The name, aster yellows, is misleading because this disease can affect not only asters but more than 300 different plant species. The namesake for the disease is the aster leafhopper, an insect that spreads the aster yellows pathogen. The bad news is that once a plant is infected, there is no treatment. But there are several things you can do to prevent aster yellows from spreading.
Which Plants Can Be Affected
Members of the aster family (Asteraceae) are most commonly affected by aster yellows. These include aster, calendula, chrysanthemum, coreopsis, cosmos, daisy, gaillardia, marigold, purple coneflower, and zinnia. But other annuals and perennials from more than 40 other plant families can get the disease as well.
Flowers susceptible to the disease are anemone, delphinium, periwinkle, petunia, snapdragon, and veronica. Many garden crops can be infected as well, including broccoli, cabbage, cauliflower, carrot, lettuce, onion, potato, pumpkin, spinach, and tomato, as well as parsley. Strawberries can also get aster yellows.
And while weeds—dandelion, cinquefoil, horseweed, ragweed, thistle, wild carrot, and wild lettuce—getting the disease might not sound problematic, it actually is because aster yellows in weeds spreads it further to desirable plants.
Symptoms of Aster Yellows
Depending on the plant species, the symptoms of aster yellows are different. What is common though is that the entire plant shows symptoms because the pathogen that causes the disease moves through the plant, from the roots to the flowers.
In infected plants you will notice stunted grown and numerous, odd-looking secondary shoots. The leaves are smaller and narrower than on a healthy plant, and they are curled or twisted. The leaves show signs of chlorosis—the leaves are yellow while the veins stay green. The foliage is pale, yellowish, or white, at a later stage also red or purple.
The flowers are small and malformed, often developing funky-looking leafy parts instead of flowers. Purple coneflower may develop a second clustered flowerhead, and marigolds may have leafy flowers. The flowers are off-colored, greenish-dull, and nowhere near their vibrant colors. The plants don’t produce seeds, and if they do, the seeds are sterile.
In infected carrots the tops have a bunch of red or yellow stunted leaves. Instead of a healthy thick root, there is only a thin taproot with lots of small, white hairy roots. They often taste bitter. Similarly, onion tops are twisted and yellow, with leaves growing into thick, stunted bunches.
In lettuce, telltale signs of aster yellows are twisted and curled inner leaves. The head never fully develops. The leaves also have pink or tan spots.
If your garden plants show a yellowing of leaves, it does not necessarily mean aster yellows, it could also be herbicide damage.
How Aster Yellow Spreads
To manage aster yellow, it’s important to understand how the pathogen is spread. Aster yellows is caused by a phytoplasma, a type of bacteria that can live only within the veins of a plant or within the aster leafhopper (Macrosteles quadrilineatus).
When the aster leafhopper feeds on a plant infected with aster yellows, it sucks up some of the aster yellows phytoplasma with the plant sap. Within about two weeks, the phytoplasma in the leafhopper’s body causes an infection of the salivary glands, and whenever the infected leafhopper feeds on other healthy plants, it transmits the phytoplasma to that plant. The phytoplasma remains in the leafhopper during its entire life without causing harm to the insect itself.
The aster leafhopper does not survive in climates with subzero winters. In the spring, aster leafhoppers move north from their overwintering locations along the Gulf of Mexico.
The occurrence of the aster yellows is directly connected to the aster leafhopper population in any given year. Cool wet summers create favorable conditions for the leafhopper, and hence for the spread of the disease, whereas aster yellows is less frequent in hot dry weather.
What to Do When a Plant Is Infected
After a plant has been infected by the aster leafhopper, symptoms will appear within ten to 40 days. There is no cure, treatment, pesticide or insecticide, to control aster yellows.
That’s why early diagnosis is crucial. Promptly remove any infected plants from your garden and dispose of them so no more aster leafhoppers can feed on them and spread the disease further.
The aster yellows pathogen won’t survive on a dead plant so you may compost infected plants.
How to Prevent Aster Yellows From Spreading
The tricky thing with aster yellows is that is does not kill a plant. Therefore, as a gardener you might be inclined to leave an infected plant in your garden, especially if it’s a perennial. Don’t—the pathogen can survive in perennial plants from one season to the next, and infected perennials can transmit aster yellows to other plants for many years. All it takes is an aster leafhopper feeding on that plant.
For that reason, you should remove any perennials—landscape plants as well as weeds—from your garden.
Mechanical barriers are also used to protect plants from aster leafhopper, such as light-colored or reflective mulches or foils to disorient the insects so they won’t feed on the plants. Vegetables may be protected by floating row covers, mesh fabrics, or fine wire mesh.
Planting only healthy cuttings and plants from a reliable source in your garden is the first step to prevent aster yellows.
If you have aster yellows, it’s a good idea to select plants that are less susceptible to the disease, such as cockscomb, geranium, impatiens, nicotiana, salvia, or verbena.