Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
6/24/02
Contacts:
Christine Baker, Plant Pathology, (515) 294-6570, cjbaker@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning June 28, 2002
Aster Yellows: A Bizarre Garden Foe
By
Christine Baker
Graduate Student
Plant Pathology
Iowa State University
Pale green or yellow twisted leaves; stunted, stiff, upright growth; beautiful blossoms replaced by bunches of deformed, sickly leaves - what could be causing these bizarre symptoms on your favorite aster? As spring turns into summer and gardens come to life, a strange plant disease called aster yellows rears its ugly little head in countless gardens, decimating the ornamental value of flowering plants and damaging vegetable crops.
Aster yellows is a devastating disease of a variety of flowers and vegetables. Despite its name, this disease attacks not only asters but also more than 300 species of plants, including garden favorites such as coneflower, daisy, marigold, zinnia, snapdragon, chrysanthemum, tomato, carrot and lettuce.
Cause
Aster yellows is an unusual plant disease caused by a tiny bacteria-like
organism without a cell wall, called a phytoplasma. This organism is impossible
to grow in the laboratory, and it is therefore very difficult to study.
In fact, scientists did not even discover it until 30 years ago.
Inside a favorable plant host, the aster yellows phytoplasma lives in the phloem, the sap-containing tissue, where it feeds and multiplies. To get from plant to plant, the phytoplasma hitches a ride on insects, especially aster leafhoppers, which feed on the phloem of a variety of plants.
A leafhopper swallows phytoplasma cells as it drinks the sap of an infected plant. The phytoplasma then multiplies within the insect and infects its salivary glands, a process that can take one to three weeks. If infected leafhoppers feed on healthy plants, the phytoplasmas in their saliva are injected into the plant phloem and disease symptoms develop in 10 to 40 days. A leafhopper carrying the aster yellows phytoplasma can continue infecting plants for the rest of its life. Hot, dry weather is unfavorable for both leafhoppers and phytoplasma, so aster yellows outbreaks tend to be worse in cool, wet summers.
Symptoms
A major symptom of aster yellows is chlorosis, or yellowing, of the leaves.
The leaf veins often appear clear, which distinguishes aster yellows from
yellowing caused by nutrient deficiencies, in which the veins remain green
as the leaf blade yellows. Leaf yellowing usually is accompanied by slowed
growth of the plant, and new leaves tend to be narrower and smaller than
normal, and sometimes curled. Infected plants often have a stiff, upright
appearance, with branches joining the stem at narrow angles. Infected
carrots roots may be excessively hairy and bitter, and in lettuce, inner
leaves may be curled and twisted, whereas outer leaves do not develop
fully and have pink or tan spots. On onion, leaves are small, twisted
and yellow.
The most bizarre but unmistakable symptom of aster yellows is the development of tufts of pale, deformed leaves, often inside or in place of flowers. Flowers that do develop may be severely deformed and may not yield fruits or seeds.
Like most plant diseases, the symptoms of aster yellows can vary greatly depending on the plant affected, the strain of aster yellows organism and environmental conditions, so plants with this disease may not show all these symptoms.
Prevention
Unfortunately, there are currently few effective control strategies for
aster yellows, though some management practices may help prevent its spread.
Once a plant is infected with aster yellows, it is impossible to cure
that particular plant, but prompt removal of infected plants may help
reduce spread of the disease.
Very susceptible plants should not be planted next to already infected plants. Because aster yellows is spread by insects, using an insecticide that is effective against leafhoppers may help commercial growers, but this approach is not usually feasible or necessary for home gardeners. Some growers have had success with mesh screens to exclude insects from small, fast growing-plants such as lettuce. Placing strips of aluminum foil between rows of plants also may help because the reflection of sunlight disorients the leafhoppers.
As more is learned about the organism that causes aster yellows, our management strategies will improve.
-30-
ml: isugarden
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.