with no symptoms developing at 10 °C. [7]

Environment

Hardly any conditions directly affect the development of aster yellows, but a few indirect factors strongly influence the rate of transmission by the leafhopper. Conditions that favor movement and spread of the leafhopper and encourage feeding assist in the spread of the phytoplasma.[ citation needed ]

Transcontinental migration begins in the spring when the prevailing winds and jet streams help carry the leafhoppers from their overwintering sites in the South to the Midwest. [2] Upon arrival in the Midwest, they begin feeding. The leafhopper may have migrated into the region already carrying the phytoplasma, which it could have acquired from infected plants along the migration or while still in the South. [2] The leafhopper could have also arrived not yet carrying the phytoplasma. If this is the case, it could feed on perennial weeds that are infected to acquire AYP. Weather conditions greatly influence leafhopper flight because they are poor flyers. Temperatures below 15 °C or rainfall temporarily halt their migration and delay the time of infection. [9] The leafhoppers then feed all summer until they migrate back to their overwintering sites in the fall.[ citation needed ]

Weather conditions of the region also greatly influence leafhopper feeding patterns. If conditions are hot and dry plants do not appear as lush and nutrient-rich to the phloem-feeding leafhopper, whereas seasons with abundant rainfall allow the plants to have much more lush growth. This means that hot and dry conditions are less conducive to the spread of aster yellows than times of abundant rainfall. [9]

In the Western United States, no migration of the vector leafhoppers occurs. This allows for transmission of the phytoplasma year round. [9]

Management

Phyllody on a purple coneflower Phyllody on Coneflower with aster yellows.jpg
Phyllody on a purple coneflower

Aster yellows phytoplasma is a difficult pathogen to control, given its wide host range. Over 300 plant species are susceptible to AYP. [10] Currently, no cure for aster yellows is known. [3] Infected plants and weeds should be removed to eliminate that source of the phytoplasma and minimize spread. [11] Unfortunately, this is the only control method that home gardeners have available.[ citation needed ]

On an agricultural level, speaking specifically about carrots, some methods can be used to manage the leafhopper populations in an attempt to control AYP spread. The aster yellows index (AYI) can be used to determine when to apply chemical controls. The AYI equals the percentage of leafhopper population containing AYP multiplied by the number of leafhoppers present per 100 sweeps. [2] The resulting number can determine when to apply insecticides based on how susceptible the crop or cultivar is to leafhopper feeding. For highly susceptible crops or cultivars, an AYI of 50 indicates the need for application, while for intermediate crops or cultivars the AYI is 75 and for crops or cultivars relatively resistant to economically harmful symptoms the AYI is 100. [2]

Importance

AYP is an economically important plant pathogen both in agricultural and nursery industries. A 25% reduction in carrot yield is common, with losses reaching 80% on occasion. [7] AYP causes symptoms that make the infected carrots unmarketable. In processed carrots, the presence of 15% of aster yellows-infected carrots results in a rejection of the entire product due to their distasteful flavor. [7] The deformation of flowers and reproductive structures causes seed not to form. This can be a problem in crops grown for seed for replanting purposes, or for consumption, such as coriander or caraway. [9] Root stunting can also result in loss of biannual crops over winter. [9]

Similar problems arise in the nursery industry. Homeowners and landscapers purchasing plants do not want to buy an aster flower that is misshapen and has the potential to cause the spread of AYP to other plants. This makes it critical for nurseries to monitor their plants to prevent initial infection of the phytoplasma. Insecticides can be used to limit leaf hopper feeding on nursery stock and as soon as infected plants are seen, they must be removed.[ citation needed ]

Related Research Articles

<i>Phytoplasma</i> Genus of bacteria

Phytoplasmas are obligate intracellular parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-like organisms. Since their discovery, phytoplasmas have resisted all attempts at in vitro culture in any cell-free medium; routine cultivation in an artificial medium thus remains a major challenge. Phytoplasmas are characterized by the lack of a cell wall, a pleiomorphic or filamentous shape, a diameter normally less than 1 μm, and a very small genome.

Barley yellow dwarf (BYD) is a plant disease caused by the barley yellow dwarf virus (BYDV), and is the most widely distributed viral disease of cereals. It affects the economically important crop species barley, oats, wheat, maize, triticale and rice.

<span class="mw-page-title-main">Phyllody</span> Abnormal development of floral parts into leafy structures

Phyllody is the abnormal development of floral parts into leafy structures. It is generally caused by phytoplasma or virus infections, though it may also be because of environmental factors that result in an imbalance in plant hormones. Phyllody causes the affected plant to become partially or entirely sterile, as it is unable to produce normal flowers.

<span class="mw-page-title-main">Curly top</span> Viral disease that affects many crops

Curly top is a viral disease that affects many crops. This disease causes plants to become smaller in size, have shriveled petals and leaves, and are twisted and pulled out of shape. They are often caused by curtoviruses, members of the virus family Geminiviridae. This disease is important in western United States, such as California, Utah, Washington, and Idaho.

<span class="mw-page-title-main">Beet leafhopper</span> Species of insect

The beet leafhopper, also sometimes known as Neoaliturus tenellus, is a species of leafhopper which belongs to the family Cicadellidae in the order Hemiptera.

<i>Albugo</i> Genus of plant-parasitic oomycetes

Albugo is a genus of plant-parasitic oomycetes. Those are not true fungi (Eumycota), although many discussions of this organism still treat it as a fungus. The taxonomy of this genus is incomplete, but several species are plant pathogens. Albugo is one of three genera currently described in the family Albuginaceae, the taxonomy of many species is still in flux.

<i>Ditylenchus dipsaci</i> Species of flowering plant

Ditylenchus dipsaci is a plant pathogenic nematode that primarily infects onion and garlic. It is commonly known as the stem nematode, the stem and bulb eelworm, or onion bloat. Symptoms of infection include stunted growth, discoloration of bulbs, and swollen stems. D. dipsaci is a migratory endoparasite that has a five-stage lifecycle and the ability to enter into a dormancy stage. D. dipsaci enters through stomata or plant wounds and creates galls or malformations in plant growth. This allows for the entrance of secondary pathogens such as fungi and bacteria. Management of disease is maintained through seed sanitation, heat treatment, crop rotation, and fumigation of fields. D. dipsaci is economically detrimental because infected crops are unmarketable.

<i>Beet curly top virus</i> Species of virus

Beet curly top virus (BCTV) is a pathogenic plant virus of the family Geminiviridae, containing a single-stranded DNA. The family Geminiviridae consists of nine genera based on their host range, virus genome structure, and type of insect vector. BCTV is a Curtovirus affecting hundreds of plants. The only known vector is the beet leafhopper, which is native to the Western United States.

<i>Impatiens necrotic spot orthotospovirus</i> Species of virus

Impatiens necrotic spot orthotospovirus(INSV) is a plant pathogenic virus of the order Bunyavirales. It was originally believed to be another strain of Tomato spotted wilt virus, but genetic investigations revealed them to be separate viruses. It is a negative-strand RNA virus which has a tripartite genome. It is largely spread by the insect vector of the western flower thrips. The virus infects more than 648 species of plants including important horticultural and agricultural species such as fuchsia, tomato, orchids, and lettuce (especially romaine). As the name implies, the main symptom on plants is necrotic spots that appear on the leaves. The INSV virus infects by injecting the RNA the virus contains into the cell which then starts using the cell resources to transcribe what the virus RNA states. Viral infection can often result in the death of the plant. The disease is mainly controlled by the elimination of the western flower thrip vector and by destroying any infected plant material.

<span class="mw-page-title-main">Elm yellows</span> Bacterial disease of elm trees

Elm yellows is a plant disease of elm trees that is spread by leafhoppers or by root grafts. Elm yellows, also known as elm phloem necrosis, is very aggressive, with no known cure. Elm yellows occurs in the eastern United States, and southern Ontario in Canada. It is caused by phytoplasmas which infect the phloem of the tree. Similar phytoplasmas, also known confusingly as 'Elm yellows', also occur in Europe. Infection and death of the phloem effectively girdles the tree and stops the flow of water and nutrients. The disease affects both wild-growing and cultivated trees.

<span class="mw-page-title-main">Grapevine yellows</span> Diseases associated to phytoplasmas

Grapevine yellows (GY) are diseases associated to phytoplasmas that occur in many grape growing areas worldwide and are of still increasing significance. The most important grapevine yellows is flavescence dorée.

Texas Phoenix palm decline, or lethal bronzing, is a plant disease caused by a phytoplasma, Candidatus Phytoplasma palmae. It takes its name from the state it was first identified in and the palm genus, Phoenix, upon which it was first identified. It is currently found in parts of Florida and Texas.

<span class="mw-page-title-main">Sugarcane grassy shoot disease</span> Phytoplasma (bacterial) disease

Sugarcane grassy shoot disease (SCGS), is associated with 'Candidatus Phytoplasma sacchari' which are small, pleomorphic, pathogenic mycoplasma that contribute to yield losses from 5% up to 20% in sugarcane. These losses are higher in the ratoon crop. A higher incidence of SCGS has been recorded in some parts of Southeast Asia and India, resulting in 100% loss in cane yield and sugar production.

The Citrus stubborn disease is a plant disease affecting species in the genus Citrus. Spiroplasma citri, a Mollicute bacterium species, is the causative agent of the disease. It is present in the phloem of the affected plant. Originally discovered transmitted by several leafhoppers including Circulifer tenellus and Scaphytopius nitridus in citrus-growing regions of California, it is now spread by the same hoppers in Arizona and Circulifer haematoceps in the Mediterranean region.

<span class="mw-page-title-main">Cherry X Disease</span> Plant disease

Cherry X disease also known as Cherry Buckskin disease is caused by a plant pathogenic phytoplasma. Phytoplasmas are obligate parasites of plants and insects. They are specialized bacteria, characterized by their lack of a cell wall, often transmitted through insects, and are responsible for large losses in crops, fruit trees, and ornamentals. The phytoplasma causing Cherry X disease has a fairly limited host range mostly of stone fruit trees. Hosts of the pathogen include sweet cherry, sour cherry, choke cherry, peaches, nectarines, almonds, clover, and dandelion. Most commonly the pathogen is introduced into economical fruit orchards from wild choke cherry and herbaceous weed hosts. The pathogen is vectored by mountain and cherry leafhoppers. The mountain leafhopper vectors the pathogen from wild hosts to cherry orchards but does not feed on the other hosts. The cherry leafhopper feeds on cherry trees and can transmit the disease from cherry orchards to peach, nectarine, and other economic crops. The Saddled Leafhopper is a vector of the disease in peaches. Control of Cherry X disease is limited to controlling the spread, vectors, and weed hosts of the pathogen. Once the pathogen has infected a tree it is fatal and removal is necessary to stop it from becoming a reservoir for vectors.

<i>Macrosteles quadrilineatus</i> Species of true bug

Macrosteles quadrilineatus, the aster leafhopper or six-spotted leafhopper, is a leafhopper species in the genus Macrosteles, found in the United States. It is the vector of aster yellows disease, which affects various vegetable plants, weeds and ornamental plants.

<i>Candidatus</i> Phytoplasma fraxini Species of bacterium

CandidatusPhytoplasma fraxini is a species of phytoplasma, a specialized group of bacteria which lack a cell wall and attack the phloem of plants. This phytoplasma causes the diseases ash yellows and lilac witches' broom.

Papaya Bunchy Top Disease was first discovered in 1931 in Puerto Rico. Early on, the identity of the pathogen was highly contested due to the inability of isolating it; thus Koch’s postulates could not be fulfilled. Scientists have previously believed that Papaya Bunchy Top Disease was caused by a virus, a mycoplasma-like organism (MLO), or a phytoplasma, but these possible pathogens have since been disproven. Since the identity of the pathogen was unknown, all diagnoses were given solely based on a list of commonly associated symptoms. Through sequencing and microscopy, scientists identified the pathogen to be a part of the genus Rickettsia in 1996. The bacterium is described as being rod-shaped, small, gram-negative, and laticifer-inhibiting. Rickettsia causes diseases in animals, such as typhus and spotted fever, as well as in other plants, such as phony disease of peach and almond leaf scorch. Papaya Bunchy Top is found throughout the American tropics and has been economically important due to its major impact on fruit production. There is little information about the current economic impact.

<span class="mw-page-title-main">Viral diseases of potato</span>

Viral diseases of potato are a group of diseases caused by different types of Viruses that affect potato crops worldwide and, although they do not affect human or animal health since they are viruses that only infect vegetables, they are a source of great economic losses annually. About 28 viruses have been reported infecting potato crops. However, potato virus X (PVX), potato virus Y (PVY), and potato leafroll virus (PLRV) are the most important viruses worldwide. Some others are of economic importance only in some regions. Such is the case of potato virus M (PVM) in some Asian and European countries.

References

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