Aster yellows
Encyclopedia
Aster yellows is a chronic, systemic plant disease caused by a bacterium-like organism called a phytoplasma
. The aster yellows phytoplasma (AYP) affects 300 species in 38 families of broad-leaf herbaceous plants, primarily in the aster family
. 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. Its economic burden is primarily felt in the carrot
(Daucus carota ssp. sativus) crop industry as well as the nursery industry. There is no cure for plants infected with Aster yellows. 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.
, coreopsis, and purple coneflower. Regarding vegetable crops, onion
, lettuce
, celery
, and carrot are affected with the latter suffering from the greatest losses.
There are a range of characteristic symptoms which vary with the phytoplasma strain, timing of infection, plant species, temperature, age, and/or size of the plant. The symptoms can be mistaken for herbicide damage. 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. Aster yellows does not typically kill perennial host plants.
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.
.
Phytoplasmas are small (0.5-1 micrometer 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. Currently AYP cannot be cultured in cell-free media, making detailed study somewhat more challenging. Interestingly, 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. AYP survives in perennial weeds, ornamentals, and vegetables. Some examples of weed host plants are thistle, wild carrot, dandelion, field daisy, black-eyed Susan, and wide-leafed plaintain.
The vector leafhopper feeds on the phloem of aster yellows infected plants by inserting their 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. At 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 degrees Celsius and 18 days at 20 degrees Celsius, with no symptoms developing at 10 degrees Celsius.
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. 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. 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 degrees Celsius or rainfall will temporarily halt their migration and delay the time of infection. The leafhoppers will then feed all summer long until they migrate back to their overwintering sites in the fall.
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.
In the Western United States, there is no migration of the vector leafhoppers. This allows for transmission of the phytoplasma year round.
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 percent of leafhopper population containing AYP multiplied by the number of leahoppers present per 100 sweeps. 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.
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.
Phytoplasma
Phytoplasma are specialised bacteria that are obligate parasites of plant phloem tissue and transmitting insects . They were first discovered by scientists in 1967 and were named mycoplasma-like organisms or MLOs. They cannot be cultured in vitro in cell-free media...
. The aster yellows phytoplasma (AYP) affects 300 species in 38 families of broad-leaf herbaceous plants, primarily in the aster family
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...
. Symptoms are variable and can include phyllody
Phyllody
Phyllody is the development of floral parts into leafy structures, generally caused by virus or phytoplasma infection such as aster yellows. Evidence suggests that the phytoplasma downregulates a gene involved in petal formation, instead causing leaves or leaflike structures to form...
, virescence, chlorosis, stunting, and sterility of flowers. The aster leafhopper vector, Macrosteles quadrilineatus, moves the aster yellows phytoplasma from plant to plant. Its economic burden is primarily felt in the carrot
Carrot
The carrot is a root vegetable, usually orange in colour, though purple, red, white, and yellow varieties exist. It has a crisp texture when fresh...
(Daucus carota ssp. sativus) crop industry as well as the nursery industry. There is no cure for plants infected with Aster yellows. 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.
Hosts and symptoms
Aster yellows affects a long list of plant species including native plants, ornamentals, weeds, and vegetables crops. The largest family affected is Asteraceae and ornamental plants commonly infected are asters, marigoldsTagetes
Tagetes is a genus of 56 species of annual and perennial mostly herbaceous plants in the sunflower family . The genus is native to North and South America, but some species have become naturalized around the world. One species, T...
, coreopsis, and purple coneflower. Regarding vegetable crops, onion
Onion
The onion , also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion The onion...
, lettuce
Lettuce
Lettuce is a temperate annual or biennial plant of the daisy family Asteraceae. It is most often grown as a leaf vegetable. It is eaten either raw, notably in salads, sandwiches, hamburgers, tacos, and many other dishes, or cooked, as in Chinese cuisine in which the stem becomes just as important...
, celery
Celery
Apium graveolens is a plant species in the family Apiaceae commonly known as celery or celeriac , depending on whether the petioles or roots are eaten: celery refers to the former and celeriac to the latter. Apium graveolens grows to 1 m tall...
, and carrot are affected with the latter suffering from the greatest losses.
There are a range of characteristic symptoms which vary with the phytoplasma strain, timing of infection, plant species, temperature, age, and/or size of the plant. The symptoms can be mistaken for herbicide damage. 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. Aster yellows does not typically kill perennial host plants.
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.
Disease cycle
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 HemipteraHemiptera
Hemiptera is an order of insects most often known as the true bugs , comprising around 50,000–80,000 species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, and others...
.
Phytoplasmas are small (0.5-1 micrometer 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. Currently AYP cannot be cultured in cell-free media, making detailed study somewhat more challenging. Interestingly, AYP has the ability to increase the fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...
and lifespan of their insect vector, thus enhancing the ability of the host to transfer AYP from plant to plant. AYP survives in perennial weeds, ornamentals, and vegetables. Some examples of weed host plants are thistle, wild carrot, dandelion, field daisy, black-eyed Susan, and wide-leafed plaintain.
The vector leafhopper feeds on the phloem of aster yellows infected plants by inserting their straw-like mouthpart, a stylet
Stylet
A stylet is a hard, sharp, anatomical structure found in some invertebrates.For example, the word stylet or stomatostyle, is used for the primitive piercing mouthparts of some nematodes and some nemerteans...
, 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. At 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 degrees Celsius and 18 days at 20 degrees Celsius, with no symptoms developing at 10 degrees Celsius.
Environment
There are hardly any conditions that directly impact the development of Aster yellows, but there are a few indirect factors that 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.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. 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. 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 degrees Celsius or rainfall will temporarily halt their migration and delay the time of infection. The leafhoppers will then feed all summer long until they migrate back to their overwintering sites in the fall.
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.
In the Western United States, there is no migration of the vector leafhoppers. This allows for transmission of the phytoplasma year round.
Management
Aster yellows phytoplasma is a difficult pathogen to control. Currently there is no cure for Aster yellows. Infected plants and weeds should be removed to eliminate that source of the phytoplasma and minimize spread. Unfortunately, this is the only control method that home gardeners have available.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 percent of leafhopper population containing AYP multiplied by the number of leahoppers present per 100 sweeps. 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.
Importance
Aster yellows phytoplasma 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. Carrot crops infected with AYP causes symptoms that make the 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. The deformation of flowers and reproductive structures causes seed not to form. This can be a problem in crops that are grown for seed for replanting purposes, or for consumption, such as coriander or caraway. Root stunting can also result in loss of biannual crops over winter.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.