"Aster Yellows" in Dahlias and Zinnias (planted together) #488367 - Ask Extension

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"Aster Yellows" in Dahlias and Zinnias (planted together) #488367

Asked October 03, 2018, 3:41 PM EDT

Through Google searches, I have determined that my dahlias and Zinnias have been subjected to Aster Yellows. In July I saw that I had tiny gnat type flies. I didn't try to amend that situation. Most of my Dahlias and many of my Zinnias have developed witches broom and other strange growth.
#! Are the Dahlia tubers compromised? Should I discard tubers of the plants that have the AY, or ALL of the tubers in the bed?
#2 Should I dispose of All of the Zinnia seeds of the now fading flowers?
#3 What should I do, if anything, to the soil in the bed once I have removed all the plants?
Thank You.

Larimer County Colorado

Expert Response

Hello,

Aster yellows are caused by a phytoplasma that is spread by leafhoppers. "Tiny gnats" wouldn't have transferred the disease. Leafhoppers are a different insect.

Aster yellows do not survive in the debris of affects plants or in the soil, but it can overwinter in the crown of perennials and weeds. Therefore, you must remove and destroy all affected plant parts. It is fine to keep the seeds of the zinnias; I would throw away the dahlia tubers.

From the U of MN Extension, How to Manage Aster Yellows:
  • Once a plant is infected with aster yellows, there is no way to cure it.
  • Completely remove infected plants from the garden.
  • Compost infected plant material. Aster yellows will not survive once the plant is dead.
  • Remove perennial weeds from the garden. If infected with aster yellows, the bacteria will survive in weeds from one season to the next.
  • Protect plants from aster leafhoppers with light colored or reflective mulches that disorient the insects and can reduce feeding on plants.
  • In the vegetable garden, floating row covers can be used to prevent leafhoppers from feeding on plants.
  • Pesticides are not effective in reducing aster yellows in the home garden.
Alison O'Connor Replied October 04, 2018, 11:41 AM EDT

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