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]

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]

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]

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]

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]}

• MBG IPM "Aster Yellows"