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Aster yellows of garlic

Quick facts

  • Small bacteria called phytoplasma cause aster yellows.
  • There must be a laboratory test to determine if a plant has the aster yellows phytoplasma.
  • At early stages of infection, plants may show no symptoms.
  • Control perennial weeds to prevent infection.
  • Protect plants from aster leafhoppers with repeat applications of a pyrethroid insecticide that is available for use on garlic.
  • Light colored or reflective mulches will disorient aster leafhoppers and can reduce feeding on the crop.

Phytoplasma, a small bacteria, cause aster yellows (AY). AY can infect over 350 plants including many common vegetables, flowers and weeds. 

In 2012, the first report of aster yellows infecting garlic in the USA was in Minnesota. Unusually high numbers of aster leafhoppers had migrated into Minnesota in 2012. Many of these leafhoppers were carrying the AY phytoplasma. Aster yellows can reduce bulb development and result in premature aging of garlic. Because the phytoplasma infects every part of the plant, garlic seed from infected plants is highly likely have AY.

Growers that planted seed from a crop infected with AY in 2012 reported very poor emergence in the spring of 2013. Infected plants that did emerge developed AY symptoms. Aster leafhopper populations in 2013 were lower than average and reports of AY in garlic and other crops have been low. 

Yellowing garlic plants in field from aster yellows infection
Garlic bulb with brown lines from aster yellows infection
Brown lines from aster yellows

Identifying aster yellows symptoms

There must be a laboratory test to determine if a plant has the AY phytoplasma. Test plants at the UMN Plant Disease Diagnostic Clinic.

At early stages of infection, plants may show no symptoms. Garlic plants infected with AY in 2012 had the below symptoms. Symptoms may vary by variety.

  • Premature yellowing and dieback of garlic plants
  • Small, often soft bulbs
  • Dark streaking or discoloration of wrapper
  • Unusual smell to bulb
  • Very poor emergence of infected seed
    Discolored garlic bulbs from aster yellows infection, with quarter coin for scale
    Wrapper discoloration

What causes aster yellows

Aster leafhopper nymph insect on leaf, close-up
Aster leafhopper nymph

Phytoplasma causes AY. It is a small bacterium that lives only within the vascular system of a plant or within the leafhopper that brings it from plant to plant. The AY phytoplasma moves systemically through the plant, infecting every part from the roots through the flowers. The pathogen affects the plant’s growth, development and ability to store nutrients.

When adult and nymph aster leafhoppers feed on plants infected with AY, they ingest some of the phytoplasma along with the plant sap. In a minimum of two weeks, the phytoplasma moves through the insect's gut into the salivary glands. At this point, when the leafhopper feeds, a small amount of phytoplasma releases into the plant. This starts a new infection. The leafhopper will transmit the pathogen every time it feeds for the rest of its life.

Only a few aster leafhoppers overwinter in Minnesota as eggs. The majority of the aster leafhopper population overwinters in southern states, feeding on grain crops and other plants. Weather systems carry the insects north early in the growing season. Only a small number of these leafhoppers carry the AY phytoplasma. The AY phytoplasma will not survive in plant debris of infected plants, but can survive in the crown and roots of infected perennial plants.

Green aster leafhopper adult insect on leaf, close-up
Aster leafhopper adult

Preventing and managing the disease

Once AY infects a plant, there is no way to cure it. Do not use cloves from AY infected garlic as seed. At this time it is unknown if there are any garlic varieties resistant to AY.

Control perennial weeds. If infected by the AY phytoplasma, these plants allow the pathogen to survive in the field from one year to the next.

Light colored or reflective mulches will disorient aster leafhoppers and can reduce feeding on the crop. Light reflected off the plastic covering of low tunnels or high tunnels will affect leafhoppers in the same way. These structures will have some protection from AY infection of crops

Covering the crop with floating row covers for the entire season will prevent leafhoppers from reaching the crop and transmitting the disease. This strategy may be useful to protect a seed crop.

Aster leafhopper populations vary in size from year to year. In addition, the percent of the population carrying the AY phytoplasma varies from year to year. In years when populations are high, use insecticides, row covers or reflective mulches to reduce disease.

Monitor leafhopper populations by casting a 15-inch diameter sweep net in an 1800 arc through the top six to eight inches of the crop 20 times in five different locations in the field ( 100 sweeps) and counting the aster leafhoppers captured. AY susceptible carrot varieties have a threshold of 20 aster leafhoppers per 100 sweeps. Use insecticide sprays when the crop reaches the threshold. There is currently no established threshold for leafhopper populations in garlic.

Protect plants from aster leafhoppers with repeat applications of a pyrethroid insecticide that is available for use on garlic (i.e. Ambush, Mustang, Pounce, Warrior). Repeat applications according to label instructions until two to three weeks prior to harvest on food and seed crops. Some chance of infection remains even with insecticide applications.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Michelle Grabowski

Reviewed in 2018

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