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Aster Yellows: Threat To More Than Asters

There are many things that can attack your garden plants and flowers, so it’s always a good idea to have a basic understanding of plant diseases so you don’t have to deal with infected plants that have the potential to cause damage to your garden. Today we’re discussing aster yellows, an infection that is found in the plant sap of a host plant; the tiny organism, called phytoplasma, is spread by infected leafhoppers.

Aster yellows can cause deformed leaves and abnormally bushy growth on a plant. Infected plants will exhibit other aster yellow symptoms that we will discuss later in this article. It is best to prevent aster yellow 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, aster yellows symptoms, what causes it, and how to control and prevent this potentially devastating plant disease. There are certain climates where aster yellows is more prevalent and it can wreak havoc on a variety of flowers, vegetables, and even grain crops. However, there are many flowers and plants that can’t be infected with aster yellows because they are immune. Let’s take a closer look at aster yellows and start with what causes plants to become infected with aster yellows. 

What Is Aster Yellows?

Coneflowers with Aster Yellows
Aster yellows symptoms on coneflowers. Source: florador

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 makes it so the plant cannot store nutrients, so it affects all parts of the plant from the taproots (i.e. carrots, potatoes), the entire leaf and other foliage, and up to the flowers and even affects the ability of the plant to produce seeds. 

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

We will go into greater detail about symptoms shortly. 

Life Cycle of The Aster Yellows Phytoplasma (AYP)

So what exactly is a phytoplasma? To keep it basic, they are single-cell organisms similar to bacteria. They live in the phloem of the plants, which is the vascular tissue that moves nutrients through the plant. 

A plant hosting phytoplasma will house these organisms for the duration of its life. Thus, the potential to spread increases if there are many vectors present. The plant hosts the phytoplasma, and aster leafhoppers come along and begin feeding on the infected plant. The aster leafhoppers are now infected and will transfer the disease through its 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. Also, the aster leafhopper carries the phytoplasma for the rest of its life, which can be as long as 90 days. That’s plenty of time to infect the plants and flowers that are healthy. 

Symptoms

Aster yellows in carrot
Aster yellows symptoms in carrot. Source: Whitney Cranshaw, CSU

One of the first aster yellows 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 and the plant exhibits stunted and abnormally bushy growth. Flowers are deformed, small, and the flower petals are lacking in color (or remain green), and 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 and 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 a 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 Effect?

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 with aster yellows. 

Controlling Aster Yellows

Leafhopper on asters
An aster leafhopper on an aster plant. Source: foxtail_1

The only sure way for controlling aster yellows is to control insects and reduce the aster leafhopper population in your garden. The leafhoppers arrive with hot, dry weather and 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 recognize 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 (pay special attention to the inner leaves and flowers to help with identification). 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 or they will spread it to other nearby plants. 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. 

Preventing Aster Yellows

The best method to prevent aster yellows 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. 

Growing flowers and plants that aren’t susceptible to the disease can seem like a harsh method of controlling aster yellows. But it is worth considering, especially in areas that have wet summers. Even though aster yellows affect a large number of plants, there are still many other families to choose from. Geraniums, salvia, and impatiens are just a few examples of flowers that aren’t susceptible. 

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

Frequently Asked Questions

Healthy and diseased coneflowers
Comparison of diseased and healthy coneflowers. Source: florador

Q: Is aster yellows a seed-borne disease?

A: No, aster yellows is transmitted through the phloem of the plant when the aster leafhopper feeds on the infected plant and then carries the aster yellows disease to another plant. Typically, aster yellows will cause the plant to become sterile. 

Q: Can echinacea get aster yellows?

A: Yes, unfortunately, echinacea, also known as coneflower, is susceptible to the aster yellows pathogen. These lovely flowers are especially prone to this disease and hot weather increases the chances of the aster leafhopper arriving and it to others. It primarily affects plants in the Asteraceae family, thus the name aster yellows. But it can also infect grasses, grains, and some weeds.


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