| Aster yellows is a viral-like disease caused by a
phytoplasma (formerly called a mycoplasma-like
organism). Insects that suck the sap of plants,
especially the aster leafhopper, vector the disease.
Aster leafhoppers are insects that annually migrate
northward from their winter home in areas along the
Gulf of Mexico. Aster yellows is a disease that
affects over 300 species of plants, including
ornamentals such as aster, coneflower, zinnia,
marigold, chrysanthemum, petunia, and snapdragon.
Edibles affected include lettuce, carrot, tomato, and
celery. Grasses and grains also are hosts. Weeds that
may harbor the disease include plantain, dandelion,
and other broad-leafed weeds.
Aster yellows is primarily transmitted by leafhoppers.
When a leafhopper feeds on a plant infected with
aster yellows it becomes "infected" with the
phytoplasma and remains infected throughout its
life. The phytoplasma cells multiply and cause
infection of the insect’s salivary glands within one to
three weeks. When the infected insects feed on
healthy plants, they inject the phytoplasma cells into
the plant phloem. Susceptible plants will be
symptomatic in 10 to 40 days.
The spread of aster yellows is worse in cool, wet
summers. Hot dry weather is not favorable for either
the phytoplasma or the leafhopper. As with many
disease and pest problems, diagnosis is perhaps the
most important factor in controlling aster yellows.
Symptoms and Diagnosis
Chlorosis, yellowing of the leaves while the veins
remain green, is a major symptom of aster yellows.
Growth slows down and leaves may be smaller and
more narrow than usual. Foliage is sometimes
curled. Flowers may be deformed and exhibit
bizarre tufts of deformed leaves inside the flower or
in place of the flower. Flowers may not produce
seeds. The symptoms of the disease will often differ
depending upon what species is infected. For
instance, carrot roots may be bitter and hairy while
lettuce may show pink or tan spots and have twisted
inner leaves.
Integrated Pest Management Strategies
1. Remove diseased plants. Once a plant is infected
with aster yellows, it is a lost cause since the
disease is incurable. Early diagnosis and prompt
removal of infected plants may help reduce the
spread of the disease. Although the disease itself
is not fatal to the plant, its presence makes it
impossible for a plant to fulfill its intended role in
the garden.
2. Plant less susceptible plant species. Controlling
aster yellows is difficult. As long as infected
leafhoppers are around, they can infect plants. A
practical way to avoid having problems with this
disease is to grow plants that are not as
susceptible to aster yellows. Verbena, salvia,
nicotiana, geranium, cockscomb, and impatiens
are among the least susceptible plants.
3. Control insects. Vegetable growers may protect
susceptible crops by using the mesh fabrics that
keep leafhoppers and other insects away from the
plants. Some growers put strips of aluminum foil
between rows because bright reflections of
sunlight confuse the leafhoppers.
4. Control weeds. Remove weeds in your lawn,
garden, and surrounding areas, including plantain
and dandelion that may harbor the disease. |
