How to Identify, Control, and Prevent Aster Yellows

Many things can attack your garden plants and flowers, so it’s important to keep an eye out for problems. One of those potential problems is aster yellows, an infection found in the plant sap of a host plant. The tiny organism, called phytoplasma, is spread by infected leafhoppers.

Aster yellows causes deformed leaves and abnormally bushy growth, among other symptoms. It is best to prevent aster yellows rather than try to treat it once you notice a plant is infected. Early diagnosis of any common symptom is key to beating this disease. 

We’ll go over what the disease is, its symptoms, its causes, and how to control and prevent this potentially devastating plant disease.

What Is Aster Yellows?

Aster yellows is a disease caused by a bacteria-like organism called phytoplasma. The phytoplasma has the potential to affect over 300 species of plants. The aster leafhopper is the most common source of transmitting this tiny organism from plant to plant. 

Infected plants display wilted and yellow foliage, stunted growth, and small malformed flowers. It affects all parts of the plant from the taproots (i.e. carrots, potatoes), to foliage, and even flowers.

You may hear other names for aster yellows. In potatoes, it is called purple top because it causes the leaves and stems of the potatoes to develop a purplish hue.  

Life Cycle

Phytoplasma are single-cell organisms similar to bacteria. They live in the phloem of the plants, the vascular tissue that moves nutrients through the plant. 

A plant hosting phytoplasma will house these organisms for the duration of its life. Aster leafhoppers may come along and begin feeding on the infected plant. The aster leafhoppers are now infected and will transfer the disease through their salivary glands as the leafhopper feeds on healthy plants.  

Once the aster leafhopper has introduced the phytoplasma into the vascular system, it takes anywhere from 10 to 40 days to spread throughout the plant. The aster leafhopper carries the phytoplasma for the rest of its life, which can be as long as 90 days.

Symptoms

One of the first symptoms is leaf veins losing their color and turning yellow. If the leaf veins remain green, this may indicate a nutrient deficiency instead. Any new inner leaves will turn yellow as the plant exhibits stunted and abnormally bushy growth.

Flowers are deformed, small, and the petals lack color (or remain green). The flowers will not produce seed. Deformed leaves can develop pink or tan spots as the disease cycle progresses.

Aster yellows symptoms can be slightly different depending on the species of the plant. For example, when the coneflower is infected, you will see small tufts of green leaves where the flowers should be growing. With potatoes, the tops will turn purple. Infected carrots will be undersized and taste bitter. The carrot roots will also form tufts of white hairs.

In hot weather, symptoms of aster yellows show up more quickly, with even more detrimental results to the infected plants. Infected perennials host the aster yellows phytoplasma through the winter, and they have the potential to infect other plants during the growth cycle. Many times, the symptoms of aster yellows are mistaken for herbicide damage. 

What Plants Does Aster Yellows Affect?

There are over 300 species of susceptible plants targeted by the aster yellows pathogen. Many of those commonly affected are from the Aster family, which is where the disease name originated from. Annual flowering plants, vegetables, grasses (such as wheat and oats), and perennial weeds and flowers can become infected with aster yellows. 

Common ornamental flowering plants affected by aster yellows are purple coneflower, black-eyed Susan, aster, petunia, chrysanthemum, snapdragon, and marigold. Edible vegetables prone to aster yellows are potato, carrot, cabbage, tomato, celery, and lettuce. Most broadleaf weeds, such as dandelion, plantain, ragweed, wild carrot, and thistle, are also vulnerable.

Unfortunately, this is not an all-inclusive list because there are many other plants that could become infected. 

Control

The only sure way to control aster yellows is to control insects and reduce the aster leafhopper population in your garden. The leafhoppers arrive with hot, dry weather. They prefer to live out their days in areas with hot climates.

Herbicides and pesticides are not an effective way to control aster yellows. Once you know the symptoms of aster yellows, you can remove infected plants and minimize the spread to neighboring plants. 

When you notice any plants infected with aster yellows, immediately remove them from your garden and destroy them. Do not compost infected material. When the plant dies, the pathogen will die with it, but you don’t want the leafhoppers or other insects to feed on the plant.

Aster yellows does not spread via the soil or through the air. An insect vector is required to transfer the disease from one plant species to another. 

Prevention

The best method to prevent this disease is to control the insect vector within your garden. Aster leafhoppers are the most common vector in spreading aster yellows. To protect your garden plants, it is important to reduce the leafhopper populations. 

You can protect plants from aster leafhoppers by placing floating row covers over the entire plant to keep the insects away. Most aster leafhoppers become disoriented when faced with light-colored or reflective mulches, so this method may reduce the chance they will feed on your plants.

Another effective method is to control weeds in your garden area. Since many broadleaf weeds can become infected with this plant disease, it will help to remove these host plants before the aster leafhoppers show up.

Clean garden tools between each use (and especially before you use them on another plant). Plants infected may not be showing symptoms when you are pruning. Using bactericidal cleaning methods will reduce the chance of spreading it to healthy plants. 

By implementing some of these methods, you can reduce the chances of this disease coming into your garden in the first place. You may find one method works better than others depending on your location and what varieties of plants you are growing.

Frequently Asked Questions