If you’ve been gardening for any amount of time you’ve had to face pests and diseases that threaten your plants. Squash bugs, hornworms, blossom end rot, powdery mildew all have prevention and treatment options that will allow you to keep your plants. However, Aster Yellows only has one treatment solution; removing the entire plant and starting over. Let’s take a look at this plant disease, its causes, symptoms, and how you should deal with it if your plants are affected.
What is Aster Yellows?
Aster Yellows is a plant disease that is spread by leafhoppers, specifically the aster leafhopper (Macrosteles quadrilineatus). It gets its name because it most commonly affects plants in the aster (Asteraceae) family.
Plants Commonly Affected
- Ornamentals - asters, coneflowers (Echinacea), marigolds, petunias, chrysanthemums, zinnias
- Vegetables - carrots, lettuce, celery, potatoes, onions
- Common weeds that serve as reservoirs for the phytoplasma - dandelion, plantain, fleabane
What Causes Aster Yellows
Aster Yellows is caused by a microscopic phytoplasma that replicates within the leafhopper body after they eat infected weeds. It takes several weeks for the phytoplasma to replicate enough to move into the insect’s salivary glands before the leafhopper becomes infectious. The insect will remain infectious for its entire life and can spread the phytoplasma to other plants as it feeds. Fortunately, the disease is not spread plant-to-plant directly, nor through seed.
Recognizing the Symptoms of Aster Yellows
This disease has multiple hallmark signs of infection on your plants. You should watch for yellowing or reddening of the foliage, stunted or bush growth (called “witches’ broom”), distorted or discolored flowers (green or deformed petals), abnormal shoot proliferation, or the flower parts turning leaf-like (called “phyllody”). Keep in mind that some of these symptoms can resemble nutrient deficiencies or herbicide damage.
Another look-alike that can occur with coneflowers is an Eriophyid mite infestation. These microscopic mites can cause green or reddish tufts on the cone of the flower. With a mite infestation, the symptoms are isolated to the seedhead and bloom, while Aster Yellows tends to be more systemic, affecting leaves and flowers often causing the witches’ broom clustering that mites do not. Making a correct diagnosis is important because Aster Yellows has no cure and requires destroying the plant, where a mite infestation can be treated by removing affected flowers and applying a horticultural oil or miticide the following spring before buds open. Look closely (or get a second opinion) before pulling an otherwise healthy plant.
How Aster Yellows Spreads in the Garden
We already mentioned how the aster leafhopper is the cause of the spread of Aster Yellows. They typically overwinter as eggs or nymphs in warmer southern regions, like the Gulf Coast, and get carried northward in the spring on storm fronts and the prevailing winds, sometimes travelling hundreds of miles. Once they settle, they feed and reproduce through multiple generations over the growing season. They typically feed on weeds like dandelion, plantain, and wild lettuce which act as reservoirs for the phytoplasma. Warmer, drier conditions tend to lead to larger leafhopper populations.
Managing and Preventing Aster Yellows
There are some strategies you can use to try to prevent Aster Yellows. One tactic is to control the weeds that host the pathogen. Survival gardeners know that dandelions and plantain have many beneficial uses, but it is wise to keep them away from your susceptible vegetables.
Monitoring and reducing leafhopper populations is the best defense. You can use row covers to prevent insect access to your plants. Attract friendly predators like lady beetles, spiders, insect-eating birds and parasitic wasps with nectar-rich flowers. Organic insecticidal soap and neem oil sprayed directly on the plants can also help reduce leafhopper populations.
If your plants are already infected, unfortunately, there is no cure or treatment. You’ll have to do one of the most painful things for a gardener; pull up the entire plant, including the root system, and destroy it (do not compost). Fortunately, the pathogen cannot survive outside of the plant, so it will not contaminate your soil. In addition, Aster Yellows cannot tolerate extended periods of hot weather. A period of 10-12 days of temperatures over 88°F is enough to destroy the phytoplasma in both plants and leafhoppers. This won’t cure your infected plants, but it can help prevent further spread of the infection.
Always practice good gardening hygiene by washing your hands and cleaning your tools after dealing with infected plants.
Aster Yellows can seem scary. There’s no cure, and watching a favorite coneflower or carrot bed afflicted by something you can’t treat is never fun. But, armed with a little knowledge, it is far from a losing battle. Learn to recognize the early signs, stay on top of weeds and garden debris where leafhoppers like to hide, and don’t hesitate to remove and destroy infected plants once you are sure of a diagnosis. Gardens are always changing, so staying on top of things now really pays off for your future flowers and harvests.
Image Credits: Aster Yellows on Marigold Whitney Cranshaw, CC BY 3.0 US , via Wikimedia, Commons Phyllody on Coneflower Estreya, CC BY-SA 3.0 , via Wikimedia Commons, Gall Mites on Purple Coneflower CriticalDrinking, CC BY 4.0 , via Wikimedia Commons, Aster Leafhopper xpda, CC BY-SA 4.0 , via Wikimedia Commons
