Putting Down Roots
Gardening Columns Putting Down Roots Book Cliff Johnson Marketing Non-Gardening Stories
 
 
  HOME > GARDENING COLUMNS > 2000 > CONEFLOWERS, MARIGOLDS MOST SUSCEPTIBLE TO ASTER YELLOWS

  CONEFLOWERS, MARIGOLDS MOST SUSCEPTIBLE TO ASTER YELLOWS

Beginning in late July, I began noticing stunted growth and yellowing on some of my plants, particularly marigolds and purple coneflowers. Since then, and after reading daily internet reports from other gardeners, I've learned the cause -- aster yellows -- a plant disease that is extremely prevalent this year.

Aster yellows can affect flowers, vegetables, and weeds. Susceptible flowers include aster, chrysanthemum, cockscomb, coreopsis, cosmos, daisy, dianthus, echinacea (coneflower), gladiolus, marigold, petunia, and phlox. Susceptible vegetables include carrots, onions, potatoes, and tomatoes. Weeds such as dandelions, plantain and thistle are also susceptible and can serve as a source of inoculum in home gardens.

The cause of aster yellows is a microscopic organism called a phytoplasma (formerly called a mycoplasma-like organism, or MLO). The disease is transmitted by the six-spotted aster leafhopper. Although its eggs can survive Minnesota winters, most adults migrate or are blown here in the spring from the south. Adults are pale green, up to 1/8 inch long, and feed on the underside of leaves. Touching a plant will cause them to hop or fly away quickly.

The phytoplasma survives winter in perennial and biennial plants. In the spring, leafhoppers feed on infected plants. During feeding, plant sap containing the phytoplasma is sucked into the leafhopper's body where multiplication of the microorganism takes place. Following an incubation period, the phytoplasma is transferred to healthy plants during leafhopper feeding.

Symptoms, which can vary from plant to plant, are more severe and appear more quickly during warm weather. At lower temperatures, plants may be infected without symptom expression. Symptoms usually include stunting or dwarfing, yellowing of the foliage, and production of many spindly stems and/or flower stalks. Flowers often fail to develop color, remain green and distorted, and seeds or fruit never develop.

Carrots and marigolds can be severely affected by aster yellows. Leaves of infected carrots grow in tight bunches. The inner leaves are yellow and stunted, while outer leaves turn rusty red to reddish purple. The roots are bitter, stunted, and deformed with tiny, hair-like roots developing all over the main root. Infected marigolds have small, yellow leaves. Infected plants may also produce a proliferation of spindly, secondary shoots, giving the plant a bushy appearance. Flowers that are produced are often irregular, deformed, and green. The whole plant is generally stunted and spindly.

Once a plant is infected with aster yellows, there is no cure. To prevent further spread, all infected plants should be removed from the garden and destroyed. There are no chemical treatments for the disease, and insecticides are not recommended for the control of leafhoppers in the home garden.

Just how widespread has aster yellows been this season?

"I've had to rip out half of my echinacea and rudbeckia because of aster yellows," writes Terry Yockey of Goodhue County.

"It's rampant in my gardens in southeast Minnesota," writes Gail Griffin of Winona.

From Sharon Smith, a Cass/CrowWing Master Gardener: "In previous years, my rudbeckia was the only plant infected. This year the coneflowers and the zinnias are being attacked."

"For me, it started in marigolds 6 or 7 years ago and spread to the coneflowers," writes MJ Smetanka of Hennepin County. "In an effort to eradicate it, I stopped planting marigolds but the aster yellows continued. I ripped out almost all my coneflowers (I hated to do it -- it is one of my favorite perennials) and started new plants from seed. They were promptly infected. This year put marigolds back in the garden. About half my coneflowers have it again! This weekend I found it in my Sir John Falstaff phlox. The flower that is supposed to be bright pink looked like a twisted, overripe hydrangea flower. Ugh. I could tolerate the coneflowers and marigolds, but this is really alarming. For me, this is the biggest garden problem I have. I wish we could get rid of the little buggers that spread the stuff but apparently it is useless to try."

Why has the problem become so prevalent, particularly on the beloved coneflower? "I suspect because coneflowers have become so hugely popular, and it seems to be extremely vulnerable to this disease," speculates Deb Brown, University of Minnesota extension horticulturist.

"It is not practical to try to keep leafhoppers away from garden plants, so aster yellows is pretty much a fact of life. For anyone who does not want to take a chance with them, the obvious answer is not to plant them."

Marge Hols of Ramsey County reports that members of the Minnesota Nursery and Landscape Association recognize the problem and complain that they're having trouble getting disease-free plants from wholesale distributors.

"Now that it's become a more serious financial issue for nurseries, I bet researchers will focus more on dealing with leafhoppers or breeding resistant varieties," concludes Smetanka.
 
  GARDENING ARCHIVE
 
1995 COLUMNS
1996 COLUMNS
1997 COLUMNS
1998 COLUMNS
1999 COLUMNS
2000 COLUMNS
2001 COLUMNS
2002 COLUMNS
2003 COLUMNS
2004 COLUMNS
 
 
PUTTING DOWN ROOTS:
A Delightful Blend of
Gardening Wisdom, Wit
and Whimsy
$10 + $2 for shipping
by Cliff Johnson

 
 
© Cliff Johnson 2004      |      Cliff@puttingdownroots.net