Aster yellows of garlic
C. Rosen, UMN Extension
Figure 1. Yellowing of AY infected garlic
Aster yellows (AY) is caused by a small bacteria called a phytoplasma. The pathogen 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 reported 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 senescence of garlic. Because the phytoplasma infects every part of the plant, garlic seed from infected plants is highly likely to be infected. Growers that planted seed from a crop infected with AY in 2012 have reported very poor emergence in the spring of 2013. Infected plants that did emerge developed AY symptoms. Aster leafhopper populations in 2013 have been observed to be lower than average and reports of AY in garlic and other crops have been low.
C. Rosen, UMN Extension
Figure 2. Wrapper discoloration on AY infected garlic bulbs.
A laboratory test must be performed to determine if a plant is infected with the AY phytoplasma. Plants can be tested at the UMN Plant Disease Diagnostic Clinic. At early stages of infection plants may show no symptoms. The symptoms below were observed on garlic plants infected with AY in 2012. 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
Aster yellows is caused by a phytoplasma, a small bacteria that lives only within the vascular system of a plant or within the leafhopper that vectors it from plant to plant. Once a plant is infected, the AY phytoplasma moves systemically through the plant, infecting every part from the roots through the flowers. The pathogen affects the plants 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 is released into the new plant starting 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 percentage 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.
Figure 3. Aster Leafhopper adult and nymph.
Once a plant is infected with AY, there is no way to cure it. Cloves from AY infected garlic should not be used as seed. At this time it is unknown if there are any garlic varieties resistant to AY.
Control perennial weeds. These plants may become infected with the AY phytoplasma and 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 and AY infection of crops in these structures will have some protection from AY infection.
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 where populations are high, insecticides, row covers or reflective mulches can be used to reduce disease. Leafhopper populations can be monitored by sweeping a 15 inch diameter sweep net in an 1800 arc through the top 6-8 inches of the crop 20 times in 5 different locations in the field (a total of 100 sweeps) and counting the aster leafhoppers captured. A threshold of 20 aster leafhoppers per 100 sweeps has been established for AY susceptible carrot varieties. When the threshold is reached insecticide sprays are recommended. There is currently no established threshold for leafhopper populations in garlic.
Plants can be protected from aster leafhoppers with repeat applications of a pyrethroid insecticide that is registered for use on garlic (i.e. Ambush, Mustang, Pounce, Warrior). Applications must be repeated according to label instructions until 2-3 weeks prior to harvest on crops to be sold as food and until harvest on seed crops. Some chance of infection remains even with insecticide applications.