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Cause of stunted and yellow alfalfa

Marvin Hall Published on 24 May 2011

I reported recently on stunted and yellow alfalfa plants that were being reported in Berks, Dauphin and Lancaster counties (see pictures).

Samples were sent to Dr. Deborah Samac, USDA-ARS alfalfa pathologist, in St Paul, Minnesota, and I received back a report that the culprit is "Candidatus Phytoplasma asteris" or more commonly known as “Witches-broom phytoplasma.”

Stunted yellow alfalfa plant

Phytoplasmas are bacteria that do not have a cell wall and live in the phloem of plants. They cannot be identified by usual disease identification techniques but can only be detected by electron microscopy or by extracting the plant’s DNA and exposing it to a sophisticated technique called “polymerase chain reaction.”

Leaf crinkling on problem alfalfa plants
Witches-broom phytoplasma are more common in clover but are transmitted from plant-to-plant by potato leafhoppers. The phytoplasma that causes witches’-broom in alfalfa in closely related to the phytoplasma that causes “aster yellows” in many other crops.

Even though most of the stunted alfalfa in Pennsylvania grew out of the symptoms, the plants are still infected and will remain infected for their entire life because the disease is systemic.

Under stressful environmental conditions the symptoms will likely start showing up again. As far as I can tell after searching the literature, there are currently no alfalfa cultivars resistant to this disease.  FG

—Excerpts from Penn State Field Crop Newsletter, Vol. 11, No. 10

Marvin Hall
Professor of Forage Management
Penn State University


PHOTOS
TOP: Stunted yellow alfalfa plant. Photo by Tim Fritz.
BOTTOM: Leaf crinkling on problem alfalfa plants. Photo by Jeff Graybill.

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