Garden Views: Aster yellows

by Mary Heie
Anoka County Master Gardener

Do your perennials such as coneflowers, daisies, cosmos and marigolds have stunted or weird growth this summer? Are the flowers failing to develop color as they should? Is there a tight rosette or “witches’ broom” where the buds should be developing. How about your vegetables such as cauliflower, pumpkins and squash? If you pull a few carrots, are the roots developing well or are they spindly with lots of hairy secondary roots?  Chances are you have a disease called “aster yellow.”Aster yellow is caused by a microscopic bacterium called a phytoplasma that is difficult to culture because it has no cell wall and it has an interesting and complicated life cycle. An infected plant cannot infect a neighboring plant. The aster leafhopper (Macrosteles fascifrons) is needed to the transfer of the disease. The leafhopper feeds on an infected plant and acquires the bacterium.

After about two weeks, the bacterium has incubated long enough that leafhopper will transmits it to other plants while feeding. The leafhopper has a long life span of about 100 days and can therefore infect many plants.

Often the cycle begins in the southern states where the leafhoppers feed on winter grains. In spring the insects are carried north by the winds bringing them into the upper Midwest. The insects continue their life cycle, feeding, laying eggs, etc.

An interesting fact is that the bacterium will winter over in infected plants, leafhoppers that are carrying it, but not in the soil or in leafhopper eggs. However, once the eggs hatch and the leafhopper feeds on infected plants the cycle begins again.

The incubation period for the bacterium in a plant is dependent on the temperature. The cooler the weather, the longer it takes before the feeding leafhopper can be infected. Once a plant is infected, it is infected for life. Meanwhile, hot spells of 10 to 12 days with temperatures in the high 80s, seems to reduce the leafhopper’s ability to transmit the bacterium. There are at least 150 plant species in 40 plant families including perennials, vegetables, shrubs and weeds that can be infected by aster yellow. Since the spread of the disease is totally dependent on the aster leafhopper, the disease will most often occur in the plants on which the insect prefers to feed. Interestingly, it appears that the leafhopper cannot acquire the bacterium from tomato or potato hosts.

With all this sometimes confusing information, what are the symptoms of the disease? The symptoms can vary with different plants species. However, watch for abnormalities: leaves grow in tight bunches instead of the plant’s normal way; inner leaves are yellow and stunted; flowers are irregular, deformed or stunted; the plant develops bizarre growth, stunted growth or stiff upright growth; yellowing.

In the home gardens and landscapes there is no good treatment for aster yellow. To avoid further infections, the best scenario is to remove and destroy the infected plants. (Do not compost them.) Since the bacterium lives in the plants and not the soil, make certain all of the plant debris is removed. This can be difficult so you may want to plant a different species in those spots where plants were removed. Weeds can be host plants for the disease especially plantain and dandelions. Another good reason to fight nasty weeds! Chemical control of the insects is not a viable option.

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