Aster yellows is incurable
Christine Michael
| Special to The News-Messenger
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USA TODAY
Aster yellows is not a new variety of flower; instead, it is a bacteria that affects over 300 species of plants, including ornamentals such as aster, coneflower, zinnia, marigold, chrysanthemum, petunia and snapdragon. In addition to flowers, aster yellows can also affect lettuce, carrots, tomatoes, potatoes and celery.
Earlier this summer, a master gardener from Central Ohio called to tell me about a problem in her flower bed. Some of her coneflowers were deformed and the leaves were yellowing, but the veins were still green.
The flower head deformities included green, stunted petals and sprouting green leaves making them look like a new plant was growing in the flower head.
This was our introduction to aster yellows. We learned that the disease is incurable. The only solution is to remove the entire plant — roots and all. Do not place this on your compost pile. Clean up any leaf debris, bagging the affected plants and place in the garbage bin.
Although aster yellows was first observed in the 1800s, it was not until the early 20th century that a phytoplasma was identified as the cause of the disease.
Officially named after the China aster, a common host plant, the disease is now known as “Candidatus Phytoplasma asteris” and commonly called aster yellows phytoplasma (AYP), which is a type of phytoplasma (a bacteria without a cell wall).
The microscopic organism takes nutrients from the plant’s phloem tissue, which transports food made during photosynthesis in the leaves.
When you notice one plant exhibiting the symptoms, most likely others will soon show the same because leafhoppers feed on the infected plant and carry AYP to healthy plants.
The phytoplasma requires a living host to complete its life cycle and replicate. It overwinters in the leafhopper and in infected plant residue, but it cannot overwinter in soil without infected plant debris. It is critical to remove plant debris.
Aster leafhoppers
Entomologists describe aster leafhoppers (Macrosteles quadrilineatus) as small insects that are about 1/8 inch when fully grown. Olive-green to tan in color, they have three pairs of spots on the top of their head. Their grey or opaque wings permit them to fly, and they are active at temperatures above 59°F. They commonly jump when disturbed.
Once they feed on infected plants, they will carry the AYP pathogen for the rest of their life, usually 60-120 days. They transmit the disease to each new plant they visit.
Leafhopper nymphs detected early in the season can be controlled by treating the undersides of leaves with neem oil, horticultural oil or insecticidal soap. Be sure to follow label instructions.
Pest management strategies
There are some steps anyone can take to reduce the spread of aster yellows in a garden. They include removing diseased plants, planting less-susceptible plants, controlling insects and controlling weeds.
Early diagnosis and prompt removal of infected plants may reduce the spread of the disease, according to specialists. Although the disease itself is not fatal to the plant, its presence is visually disturbing.
A practical way to avoid having problems is to grow plants that are not as susceptible to aster yellows. Those plants include verbena, salvia, nicotiana, geranium, cockscomb and impatiens.Vegetable growers can protect susceptible crops by using mesh fabrics that keep leafhoppers and other insects away from their plants. Some growers have placed strips of aluminum foil between rows of their plants because bright reflections of sunlight tend to confuse the leafhoppers.
Remove broad-leaf weeds such as plantain and dandelion from your lawn, garden and surrounding areas because they may harbor the disease.
While aster yellows is more pervasive in cool, rainy summers, the pathogen cannot tolerate prolonged periods of hot weather. Reportedly temperatures of at least 88°F for 10-12 days inactivate the pathogen in both the leafhopper and the infected plants.
My friend wisely chose to remove the groupings of coneflowers from her garden and replaced them with salvia and cockscomb.
Christine Michael is a Master Gardener for The Ohio State University Extension Offices in Sandusky and Ottawa Counties.
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