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 do harm to your garden. Today we are going to talk about yellow aster, an infection found in the sap of a host plant; The tiny organism called phytoplasma is spread by infected planthoppers.
Aster yellows can cause deformed leaves and unusually bushy growth on a plant. Infected plants will show other aster yellow symptoms which we will discuss later in this article. It's best to prevent aster yellow rather than try to treat it as soon as you notice a plant is infected. Early diagnosis of a common symptom is key to combating this disease.
We'll cover what the disease is, the symptoms of aster yellow, what causes it, and how to control and prevent this potentially devastating plant disease. There are certain climates where aster yellow is more common and can wreak havoc on a variety of flowers, vegetables, and even crops. However, there are many flowers and plants that cannot be infected with aster yellow because they are immune. Let's take a closer look at aster yellow and start with what causes plants to become infected with aster yellow.
What is aster yellow?
Aster yellowing symptoms on coneflowers. Source: Florador
Aster Yellows is a disease caused by a bacteria-like organism called phytoplasma. The phytoplasm has the potential to affect over 300 plant species. The aster leafhopper is the most common source of plant-to-plant transmission of this tiny organism.
Infected plants show wilted and yellow foliage, stunted growth, and small malformed flowers. As a result, the plant cannot store nutrients, so it affects all parts of the plant from the taproots (e.g. carrots, potatoes) through the entire leaf and other leaves to the flowers and even the ability of the plant to produce seeds.
You may hear other names for aster yellow. For example, aster yellow in potatoes is called purple spike because it causes the leaves and stems of the potatoes to develop a purple hue.
We'll go into the symptoms in more detail shortly.
Life cycle of the Aster Yellows Phytoplasma (AYP)
What exactly is a phytoplasm? To keep it simple, they are unicellular organisms, much like bacteria. They live in plants' phloem, the vascular tissue that transports nutrients throughout the plant.
A plant that hosts phytoplasma will host these organisms for the duration of its life. Thus, the propagation potential increases when there are many vectors. The plant houses the phytoplasm, and aster planthoppers come along and start feeding on the infected plant. The aster cicada is now infected and transmits the disease through its salivary glands when the cicada feeds on healthy plants.
Once the aster plant has introduced the phytoplasm into the vascular system, it takes between 10 and 40 days for it to spread throughout the plant. Also, the aster cicada carries the phytoplasm for the rest of its life, which can last up to 90 days. That is enough time to infect the plants and flowers that are healthy.
Aster Yellows Symptoms on Carrots. Source: Whitney Cranshaw, CSU
One of the first symptoms of aster yellow is leaf veins losing their color and turning yellow (if the leaf veins remain green, this may instead indicate a nutrient deficiency). Any new inner leaves will turn yellow and the plant will exhibit stunted and unusually bushy growth. Flowers are deformed, small, and the petals are colorless (or remain green) and do not produce seeds. Deformed leaves may develop pink or brown spots as the disease cycle progresses.
The symptoms of aster yellow can vary slightly depending on the plant species. For example, when coneflowers are infected, you will see small clumps of green leaves where the flowers should be growing. In potatoes, the tops turn purple and infected carrots become too small and taste bitter. The carrot roots also produce clumps of white hairs.
In hot weather, the symptoms of aster yellow show up more quickly, with even more damaging consequences for the infected plants. Infected perennials harbor the aster-yellow phytoplasm during the winter and have the potential to infect other plants during the growth cycle. Often the symptoms of aster yellow are mistaken for herbicide damage.
What plants does Aster Yellows work on?
There are over 300 species of susceptible plants that are attacked by the Aster Yellow pathogen. Many of those commonly affected are from the daisy family, which is where the disease's name comes from. Annual flowering plants, vegetables, grasses (such as wheat and oats), and perennial weeds and flowers can become infected with aster yellow.
Common ornamental plants affected by aster yellow include coneflower, black-eyed susan, aster, petunia, chrysanthemum, snapdragon, and marigold. Edible vegetables that are susceptible to aster yellow include potatoes, carrots, cabbage, tomatoes, celery and lettuce. Most broadleaf weeds such as dandelion, plantain, ragweed, wild carrot and thistle are also susceptible. Unfortunately, this is not an exhaustive list as there are many other plants that could become infected with aster yellows.
Control Aster Yellows
An aster leafhopper on an aster plant. Source: Foxtail_1
The only surefire way to control aster yellows is to control insects and reduce the aster leafhopper population in your garden. The cicadas arrive in hot, dry weather and prefer to spend their days in hot climate areas. Herbicides and pesticides are not an effective means of controlling aster yellow. Once you know the symptoms of aster yellow, you can identify infected plants and minimize spread to neighboring plants.
If you notice plants infected with aster yellow, remove them from your garden immediately and destroy them (pay special attention to the inner leaves and flowers to aid in identification). Do not compost infected material. When the plant dies, the pathogen dies with it, but you don't want the planthoppers or other insects to feed on the plant or they'll spread it to other nearby plants. Aster yellow does not spread over the ground or through the air. An insect vector is required to transmit the disease from one plant species to another.
Preventing aster yellow
The best way to prevent aster yellow disease is to control the insect vector in your garden. Aster leafhoppers are the most common vector in spreading aster yellow. To protect your garden plants, it is important to reduce cicada populations.
You can protect plants from aster cicadas by placing floating row covers over the entire plant to keep the insects out. Most aster cicadas become disoriented when confronted with light-colored or reflective mulch, so this method can reduce the likelihood of them feeding on your plants.
Another effective method is weed control in your garden area. Since many broadleaf weeds can be infected with this plant disease, removing these host plants before the aster leafhopper shows up helps. Clean the gardening tools between each use (and especially before using them on another plant). Infected plants may not show any symptoms when pruned. Using bactericidal cleaning methods reduces the chance of it spreading to healthy plants.
Growing flowers and plants that are not susceptible to the disease can seem like a tough way to combat aster yellow. But it's worth considering, especially in areas with wet summers. Although aster yellow affects a large number of plants, there are many other families to choose from. Geraniums, sage and impatiens are just a few examples of flowers that are not susceptible.
By implementing some of these methods, it will help reduce the chances of this disease getting into your garden in the first place. Depending on your location and the types of plants you grow, you may find that one method works better for you than others.
frequently asked Questions
Comparison of diseased and healthy coneflowers. Source: Florador
Q: Is aster yellow a seed borne disease?
A: No, aster yellow is transmitted through the phloem of the plant when the aster leafhopper feeds on the infected plant and then transmits the aster yellow disease to another plant. Typically, aster yellow causes the plant to become sterile.
Q: Can Echinacea turn aster yellow?
A: Yes, unfortunately Echinacea, also known as coneflowers, is susceptible to the aster yellow pathogen. These beautiful flowers are particularly susceptible to this disease, and hot weather increases the likelihood that the aster leafhopper will arrive and it will encounter others. Plants from the Asteraceae family are particularly affected, hence the name yellow aster. But it can also infect grasses, crops and some weeds.
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