Aster yellows is a phytoplasma disease that affects over 300 plant species, including many ornamental and vegetable crops. The disease is spread by insect vectors and can cause significant economic losses in commercial plantings. Identifying aster yellows early and taking measures to prevent infection are the best ways to manage this disease.
What Causes Aster Yellows
Aster yellows is caused by a phytoplasma, a bacterium that lacks cell walls and infects the phloem tissue of plants. The phytoplasma that causes aster yellows is transmitted by insects in the order Hemiptera, primarily leafhoppers. Once a plant is infected, the phytoplasma invades the phloem tissues and disrupts the normal hormone balance and nutrient flow in the plant.
Some common insect vectors that can spread aster yellows include:
- Leafhoppers such as the six-spotted leafhopper, aster leafhopper, and potato leafhopper
- Plant hoppers such as the beet leafhopper
- Cicadas
- Spittle bugs
These insects feed on the sap from the phloem tissues of an infected plant. When they move to an uninfected plant, they can transmit the phytoplasma that causes aster yellows. The phytoplasma essentially hitchhikes in the insect’s body until it can be transmitted to a new host plant.
Recognizing Aster Yellows Symptoms
Some of the most characteristic symptoms of aster yellows include:
Abnormal Growth
- Stunting and smaller leaves
- Distorted shoots with fasciation (ribbon-like stems)
- Witches’ brooms or dense masses of shoots and leaves
- Abnormal flower structures and sterile flowers that do not produce seeds
Yellowing
- Overall yellowing or chlorosis of leaves, shoots, and flowers
- Yellowing in spots, stripes, or patterns throughout the plant
Other Changes
- Premature budding and flowering
- Red or purple leaf discoloration
- Excessive branching and tufted growth at branch ends
- Defoliation
- Plant death
The symptoms can vary widely between plant species and individual specimens. In general, aster yellows causes abnormal growth patterns, distorted foliage and shoots, chlorosis, and affects flowering and seed production. The symptoms are systemic, affecting the entire plant.
Symptoms on Common Host Plants
- Lettuce: Stunting, chlorosis, small heads that do not form properly
- Carrots: Hairy, rough roots with off tastes; shoots are chlorotic and brittle
- Potatoes: aerial tubers, leaf curling and chlorosis
- Asters: Witches’ brooms, distorted leaves, green petals
- Zinnias: Green streaks and malformed flowers, sterile blooms
How to Identify Aster Yellows
Aster yellows can sometimes be difficult to diagnose conclusively, as the symptoms overlap with nutrient deficiencies, other diseases, and environmental stresses. Here are some tips for identifying aster yellows:
- Look at the overall symptom pattern on the plant – systemic abnormal growth and yellowing points to aster yellows.
- Check for insect vectors – are leafhoppers, planthoppers, or other phloem-feeding insects present? Their presence helps support an aster yellows diagnosis.
- Compare to healthy plants – differences in growth, flowering, and leaf color indicate aster yellows.
- Monitor symptom progression – early stunting and virescence followed by chlorosis points to this disease.
- Consider testing – PCR, ELISA, or other tests can detect the phytoplasma in infected plant tissues.
- Review plant history – have other susceptible plants become infected in the area recently?
- Rule out lookalikes – check nutrient levels and exclude other potential causes like viruses, root rot, etc.
PCR Testing
Polymerase chain reaction (PCR) tests can detect the presence of the aster yellows phytoplasma in plant sap. PCR testing is the most reliable way to conclusively diagnose aster yellows. Samples of symptomatic shoot tips and leaves can be sent to a plant diagnostic lab for PCR testing.
ELISA Testing
ELISA (enzyme-linked immunosorbent assay) serological tests can also detect aster yellows phytoplasmas from plant sap. ELISA is less sensitive than PCR testing but can provide supporting evidence of infection.
How to Treat Aster Yellows
Unfortunately, there are no chemical treatments available to eliminate or cure aster yellows infections in plants. Once infected, plants cannot be cured. However, some cultural practices may help reduce disease severity:
- Improve plant vigor – provide adequate water and fertilizer based on soil test results to keep plants as healthy as possible.
- Prune out infection – remove symptomatic shoots well below affected tissues to potentially help reduce spread in the plant. Sterilize pruners between each cut.
- Roguing – pull up and destroy severely infected plants to prevent disease spread.
- Protect new growth – new foliage and shoots may be free of infection if protected from insect vectors. Cover young growth with row covers or apply insecticides.
- Crop rotation – avoid planting related susceptible species in the same location the following year.
- Weed control – remove weed hosts around crop fields that could harbor the pathogens.
- Resistant varieties – plant resistant or tolerant varieties when available. Certain asters, zinnias, carrots, lettuces, etc. have better resistance.
- Sanitation – disinfect benches, tools, and equipment regularly in greenhouses and nurseries.
Even with good management, reinfection is common once aster yellows becomes established in an area. Preventive measures are essential.
How to Prevent Aster Yellows
Preventing the introduction and spread of aster yellows is key, as treatment options are very limited. Focus on the following practices:
Exclusion
- Inspect all new plant materials thoroughly for signs of infection before bringing them onto your property
- Quarantine incoming plants for 2-4 weeks and monitor for symptom development
- Purchase plant materials only from reputable, disease-free sources
Managing Insect Vectors
- Monitor for the presence of leafhoppers, planthoppers, spittlebugs, and other sap-feeding insects
- Remove weeds that can harbor the insects
- Use reflective mulches early in crop establishment to deter insects
- Apply registered insecticides targeted at the sap-feeding insects to protect crops
- Use row covers to form a physical barrier against insect transmission
Crop Hygiene
- Scout fields frequently and rogue out any infected plants immediately
- Disinfect tools and equipment between uses in different areas of the field
- Allow adequate time between rotations with susceptible species
Creating Unfavorable Conditions
- Avoid excess nitrogen fertilization that produces soft, succulent growth attractive to insects
- Time plantings so vulnerable new growth stages don’t coincide with peak vector activity
- Remove plant debris and weed hosts that provide overwintering sites
- Irrigate in the morning to allow foliage to dry out during the day
Using Resistant Varieties
- Choose resistant or tolerant varieties when available – check with your local extension office for recommendations
- For valuable plants, consider preventive applications of systemic acquired resistance (SAR) inducers
Testing
- Submit samples to a diagnostic lab for PCR testing to monitor for pathogens
- Increase sampling when vectors are active or disease is present in the area
How to Maintain Plant Health and Vigor
To help minimize impacts of aster yellows, focus on cultural practices that promote vigorous, healthy plant growth:
Provide Adequate Sunlight
- Most vegetables and ornamentals prone to aster yellows need full sun – minimum 6-8 hours direct sunlight daily
- Proper sunlight encourages robust growth better able to withstand infections
Use Drip Irrigation
- Drip irrigation minimizes leaf wetness, reducing risk of foliar diseases
- Maintains even soil moisture levels without oversaturation
- Can be used to deliver soluble fertilizers directly to roots
Follow Recommended Plant Spacing
- Avoid overcrowding plants to minimize humidity levels and air circulation issues
- Proper spacing enables good light penetration for all plants
Monitor Soil Nutrients and pH
- Take periodic soil tests to determine fertilizer needs and maintain optimal pH
- Avoid over-fertilization with nitrogen that causes excessive leafy growth
- Address nutrient deficiencies which cause added plant stress
Control Weeds
- Reduce competition for water, light, and nutrients by keeping area weed-free
- Eliminate alternate weed hosts that can harbor insect vectors
Allow for Good Air Movement
- Space rows to allow airflow through foliage
- Avoid planting locations with poor air circulation
- Stake or trellis plants to open up canopy
Scout Regularly
- Monitor plants frequently for symptoms or pest buildup
- Early detection allows for prompt treatment or removal
With a combination of preventive insect management, cultural practices to encourage plant vigor, and early disease detection, the impacts of aster yellows can be minimized. However, it is difficult to achieve complete control once this destructive phytoplasma disease becomes established in an area. Diligent monitoring and preventive measures offer the best results.
Frequently Asked Questions About Aster Yellows
Can aster yellows spread to all plants in a garden?
Unfortunately yes, aster yellows can spread to infect many different ornamentals, vegetables, and weeds once it is transmitted by insect vectors in a garden. Susceptible plants growing near an infected plant are at high risk.
Are some types of asters resistant?
Yes, certain aster species and cultivars have better tolerance or resistance to aster yellows. Examples include Aster ericoides, A. lateriflorus, A. novae-angliae, and A. novi-belgii. Ask your local nursery for resistant variety recommendations.
Can aster yellows affect vegetable crops?
Yes, many vegetables are susceptible to aster yellows, especially leafy greens and root crops. Lettuce, carrots, celery, potatoes, and fennel are commonly impacted vegetable hosts.
Can insecticides cure aster yellows?
No, insecticides only prevent transmission of the disease, they cannot cure plants already infected. Once in the plant’s phloem, the phytoplasma cannot be killed with insecticide applications.
How is aster yellows spread from plant to plant?
Aster yellows spreads from plant to plant mainly through phloem-feeding insects in the order Hemiptera. Leafhoppers, planthoppers, and cicadas can acquire the pathogen while feeding on an infected plant and transmit it to a healthy plant.
Can I compost plant material infected with aster yellows?
It’s best not to compost any plant matter from aster yellows infected plants. The phytoplasma may be able to overwinter in compost. Deep burial under at least 2 feet of soil is recommended.
Conclusion
Aster yellows is a challenging disease to manage once established in an area. The systemic phytoplasma infects over 300 plant species and causes significant damage and losses, especially in leafy and root vegetables. By identifying symptoms early, monitoring for insect vectors, and implementing preventive measures, the spread and impacts of aster yellows can be reduced. Careful inspection of new plant materials, weed control, vector management, and other good cultural practices are critical. If aster yellows becomes problematic, work with local agricultural experts to adopt an integrated disease management plan. With diligent monitoring and prevention efforts, gardeners and growers can help protect their plants from this destructive pathogen.