Phytoplasma
Aster yellows is a devastating disease that can affect more than 300 plant species. And that only includes broad-leaved plants! Aster yellows also affects grain crops like wheat and barley.
Without effective management, the disease can rapidly spiral out of control.
Aster yellows is found throughout North America, Europe, and much of the world’s temperate zones.
Unfortunately, it’s not a treatable condition.
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The best thing to do with an infected plant is to purge it from your garden, so the disease will not spread.
Read on for ways to prevent your plants from becoming infected with this pernicious organism, and manage it if it does occur.
What You’ll Learn
What Is Aster Yellows Disease?
Aster yellows is caused by microscopic organisms called phytoplasmas that are very similar to bacteria.
Your plants would not be threatened by this disease, were it not for the aster leafhopper (Macrosteles quadrilineatus). As its name suggests, this is a lively insect that hops from plant to plant, spreading the disease.
The pathogen responsible is found in the sap of infected plants. When the insect feeds, it sucks up this phytoplasma, transmitting it to nearby plants. The leafhopper can cause infection in just 1-3 weeks and transmits aster yellows every time it feeds for the rest of its life.
Once in the plant, the phytoplasma spreads throughout the entire plant from the root to the flowers. It will take a while for you to realize that your plants are infected, since they typically don’t start showing symptoms for 10-40 days after infection.
While the organism that causes aster yellows does not kill its hosts (unless they are young), it disfigures them so badly that you will no longer want the plants in your garden. If you grow susceptible vegetables, your crop will be damaged.
Symptoms of Aster Yellows
This disease gets its name from asters, since they are one type of plant on which aster yellows shows strong distinctive symptoms. The daisy-like flowers can be transformed into grotesque structures.
For example, aster, daisy, coneflower, and chrysanthemum flowers can have tufts of deformed leaves in them or in place of them. Another common symptom is leaves that turn yellow, while their veins remain green.
Other symptoms include stunting, abnormally bushy growth, new leaves that turn yellow, and twisting or curling of the foliage. In addition, flowers may not produce seed.
Symptoms May Vary
To make things even more difficult, this pathogen produces different symptoms on various types of plants.
Carrot roots can be hairy and bitter, while the inner leaves of lettuce can be twisted, and any of the leaves may show tan or pink spots. The disease is often called “purple top” in potatoes, because the foliage frequently turns purple.
The symptoms are more noticeable in hot weather, and plants can be infected in cool weather without showing any symptoms. This is particularly unfortunate, since the disease can spread widely in cool and wet summers.
Other factors that can cause variations in symptoms include the age and/or size of the plant when it was infected. Plants that are infected when they are small tend to be stunted and have different leaves that can have rosettes or are narrower than healthy leaves.
Factors That Can Cause Similar Symptoms
Not every plant that looks deformed is infected with aster yellows.
Damage from herbicides can also present these symptoms. This is particularly true when plants have been treated with Weed ‘n’ Feed, a combination fertilizer and herbicide commonly used on lawns to tackle broadleaf weeds.
Witches’-broom, a deformity caused by parasitic plants, can produce similar symptoms.
And coneflowers can be attacked by eriophyid mites that cause tufts of small distorted flower parts that sprout from the cone.
Aster Leafhoppers Love to Travel
The leafhoppers that transmit this disease cannot survive cold winters, like what gardeners experience in Minnesota (although their eggs can survive in the upper Midwest). However, brutally cold winters do not lead to their demise.
Within North America, the insects travel to the Gulf of Mexico, where populations build to high levels in grain fields in Missouri and Arkansas.
After overwintering there, they will fly into the upper atmosphere, where storm systems can blow them hundreds of miles north to plague northern gardeners.
In some years, infected leafhoppers arrive early in spring, even before the local insect population has had a chance to become infected. However, given the vagaries of the weather, in other years they are not a problem.
Hot Weather Can Inhibit the Disease
Fortunately for gardeners in hot climates, hot temperatures inactivate the pathogen in both the leafhopper and infected plants. Temperatures of 88°F for 10-12 days are required to limit the growth of the phytoplasma.
How to Manage Aster Yellows
There is no treatment for this disease. The first and most critical thing to do is purge any infected plants from your garden, and destroy all plant matter. This will prevent them from being a source of phytoplasma for the leafhoppers to spread.
Control Weeds
The phytoplasma can survive northern winters in the roots and crowns of infected perennials, including weeds.
Many weeds can harbor this pathogen, so it’s important to remove them from your garden, lawn, and surrounding areas.
You should focus on dandelions and plantain, since they are such common hosts.
Consider Less Susceptible Types of Plants
If aster yellows is a persistent problem in your garden, you can plant alternate types of flowering plants.
Plants that are not susceptible to this disease include:
- Cockscomb
- Storkbill geranium
- Impatiens
- Flowering tobacco (Nicotiana)
- Salvia
- Verbena
Control Insects
While purging the leafhoppers from your yard may seem like an excellent solution, this can be difficult to do – unless you are growing susceptible plants on a large scale.
If you know you are likely to have a problem, you can plant crops like lettuce under floating row covers to keep the insects away.
Another option used by some farmers is to put strips of aluminum foil between the rows. This serves to confuse the leafhoppers, who are then less likely to land on your crops.
Aster Leafhoppers Leave Misshapen Plants in Their Wake
Unless you live in a very hot area, your plants in the aster and composite family are vulnerable to the insidious disease aster yellows.
However, the disease will not spread unless leafhoppers carrying the pathogen are on the loose in your garden. If you know they are a problem, you can take measures to prevent them.
If you have grotesque misshapen flowers, you will need to quickly remove these plants from your garden to prevent the disease from spreading.
Have you encountered aster yellows in your garden? Were you able to keep it from spreading? Let us know how you fared in the comments.
And read on for more common diseases and pests of garden plants including:
